NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions and Equations
These NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions and Equations are prepared according to the latest NCERT textbook. The solutions provide clear explanations, balanced chemical equations, and important concepts to help students prepare for school examinations and board exams.
In this chapter, students learn about chemical reactions, chemical equations, balancing equations, types of chemical reactions, oxidation and reduction, corrosion, and rancidity. The answers are written in a simple and easy-to-understand format for quick revision and concept clarity.
Topics Covered
- Chemical Equations
- Balanced Chemical Equations
- Types of Chemical Reactions
- Combination Reactions
- Decomposition Reactions
- Displacement Reactions
- Double Displacement Reactions
- Oxidation and Reduction
- Corrosion
- Rancidity
Chemical Reactions and Equations NCERT Solutions
Page No. 6
1. Why should a magnesium ribbon be cleaned before burning in air?
Solution
Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a thin layer of magnesium oxide on its surface. This layer is stable and prevents further reaction of magnesium with oxygen. Therefore, the magnesium ribbon is cleaned with sandpaper to remove the oxide layer so that the fresh metal surface is exposed to air and burns easily.
Many reactive metals form a protective oxide layer when exposed to air. Removing this layer helps the metal react more effectively.
2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Solution
(i)
\(H_2(g)+Cl_2(g)\rightarrow2HCl(g)\)
(ii)
\(3BaCl_2(aq)+Al_2(SO_4)_3(aq)\rightarrow2AlCl_3(aq)+3BaSO_4(s)\)
(iii)
\(2Na(s)+2H_2O(l)\rightarrow2NaOH(aq)+H_2(g)\)
3. 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.
Solution
(i)
\(BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)+2NaCl(aq)\)
(ii)
\(NaOH(aq)+HCl(aq)\rightarrow NaCl(aq)+H_2O(l)\)
Page No. 10
1. 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.
Solution
(i) The substance ‘X’ is calcium oxide (quick lime). Its chemical formula is \(CaO\).
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
\(CaO(s)+H_2O(l)\rightarrow Ca(OH)_2(aq)\)
This reaction is highly exothermic because a large amount of heat is released during the reaction.
The calcium hydroxide formed during whitewashing slowly reacts with carbon dioxide present in the air to form calcium carbonate, which gives the walls a smooth and shiny finish after a few days.
2. 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
When electricity is passed through water, it decomposes into hydrogen and oxygen gases.
\(2H_2O(l)\rightarrow2H_2(g)+O_2(g)\)
According to the balanced chemical equation, two molecules of hydrogen are produced for every one molecule of oxygen. Therefore, the volume of hydrogen collected is double the volume of oxygen collected.
The gas collected in larger quantity is hydrogen.
Page No. 13
1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Solution
Iron is more reactive than copper. Therefore, when an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution and forms iron sulphate.
\(Fe(s)+CuSO_4(aq)\rightarrow FeSO_4(aq)+Cu(s)\)
As a result, the blue colour of copper sulphate solution gradually changes to green due to the formation of iron sulphate solution.
2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Solution
When aqueous solutions of lead nitrate and potassium iodide are mixed, a yellow precipitate of lead iodide is formed.
\(Pb(NO_3)_2(aq)+2KI(aq)\rightarrow PbI_2(s)+2KNO_3(aq)\)
This is a double displacement reaction because the ions of the reactants exchange places to form new compounds.
3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) \(4Na(s)+O_2(g)\rightarrow2Na_2O(s)\)
(ii) \(CuO(s)+H_2(g)\rightarrow Cu(s)+H_2O(l)\)
Solution
(i)
Sodium gains oxygen and forms sodium oxide. Therefore, sodium is oxidised.
Oxygen is reduced during the reaction.
Oxidised: Sodium (Na)
Reduced: Oxygen (O₂)
(ii)
Copper oxide loses oxygen and gets converted into copper. Therefore, copper oxide is reduced.
Hydrogen gains oxygen and forms water. Therefore, hydrogen is oxidised.
Oxidised: Hydrogen (H₂)
Reduced: Copper oxide (CuO)
Oxidation means gain of oxygen or loss of hydrogen, whereas reduction means loss of oxygen or gain of hydrogen. Both processes occur simultaneously in a redox reaction.
Exercise
1. Which of the statements about the reaction below are incorrect?
\(2PbO(s)+C(s)\rightarrow2Pb(s)+CO_2(g)\)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
Solution
The incorrect statement is (b) Carbon dioxide is getting oxidised.
