--- title: "NCERT Solutions Class 10 Science Chemical Reactions and Equations" url: https://www.swavid.com/science/class/10/chapter/chemical-reactions-and-equations/ncert-solutions dateModified: 2026-10-07T16:41:26+00:00 --- # NCERT Solutions Class 10 Science Chemical Reactions and Equations This chapter's questions cover fundamental concepts of chemical reactions, including writing and balancing chemical equations, identifying different types of chemical reactions, and understanding everyday phenomena like oxidation, corrosion, and rancidity. Free PDF (16 pages): https://www.swavid.com/api/seo/pdf/ncert/science/class-10/swavid-ncert-solutions-class-10-science-chapter-1-chemical-reactions-and-equations-81e0887e50.pdf ## QUESTIONS ### Question 1 *3 marks · Short answer* Why should a magnesium ribbon be cleaned before burning in air? **Solution** 1. Magnesium metal reacts with atmospheric oxygen over time to form a protective layer of magnesium oxide on its surface. 2. This layer of magnesium oxide acts as a barrier and prevents the underlying metal from burning easily. 3. Rubbing the ribbon with sandpaper removes this protective layer of magnesium oxide so that pure magnesium can readily burn in air. **Answer:** A magnesium ribbon is cleaned with sandpaper to remove the protective layer of magnesium oxide formed on its surface so that it can readily burn in air. > Common mistake: Students often write that it is cleaned to remove dust or dirt, missing the chemical reason about the magnesium oxide layer. ### Question 2 *3 marks · Short answer* Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine $\rightarrow$ Hydrogen chloride (ii) Barium chloride + Aluminium sulphate $\rightarrow$ Barium sulphate + Aluminium chloride (iii) Sodium + Water $\rightarrow$ Sodium hydroxide + Hydrogen **Part (i)** 1. Write the skeletal equation: $\text{H}_2 + \text{Cl}_2 \rightarrow \text{HCl}$ 2. Balance the chlorine and hydrogen atoms by placing a coefficient of 2 on the right-hand side. 3. The balanced equation is $\text{H}_2(\text{g}) + \text{Cl}_2(\text{g}) \rightarrow 2\text{HCl}(\text{g})$. Answer (i): $$\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$$ **Part (ii)** 1. Write the skeletal equation: $\text{BaCl}_2 + \text{Al}_2(\text{SO}_4)_3 \rightarrow \text{BaSO}_4 + \text{AlCl}_3$ 2. Balance the sulphate groups and aluminium atoms by placing appropriate coefficients. 3. The balanced equation is $3\text{BaCl}_2 + \text{Al}_2(\text{SO}_4)_3 \rightarrow 3\text{BaSO}_4 + 2\text{AlCl}_3$. Answer (ii): $$3\text{BaCl}_2 + \text{Al}_2(\text{SO}_4)_3 \rightarrow 3\text{BaSO}_4 + 2\text{AlCl}_3$$ **Part (iii)** 1. Write the skeletal equation: $\text{Na} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2$ 2. Balance the hydrogen and sodium atoms by placing coefficients. 3. The balanced equation is $2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$. Answer (iii): $$2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$$ **Answer:** The balanced chemical equations for the given reactions are written for each part. > Common mistake: Forgetting to balance polyatomic ions or individual atoms correctly across both sides. ### Question 3 *3 marks · Short answer* Write a balanced chemical equation with state symbols for the following reactions. (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride. (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water. **Part (i)** 1. Write the reactants and products with their chemical formulae and state symbols: aqueous barium chloride and aqueous sodium sulphate react to form solid barium sulphate and aqueous sodium chloride. 2. Write the skeletal equation: $\text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \rightarrow \text{BaSO}_4(\text{s}) + \text{NaCl}(\text{aq})$ 3. Balance the sodium and chlorine atoms to get the final balanced equation: $\text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \rightarrow \text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq})$. Answer (i): $$\text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \rightarrow \text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq})$$ **Part (ii)** 1. Write the reactants and products: aqueous sodium hydroxide and aqueous hydrochloric acid react to form aqueous sodium chloride and liquid water. 