--- title: "NCERT Solutions Class 10 Science Chapter 2 Acids, Bases and Salts" url: https://www.swavid.com/science/class/10/chapter/acids-bases-and-salts/ncert-solutions dateModified: 2026-10-07T16:43:53+00:00 --- # NCERT Solutions Class 10 Science Chapter 2 Acids, Bases and Salts This chapter's questions cover the chemical and physical properties of acids, bases, and salts, including indicators, neutralization, pH scale, and common chemical compounds derived from salt. Free PDF (34 pages): https://www.swavid.com/api/seo/pdf/ncert/science/class-10/swavid-ncert-solutions-class-10-science-chapter-2-acids-bases-and-salts-f9e04fb4c3.pdf ## Page 1 ### Question 1 *2 marks · Very short answer* If someone in the family is suffering from a problem of acidity after overeating, which of the following would you suggest as a remedy– lemon juice, vinegar or baking soda solution? **Solution** 1. Baking soda solution is suggested as a remedy for acidity after overeating. 2. It is a mild base that neutralises the excess acid produced in the stomach. **Answer:** Baking soda solution > Common mistake: Suggesting lemon juice or vinegar, which are acidic and would worsen the acidity. ### Question n *2 marks · Very short answer* Which property did you think of while choosing the remedy? **Solution** 1. The property considered is the ability of acids and bases to nullify each other's effect. 2. A base neutralises an excess acid to form salt and water, providing relief from acidity. **Answer:** Ability of acids and bases to nullify each other's effect > Common mistake: Stating taste instead of the chemical nullification property. ### Question n *2 marks · Very short answer* Recall how we tested sour and bitter substances without tasting them. **Solution** 1. Sour and bitter substances were tested without tasting by using indicators. 2. Indicators like litmus paper or natural indicators change colour in acidic and basic media respectively. **Answer:** Using indicators such as litmus paper > Common mistake: Omitting the mention of indicators. ## QUESTION ### Question 1 *3 marks · Short answer* You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? **Solution** 1. Take a piece of red litmus paper and dip it into each of the three test tubes. 2. The solution in which the red litmus turns blue is the basic solution, and set this blue litmus paper aside. 3. Using this newly obtained blue litmus paper, test the remaining two solutions; the solution that turns the blue litmus red is the acidic solution, while the one that causes no colour change contains distilled water. **Answer:** The basic solution turns red litmus blue, and the resulting blue litmus turns red in the acidic solution, leaving distilled water unaffected. > Common mistake: Forgetting that distilled water produces no colour change on either red or blue litmus paper. ## Activity 2.1 ### Question n *2 marks · Very short answer* What change in colour did you observe with red litmus, blue litmus, phenolphthalein and methyl orange solutions for each of the solutions taken? **Solution** 1. Red litmus turns blue in basic solutions and remains red in acidic solutions. 2. Blue litmus turns red in acidic solutions and remains blue in basic solutions. 3. Phenolphthalein turns pink in basic solutions and remains colourless in acidic solutions. 4. Methyl orange turns red in acidic solutions and yellow in basic solutions. **Answer:** Acids turn blue litmus red, phenolphthalein colourless, and methyl orange red; bases turn red litmus blue, phenolphthalein pink, and methyl orange yellow. > Common mistake: Confusing the colour changes of methyl orange and phenolphthalein in acidic and basic media. ### Question n *2 marks · Very short answer* Tabulate your observations in Table 2.1. **Solution** 1. Acids turn red litmus blue? No, acids turn blue litmus red and show no colour change with red litmus. 2. Bases turn red litmus blue and phenolphthalein pink, while methyl orange turns red in acids and yellow in bases. **Answer:** Indicators show characteristic colour changes to distinguish between acidic and basic solutions as detailed in Table 2.1. > Common mistake: Confusing the colour changes of litmus and synthetic indicators in acidic and basic media. ## Activity 2.2 ### Question n *2 marks · Very short answer* Take two of these cloth strips and check their odour. **Solution** 1. The cloth strips treated with onion juice retain their characteristic onion smell when checked. **Answer:** The onion-treated cloth strips have a distinct onion odour. > Common mistake: Failing to store the cloth strips properly in the sealed bag. ### Question n *2 marks · Very short answer* Rinse both cloth strips with water and again check their odour. **Solution** 1. After rinsing the cloth strips with water and checking their odour again, the characteristic smell of onion persists. **Answer:** Rinsing with water does not remove the characteristic odour of onion from the cloth strips. > Common mistake: Assuming water washes away the onion odour completely. ### Question n *2 marks · Very short answer* Note your observations. **Solution** 1. Dilute HCl destroys or retains the characteristic onion odour, while dilute NaOH causes the onion smell to disappear. **Answer:** Acid retains the onion odour, whereas base destroys the onion odour. > Common mistake: Confusing the effect of acid and base on the odour. ### Question n *2 marks · Very short answer* Now take some dilute vanilla essence and clove oil and check their odour. **Solution** 1. Dilute vanilla essence has a distinct pleasant smell, and dilute HCl retains this vanilla odour while dilute NaOH destroys it. **Answer:** Vanilla essence retains its odour in acidic medium but loses it in basic medium. > Common mistake: Not noting the loss of odour in basic medium. ### Question n *2 marks · Very short answer* Similarly, test the change in the odour of clove oil with dilute HCl and dilute NaOH solutions and record your observations. **Solution** 1. Clove oil has a characteristic strong smell which is retained in acidic medium like dilute HCl but destroyed in basic medium like dilute NaOH. **Answer:** Clove oil retains its odour with dilute HCl and loses its odour with dilute NaOH. > Common mistake: Mixing up clove oil odour with vanilla essence. ### Question 6 *2 marks · Very short answer* Which of these – vanilla, onion and clove, can be used as olfactory indicators on the basis of your observations? **Solution** 1. Vanilla, onion and clove all show a change in odour in acidic and basic media respectively. **Answer:** Vanilla, onion and clove can all be used as olfactory indicators. > Common mistake: Omitting one of the three substances. ## Activity 2.3 ### Question n *2 marks · Very short answer* What do you observe on the surface of zinc granules? **Solution** 1. When dilute sulphuric acid reacts with zinc granules, bubbles of hydrogen gas are observed on the surface of the zinc granules. **Answer:** Bubbles of hydrogen gas are observed on the surface of the zinc