In this reaction, carbon gains oxygen and gets oxidised to carbon dioxide, whereas lead oxide loses oxygen and gets reduced to lead.
Therefore, the correct answer is:
(i) (a) and (b)
2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Solution
(d) displacement reaction
Aluminium is more reactive than iron and displaces iron from iron(III) oxide to form aluminium oxide and iron.
3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Solution
(a) Hydrogen gas and iron chloride are produced.
Iron reacts with dilute hydrochloric acid to form iron chloride and hydrogen gas.
\(Fe(s)+2HCl(aq)\rightarrow FeCl_2(aq)+H_2(g)\)
4. What is a balanced chemical equation? Why should chemical equations be balanced?
Solution
A balanced chemical equation is an equation in which the number of atoms of each element is the same on both the reactant side and the product side.
Chemical equations should be balanced to satisfy the Law of Conservation of Mass. According to this law, mass can neither be created nor destroyed during a chemical reaction. Therefore, the total number of atoms of each element must remain the same before and after the reaction.
While balancing a chemical equation, only the coefficients are changed. The chemical formulae of reactants and products should never be changed.
5. 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.
Solution
(a)
\(N_2(g)+3H_2(g)\rightarrow2NH_3(g)\)
(b)
\(2H_2S(g)+3O_2(g)\rightarrow2H_2O(l)+2SO_2(g)\)
(c)
\(3BaCl_2(aq)+Al_2(SO_4)_3(aq)\rightarrow2AlCl_3(aq)+3BaSO_4(s)\)
(d)
\(2K(s)+2H_2O(l)\rightarrow2KOH(aq)+H_2(g)\)
6. Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Solution
(a)
\(2HNO_3+Ca(OH)_2\rightarrow Ca(NO_3)_2+2H_2O\)
(b)
\(2NaOH+H_2SO_4\rightarrow Na_2SO_4+2H_2O\)
(c)
\(NaCl+AgNO_3\rightarrow AgCl+NaNO_3\)
The equation is already balanced.
(d)
\(BaCl_2+H_2SO_4\rightarrow BaSO_4+2HCl\)
7. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Solution
(a)
\(Ca(OH)_2+CO_2\rightarrow CaCO_3+H_2O\)
(b)
\(Zn+2AgNO_3\rightarrow Zn(NO_3)_2+2Ag\)
(c)
\(2Al+3CuCl_2\rightarrow2AlCl_3+3Cu\)
(d)
\(BaCl_2+K_2SO_4\rightarrow BaSO_4+2KCl\)
8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Solution
(a)
\(2KBr(aq)+BaI_2(aq)\rightarrow2KI(aq)+BaBr_2(aq)\)
Type of reaction: Double Displacement Reaction
(b)
\(ZnCO_3(s)\rightarrow ZnO(s)+CO_2(g)\)
Type of reaction: Decomposition Reaction
(c)
\(H_2(g)+Cl_2(g)\rightarrow2HCl(g)\)
Type of reaction: Combination Reaction
(d)
\(Mg(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2(g)\)
Type of reaction: Displacement Reaction
9. What does one mean by exothermic and endothermic reactions? Give examples.
Solution
Exothermic reactions are chemical reactions in which heat is released along with the formation of products.
Example:
\(CH_4(g)+2O_2(g)\rightarrow CO_2(g)+2H_2O(g)+\text{Heat}\)
Endothermic reactions are chemical reactions in which energy is absorbed from the surroundings.
Example:
\(CaCO_3(s)\xrightarrow{\text{heat}}CaO(s)+CO_2(g)\)
A simple way to remember this is: Exo = Exit (heat goes out) and Endo = Enter (heat comes in). Combustion reactions are generally exothermic, while most thermal decomposition reactions are endothermic.
10. Why is respiration considered an exothermic reaction? Explain.
Solution
During respiration, glucose combines with oxygen in the cells of our body to form carbon dioxide, water, and energy.
\(C_6H_{12}O_6(aq)+6O_2(aq)\rightarrow6CO_2(aq)+6H_2O(l)+\text{Energy}\)
Since energy is released during this process, respiration is considered an exothermic reaction.
The energy released is used by living organisms to carry out various life processes.
Board exams frequently ask why respiration is exothermic. Always mention that energy is released when glucose reacts with oxygen, which is then used by the body for various activities.
11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Solution
In a combination reaction, two or more substances combine to form a single product.
Example:
\(CaO(s)+H_2O(l)\rightarrow Ca(OH)_2(aq)\)
In a decomposition reaction, a single compound breaks down into two or more simpler substances.
