1. Chemical Reactions and Equations Class 10 Science NCERT – Chapterwise Important Questions & Answers for CBSE Exam Preparation

Class 10 Science Chapter 1: Chemical Reactions and Equations NCERT Important Questions and Answers for CBSE

Chapter 1: Chemical Reactions and Equations

Multiple Choose Questions and Answers :

Question: Calcium oxide reacts vigorously with water to produce slaked lime.

This reaction can be classified as:

(a) Combination reaction (b) Exothermic reaction (c) Endothermic reaction (d) Oxidation reaction

Which of the following is a correct option? [2020 1M]

Answer: (b) Exothermic reaction .

[When calcium oxide reacts with water, it forms a single product (slaked lime), so it is a combination reaction. Again, a large amount of heat is released during this process, so it is an exothermic reaction. Also, a lot of heat is released during this reaction (you can even feel the mixture becoming hot), so it is an exothermic reaction.]

Question: When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. The reaction is an example of a : [2020]

(a) Combination reaction (b) Displacement reaction

Answer: (d) Double displacement reaction

[ When hydrogen sulphide gas reacts with copper sulphate solution, ions exchange between the two compounds. As a result, black copper sulphide (CuS) is formed as a precipitate, and sulphuric acid remains in solution.

This type of reaction involves exchange of ions between two compounds, so it is a double displacement reaction. ]

Question: In a double displacement reaction such as the reaction between sodium sulphate solution and barium chloride solution : [2020]

(i) Exchange of atoms takes place

(ii) Exchange of ions takes place .

(iii) a precipitate is produced .

(iv) an insoluble salt is produced

The correct option is :

(a) (ii) and (iv) (b) (i) and (iii) (c) only (ii) (d) (ii) , (iii) and (iv)

Answer: (d) (ii), (iii), and (iv)

[ In a double displacement reaction, the ions of the reacting compounds exchange places, not the atoms.

For the reaction: Sodium sulphate + Barium chloride → Barium sulphate (precipitate) + Sodium chloride

(i) Exchange of atoms → Wrong

(ii) Exchange of ions → Correct

(iii) A precipitate is produced → Correct (BaSO₄ is a white precipitate)

(iv) An insoluble salt is produced → Correct (Barium sulphate is insoluble)

So, the correct answer is: (d) (ii), (iii) and (iv) ]

Question: Which of the statements about the reaction below are incorrect?

(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all

Answer : (i) (a) and (b)
Question:
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction
(c) decomposition reaction.
(d) displacement reaction.

Answer : (d) displacement reaction.

Question: What happens when dilute hydrochloric acid is added to iron fillings? 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.

Answer : (a) Hydrogen gas and iron chloride are produced.

Question: A student took sodium sulphate solution in a test tube and added barium chloride solution to it . He observed that an insolution substance has formed . The colour and molecular formula of the insoluble substance is : [2021]

(a) Grey , (b) Yellow , (c) White , (d) Pink ,

Answer: (C) White , .

[ When sodium sulphate solution reacts with barium chloride solution, the reaction is:

Na₂SO₄ (aq)+BaCl₂ (aq)→BaSO₄ (s)+2NaCl (aq)

Barium sulphate (BaSO₄) is formed as an insoluble precipitate, and its colour is white.

So, the correct answer is: (c) White, BaSO₄ ]

Question: Sodium reacts with water to form sodium hydroxide and hydrogen gas . The balanced equation which represents the above reaction is : [2021]

(a)

(b)

(c)

(d)

Answer: (b) .

[Reaction given: Sodium + Water → Sodium hydroxide + Hydrogen

Correct balanced equation is: 2Na (s)+2H₂O (l)→2NaOH (aq)+H₂ (g) ]

Question:

The above reaction is a/an [2021]

(a) displacement reaction

(b) endothermic reaction

(d) neutralization reaction

Answer: (c) exothermic reaction .

[ The reaction : represents respiration (combustion of glucose).

In this process, energy is released in the form of heat. So, it is an exothermic reaction. ]

10. Which of the following statements about the reaction given below are correct ? [2021]

(i) HCl is oxidised to

(ii) is reduced to

(iii) acts as an oxidising agent

(iv) HCl acts as an oxidising agent .