granules. > Common mistake: Writing that a precipitate is formed instead of gas bubbles. ### Question n *2 marks · Very short answer* Pass the gas being evolved through the soap solution. **Solution** 1. When the gas evolved during the reaction is passed through the soap solution, gas-filled bubbles rise into the air. **Answer:** Gas-filled bubbles are formed and rise into the air. > Common mistake: Forgetting that the gas is trapped in soap to form bubbles. ### Question n *2 marks · Very short answer* Why are bubbles formed in the soap solution? **Solution** 1. Bubbles are formed in the soap solution because the hydrogen gas being evolved during the reaction gets trapped inside the soap solution. **Answer:** Hydrogen gas gets trapped inside the soap solution to form bubbles. > Common mistake: Stating air causes the bubbles instead of hydrogen gas. ### Question n *2 marks · Very short answer* Take a burning candle near a gas filled bubble. **Solution** 1. Bringing a burning candle near a hydrogen gas-filled bubble tests for the presence of hydrogen gas. **Answer:** It is used to test for the presence of hydrogen gas. > Common mistake: Confusing the test for carbon dioxide with hydrogen gas. ### Question n *2 marks · Very short answer* What do you observe? **Solution** 1. When a burning candle is brought near the gas-filled bubble, the hydrogen gas burns with a pop sound. **Answer:** The hydrogen gas burns with a pop sound. > Common mistake: Not mentioning the characteristic pop sound. ### Question n *2 marks · Very short answer* Repeat this Activity with some more acids like $\text{HCl}$, $\text{HNO}_3$ and $\text{CH}_3\text{COOH}$. **Solution** 1. When zinc granules react with other acids like $\text{HCl}$, $\text{HNO}_3$, and $\text{CH}_3\text{COOH}$, hydrogen gas is evolved in each case. 2. The metal displaces hydrogen from the acids to form a salt and hydrogen gas. **Answer:** Hydrogen gas is evolved when zinc reacts with $\text{HCl}$, $\text{HNO}_3$, and $\text{CH}_3\text{COOH}$. > Common mistake: Forgetting that weak acids also react with reactive metals like zinc to produce hydrogen gas. ### Question n *2 marks · Very short answer* Are the observations in all the cases the same or different? **Solution** 1. The observations are similar in all cases as hydrogen gas is evolved when various acids react with zinc granules. 2. However, the rate of reaction can vary depending on whether the acid is strong or weak. **Answer:** The observations are similar as hydrogen gas is evolved in all cases. > Common mistake: Stating that different gases are produced with different acids. ### Question 8 *2 marks · Very short answer* Can you now write the equations for the reactions you have observed? **Solution** 1. With hydrochloric acid: $\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$ 2. With acetic acid: $\text{Zn}(s) + 2\text{CH}_3\text{COOH}(aq) \rightarrow (\text{CH}_3\text{COO})_2\text{Zn}(aq) + \text{H}_2(g)$ **Answer:** $\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$ and $\text{Zn}(s) + 2\text{CH}_3\text{COOH}(aq) \rightarrow (\text{CH}_3\text{COO})_2\text{Zn}(aq) + \text{H}_2(g)$. > Common mistake: Failing to balance the chemical equations correctly. ## Activity 2.4 ### Question n *2 marks · Very short answer* Repeat the rest of the steps as in Activity 2.3 and record your observations. **Solution** 1. Zinc metal reacts with sodium hydroxide solution on warming to form sodium zincate and hydrogen gas is evolved. 2. The reaction is represented as: $2\text{NaOH(aq)} + \text{Zn(s)} \rightarrow \text{Na}_2\text{ZnO}_2\text{(s)} + \text{H}_2\text{(g)}$. **Answer:** Hydrogen gas is evolved when zinc metal is warmed with sodium hydroxide solution. > Common mistake: Forgetting to mention that the mixture needs to be warmed for the reaction to proceed. ## Activity 2.5 ### Question n *2 marks · Very short answer* What do you observe? **Solution** 1. Brisk effervescence is observed in both test tubes containing sodium carbonate and sodium hydrogencarbonate upon the addition of dilute hydrochloric acid. 2. This indicates the evolution of a gas rapidly. **Answer:** Brisk effervescence due to the evolution of carbon dioxide gas is observed. > Common mistake: Stating only that bubbles are seen without mentioning brisk effervescence. ### Question n *2 marks · Very short answer* Pass the gas produced in each case through lime water (calcium hydroxide solution) as shown in Fig. 2.2 and record your observations. **Solution** 1. When the evolved gas is passed through calcium hydroxide solution (lime water), the lime water turns milky. 2. On passing excess carbon dioxide, the milkiness disappears due to the formation of soluble calcium hydrogencarbonate. **Answer:** Lime water turns milky due to the formation of a white precipitate of calcium carbonate. > Common mistake: Forgetting to mention the white precipitate of calcium carbonate. ## Activity 2.6 ### Question n *2 marks · Very short answer* What is the colour of the solution? **Solution** 1. Phenolphthalein turns pink in basic medium. 2. Therefore, the colour of the sodium hydroxide solution becomes pink upon adding phenolphthalein. **Answer:** The colour of the solution becomes pink. > Common mistake: Confusing the colour of phenolphthalein in acids and bases. ### Question n *2 marks · Very short answer* Is there any colour change for the reaction mixture? **Solution** 1. Dilute hydrochloric acid is acidic in nature and neutralises the base. 2. The reaction mixture turns colourless as the pink colour disappears. **Answer:** Yes, the pink solution turns colourless. > Common mistake: Stating that the solution turns red instead of colourless. ### Question n *2 marks · Very short answer* Why did the colour of phenolphthalein change after the addition of an acid? **Solution** 1. The addition of an acid neutralises the basic effect of sodium hydroxide. 2. Phenolphthalein is colourless in acidic medium, causing the colour change. **Answer:** The colour changed because the acid nullified the effect of the base, making the medium acidic where phenolphthalein is colourless. > Common mistake: Not mentioning the neutralisation of the base. ### Question n *2 marks · Very short answer* Does the pink colour of phenolphthalein reappear? **Solution** 1. Adding sodium hydroxide introduces excess base into the mixture. 2. The basic medium causes the pink colour of phenolphthalein to reappear. **Answer:** Yes, the pink colour of phenolphthalein reappears. > Common mistake: Forgetting that excess base restores the alkaline medium. ### Question n *2 marks · Very short answer* Why do you think this has happened? **Solution** 1. The added sodium hydroxide is a base that overcomes the acid present. 2. The solution becomes basic again, restoring the pink colour of phenolphthalein. **Answer:** This happens because the excess base makes the solution alkaline again, where phenolphthalein turns pink. > Common mistake: Stating that a salt is formed without mentioning the excess base. ## Activity 2.7 ### Question n *2 marks · Very short answer* Note the colour of the solution. What has happened to the copper oxide? **Solution** 1. The colour of the solution becomes blue-green due to the formation of copper(II) chloride. 