Example:
\(CaCO_3(s)\xrightarrow{\text{heat}}CaO(s)+CO_2(g)\)
Therefore, decomposition reactions are called the opposite of combination reactions because the process involved is exactly reverse.
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Solution
(i) Decomposition by Heat (Thermal Decomposition)
\(CaCO_3(s)\xrightarrow{\text{heat}}CaO(s)+CO_2(g)\)
(ii) Decomposition by Light (Photochemical Decomposition)
\(2AgCl(s)\xrightarrow{\text{sunlight}}2Ag(s)+Cl_2(g)\)
(iii) Decomposition by Electricity (Electrolytic Decomposition)
\(2H_2O(l)\xrightarrow{\text{electricity}}2H_2(g)+O_2(g)\)
13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Solution
In a displacement reaction, a more reactive element displaces a less reactive element from its compound.
Example:
\(Zn(s)+CuSO_4(aq)\rightarrow ZnSO_4(aq)+Cu(s)\)
In a double displacement reaction, two compounds react by exchanging their ions to form two new compounds.
Example:
\(Na_2SO_4(aq)+BaCl_2(aq)\rightarrow BaSO_4(s)+2NaCl(aq)\)
In displacement reactions, only one element is replaced. In double displacement reactions, ions from both compounds exchange places, often forming a precipitate.
14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Solution
Copper is more reactive than silver. Therefore, copper displaces silver from silver nitrate solution.
\(Cu(s)+2AgNO_3(aq)\rightarrow Cu(NO_3)_2(aq)+2Ag(s)\)
15. What do you mean by a precipitation reaction? Explain by giving examples.
Solution
A reaction in which two aqueous solutions react to form an insoluble solid called a precipitate is known as a precipitation reaction.
Example:
\(Na_2SO_4(aq)+BaCl_2(aq)\rightarrow BaSO_4(s)+2NaCl(aq)\)
In this reaction, white insoluble barium sulphate \((BaSO_4)\) is formed as a precipitate.
16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
Solution
Oxidation is the process in which a substance gains oxygen or loses hydrogen.
Examples:
\(2Cu+O_2\rightarrow2CuO\)
\(2Mg+O_2\rightarrow2MgO\)
(b) Reduction
Solution
Reduction is the process in which a substance loses oxygen or gains hydrogen.
Examples:
\(CuO+H_2\rightarrow Cu+H_2O\)
\(ZnO+C\rightarrow Zn+CO\)
Oxidation and reduction always occur together. When one substance gains oxygen, another substance loses oxygen. Such reactions are called redox reactions.
17. 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
The shiny brown coloured element 'X' is copper (Cu).
When copper is heated in air, it reacts with oxygen to form black coloured copper(II) oxide (CuO).
\(2Cu(s)+O_2(g)\rightarrow2CuO(s)\)
18. Why do we apply paint on iron articles?
Solution
Paint is applied on iron articles to prevent rusting. The layer of paint prevents iron from coming into contact with air and moisture, which are essential for corrosion.
As a result, the iron remains protected from rust and its life is increased.
19. Oil and fat containing food items are flushed with nitrogen. Why?
Solution
Nitrogen is an inert gas and does not react easily with food substances.
Food items containing oils and fats are flushed with nitrogen to prevent their oxidation. This helps in preventing rancidity and increases the shelf life of the food products.
20. Explain the following terms with one example each.
(a) Corrosion
Solution
Corrosion is the gradual deterioration of a metal due to its reaction with air, moisture, or other substances present in the environment.
A common example is the rusting of iron.
\(4Fe+3O_2+nH_2O\rightarrow2Fe_2O_3.nH_2O\)
(b) Rancidity
Solution
Rancidity is the process in which fats and oils are oxidised, causing an unpleasant smell and taste in food.
For example, food items such as butter, chips, and fried snacks develop a bad smell when kept for a long time due to oxidation.
Corrosion causes damage to metals and leads to economic losses. Rancidity can be reduced by storing food in airtight containers, refrigeration, vacuum packing, or by flushing packets with nitrogen gas.
Related Readings
Practice MCQ Quiz
Check your understanding of the chapter by attempting the following important multiple-choice questions.
1. Which of the following is a combination reaction?
2. Which gas is produced when zinc reacts with dilute hydrochloric acid?
3. Fe + CuSO₄ → FeSO₄ + Cu is an example of:
4. What is the black substance formed when copper is heated in air?
5. Which process causes oils and fats to develop an unpleasant smell and taste?