2. Write the balanced equation directly with state symbols: $\text{NaOH}(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l})$. Answer (ii): $$\text{NaOH}(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l})$$ **Answer:** The balanced chemical equations with state symbols are provided for each part. > Common mistake: Omitting or incorrectly writing state symbols like (aq), (s), and (l). ## QUESTIONS ### Question 1 *3 marks · Short answer* A solution of a substance 'X' is used for whitewashing. (i) Name the substance 'X' and write its formula. (ii) Write the reaction of the substance 'X' named in (i) above with water. **Part (i)** 1. The substance used for whitewashing is calcium oxide, also known as quick lime. 2. The chemical formula of calcium oxide is $\text{CaO}$. Answer (i): Substance X is calcium oxide with the formula $\text{CaO}$. **Part (ii)** 1. Calcium oxide reacts vigorously with water to form slaked lime (calcium hydroxide), releasing a large amount of heat. 2. The balanced chemical equation is $\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}$. Answer (ii): $\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}$ **Answer:** (i) Substance X is calcium oxide, formula $\text{CaO}$. (ii) $\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{aq} + \text{Heat}$ > Common mistake: Writing slaked lime instead of quick lime for substance X. ### Question 2 *3 marks · Short answer* Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas. **Solution** 1. Water ($_\text{H}_2\text{O}$) consists of two hydrogen atoms for every one oxygen atom combined chemically. 2. During the electrolysis of acidified water, water decomposes to give hydrogen gas at the cathode and oxygen gas at the anode. 3. Since the ratio of hydrogen to oxygen elements in water is $2:1$ by volume, the volume of hydrogen gas collected is double the volume of oxygen gas. 4. The gas whose volume is double is hydrogen. **Answer:** The amount of hydrogen gas collected is double because water contains two parts of hydrogen for every one part of oxygen by volume. > Common mistake: Confusing hydrogen and oxygen collection electrodes or volume ratios. ## QUESTIONS ### Question 1 *3 marks · Short answer* Why does the colour of copper sulphate solution change when an iron nail is dipped in it? **Solution** 1. When an iron nail is dipped in a copper sulphate solution, iron displaces copper from the copper sulphate solution because iron is more reactive than copper. 2. This forms a green-coloured solution of iron(II) sulphate ($\text{FeSO}_4$) and brown copper metal gets deposited on the iron nail. 3. The reaction is represented as: $\text{Fe}(s) + \text{CuSO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{Cu}(s)$. **Answer:** The blue colour of copper sulphate solution fades and changes to green due to the formation of iron sulphate, and a brown coating of copper forms on the iron nail. > Common mistake: Students often write that copper displaces iron instead of iron displacing copper. ### Question 2 *3 marks · Short answer* Give an example of a double displacement reaction other than the one given in Activity 1.10. **Solution** 1. A double displacement reaction is a reaction in which there is an exchange of ions between the reactants. 2. An example is the reaction between sodium chloride and silver nitrate to form a white precipitate of silver chloride. 3. The balanced chemical equation is: $\text{NaCl}(aq) + \text{AgNO}_3(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$. **Answer:** $\text{NaCl}(aq) + \text{AgNO}_3(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$ > Common mistake: Forgetting to write the state symbols or failing to balance the equation. ### Question 3 *3 marks · Short answer* Identify the substances that are oxidised and the substances that are reduced in the following reactions. (i) $4\text{Na}(s) + \text{O}_2(g) \rightarrow 2\text{Na}_2\text{O}(s)$ (ii) $\text{CuO}(s) + \text{H}_2(g) \rightarrow \text{Cu}(s) + \text{H}_2\text{O}(l)$ **Part (i)** 1. In the reaction $4\text{Na}(s) + \text{O}_2(g) \rightarrow 2\text{Na}_2\text{O}(s)$, sodium gains oxygen to form sodium oxide. 2. Therefore, sodium ($\text{Na}$) is oxidised and oxygen ($\text{O}_2$) is reduced. Answer (i): Sodium ($\text{Na}$) is oxidised and oxygen ($\text{O}_2$) is reduced. **Part (ii)** 1. In the reaction $\text{CuO}(s) + \text{H}_2(g) \rightarrow \text{Cu}(s) + \text{H}_2\text{O}(l)$, hydrogen gains oxygen while copper(II) oxide loses oxygen. 