2. The black copper oxide dissolves completely in the dilute hydrochloric acid. **Answer:** The solution becomes blue-green due to the formation of copper(II) chloride, and the copper oxide dissolves. > Common mistake: Stating that the solution becomes blue instead of blue-green. ## QUESTIONS ### Question 1 *3 marks · Short answer* Why should curd and sour substances not be kept in brass and copper vessels? **Solution** 1. Curd and sour substances contain acids. 2. Brass and copper vessels contain metals such as copper and brass (copper-zinc alloy). 3. Acids present in these substances react with the metal vessels to form soluble salts, which can be toxic and spoil the food. **Answer:** Curd and sour substances contain acids which react with the metal of brass and copper vessels to form poisonous metal salts, making the food unfit for consumption. > Common mistake: Stating only that food gets spoiled without mentioning the formation of toxic metal salts due to reaction between acids and metals. ### Question 2 *3 marks · Short answer* Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas? **Solution** 1. Hydrogen gas is usually liberated when an acid reacts with a metal. 2. Example: $\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$. 3. To test for hydrogen gas, bring a burning candle near the gas-filled soap bubbles; the gas burns with a pop sound. **Answer:** Hydrogen gas is liberated, tested by bringing a burning candle near soap bubbles containing the gas, which burns with a pop sound. > Common mistake: Forgetting to mention the pop sound test for hydrogen gas. ### Question 3 *3 marks · Short answer* Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. **Solution** 1. Metal compound A reacts with dilute $\text{HCl}$ to produce effervescence of carbon dioxide gas, which extinguishes a burning candle. 2. Since one of the products is calcium chloride, the metal compound A is calcium carbonate ($\text{CaCO}_3$). 3. Balanced chemical equation: $\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$. **Answer:** $\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$ > Common mistake: Failing to balance the chemical equation properly or misidentifying compound A. ## Activity 2.8 ### Question n *2 marks · Very short answer* What do you observe? **Solution** 1. When dilute hydrochloric acid or sulphuric acid is taken in the beaker and the current is switched on, the bulb starts glowing. 2. This indicates that acids provide ions in aqueous solution which conduct electricity. **Answer:** The bulb starts glowing when dilute acid is used, showing that electric current flows through the solution. > Common mistake: Stating that the bulb glows due to the metal wires alone without mentioning the ions in the solution. ### Question n *2 marks · Very short answer* Repeat the experiment separately with glucose and alcohol solutions. What do you observe now? **Solution** 1. When the experiment is repeated separately with glucose and alcohol solutions, the bulb does not glow. 2. This shows that glucose and alcohol do not produce ions in water and therefore do not conduct electricity. **Answer:** The bulb does not glow with glucose and alcohol solutions, indicating the absence of ions. > Common mistake: Assuming all compounds containing hydrogen will conduct electricity. ### Question n *2 marks · Very short answer* Does the bulb glow in all cases? **Solution** 1. No, the bulb does not glow in all cases. 2. It glows only in the case of acid solutions like hydrochloric acid and sulphuric acid, but not with glucose or alcohol solutions. **Answer:** No, the bulb glows only for acid solutions and does not glow for glucose and alcohol solutions. > Common mistake: Concluding that all aqueous solutions conduct electricity. ### Question 4 *2 marks · Very short answer* What can you conclude from the results of this Activity? **Solution** 1. From the results, we can conclude that the electric current through the solution is carried by ions. 2. Acids produce hydrogen ions, $\text{H}^+(\text{aq})$, in solution which are responsible for conducting electricity and for their acidic properties. **Answer:** Acids produce ions in aqueous solutions which carry electric current, confirming that hydrogen ions are responsible for acidic properties. > Common mistake: Failing to connect the glowing of the bulb to the presence of free ions in the solution. ## Activity 2.9 ### Question n *2 marks · Very short answer* What do you observe? Is there a gas coming out of the delivery tube? **Solution** 1. On adding concentrated sulphuric acid to solid sodium chloride in a test tube, a chemical reaction takes place. 2. A colourless gas is observed coming out of the delivery tube. **Answer:** A colourless gas (HCl gas) is observed coming out of the delivery tube. > Common mistake: Confusing the gas with water vapour or not mentioning that it is a colourless gas. ### Question n *2 marks · Very short answer* Test the gas evolved successively with dry and wet blue litmus paper. **Solution** 1. The evolved HCl gas is tested first with dry blue litmus paper, where no colour change is observed. 2. When the gas is tested with wet (moist) blue litmus paper, the paper turns red. **Answer:** Dry blue litmus paper shows no colour change, whereas wet blue litmus paper turns red. > Common mistake: Stating that dry litmus paper changes colour in the absence of moisture. ### Question n *2 marks · Very short answer* In which case does the litmus paper change colour? **Solution** 1. The blue litmus paper changes colour only in the case of wet blue litmus paper. 2. Water is required to release hydrogen ions from HCl gas, which is responsible for the acidic colour change. **Answer:** The litmus paper changes colour only in the case of wet blue litmus paper. > Common mistake: Ignoring the role of water in generating hydrogen ions. ### Question n *3 marks · Short answer* On the basis of the above Activity, what do you infer about the acidic character of: (i) dry HCl gas (ii) HCl solution? **Part (i)** 1. Dry HCl gas does not show acidic character. 2. Reason: Separation of hydrogen ions ($H^+$) from HCl molecules cannot occur in the absence of water. Answer (i): Dry HCl gas does not show acidic character as it does not release hydrogen ions without water. **Part (ii)** 1. HCl solution shows acidic character. 