class 10 chemical reactions and equations important questions

Chemical Equations

Chemical reactions play a crucial role in our daily lives. From cooking food to rusting iron, chemical reactions are happening all around us. In CBSE Class 10 Science, the chapter “Chemical Reactions and Equations” introduces students to the fundamentals of how substances interact and transform.

This blog post provides detailed notes, explanations, and examples to help students understand the topic effectively and score full marks in their exams.

Activity 1.1

1. Mg Ribbon (3-4 cm) rubbed with sandpaper

2. Hold it with tongs.

3. Burn it using a burner and collect the ash.

Inference:

Burning of Mg in air forms Magnesium oxide.
Equation:
2Mg + O₂ → 2MgO
(Reactants) → (Product)

A chemical reaction has taken place and resulted in the formation of MgO.

If there is:

Change in state

Change in colour

Evolution of gas

Change in temperature

Then a chemical reaction has taken place.

Chemical Equation:

It represents a chemical reaction.
LHS: Reactants
RHS: Products

Balanced Equation:

The number of atoms of each element should be equal on both sides.

2Mg + O₂ → 2MgO

Law of Conservation of Mass:

Mass can neither be created nor destroyed.
The total mass of elements present in the product of a chemical reaction has to be equal to the total mass of elements present in the reactants.
In other words, the number of atoms of each element remains the same before and after the chemical reaction.

Example 1:

Zn + H₂SO₄ → ZnSO₄ + H₂↑

Reactants	Products
Zn – 1	Zn – 1
H – 2	H – 2
S – 1	S – 1
O – 4	O – 4

So this is a balanced chemical equation.

Example 2:

Fe + H₂O → Fe₃O₄ + H₂

Reactants	Products
Fe – 1	Fe – 3
H – 2	H – 2
O – 1	O – 4

Steps to balance the equation:

1. Fe + H₂O → Fe₃O₄ + H₂

2. 3Fe + H₂O → Fe₃O₄ + H₂

3. 3Fe + 4H₂O → Fe₃O₄ + H₂

4. 3Fe + 4H₂O → Fe₃O₄ + 4H₂

Now, the equation is balanced.

Physical States in Chemical Equations:

To make the chemical equation more informative, the physical states of reactants and products are mentioned as:

(s) = Solid

(g) = Gaseous

(aq) = Aqueous (Liquid)

When (g) is used in H₂O, it indicates that a chemical reaction takes place using steam instead of liquid water.

Indicating Conditions in Reactions:

Sometimes, temperature, pressure, and catalysts are indicated above or below the arrow in the equation.

Example:

CO (g) + H₂ (g) → CH₃OH (at 340 atm)

Photosynthesis Reaction:

CO₂ (aq) + H₂O (aq) → C₆H₁₂O₆ (aq) + O₂ (g) + H₂O (aq)

(Occurs in sunlight with chlorophyll)

Catalyst in Chemical Reactions:

A catalyst is a chemical substance that speeds up a chemical reaction without reacting with the reactants or products.

Example:

Potassium permanganate (KMnO₄) is used in the decomposition of hydrogen peroxide into water and oxygen.

Reaction:

H₂O₂ (aq) → H₂O (l) + O₂ (g) (KMnO₄ as catalyst)

Types of Chemical Reactions

Combination Reactions:

When two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.

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

Quicklime + Water → Slaked Lime

C (s) + O₂ (g) → CO₂ (g)

2H₂ (g) + O₂ (g) → 2H₂O (g)

Exothermic Reactions:

Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Burning of Natural Gas:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

(Methane)

Respiration is an Exothermic Reaction

How?
During digestion, food is broken down into simpler substances.

Example:

Rice, potatoes contain carbohydrates.
Carbohydrates are broken down to form glucose.
Glucose combines with O₂ in the cells of our body and provides energy.

The special name of this reaction is Respiration.

Decomposition Reaction

Example 1:

2FeSO₄ → (Heat) Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

Ferrous Sulphate → Ferric Oxide + Sulphur Dioxide + Sulphur Trioxide

A single reactant breaks down to give simpler products. This is a decomposition reaction.

FeSO₄.7H₂O loses water when heated, and the colour of the crystals changes.

Example 2:
When a decomposition reaction is carried out by heating, it is called thermal decomposition.

Decomposition of Calcium Carbonate

CaCO₃ → (Heat) CaO (s) + CO₂ (g)

Limestone → Quicklime + Carbon Dioxide

Example 3:

Lead Nitrate Decomposition:
2Pb(NO₃)₂ → (Heat) 2PbO (s) + 4NO₂ (g) + O₂ (g)

Lead Nitrate → Lead Oxide + Nitrogen Dioxide + Oxygen

Decomposition of silver chloride turns grey into silver and chlorine by sunlight

2AgCl 2Ag + Cl₂(Sunlight )

(s) (s) (g)

2AgBr 2Ag + Br₂It is used in Black and white

(s) (s) (g) Photography

Endothermic Reaction:

Decomposition reactions require energy either in the form of heat, energy or electricity for breaking down the reactants.