2. Therefore, hydrogen ($\text{H}_2$) is oxidised and copper(II) oxide ($\text{CuO}$) is reduced. Answer (ii): Hydrogen ($\text{H}_2$) is oxidised and copper(II) oxide ($\text{CuO}$) is reduced. **Answer:** (i) Sodium is oxidised and oxygen is reduced. (ii) Hydrogen is oxidised and copper(II) oxide is reduced. > Common mistake: Confusing which substance is oxidised and which substance is reduced based on oxygen gain or loss. ## EXERCISES ### Question 1 *1 mark · MCQ* Which of the statements about the reaction below are incorrect? $2\text{PbO}(s) + \text{C}(s) \rightarrow 2\text{Pb}(s) + \text{CO}_2(g)$ (a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced. - (a) and (b) - (a) and (c) - (a), (b) and (c) - all **Solution** 1. Lead oxide ($\text{PbO}$) loses oxygen to form lead ($\text{Pb}$), so lead oxide is getting reduced making statement (a) incorrect. 2. Carbon gains oxygen to form carbon dioxide ($\text{CO}_2$), so carbon is getting oxidised making statement (b) incorrect since it says carbon dioxide is getting oxidised. 3. Thus, statements (a) and (b) are incorrect. **Answer:** (i) (a) and (b) > Common mistake: Confusing which substance is getting oxidised and which substance is acting as the reducing agent. ### Question 2 *1 mark · MCQ* $\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$ The above reaction is an example of a - combination reaction. - double displacement reaction. - decomposition reaction. - displacement reaction. **Solution** 1. In the given reaction, aluminium ($\text{Al}$) displaces iron ($\text{Fe}$) from its oxide ($\text{Fe}_2\text{O}_3$). 2. An element displacing another element from its compound is a displacement reaction. **Answer:** (d) displacement reaction. > Common mistake: Confusing displacement reaction with double displacement reaction. ### Question 3 *1 mark · MCQ* What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer. - Hydrogen gas and iron chloride are produced. - Chlorine gas and iron hydroxide are produced. - No reaction takes place. - Iron salt and water are produced. **Solution** 1. When dilute hydrochloric acid is added to iron fillings, iron reacts with hydrochloric acid to form iron(II) chloride and hydrogen gas. 2. Therefore, hydrogen gas and iron chloride are produced. **Answer:** (a) Hydrogen gas and iron chloride are produced. > Common mistake: Assuming no reaction takes place because iron is less reactive than some other metals. ### Question 4 *3 marks · Short answer* What is a balanced chemical equation? Why should chemical equations be balanced? **Solution** 1. A balanced chemical equation is an equation in which the total number of atoms of each element is equal on both the reactant side (LHS) and the product side (RHS). 2. Chemical equations must be balanced to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. 3. Thus, the total mass of the elements present in the products must equal the total mass of the elements present in the reactants. **Answer:** A balanced equation has an equal number of atoms of each element on both sides, satisfying the law of conservation of mass. > Common mistake: Forgetting to mention the law of conservation of mass. ### Question 5 *3 marks · Short answer* Translate the following statements into chemical equations and then balance them. (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas. **Part (a)** 1. Write the skeletal equation: $\text{N}_2 + \text{H}_2 \rightarrow \text{NH}_3$ 2. Balance the equation: $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$ Answer (a): $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$ **Part (b)** 1. Write the skeletal equation: $\text{H}_2\text{S} + \text{O}_2 \rightarrow \text{H}_2\text{O} + \text{SO}_2$ 2. Balance the equation: $2\text{H}_2\text{S}(g) + 3\text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l) + 2\text{SO}_2(g)$ Answer (b): $2\text{H}_2\text{S}(g) + 3\text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l) + 2\text{SO}_2(g)$ **Part (c)** 1. Write the skeletal equation: $\text{BaCl}_2 + \text{Al}_2(\text{SO}_4)_3 \rightarrow \text{AlCl}_3 + \text{BaSO}_4$ 2. Balance the equation: $3\text{BaCl}_2(aq) + \text{Al}_2(\text{SO}_4)_3(aq) \rightarrow 2\text{AlCl}_3(aq) + 3\text{BaSO}_4(s)$ Answer (c): $3\text{BaCl}_2(aq) + \text{Al}_2(\text{SO}_4)_3(aq) \rightarrow 2\text{AlCl}_3(aq) + 3\text{BaSO}_4(s)$ **Part (d)** 1. Write the skeletal equation: $\text{K} + \text{H}_2\text{O} \rightarrow \text{KOH} + \text{H}_2$ 2. Balance the equation: $2\text{K}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{KOH}(aq) + \text{H}_2(g)$ Answer (d): $2\text{K}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{KOH}(aq) + \text{H}_2(g)$ **Answer:** Balanced equations obtained for all statements. > Common mistake: Incorrect formulas for compounds like aluminium sulphate or hydrogen sulphide. ### Question 6 *3 marks · Short answer* Balance the following chemical equations. (a) $\text{HNO}_3 + \text{Ca}(\text{OH})_2 \rightarrow \text{Ca}(\text{NO}_3)_2 + \text{H}_2\text{O}$ (b) $\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}$ (c) $\text{NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{NaNO}_3$ (d) $\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{HCl}$ **Part (a)** 1. Count atoms of each element on both sides of $\text{HNO}_3 + \text{Ca}(\text{OH})_2 \rightarrow \text{Ca}(\text{NO}_3)_2 + \text{H}_2\text{O}$. 2. Place coefficient 2 before $\text{HNO}_3$ and $2$ before $\text{H}_2\text{O}$ to balance hydrogen and nitrate ions. Answer (a): $2\text{HNO}_3 + \text{Ca}(\text{OH})_2 \rightarrow \text{Ca}(\text{NO}_3)_2 + 2\text{H}_2\text{O}$ **Part (b)** 1. Count atoms of each element on both sides of $\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}$. 2. Place coefficient 2 before $\text{NaOH}$ and $2$ before $\text{H}_2\text{O}$. Answer (b): $2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$ **Part (c)** 1. Examine $\text{NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{NaNO}_3$. 2. The equation is already balanced as the number of atoms of each element is equal on both sides. Answer (c): $\text{NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{NaNO}_3$ **Part (d)** 1. Count atoms in $\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{HCl}$. 2. Place coefficient 2 before $\text{HCl}$ to balance hydrogen and chlorine atoms. Answer (d): $\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{HCl}$ **Answer:** All given chemical equations balanced. > Common mistake: Altering subscripts inside chemical formulas instead of changing coefficients. ### Question 7 *3 marks · Short answer* Write the balanced chemical equations for the following reactions. (a) Calcium hydroxide + Carbon dioxide $\rightarrow$ Calcium carbonate + Water (b) Zinc + Silver nitrate $\rightarrow$ Zinc nitrate + Silver (c) Aluminium + Copper chloride $\rightarrow$ Aluminium chloride + Copper (d) Barium chloride + Potassium sulphate $\rightarrow$ Barium sulphate + Potassium chloride **Part (a)** 1. Write the chemical formulae for reactants and products: $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$. 2. Count the number of atoms of each element on both sides to check balancing. 3. The balanced equation is $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$. Answer (a): $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$ **Part (b)** 1. Write the skeletal equation: $\text{Zn} + \text{AgNO}_3 \rightarrow \text{Zn(NO}_3)_2 + \text{Ag}$. 2. Balance the nitrate ions by placing a coefficient of 2 before $\text{AgNO}_3$: $\text{Zn} + 2\text{AgNO}_3 \rightarrow \text{Zn(NO}_3)_2 + \text{Ag}$. 3. Balance the silver atoms by placing a coefficient of 2 before $\text{Ag}$: $\text{Zn} + 2\text{AgNO}_3 \rightarrow \text{Zn(NO}_3)_2 + 2\text{Ag}$. Answer (b): $\text{Zn} + 2\text{AgNO}_3 \rightarrow \text{Zn(NO}_3)_2 + 2\text{Ag}$ **Part (c)** 1. Write the skeletal equation: $\text{Al} + \text{CuCl}_2 \rightarrow \text{AlCl}_3 + \text{Cu}$. 2. Balance chlorine atoms by using lowest common multiple 6: $2\text{Al} + 3\text{CuCl}_2 \rightarrow 2\text{AlCl}_3 + \text{Cu}$. 3. Balance aluminium and copper atoms: $2\text{Al} + 3\text{CuCl}_2 \rightarrow 2\text{AlCl}_3 + 3\text{Cu}$. Answer (c): $2\text{Al} + 3\text{CuCl}_2 \rightarrow 2\text{AlCl}_3 + 3\text{Cu}$ **Part (d)** 1. Write the skeletal equation: $\text{BaCl}_2 + \text{K}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{KCl}$. 2. Balance potassium and chlorine atoms by placing a coefficient of 2 before $\text{KCl}$: $\text{BaCl}_2 + \text{K}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{KCl}$. Answer (d): $\text{BaCl}_2 + \text{K}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{KCl}$ **Answer:** Balanced equations written for all four chemical reactions. > Common mistake: Failing to check the valency of radicals like nitrate or sulphate while writing chemical formulae. ### Question 8 *3 marks · Short answer* Write the balanced chemical equation for the following and identify the type of reaction in each case. (a) Potassium bromide(aq) + Barium iodide(aq) $\rightarrow$ Potassium iodide(aq) + Barium bromide(s) (b) Zinc carbonate(s) $\rightarrow$ Zinc oxide(s) + Carbon dioxide(g) (c) Hydrogen(g) + Chlorine(g) $\rightarrow$ Hydrogen chloride(g) (d) Magnesium(s) + Hydrochloric acid(aq) $\rightarrow$ Magnesium chloride(aq) + Hydrogen(g) **Part (a)** 1. Write the balanced equation: $2\text{KBr(aq)} + \text{BaI}_2\text{(aq)} \rightarrow 2\text{KI(aq)} + \text{BaBr}_2\text{(s)}$. 