2. Reason: In the presence of water, HCl dissociates to form hydrogen ions or hydronium ions ($H_3O^+$), which are responsible for the acidic property. Answer (ii): HCl solution shows acidic character due to the formation of hydronium ions in water. **Answer:** Dry HCl gas does not show acidic character because hydrogen ions are not separated without water, whereas HCl solution shows acidic character due to the presence of hydronium ions. > Common mistake: Concluding that dry acids release hydrogen ions without a solvent. ## Activity 2.10 ### Question n *2 marks · Very short answer* Is there a change in temperature? **Solution** 1. Yes, there is a change in temperature and the base of the beaker becomes warm. **Answer:** The temperature of the mixture increases, making the beaker warm. > Common mistake: Stating that temperature decreases or remains constant. ### Question n *2 marks · Very short answer* Is this an exothermic or endothermic process? **Solution** 1. The process of dissolving a concentrated acid or base in water releases a large amount of heat. **Answer:** It is an exothermic process. > Common mistake: Confusing exothermic process with endothermic process. ### Question n *2 marks · Very short answer* Repeat the above Activity with sodium hydroxide pellets and record your observations. **Solution** 1. When sodium hydroxide pellets are dissolved in water, the base of the beaker becomes warm. **Answer:** The process is exothermic, releasing heat and increasing the temperature of the solution. > Common mistake: Forgetting that dissolving bases like sodium hydroxide in water is also highly exothermic. ## QUESTIONS ### Question 1 *3 marks · Short answer* Why do $\text{HCl}$, $\text{HNO}_3$, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character? **Solution** 1. Compounds like $\text{HCl}$ and $\text{HNO}_3$ dissociate in aqueous solution to form hydrogen ions, $\text{H}^+(\text{aq})$ or hydronium ions, $\text{H}_3\text{O}^+(\text{aq})$, which are responsible for their acidic character. 2. On the other hand, compounds like alcohol and glucose contain hydrogen, but they do not dissociate to form hydrogen ions in water. 3. Therefore, solutions of alcohol and glucose do not show acidic character. **Answer:** Acids dissociate in water to produce $\text{H}^+(\text{aq})$ ions, whereas alcohol and glucose do not release hydrogen ions in water. > Common mistake: Students often think that any compound containing hydrogen is an acid. ### Question 2 *3 marks · Short answer* Why does an aqueous solution of an acid conduct electricity? **Solution** 1. An aqueous solution of an acid dissociates into ions when dissolved in water. 2. These ions are charged particles, specifically hydrogen ions ($\text{H}^+(\text{aq})$ or $\text{H}_3\text{O}^+(\text{aq})$) and anions. 3. The flow of these charged ions through the solution carries the electric current, allowing the aqueous solution to conduct electricity. **Answer:** An aqueous solution of an acid conducts electricity due to the presence of mobile ions formed by the dissociation of the acid in water. > Common mistake: Stating that electrons conduct electricity through the solution instead of ions. ### Question 3 *3 marks · Short answer* Why does dry $\text{HCl}$ gas not change the colour of the dry litmus paper? **Solution** 1. Colour change of litmus paper is produced by the presence of hydrogen ions, $\text{H}^+(\text{aq})$. 2. Hydrogen ions are produced only in the presence of water, as water helps in the separation of $\text{H}^+$ ions from $\text{HCl}$ molecules. 3. Since both the $\text{HCl}$ gas and the litmus paper are dry, no $\text{H}^+(\text{aq})$ ions can be formed, and hence there is no change in colour. **Answer:** Dry $\text{HCl}$ gas does not form hydrogen ions in the absence of water, which are required to change the colour of litmus paper. > Common mistake: Forgetting that $\text{HCl}$ molecules need water to dissociate into ions. ### Question 4 *3 marks · Short answer* While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? **Solution** 1. The process of dissolving an acid in water is a highly exothermic process. 2. If water is added to a concentrated acid, the heat generated causes the mixture to splash out and cause severe burns, and the glass container may also break due to excessive local heating. 3. Therefore, acid must always be added slowly to water with constant stirring. **Answer:** Acid is added to water and not water to acid to safely absorb the large amount of heat evolved in the exothermic process and prevent splashing. > Common mistake: Reversing the order and stating that water should be added to acid. ### Question 5 *3 marks · Short answer* How is the concentration of hydronium ions ($\text{H}_3\text{O}^+$) affected when a solution of an acid is diluted? **Solution** 1. Dilution is the process of adding water to an acid or a base, which results in a decrease in the concentration of ions per unit volume. 2. When an acid solution is diluted, the number of hydronium ions ($\text{H}_3\text{O}^+$) present per unit volume decreases. 3. Thus, the concentration of hydronium ions decreases upon dilution. **Answer:** The concentration of hydronium ions ($\text{H}_3\text{O}^+$) decreases per unit volume when an acid solution is diluted. > Common mistake: Thinking the total number of ions decreases instead of the concentration per unit volume. ### Question 6 *3 marks · Short answer* How is the concentration of hydroxide ions ($\text{OH}^-$) affected when excess base is dissolved in a solution of sodium hydroxide? **Solution** 1. Sodium hydroxide is a strong base that completely dissociates in water to give sodium ions and hydroxide ions ($\text{OH}^-$). 2. When excess base is dissolved in a sodium hydroxide solution, the total amount of $\text{OH}^-$ ions in the solution increases. 3. Consequently, the concentration of hydroxide ions ($\text{OH}^-$) per unit volume increases. **Answer:** The concentration of hydroxide ions ($\text{OH}^-$) increases when excess base is dissolved in the sodium hydroxide solution. > Common mistake: Confusing hydroxide ion concentration with hydronium ion concentration. ## Activity 2.11 ### Question n *2 marks · Very short answer* Test the pH values of solutions given in Table 2.2. **Solution** 1. The pH values of various solutions can be tested using pH paper or a universal indicator. 2. The colour produced on the pH paper is matched with the standard pH colour chart to determine the approximate pH value. **Answer:** The pH values of solutions are tested by observing the colour change on pH paper and comparing it with the standard pH chart. > Common mistake: Confusing universal indicator paper with ordinary litmus paper. ### Question n *2 marks · Very short answer* Record your observations. **Solution** 1. Observations are recorded by noting the specific colour developed on the pH paper for each given solution. 2. The corresponding approximate pH value is then read from the pH colour scale. **Answer:** Observations are recorded by noting the colour change and the corresponding approximate pH value for each solution. > Common mistake: Not matching the colour accurately with the standard colour chart. ### Question n *2 marks · Very short answer* What is the nature of each substance on the basis of your observations? **Solution** 1. Substances with a pH value less than 7 are acidic in nature. 