(a) (ii) , (iii) and (iv) (b) (i) , (ii) and (iii) (c) (i) and (ii) only (d) (iii) and (iv) only

Answer: (c) (i) and (ii) only .

[ The reaction :

(i) HCl is oxidised to Cl₂ → Correct
(Chloride ions from HCl lose electrons to form Cl₂ → oxidation)

(ii) MnO₂ is reduced to MnCl₂ → Correct
(Mn gains electrons → reduction)

(iii) MnCl₂ acts as an oxidising agent → Wrong
(It is just a product, not acting as oxidising agent)

(iv) HCl acts as an oxidising agent → Wrong
(HCl is actually reducing agent, since it gets oxidised)

Correct answer: (c) (i) and (ii) only ]

Question:

In order to balance the above chemical equation , the values of and respectively are : [2023]

(a) 6 , 2 , 2 (b) 4 , 1 , 2 (c) 4 , 2 , 1 (d) 2 , 2 , 1

Answer: (c) 4, 2, 1

[ Balance the equation:

So, values are: x = 4, y = 2, z = 1

Correct answer: (c) 4, 2, 1 ]

Assertion-Reason based questions.

These consist of two statements – Assertion (A) and Reason (R) . Answer these questions selecting the appropriate option given below : [2023]

(a) Both (A) and (R) are true and (R) is the correct explanation of (A) .

(b) Both (A) and (R) are true but (R) is not the correct explanation of (A) .

(d) (A) is false , but (R) is true .

Question: Assertion (A) : The colour of aqueous solution of copper sulphate turns colourless when a piece of lead is added to it .

Reason (R) : Lead is more reactive than copper and hence displaces copper from its salt solution.

Answer: (d) (A) is false, but (R) is true .

[ Assertion (A): The blue colour of copper sulphate solution turns colourless when lead is added → False
When lead reacts with copper sulphate, lead sulphate (PbSO₄) is formed, which is insoluble (white precipitate). The solution does not become colourless in the proper sense because the reaction is not a simple clear displacement like with zinc or iron.

Reason (R): Lead is more reactive than copper and displaces it → True
Lead is above copper in the reactivity series, so it can displace copper.

So, the correct answer is: (d) (A) is false, but (R) is true ]

Short Question and Answer (1 Mark) :

Question: Balance the following chemical equations : [2008 1M]

Answer: The balance chemical equation is:

Question: Balance the following chemical equation : [2009 1M]

Answer: The balance chemical equation is :

Question: Why is respiration considered an exothermic process ? [2008 1M]

Answer: Respiration is exothermic because it involves the breakdown of carbohydrates into glucose, which then combines with oxygen in cells, releasing a significant amount of heat energy.

Question: What change in the colour of iron nails and copper sulphate solution you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes ? [ 2010 1M]

Answer: The colour of iron nails changes to brownish due to the deposition of copper on the nails. The blue colour of copper sulphate solution fades as copper ions transfer to the iron nails.

Short Question and Answer (2 Marks)

Question: What happen when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride ? State the physical conditions of reactants in which the reaction between them will not take place . Write the balanced chemical equation for the reaction and name the type of reaction. [2010 2M]

Answer: When an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride, a white insoluble substance is formed. This substance is barium sulphate, which settles down as a precipitate. Sodium chloride remains dissolved in the solution.

This reaction will not take place if the substances are not in aqueous form (for example, if they are in solid state), because the ions will not be free to move and react.

Balanced chemical equation: Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) ↓ + 2NaCl (aq)
This is a double displacement reaction and also a precipitation reaction, because an insoluble solid (precipitate) is formed.

Question: What is a balanced chemical equation? Why should chemical equations be balanced? [2M]
Answer: A balanced chemical equation is one in which the number of atoms of each element is equal on both the reactant and product sides.

Chemical equations should be balanced to satisfy the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction. It also ensures the equation correctly represents the actual chemical change.