2. Identify the type of reaction: Since an exchange of ions takes place between the reactants, it is a double displacement reaction. Answer (a): $2\text{KBr(aq)} + \text{BaI}_2\text{(aq)} \rightarrow 2\text{KI(aq)} + \text{BaBr}_2\text{(s)}$; Double displacement reaction **Part (b)** 1. Write the balanced equation: $\text{ZnCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{ZnO(s)} + \text{CO}_2\text{(g)}$. 2. Identify the type of reaction: Since a single reactant breaks down to give simpler products upon heating, it is a thermal decomposition reaction. Answer (b): $\text{ZnCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{ZnO(s)} + \text{CO}_2\text{(g)}$; Decomposition reaction **Part (c)** 1. Write the balanced equation: $\text{H}_2\text{(g)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{HCl(g)}$. 2. Identify the type of reaction: Since two substances combine to form a single product, it is a combination reaction. Answer (c): $\text{H}_2\text{(g)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{HCl(g)}$; Combination reaction **Part (d)** 1. Write the balanced equation: $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$. 2. Identify the type of reaction: Since magnesium displaces hydrogen from hydrochloric acid, it is a displacement reaction. Answer (d): $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$; Displacement reaction **Answer:** Balanced equations and reaction types identified for all four cases. > Common mistake: Confusing double displacement reactions with displacement reactions. ### Question 9 *3 marks · Short answer* What does one mean by exothermic and endothermic reactions? Give examples. **Solution** 1. Reactions in which heat is released along with the formation of products are called exothermic reactions. 2. Example of an exothermic reaction is the burning of natural gas: $\text{CH}_4(\text{g}) + 2\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{g})$. 3. Reactions in which energy is absorbed are known as endothermic reactions. 4. Example of an endothermic reaction is the decomposition of calcium carbonate: $\text{CaCO}_3(\text{s}) \xrightarrow{\text{Heat}} \text{CaO}(\text{s}) + \text{CO}_2(\text{g})$. **Answer:** Exothermic reactions release heat, while endothermic reactions absorb energy. > Common mistake: Forgetting to mention examples along with the definitions. ### Question 10 *3 marks · Short answer* Why is respiration considered an exothermic reaction? Explain. **Solution** 1. During digestion, food containing carbohydrates is broken down into simpler substances like glucose. 2. This glucose combines with oxygen in the cells of our body and provides energy. 3. Since energy is released during this process, respiration is considered an exothermic reaction. 4. The reaction is represented as: $\text{C}_6\text{H}_{12}\text{O}_6(\text{aq}) + 6\text{O}_2(\text{aq}) \rightarrow 6\text{CO}_2(\text{aq}) + 6\text{H}_2\text{O}(\text{l}) + \text{energy}$. **Answer:** Respiration is considered an exothermic reaction because energy is released during the breakdown of glucose combined with oxygen in body cells. > Common mistake: Not mentioning the release of energy or omitting the chemical equation. ### Question 11 *3 marks · Short answer* Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions. **Solution** 1. In a combination reaction, two or more reactants combine to form a single product, whereas in a decomposition reaction, a single reactant breaks down to give two or more simpler products. 2. Therefore, decomposition reactions are opposite to combination reactions. 