2. Substances with a pH value of 7 are neutral, and those with a pH value greater than 7 are basic in nature. **Answer:** Substances with pH less than 7 are acidic, pH equal to 7 are neutral, and pH greater than 7 are basic. > Common mistake: Inverting the pH ranges for acidic and basic solutions. ### Question 4 *2 marks · Very short answer* Can you now say what weak and strong bases are? **Solution** 1. Bases that produce a large number of hydroxide ions ($OH^-$) in aqueous solution are called strong bases. 2. Bases that produce fewer hydroxide ions ($OH^-$) in aqueous solution are called weak bases. **Answer:** Bases that give rise to more $OH^-$ ions are strong bases, and bases that give less $OH^-$ ions are weak bases. > Common mistake: Confusing strong and weak bases with concentrated and dilute solutions. ## Activity 2.12 ### Question n *2 marks · Very short answer* What can you conclude about the ideal soil pH for the growth of plants in your region? **Solution** 1. Most plants require a specific pH range which is near neutral, typically between $6.5$ and $7.3$, for their healthy growth. 2. Thus, the ideal soil pH for plant growth in a region is generally neutral or slightly acidic to slightly basic, depending on the specific crops suited to that area. **Answer:** The ideal soil pH for the healthy growth of plants is generally around a neutral range of $6.5$ to $7.3$. > Common mistake: Writing an extreme pH value like very acidic or very basic instead of the near-neutral range required by plants. ## QUESTIONS ### Question 1 *3 marks · Short answer* You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic? **Part (i)** 1. Solution A has a pH of 6, and solution B has a pH of 8. 2. Higher the hydronium ion concentration, lower is the pH value. 3. Therefore, solution A has more hydrogen ion concentration than solution B. Answer (i): Solution A has more hydrogen ion concentration. **Part (ii)** 1. Values less than 7 on the pH scale represent an acidic solution, so solution A (pH 6) is acidic. 2. Values greater than 7 represent a basic solution, so solution B (pH 8) is basic. Answer (ii): Solution A is acidic and solution B is basic. **Answer:** Solution A has more hydrogen ion concentration. Solution A is acidic and solution B is basic. > Common mistake: Confusing pH values where lower pH means lower acidity or lower hydrogen ion concentration. ### Question 2 *3 marks · Short answer* What effect does the concentration of $\text{H}^+(\text{aq})$ ions have on the nature of the solution? **Solution** 1. The concentration of $\text{H}^+(\text{aq})$ ions determines whether a solution is acidic, neutral, or basic. 2. An increase in the concentration of $\text{H}^+(\text{aq})$ ions makes the solution more acidic, which decreases the pH value. 3. A decrease in the concentration of $\text{H}^+(\text{aq})$ ions (or increase in $\text{OH}^-$ ions) makes the solution less acidic or basic, which increases the pH value. **Answer:** Higher concentration of $\text{H}^+(\text{aq})$ ions makes the solution more acidic, while a lower concentration makes it less acidic or basic. > Common mistake: Stating that hydrogen ions are only present in acids without relating concentration to the degree of acidity. ### Question 3 *3 marks · Short answer* Do basic solutions also have $\text{H}^+(\text{aq})$ ions? If yes, then why are these basic? **Solution** 1. Yes, basic solutions do have $\text{H}^+(\text{aq})$ ions present in them. 2. However, the concentration of these hydrogen ions is much less than the concentration of hydroxide ions ($\text{OH}^-$). 3. They are basic because the overall concentration of hydroxide ions dominates over the hydrogen ions. **Answer:** Basic solutions contain $\text{H}^+(\text{aq})$ ions, but they are basic because hydroxide ions ($\text{OH}^-$) are present in excess. > Common mistake: Students often think basic solutions have zero hydrogen ions. ### Question 4 *3 marks · Short answer* Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)? **Solution** 1. Plants require a specific pH range for their healthy growth, and soil can sometimes become too acidic due to various factors like acid rain. 2. Quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) are basic substances. 3. A farmer would treat the soil with these basic compounds when the soil is overly acidic, to neutralise the excess acid and restore the proper pH for crop growth. **Answer:** A farmer would treat the soil with these basic substances when the soil is too acidic, in order to neutralise the excess acidity. > Common mistake: Thinking that lime is added to increase acidity rather than to neutralise acidic soil. ## Activity 2.13 ### Question n *2 marks · Very short answer* Write the chemical formulae of the salts given below. Potassium sulphate, sodium sulphate, calcium sulphate, magnesium sulphate, copper sulphate, sodium chloride, sodium nitrate, sodium carbonate and ammonium chloride. **Solution** 1. Potassium sulphate: $\text{K}_2\text{SO}_4$, sodium sulphate: $\text{Na}_2\text{SO}_4$, calcium sulphate: $\text{CaSO}_4$, magnesium sulphate: $\text{MgSO}_4$, and copper sulphate: $\text{CuSO}_4$. 2. Sodium chloride: $\text{NaCl}$, sodium nitrate: $\text{NaNO}_3$, sodium carbonate: $\text{Na}_2\text{CO}_3$, and ammonium chloride: $\text{NH}_4\text{Cl}$. **Answer:** The chemical formulae are $\text{K}_2\text{SO}_4$, $\text{Na}_2\text{SO}_4$, $\text{CaSO}_4$, $\text{MgSO}_4$, $\text{CuSO}_4$, $\text{NaCl}$, $\text{NaNO}_3$, $\text{Na}_2\text{CO}_3$, and $\text{NH}_4\text{Cl}$. > Common mistake: Writing incorrect valencies for radicals leading to wrong chemical formulae. ### Question n *2 marks · Very short answer* Identify the acids and bases from which the above salts may be obtained. **Solution** 1. Potassium sulphate is obtained from sulphuric acid ($H_2SO_4$) and potassium hydroxide ($KOH$). 2. Sodium sulphate, sodium chloride, sodium nitrate, and sodium carbonate are obtained from $NaOH$, while copper, calcium, and magnesium sulphates use respective metal hydroxides and sulphuric acid. **Answer:** Salts are formed from their respective acids and bases, such as $H_2SO_4$ and $NaOH$ for sodium sulphate. > Common mistake: Writing incorrect formulas for the parent bases or acids. ### Question n *2 marks · Very short answer* How many families can you identify among the salts given in this Activity? **Solution** 1. Salts having the same positive or negative radicals belong to a family. 2. Based on the given salts, we can identify various families of positive radicals (such as sodium salts, potassium salts) and negative radicals (such as sulphate salts, chloride salts), making a total of several families. **Answer:** Multiple families of salts can be identified based on shared positive or negative radicals. > Common mistake: Confusing positive and negative radicals while grouping salt families. ## Activity 2.14 ### Question n *2 marks · Very short answer* Which of the salts are acidic, basic or neutral? **Solution** 1. Salts of strong acids and strong bases are neutral with a pH of 7. 