Reactions in which energy is absorbed are known as Endothermic reactions.

Exothermic reaction	Endothermic reaction
Exo means to release. Thermic means Heat	Endo means to absorb thermic means Heat
Exothermic reactions are chemical reactions that release heat energy into the surroundings	Endothermic reactions are chemical reactions that absorb heat energy into the surroundings
Temperature increases with the speed up of the reaction	Temperature decreases with the speed up of the reaction
Energy is released from the system	Energy should be given to the system

Displacement reaction:

It is a chemical reaction that occurs when a more reactive element replaces less reactive element in a compound

Ex1:

Fe + CuSO₄ FeSO₄+ Cu

(s) (aq) (aq) (s)

Iron has displaced Copper from Copper sulphate and formed as Ferrous sulphate

Ex 2:

Zn + CuSO₄ ZnSO₄+ Cu

(s) (aq) (aq) (s)

Pb + Cl₂PbCl₂ + Cu

(s) (g)

Here Fe, Zn, Pb are more reactive element , they have displayed Copper less reactive element

Double Displacement Reaction / Metathesis

What is it?

It is a chemical reaction where two compounds react, and two ions or radicals interchange places and form new compounds.

Example 1:
Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

Sodium Sulphate + Barium Chloride → Barium Sulphate + Sodium Chloride
SO₄²⁻ & Cl⁻ : Both interchange

Precipitate

It is an insoluble compound.
Any reaction that produces a precipitate is called a precipitation reaction.

BaSO₄ = White precipitate

Example 2:
Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (s) + 2KNO₃ (aq)

Lead Nitrate + Potassium Iodide → Lead Iodide + Potassium Nitrate

PbI₂ = Yellow precipitate

Oxidation & Reduction Reaction

If a substance gains O₂ during reaction, it is said to be oxidized.
If a substance loses O₂ during reaction, it is said to be reduced.

When one reactant get oxidised and other gets reduced. It is called as Oxidation reduction reaction or redox reaction

Ex:

CuO + H₂ Cu + H₂O

CuO Cu 🡪 Reduction ( loses Oxygen)

Copper oxide Copper

H₂ H₂O 🡪 Oxidation ( gains Oxygen)

Other Examples:

ZnO + C Zn + CO

Zinc Carbon Zinc Carbon

Oxide Monoxide

MnO₂ + 4HCl MnCl₂+ 2H₂O + Cl₂

MnO₂ – Manganese Oxide

HCl – Hydrochloric acid

MnCl₂– Manganese Chloride

2H₂O – Water

Cl₂– Chlorine

Oxidised – Gains oxygen/ loses Hydrogen

Reduced – Loses oxygen/ gains Hydrogen

Effect of oxidation in everyday life:

Corrosion :

The process in which metal is eaten up by air, moisture or chemical on their surface is called corrosion

Ex: Rusting of Iron

When Iron object is left in a place of high moisture for considerable amount of time it result in the formation of reddish brown substance

4Fe + 3O₂+ 6 H₂O 🡪 4Fe(OH)₃

Fe(OH)₃– Hydrated Iron III Oxide (Rust)

Corrosion causes damage to car bodies, bridges, iron railing, ships etc.
It can be prevented by galvanisation (coating of Zn)
Apply oil/grease/ paint

Copper reacting with moisture:

2Cu + O₂ + H₂0 + CO₂ 🡪 Cu(OH)₂+ CuCO₃

Cu(OH)₂+ CuCO₃🡪 Copper hydroxide and copper carbonate together forms the green coating on the object

Silver kept in air get tarnished and turns black

2Ag + H₂S 🡪 Ag₂S + H₂

(s) (g) (s) (g)

Ag – Silver

H₂S – Hydrogen Sulphide

Ag₂S – Silver sulphide

Rancidity:

Rancid – Fat/oil containing food waste /unpleasant smell when it is kept for a long time
This happens due to oxidation reaction
Substances that prevent oxidation are called as antioxidant are added to foods containing fat/oil
Keep food in air tight container will slow down the process of oxidation
Chips company flush the chips packet with nitrogen to prevent chips from being oxidised

Class 10 Chemical Reactions and Equations Activity Questions with answers

Activity 1.1

Burning of Magnesium Ribbon in air

Observation:

When magnesium ribbon is burned, it burns with a bright white flame.
After burning, it leaves behind a white powdery ash in the watch-glass.

Explanation:

This is a combination reaction where magnesium reacts with oxygen in the air to form magnesium oxide (MgO).
The white ash is magnesium oxide, a basic oxide.