3. Combination reaction example: $\mathrm{CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)}$ 4. Decomposition reaction example: $\mathrm{CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g)}$ **Answer:** Decomposition reactions break down a single reactant into multiple products, which is the exact reverse of combination reactions where multiple reactants form a single product. > Common mistake: Forgetting to write balanced chemical equations for both types of reactions as examples. ### Question 12 *3 marks · Short answer* Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity. **Solution** 1. Thermal decomposition (using heat): $\mathrm{CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g)}$ 2. Decomposition by light (photolytic decomposition): $\mathrm{2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g)}$ 3. Decomposition by electricity (electrolytic decomposition): $\mathrm{2H_2O(l) \xrightarrow{\text{Electricity}} 2H_2(g) + O_2(g)}$ **Answer:** Thermal decomposition: $\mathrm{CaCO_3 \xrightarrow{\text{Heat}} CaO + CO_2}$, Photolytic decomposition: $\mathrm{2AgCl \xrightarrow{\text{Sunlight}} 2Ag + Cl_2}$, Electrolytic decomposition: $\mathrm{2H_2O \xrightarrow{\text{Electricity}} 2H_2 + O_2}$. > Common mistake: Writing unbalanced equations or missing the state symbols. ### Question 13 *3 marks · Short answer* What is the difference between displacement and double displacement reactions? Write equations for these reactions. **Part (a)** 1. Definition: A reaction in which a more reactive element displaces another element from its compound. 2. Equation: $\text{Fe}(s) + \text{CuSO}_4(aq) \to \text{FeSO}_4(aq) + \text{Cu}(s)$ Answer (a): Displacement reaction definition and example equation. **Part (b)** 1. Definition: A reaction in which there is an exchange of ions between the reactants. 2. Equation: $\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \to \text{BaSO}_4(s) + 2\text{NaCl}(aq)$ Answer (b): Double displacement reaction definition and example equation. **Answer:** In a displacement reaction, a more reactive element displaces a less reactive element from its compound. In a double displacement reaction, two different atoms or groups of atoms are exchanged between two reactants. > Common mistake: Forgetting to write the physical states in the equations. ### Question 14 *3 marks · Short answer* In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved. **Solution** 1. Copper is more reactive than silver and displaces silver from silver nitrate solution. 2. The chemical reaction involved is: 3. $\text{Cu}(s) + 2\text{AgNO}_3(aq) \to \text{Cu(NO}_3)_2(aq) + 2\text{Ag}(s)$ **Answer:** $\text{Cu}(s) + 2\text{AgNO}_3(aq) \to \text{Cu(NO}_3)_2(aq) + 2\text{Ag}(s)$ > Common mistake: Writing an unbalanced equation or incorrect formula for copper nitrate. ### Question 15 *3 marks · Short answer* What do you mean by a precipitation reaction? Explain by giving examples. **Solution** 1. Any reaction that produces an insoluble substance (precipitate) is called a precipitation reaction. 2. Example 1: $\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \to \text{BaSO}_4(s) + 2\text{NaCl}(aq)$ (gives a white precipitate of barium sulphate). 3. Example 2: $\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq) \to \text{PbI}_2(s) + 2\text{KNO}_3(aq)$ (gives a yellow precipitate of lead iodide). **Answer:** A reaction producing an insoluble precipitate, e.g., formation of barium sulphate. > Common mistake: Failing to mention the state symbol $(s)$ for the precipitate. ### Question 16 *3 marks · Short answer* Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction **Part (a)** 1. Oxidation is the gain of oxygen by a substance. 2. Example 1: $2\text{Cu} + \text{O}_2 \xrightarrow{\text{Heat}} 2\text{CuO}$ 3. Example 2: $2\text{Mg} + \text{O}_2 \to 2\text{MgO}$ Answer (a): Oxidation definition with two examples. **Part (b)** 1. Reduction is the loss of oxygen by a substance. 2. Example 1: $\text{CuO} + \text{H}_2 \xrightarrow{\text{Heat}} \text{Cu} + \text{H}_2\text{O}$ 3. Example 2: $\text{ZnO} + \text{C} \to \text{Zn} + \text{CO}$ Answer (b): Reduction definition with two examples. **Answer:** Oxidation is the gain of oxygen and reduction is the loss of oxygen during a reaction. > Common mistake: Confusing oxidation with reduction or giving examples based on hydrogen instead of oxygen. ### Question 17 *3 marks · Short answer* A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed. **Solution** 1. The shiny brown coloured element 'X' is copper (Cu). 2. When copper is heated in air, it combines with oxygen to form copper(II) oxide, which is black in colour. 3. Reaction: $2\text{Cu}(s) + \text{O}_2(g) \xrightarrow{\text{Heat}} 2\text{CuO}(s)$ **Answer:** Element 'X' is Copper (Cu) and the black compound is Copper(II) oxide (CuO). > Common mistake: Writing copper oxide without specifying copper(II) oxide or formula CuO. ### Question 18 *3 marks · Short answer* Why do we apply paint on iron articles? **Solution** 1. Iron articles get coated with a reddish-brown powder when left exposed to humid atmosphere, a process known as rusting. 