2. Salts of strong acids and weak bases are acidic with a pH less than 7, while salts of strong bases and weak acids are basic with a pH more than 7. **Answer:** Salts formed from strong acid and strong base are neutral, those from strong acid and weak base are acidic, and those from strong base and weak acid are basic. > Common mistake: Confusing the pH values and acid-base strengths for determining salt nature. ### Question n *2 marks · Very short answer* Identify the acid or base used to form the salt. **Solution** 1. The acid used to form a salt is identified from its negative radical or anion. 2. The base used to form the salt is identified from its positive radical or metal cation. **Answer:** The parent acid is identified from the anion part and the parent base from the cation part of the salt. > Common mistake: Reversing the roles of anions and cations in identifying acids and bases. ### Question n *2 marks · Very short answer* Report your observations in Table 2.4. **Solution** 1. Test the solubility of each salt sample using distilled water. 2. Check the action of the salt solutions on litmus paper and find their pH values to record them in Table 2.4. **Answer:** Solubility, action on litmus, pH value, acid used, and base used are recorded for each salt in Table 2.4. > Common mistake: Using tap water instead of distilled water for solubility and pH testing. ## Activity 2.15 ### Question n *2 marks · Very short answer* What is the colour of the copper sulphate after heating? **Solution** 1. Copper sulphate crystals lose their water of crystallisation on heating. 2. The salt turns white after heating due to the loss of water molecules. **Answer:** The copper sulphate turns white after heating. > Common mistake: Writing that copper sulphate melts instead of noting the colour change to white. ### Question n *2 marks · Very short answer* Do you notice water droplets in the boiling tube? Where have these come from? **Solution** 1. Water droplets are observed on the inner walls of the boiling tube. 2. These water droplets come from the water of crystallisation present in the copper sulphate crystals. **Answer:** Yes, water droplets are noticed in the boiling tube which come from the water of crystallisation present in copper sulphate crystals. > Common mistake: Thinking atmospheric moisture condensed inside the tube. ### Question n *2 marks · Very short answer* Add 2-3 drops of water on the sample of copper sulphate obtained after heating. **Solution** 1. Adding 2-3 drops of water to the white anhydrous copper sulphate sample hydrates the salt. 2. The blue colour of copper sulphate is restored due to the formation of hydrated copper sulphate pentahydrate. **Answer:** The white copper sulphate sample regains its water of crystallisation when water is added. > Common mistake: Failing to mention the restoration of the blue colour. ### Question n *2 marks · Very short answer* What do you observe? Is the blue colour of copper sulphate restored? **Solution** 1. Upon adding water to the heated copper sulphate, it absorbs water molecules to form hydrated copper sulphate. 2. Yes, the blue colour of copper sulphate is restored. **Answer:** Yes, the blue colour of copper sulphate is restored as it combines with water to form hydrated copper sulphate ($CuSO_4 \cdot 5H_2O$). > Common mistake: Not relating the blue colour to the presence of water of crystallisation. ## QUESTIONS ### Question 1 *2 marks · Very short answer* What is the common name of the compound $\text{Ca}(\text{ClO})_2$? **Solution** 1. The chemical formula $\text{Ca}(\text{ClO})_2$ represents bleaching powder. 2. Its common name is bleaching powder. **Answer:** Bleaching powder > Common mistake: Writing the chemical name instead of the common name. ### Question 2 *2 marks · Very short answer* Name the substance which on treatment with chlorine yields bleaching powder. **Solution** 1. Bleaching powder is produced by the action of chlorine on dry slaked lime. 2. Therefore, the substance is dry slaked lime, $\text{Ca}(\text{OH})_2$. **Answer:** Dry slaked lime [$\text{Ca}(\text{OH})_2$] > Common mistake: Writing lime water instead of dry slaked lime. ### Question 3 *2 marks · Very short answer* Name the sodium compound which is used for softening hard water. **Solution** 1. Washing soda is used for removing permanent hardness of water. 2. Therefore, the sodium compound is washing soda (sodium carbonate decahydrate). **Answer:** Washing soda ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$) > Common mistake: Confusing baking soda with washing soda. ### Question 4 *3 marks · Short answer* What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved. **Solution** 1. When a solution of sodium hydrogencarbonate is heated, it decomposes to form sodium carbonate, water and carbon dioxide gas. 2. The balanced chemical equation for the reaction is given as follows: 3. $2\text{NaHCO}_3 \xrightarrow{\text{Heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$ **Answer:** Sodium hydrogencarbonate on heating gives sodium carbonate, water and carbon dioxide. > Common mistake: Forgetting to balance the equation or writing the wrong products. ### Question 5 *3 marks · Short answer* Write an equation to show the reaction between Plaster of Paris and water. **Solution** 1. Plaster of Paris is a white powder which on mixing with water changes to gypsum giving a hard solid mass. 2. The reaction involved can be represented by the following chemical equation: 3. $\text{CaSO}_4\cdot\frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4\cdot2\text{H}_2\text{O}$ **Answer:** Plaster of Paris reacts with water to form gypsum. > Common mistake: Writing incorrect water of crystallisation stoichiometry for Plaster of Paris or gypsum. ## EXERCISES ### Question 1 *1 mark · MCQ* A solution turns red litmus blue, its pH is likely to be - 1 - 4 - 5 - 10 **Solution** 1. A solution that turns red litmus blue is basic in nature. 2. Bases have a pH value greater than 7, so the pH is likely to be 10. **Answer:** (d) 10 > Common mistake: Confusing red litmus turning blue with acidic solutions. ### Question 2 *1 mark · MCQ* A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains - $\text{NaCl}$ - $\text{HCl}$ - $\text{LiCl}$ - $\text{KCl}$ **Solution** 1. Crushed egg-shells contain calcium carbonate, which reacts with an acid to release carbon dioxide gas that turns lime-water milky. 2. Therefore, the solution contains hydrochloric acid ($\text{HCl}$). **Answer:** (b) $\text{HCl}$ > Common mistake: Selecting $\text{NaCl}$ assuming neutral salts react with carbonates. ### Question 3 *1 mark · MCQ* $10\text{ mL}$ of a solution of $\text{NaOH}$ is found to be completely neutralised by $8\text{ mL}$ of a given solution of $\text{HCl}$. If we take $20\text{ mL}$ of the same solution of $\text{NaOH}$, the amount $\text{HCl}$ solution (the same solution as before) required to neutralise it will be - $4\text{ mL}$ - $8\text{ mL}$ - $12\text{ mL}$ - $16\text{ mL}$ **Solution** 1. $10\text{ mL}$ of $\text{NaOH}$ solution is neutralised by $8\text{ mL}$ of $\text{HCl}$ solution. 