Question: Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactions . Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity . [2018 2M]

Answer : Decomposition reactions need energy to break a compound into simpler substances. This energy can be supplied in different forms:

(a) By heat (Thermal decomposition)

When heat is supplied:

(b) By light (Photochemical decomposition)

When sunlight is supplied:

When electric current is passed:

Question: Translate the following statements into chemical equations and then balance them. [2M]
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulpur 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.
Answer: (a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Question: Balance the following chemical equations. [2M]

(a)
(b)
(c)
(d)
Answer: (a)
(b)
(c)
(d)

Question: Write the balanced chemical equations for the following reactions. [2M]
(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
Answer: (a)

(b)

(c)

(d)

Question: Write the balanced chemical equation for the following and identify the type of reaction in each case. [2M]
(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)

Answer: (a)

Double displacement reaction .

(b)

Decomposition reaction .

(c)

Combination reaction .

(d)

Combination reaction (Exothermic reaction) .
Question: What does one mean by exothermic and endothermic reactions? Give examples. [2M]
Answer: Exothermic reactions : Reaction in which heat is released along with the formation of products are called exothermic chemical reactions.

Example:

Endothermic reactions : Reaction in which energy is absorbed are known as endothermic reactions.

Example: Melting ice , Dry ice

Long Questions and Answers : (3 Marks)

Question: 2 g of silver chloride is taken in a china dish and the china dish is placed in sunlight for sometime. What will be your observation in this case ? write the chemical reaction involved in the form of a balanced chemical equation . Identify the type of chemical reaction . [3M 2019]

Answer: When 2 g of silver chloride is kept in a china dish and exposed to sunlight, its white colour slowly changes to grey. This happens because silver chloride decomposes and forms metallic silver.

Balanced chemical equation:

Type of reaction: This is a decomposition reaction, because the compound breaks down in the presence of sunlight.

Question: (i) What is the colour of ferrous sulphate crystals ? How does this colour change after heating ?

(ii) Name the products formed on strongly heating ferrous sulphate crystals . What type of chemical reaction occurs in this change ? [2009 3M]

Answer: (i) The ferrous sulphate crystals is green colour . When ferrous sulphate crystals are heated, the colour changes from green to white due to the formation of ferric oxide (Fe₂O₃) during the decomposition reaction.

(ii) On strongly heating ferrous sulphate crystals, the products formed are ferric oxide (Fe₂O₃), sulphur dioxide (SO₂), and sulphur trioxide (SO₃).

The chemical reaction that occurs is a decomposition reaction.

The reaction chemical reaction :

Question: Explain the following terms with one example each. [3M]
(a) Corrosion
(b) Rancidity

Answer: (a) Corrosion : 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. The black coating on silver and the green coating on copper are other examples of corrosion.

Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron. Corrosion of iron is a serious problem. Every year an enormous amount of money is spent to replace damaged iron.
(b) Rancidity : When fats and oils are oxidized , they become rancid and their smell and taste change. Usually some substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. So, Keeping food in air tight containers helps to slow down oxidation.

For example : The chip manufacturers usually do flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidized .

Question: Lead nitrate solution is added to a test tube containing potassium iodine solution .

(a) Write the name and colour of the compound precipitated.

(b) Write the balance chemical equation for the reaction involved .

Answer: (a) Name and colour of precipitate:
The compound formed is lead iodide (PbI₂), and its colour is bright yellow.

(b) Balanced chemical equation:

(c) Type of reaction (with reason):
This is a double displacement reaction because the ions exchange their partners between the two compounds.
It is also a precipitation reaction because an insoluble solid (PbI₂) is formed.

Question: (a) Identify the reducing agent in the following reactions : [2023]

(i)

(ii)

(iii)

(iv)

(b) Define a redox reaction in terms of gain or loss of oxygen.

Answer: (i)
NH₃ is the reducing agent
( NH₃ gets oxidised to NO)

(ii)
H₂O is the reducing agent
( Oxygen in H₂O gets oxidised)

(iii)
CO (carbon monoxide) is the reducing agent
( CO gets oxidised to CO₂)

(iv)
H₂ (hydrogen) is the reducing agent
( H₂ gets oxidised to H₂O)

(b) A chemical reaction in which one substance gains oxygen (oxidation) and another substance loses oxygen (reduction) at the same time is called a redox reaction.