class 10 - Chapter 1 [Chemical Reactions and Equations]

NCERT Solutions for Class 10 Science Chapter 1

Intext Questions

[Page Number: 6]

Question 1 

Why should a magnesium ribbon be cleaned before burning in air?

 Magnesium ribbon is cleaned by rubbing with sandpaper before burning to remove the protective layer of magnesium oxide (MgO) that forms on its surface when it's exposed to air.

Reason:

• Magnesium reacts with oxygen in the air and forms magnesium oxide:

2Mg + O₂ → 2MgO

• This oxide layer prevents proper burning of magnesium.

Cleaning exposes the fresh magnesium metal, allowing it to burn brightly and react properly with oxygen

Question 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

(i) H₂ + Cl₂ → 2HCl
(ii) 3 BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + 2 AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂↑

Question 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.
Answer:
(i) BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄(s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl(aq) → NaCl(aq) + H₂O(l)

[Page Number: 10]

Question 1 
A solution of a substance ‘X’ is used for white washing.

(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

(i) Name the substance ‘X’ and write its formula.

• The substance ‘X’ is quicklime.
• Name: Calcium oxide
• Formula: CaO

This is the substance used for whitewashing.

(ii) Reaction of ‘X’ (Calcium oxide) with water:

When calcium oxide (CaO) is mixed with water, it reacts to form calcium hydroxide (also called slaked lime), which is actually used in whitewashing.

CaO (s)  +  H₂O (l) → Ca(OH)₂ (aq) +Heat

This is an exothermic reaction (releases heat).

How it works in whitewashing:

• Calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate (CaCO₃), which gives a shiny white finish on walls:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O 

Question 2 

Why is the amount of gas collected in one of the test tubes in text book Activity 1.7 (i.e., electrolysis of water) double of the amount collected in the other? Name this gas

Activity 1.7: Electrolysis of Water

In this activity, water is broken down into hydrogen and oxygen gases using electricity. This process is called electrolysis.

The amount of gas in one test tube double that of the other

Because water (H₂O) contains 2 parts of hydrogen and 1 part of oxygen, during electrolysis:

• Hydrogen gas (H₂) is released at the cathode.
• Oxygen gas (O₂) is released at the anode.

So, the volume of hydrogen collected is twice the volume of oxygen.

Reaction:

2H₂O(l) → Electricity 2H₂(g) + O₂(g)

[Page Number: 13]

Question 1

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

When an iron nail is dipped into copper sulphate solution (CuSO₄), a chemical reaction takes place:

Fe (s) + CuSO₄(aq) → FeSO₄(aq) + Cu (s) 

What happens in this reaction?

• Iron (Fe) is more reactive than copper (Cu).
• Iron displaces copper from copper sulphate solution.
• As a result:
• Copper gets deposited on the nail.
• The blue colour of CuSO₄ fades, forming greenish FeSO₄ (iron sulphate).

Question 2
Give an example of a double displacement reaction other than the one given in Activity 1.10 (NCERT Text Book)

Reaction between Sodium carbonate and Calcium chloride

CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s)↓ + 2NaCl(aq)

•    Calcium carbonate (CaCO₃) is a white precipitate.

Question 3
Identify the substances that are oxidised and the substances which are reduced in the following reactions.
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO (s) + H₂(g) → Cu (s) + H₂O(l)

 

(i) 4Na(s) + O₂(g) → 2Na₂O(s)

Explanation:

• Sodium (Na) loses electrons to form Na⁺ → Oxidation
• Oxygen (O₂) gains electrons to form O²⁻ → Reduction

Substances:

• Oxidised: Sodium (Na)
• Reduced: Oxygen (O₂)

 

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Explanation:

• Hydrogen (H₂) gains oxygen → forms H₂O → Oxidation
• Copper oxide (CuO) loses oxygen → forms Cu → Reduction

 Substances:

• Oxidised: Hydrogen (H₂)
• Reduced: Copper oxide (CuO)

Textbook Chapter End Questions

Question 1
Which of the statements about the reaction below are incorrect ?
2 PbO(s) + C(s) → 2Pb (s) + CO₂(g)
(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)

Species                      What Happened                  Correct to Say?