2. When the amount of $\text{NaOH}$ is doubled to $20\text{ mL}$, the volume of $\text{HCl}$ required will also be doubled, which is $2 \times 8\text{ mL} = 16\text{ mL}$. **Answer:** (d) $16\text{ mL}$ > Common mistake: Halving the volume instead of doubling it. ### Question 4 *1 mark · MCQ* Which one of the following types of medicines is used for treating indigestion? - Antibiotic - Analgesic - Antacid - Antiseptic **Solution** 1. Indigestion is caused by the production of excess acid in the stomach. 2. Medicines used to neutralise this excess acid are called antacids. **Answer:** (c) Antacid > Common mistake: Choosing antibiotics or antiseptics instead of antacids. ### Question 5 *3 marks · Short answer* Write word equations and then balanced equations for the reaction taking place when - (a) dilute sulphuric acid reacts with zinc granules. (b) dilute hydrochloric acid reacts with magnesium ribbon. (c) dilute sulphuric acid reacts with aluminium powder. (d) dilute hydrochloric acid reacts with iron filings. **Part (a)** 1. Word equation: Sulphuric acid + Zinc $\rightarrow$ Zinc sulphate + Hydrogen 2. Balanced equation: $\text{H}_2\text{SO}_4(\text{aq}) + \text{Zn}(\text{s}) \rightarrow \text{ZnSO}_4(\text{aq}) + \text{H}_2(\text{g})$ Answer (a): $\text{H}_2\text{SO}_4(\text{aq}) + \text{Zn}(\text{s}) \rightarrow \text{ZnSO}_4(\text{aq}) + \text{H}_2(\text{g})$ **Part (b)** 1. Word equation: Hydrochloric acid + Magnesium $\rightarrow$ Magnesium chloride + Hydrogen 2. Balanced equation: $2\text{HCl}(\text{aq}) + \text{Mg}(\text{s}) \rightarrow \text{MgCl}_2(\text{aq}) + \text{H}_2(\text{g})$ Answer (b): $2\text{HCl}(\text{aq}) + \text{Mg}(\text{s}) \rightarrow \text{MgCl}_2(\text{aq}) + \text{H}_2(\text{g})$ **Part (c)** 1. Word equation: Sulphuric acid + Aluminium $\rightarrow$ Aluminium sulphate + Hydrogen 2. Balanced equation: $3\text{H}_2\text{SO}_4(\text{aq}) + 2\text{Al}(\text{s}) \rightarrow \text{Al}_2(\text{SO}_4)_3(\text{aq}) + 3\text{H}_2(\text{g})$ Answer (c): $3\text{H}_2\text{SO}_4(\text{aq}) + 2\text{Al}(\text{s}) \rightarrow \text{Al}_2(\text{SO}_4)_3(\text{aq}) + 3\text{H}_2(\text{g})$ **Part (d)** 1. Word equation: Hydrochloric acid + Iron $\rightarrow$ Iron(II) chloride + Hydrogen 2. Balanced equation: $6\text{HCl}(\text{aq}) + 2\text{Fe}(\text{s}) \rightarrow 2\text{FeCl}_3(\text{aq}) + 3\text{H}_2(\text{g})$ or $2\text{HCl}(\text{aq}) + \text{Fe}(\text{s}) \rightarrow \text{FeCl}_2(\text{aq}) + \text{H}_2(\text{g})$ Answer (d): $2\text{HCl}(\text{aq}) + \text{Fe}(\text{s}) \rightarrow \text{FeCl}_2(\text{aq}) + \text{H}_2(\text{g})$ **Answer:** Word and balanced equations for acid-metal reactions. > Common mistake: Incorrect balancing of aluminium sulphate or iron chloride formulas. ### Question 6 *3 marks · Short answer* Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it. **Solution** 1. Set up an apparatus with a beaker, two nails on a cork, a 6-volt battery, a bulb, and a switch as shown in Activity 2.8. 2. Pour glucose or alcohol solution into the beaker and switch on the current; observe that the bulb does not glow. 3. This proves that glucose and alcohol do not produce ions in solution and are not categorised as acids, unlike acids which conduct electricity. **Answer:** The bulb does not glow with glucose or alcohol solutions, proving they do not release ions. > Common mistake: Omitting the mention of ion formation in aqueous solution. ### Question 7 *3 marks · Short answer* Why does distilled water not conduct electricity, whereas rain water does? **Solution** 1. Distilled water is pure and does not contain any ionic compounds like acids, bases, or salts dissolved in it. 2. Rain water, while falling through the atmosphere, dissolves acidic gases such as carbon dioxide from the air to form carbonic acid, producing ions. 3. Due to the presence of these dissolved ions, rain water conducts electricity, whereas pure distilled water cannot. **Answer:** Distilled water lacks ions, while rain water contains dissolved acidic gases that form ions to conduct electricity. > Common mistake: Thinking that pure water itself contains ions. ### Question 8 *3 marks · Short answer* Why does dry $\text{HCl}$ gas not show acidic behaviour in the absence of water? **Solution** 1. Dry $\text{HCl}$ gas does not contain hydrogen ions in free form as its separation from $\text{HCl}$ molecules cannot occur in the absence of water. 2. Water is required to produce hydronium ions ($\text{H}_3\text{O}^+$) or hydrogen ions ($\text{H}^+(\text{aq})$) from $\text{HCl}$. 3. Since acidic behaviour is due to the presence of hydrogen ions, dry $\text{HCl}$ gas does not show acidic behaviour without water. **Answer:** Dry HCl gas does not form hydrogen ions in the absence of water. > Common mistake: Forgetting that water is essential for the ionization of acids. ### Question 9 *3 marks · Short answer* Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration. **Solution** 1. Solution D with pH 7 is neutral. 2. Solution C with pH 11 is strongly alkaline. 3. Solution B with pH 1 is strongly acidic. 4. Solution A with pH 4 is weakly acidic. 5. Solution E with pH 9 is weakly alkaline. 6. The increasing order of hydrogen-ion concentration is: $\text{pH } 11 < \text{pH } 9 < \text{pH } 7 < \text{pH } 4 < \text{pH } 1$. **Answer:** (a) D (b) C (c) B (d) A (e) E. Increasing order of hydrogen-ion concentration: $11 < 9 < 7 < 4 < 1$. > Common mistake: Confusing pH value with hydrogen ion concentration (lower pH means higher hydrogen ion concentration). ### Question 10 *3 marks · Short answer* Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid ($\text{HCl}$) is added to test tube A, while acetic acid ($\text{CH}_3\text{COOH}$) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why? **Solution** 1. Hydrochloric acid ($\text{HCl}$) is a strong acid, whereas acetic acid ($\text{CH}_3\text{COOH}$) is a weak acid. 2. Strong acids produce a higher concentration of hydrogen ions ($\text{H}^+$) compared to weak acids of the same concentration. 3. Therefore, fizzing will occur more vigorously in test tube A containing $\text{HCl}$ due to the faster rate of hydrogen gas evolution. **Answer:** Fizzing will occur more vigorously in test tube A because hydrochloric acid is a strong acid and produces more hydrogen ions than acetic acid. > Common mistake: Assuming that equal amounts and concentrations of different acids produce the same amount of ions. ### Question 11 *3 marks · Short answer* Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. **Solution** 1. Fresh milk turns into curd due to the formation of lactic acid by bacteria. 2. As acid is produced, the concentration of hydrogen ions in the solution increases. 