2. Applying paint on iron articles keeps their surface away from direct contact with air and moisture. 3. This prevents the iron from rusting and undergoing corrosion. **Answer:** Paint is applied to iron articles to prevent them from coming in contact with moisture and oxygen, thereby avoiding rusting. > Common mistake: Writing only that it makes the iron look good, missing the chemical prevention of rusting. ### Question 19 *3 marks · Short answer* Oil and fat containing food items are flushed with nitrogen. Why? **Solution** 1. Oil and fat containing food items get oxidised when left exposed to air, becoming rancid and changing their smell and taste. 2. To prevent this oxidation, manufacturers flush bags of chips and other food items with an unreactive gas like nitrogen. 3. Nitrogen creates an inert atmosphere around the food, preventing oxygen from coming into contact with the fats and oils. **Answer:** Oil and fat containing food items are flushed with nitrogen to prevent them from getting oxidised and becoming rancid. > Common mistake: Writing that nitrogen reacts with the food instead of realizing it acts as an inert gas to prevent oxidation. ### Question 20 *3 marks · Short answer* Explain the following terms with one example each. (a) Corrosion (b) Rancidity **Part (a)** 1. When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. 2. Example: The rusting of iron articles leaving a reddish-brown powder, or the green coating formed on copper. Answer (a): Corrosion is the gradual deterioration of metals by environmental factors like moisture and acids; Example: Rusting of iron. **Part (b)** 1. When fats and oils are oxidised, they become rancid and their smell and taste change. 2. Example: The unpleasant smell and taste of butter or cooking oil left exposed to air for a long time. Answer (b): Rancidity is the process where fats and oils get oxidised, leading to a change in smell and taste; Example: Spoilage of chips or butter left in the open. **Answer:** Corrosion is the attack of metals by substances around them such as moisture and acids, and rancidity is the oxidation of fats and oils causing foul smell and taste. > Common mistake: Failing to give examples for each term as required by the question. ## Frequently asked questions ### How many total questions are there in SwaVid NCERT Solutions for Class 10 Science Chapter 1? This chapter has a total of 23 questions divided into in-text and end-of-chapter exercises based on the 2026-27 session NCERT textbook. SwaVid provides free PDF and step-by-step solutions for all of them on this page only. ### Which specific topics do the questions in this chapter cover? The questions cover important concepts like balancing chemical equations with state symbols, the electrolysis of water, displacement and double displacement reactions, oxidation and reduction, and corrosion and rancidity. You can access SwaVid's free PDF and step-by-step solutions for these topics on this page only. ### What are the hardest question types in these solutions and how should I approach them? The hardest questions are usually the descriptive ones on balancing chemical equations and identifying reaction types, which require careful application of the Law of Conservation of Mass. To approach these, break down the reactants and products step by step as explained in SwaVid's free PDF and step-by-step solutions available on this page only. ### How do I write answers in the Class 10 board exam to score full marks? To score full marks, always write balanced chemical equations with proper physical state symbols like $(s)$, $(l)$, $(g)$, and $(aq)$ where applicable. SwaVid's free PDF and step-by-step solutions on this page only demonstrate the exact presentation format required by examiners. ### Is the free PDF for Class 10 Science Chapter 1 Chemical Reactions and Equations available here? Yes, you can easily download the complete study material for this chapter. SwaVid's free PDF and step-by-step solutions are on this page only to help you prepare effectively for your exams. ## Related pages - [Chemical Reactions and Equations: CBSE previous year questions](https://www.swavid.com/cbse/class-10/science/pyq/chemical-reactions-and-equations) - [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