PbO → Pb                 Pb²⁺ gains e⁻                         PbO is reduced ✅

C → CO₂                    C loses e⁻                              C is oxidized ✅

CO₂ (product)          Already formed                   CO₂ is oxidized ❌ (incorrect)

Pb (product)             Already formed                   Pb is reduced ❌ (incorrect)

Question 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

Correct answer:

(d) Displacement reaction

 Fe₂O₃ (Iron(III) oxide) reacts with Al (Aluminium).

 Aluminium displaces iron from its compound.

Question 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.

the correct answer is:

(a) Hydrogen gas and iron chloride are produced.

When dilute hydrochloric acid (HCl) is added to iron filings (Fe), the following reaction takes place:

Fe (s) + 2HCl (aq) → FeCl₂(aq) + H₂(g) 

Question 4
What is a balanced chemical equation ? Why should chemical equations be balanced ?

What is a Balanced Chemical Equation?

A balanced chemical equation is a chemical equation in which the number of atoms of each element is the same on both sides of the equation — reactants and products.

This means mass is conserved during the reaction.

Why Should Chemical Equations Be Balanced?

1.    To obey the Law of Conservation of Mass:
Matter cannot be created or destroyed in a chemical reaction. So the total mass of reactants must equal the total mass of products.

2.    To show accurate proportions:
It tells us the exact ratio of reactants and products involved in the reaction.

Question 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.

Answer:
(a) 3H₂ (g) + N₂ (g) → 2NH₃ (g)
(b) H₂S (g) + 3O₂ (g) → SO₂ (g) + 2H₂O(l)
(c) 3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) → 2AlCl₃ (aq) + 3BaSO₄ ↓(s)
(d) 2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

Question 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
Answer:

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Question 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

Answer:
(a) Ca (OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2 Ag
(c) 2Al + 3 CuCl₂ → 2AlCl₃ + 3 Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

Question 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) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen 

Answer:

(a) 2KBr (aq) + Bal₂(aq) → 2Kl(aq) + BaBr₂(s)
Type : Double displacement reaction

(b) ZnCO₃ (s) → ZnO (s) + CO₂ (g)
Type : Decomposition reaction

(c) H₂ (g) + Cl₂ (g) → 2HCl(g)
Type : Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)
Type : Displacement reaction

Question 9
What does one mean by exothermic and endothermic reactions ? Give examples.

Exothermic Reaction:

An exothermic reaction is a chemical reaction that releases heat energy to the surroundings.

• The temperature of the surroundings increases.
• Energy is given out.

🔹 Example:

C + O₂ → CO₂ + Heat

Another example:

CH₄ + 2O₂ → CO₂ + 2H₂O +Heat

(Burning of methane – a common fuel)

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❄️ Endothermic Reaction:

An endothermic reaction is a chemical reaction that absorbs heat energy from the surroundings.

• The temperature of the surroundings decreases.
• Energy is taken in.

🔹 Example:

Ba(OH)₂ + NH₄Cl → BaCl₂ + NH₃ + H₂O −

Another common example:

Photosynthesis: 6CO₂ + 6H₂O → Sunlight → C₆H₁₂O₆ + 6O₂

Question 10
Why is respiration considered an exothermic reaction ? Explain.

Why is Respiration Considered an Exothermic Reaction?

Respiration is considered an exothermic reaction because it releases energy in the form of heat when glucose is broken down in the presence of oxygen.

The Reaction:

C₆H₁₂O₆ +6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat) .

Why is it Exothermic?

• The chemical bonds in glucose store a lot of potential energy.
• During respiration, these bonds are broken, and energy is released.
• This energy is used for various life processes like walking, thinking, digesting, etc

Question 11
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Why Are Decomposition Reactions the Opposite of Combination Reactions?

Because:

• In a combination reaction, two or more substances combine to form a single product.
• In a decomposition reaction, a single substance breaks down into two or more products.

 Examples:

Combination Reaction:

2H₂ + O₂ → 2H₂O₂
(Hydrogen and oxygen combine to form water)

Decomposition Reaction:

2H₂O → Electricity → 2H₂ + O₂ ​
(Water decomposes into hydrogen and oxygen)

Decomposition reactions are called the opposite of combination reactions because they reverse the process — breaking things down instead of building them up.