3. Consequently, the pH will decrease to a value lower than 6, making the solution more acidic. **Answer:** The pH will decrease below 6 because lactic acid is produced during the formation of curd. > Common mistake: Stating that pH increases as milk turns sour. ### Question 12 *3 marks · Short answer* A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd? **Part (a)** 1. Fresh milk has a pH of 6, which becomes acidic when it turns into curd due to the formation of lactic acid. 2. The milkman shifts the pH to slightly alkaline so that the milk does not easily become acidic and spoil into curd. Answer (a): He shifts the pH to slightly alkaline to prevent the milk from turning sour and spoiling into curd easily. **Part (b)** 1. The added baking soda makes the milk basic. 2. When the milk is set as curd, the lactic acid produced by bacteria must first neutralise the added alkali before the pH can drop and curdle the milk, which takes a longer time. Answer (b): It takes a long time to set as curd because the lactic acid produced first has to neutralise the added base. **Answer:** Milkman adds baking soda to make milk slightly alkaline so it resists turning into curd; this milk takes longer to set because the acid produced must first neutralise the added base. > Common mistake: Stating that baking soda directly helps in setting the curd faster instead of realising it delays curd formation by neutralising the initial acid produced. ### Question 13 *3 marks · Short answer* Plaster of Paris should be stored in a moisture-proof container. Explain why? **Solution** 1. Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$) is a white powder that sets into a hard solid mass, gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$), when it comes in contact with water. 2. Moisture present in the air can cause this slow setting reaction. 3. Therefore, it must be stored in moisture-proof containers to prevent it from losing its properties and hardening prematurely. **Answer:** Plaster of Paris reacts with moisture to form a hard solid mass called gypsum, rendering it useless. > Common mistake: Stating that it absorbs moisture to form a solution instead of a solid mass. ### Question 14 *3 marks · Short answer* What is a neutralisation reaction? Give two examples. **Solution** 1. The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. 2. Example 1: $\text{NaOH}(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l})$ 3. Example 2: $\text{KOH}(\text{aq}) + \text{HNO}_3(\text{aq}) \rightarrow \text{KNO}_3(\text{aq}) + \text{H}_2\text{O}(\text{l})$ **Answer:** Neutralisation is the reaction of an acid with a base to form salt and water, for example, the reaction between $\text{NaOH}$ and $\text{HCl}$. > Common mistake: Forgetting to balance the chemical equations for the examples. ### Question 15 *3 marks · Short answer* Give two important uses of washing soda and baking soda. **Solution** 1. Uses of washing soda ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$): It is used in glass, soap, and paper industries, and for removing permanent hardness of water. 2. Uses of baking soda ($\text{NaHCO}_3$): It is used for making baking powder (mixture of baking soda and a mild edible acid like tartaric acid) and as an ingredient in antacids to neutralise stomach acidity. **Answer:** Washing soda is used in glass and soap industries and for softening hard water; baking soda is used in making baking powder and in antacids. > Common mistake: Confusing the chemical formulas or uses of washing soda and baking soda. ## Group Activity ### Question n *2 marks · Very short answer* Put 2-3 drops of the beetroot extract in each test tube and note the colour change if any. Write your observation in a Table. **Solution** 1. Beetroot extract acts as a natural indicator that changes colour in acidic and basic solutions. 2. In acidic solutions like lemon juice, soda-water, and vinegar, the extract remains reddish, while in a basic solution like baking soda solution, it turns yellow or greenish-yellow. **Answer:** Beetroot extract remains reddish in acidic solutions (lemon juice, soda-water, vinegar) and turns yellow in basic solutions (baking soda solution). > Common mistake: Confusing the colour change of beetroot extract with that of litmus or other indicators like methyl orange. ## (II) Preparing a soda-acid fire extinguisher ### Question n *2 marks · Very short answer* Direct this discharge on a burning candle. What happens? **Solution** 1. The discharge coming out of the nozzle contains carbon dioxide gas produced by the reaction between sodium hydrogencarbonate and dilute sulphuric acid. 2. When directed on a burning candle, the carbon dioxide gas extinguishes the fire because it is heavier than air and does not support combustion. **Answer:** The burning candle gets extinguished by the carbon dioxide gas present in the discharge. > Common mistake: Stating that oxygen is produced instead of carbon dioxide. ## Frequently asked questions ### How many total questions and exercises are covered in the NCERT solutions for Class 10 Science Chapter 2? The NCERT solutions cover various activities, in-text questions, and an exercise section containing 15 questions of MCQ and short answer types for the 2026-27 session. You can access all these step-by-step solutions and the free PDF right here on this page. ### Which important topics and concepts are covered in the exercise and question sets of this chapter? The questions cover crucial concepts like the acidic behaviour of HCl, neutralisation reactions, electrical conductivity of acids in water, and the action of antacids. They also explore reactions of acids with metals and metal carbonates as per the latest NCERT textbook. ### What are the hardest question types in this chapter and how should students approach them? The most challenging questions usually involve writing balanced chemical equations for reactions such as metal-acid interactions or explaining the electrical conductivity of acid solutions using ions like $H^+$. Students should approach them by first identifying the reactants and products, and then applying proper stoichiometry and core chemical principles. ### How can students write answers to score full marks in Class 10 board exams for these chemistry questions? To score full marks, students must write clear chemical equations with proper physical states, explain the underlying concepts step by step, and mention specific observations like gas evolution or colour changes. Referring to our structured solutions on this page will help you understand the ideal answer-writing format. ### Is a free PDF of these NCERT solutions available for download? Yes, the complete and free PDF of the NCERT solutions for Class 10 Science Chapter 2, based on the 2026-27 session textbook, is available on this page. You can easily download or view the detailed solutions to prepare effectively for your exams. ## Related pages - [Acids, Bases and Salts: CBSE previous year questions](https://www.swavid.com/cbse/class-10/science/pyq/acids-bases-and-salts) - [Class 10 Science chapters](https://www.swavid.com/science/class/10) Solutions written by SwaVid, a personal AI tutor for Class 6 to 10 Maths and Science. Practise this chapter free: https://www.swavid.com/start/student