Chemical Equation:

2Mg (s) + O₂ (g) → 2MgO (s)

Activity 1.2

Lead nitrate and Potassium Iodide

Observation:

When potassium iodide solution is added to lead nitrate solution, a bright yellow precipitate is formed immediately.

Explanation:

This is a double displacement reaction.
Lead(II) iodide (PbI₂) is formed as an insoluble yellow precipitate, and potassium nitrate (KNO₃) remains in solution.
This is also called a precipitation reaction.

Activity 1.3

Zinc and Dilute Sulphuric acid

Observation:

When dilute hydrochloric acid or sulphuric acid is added to zinc granules, bubbles (effervescence) are seen around the zinc.
A gas is evolved.
On touching the flask or test tube, it feels warm.

Explanation:

This is a chemical reaction between zinc and dilute acid, producing hydrogen gas.
The reaction is exothermic, so heat is released, causing a rise in temperature.
The bubbles are due to the evolution of hydrogen gas.

Chemical Equation:

With hydrochloric acid:
Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

With sulphuric acid:
Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)

Chemical Equation:

Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (s) ↓ + 2KNO₃ (aq)

Activity 1.4

Formation of slaked lime by the reaction of calcium oxide with water

Observation:

When water is added to calcium oxide (quick lime), a hissing sound is heard.
A white, slaked suspension is formed.
On touching the beaker, it feels hot

Explanation:

This is an exothermic reaction, which releases heat.
Calcium oxide reacts with water to form calcium hydroxide (slaked lime), and heat is produced in the process.

Chemical Equation:

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

Activity 1.5

Heating of Ferrous sulphate

Observation:

Before heating: Ferrous sulphate crystals are green in colour.
After heating:
- The crystals first turn white as water of crystallization is lost.
- On further heating, they turn brown and emit a smell of burning sulphur (like rotten eggs).

Explanation:

This is a thermal decomposition reaction.
On heating, ferrous sulphate (FeSO₄·7H₂O) loses water and decomposes to form:
- Ferric oxide (Fe₂O₃) – brown solid
- Sulphur dioxide (SO₂) and sulphur trioxide (SO₃) – gases with a pungent smell

Chemical Equation:

2FeSO₄ (s) → Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)
(on heating)

Activity 1.6

Heating of Lead nitrate and emission of Nitrogen dioxide

Observation:

On heating, lead nitrate starts to decompose.
A brown gas is released.
A yellow residue is left behind in the boiling tube.

Explanation:

This is a thermal decomposition reaction.
On heating, lead nitrate (Pb(NO₃)₂) breaks down to form:
- Lead oxide (PbO) – yellow solid
- Nitrogen dioxide (NO₂) – brown gas
- Oxygen (O₂) – colourless gas

Chemical Equation:

2Pb(NO₃)₂ (s) → 2PbO (s) + 4NO₂ (g) + O₂ (g)
(on heating)

Activity 1.7

Electrolyis of water

Observation:

Bubbles are formed at both electrodes after switching on the current.
The volume of gas collected is different in the two test tubes:
- One test tube collects twice the volume of gas as the other.
When a burning candle is brought near the gas:
- In one test tube, the gas burns with a ‘pop’ sound.
- In the other, the candle glows brighter.

Explanation:

This is an example of electrolysis of water.
Water (H₂O) breaks down into hydrogen and oxygen gases.
The gas that burns with a pop sound is hydrogen.
The gas that supports combustion and makes the flame brighter is oxygen.
Since water has two parts of hydrogen and one part of oxygen, the volume of hydrogen collected is twice that of oxygen.

Chemical Equation:

2H₂O (l) → 2H₂ (g) + O₂ (g)
(by electrolysis)

Activity 1.8

Silver chloride turns grey in sunlight to form silver metal

Observation:

Initially, silver chloride is white in colour.
After exposure to sunlight, it turns grey.

Explanation:

This is a photochemical decomposition reaction.
In sunlight, silver chloride breaks down into silver metal and chlorine gas.
The grey colour appears because of the formation of metallic silver.

Chemical Equation:
2AgCl (s) → 2Ag (s) + Cl₂ (g) [in presence of sunlight]

Activity 1.9

Iron nails dipped in copper sulphate solution
Iron nails and copper sulphate solutions compared before and after the experiment

:Observation:

The blue colour of copper sulphate solution in test tube B becomes fainter compared to test tube A.
The iron nails kept in test tube B get a reddish-brown coating.
The iron nail kept aside remains unchanged.

Explanation:

This is a displacement reaction.
Iron is more reactive than copper, so it displaces copper from copper sulphate solution.
The displaced copper gets deposited on the iron nails, giving them a reddish-brown coating.
Due to the removal of copper from the solution, the blue colour of copper sulphate fades.

Chemical Equation:

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

Activity 1.10

Formation of Barium sulphate and Sodium Chloride

Observation:

When sodium sulphate solution is mixed with barium chloride solution, a white precipitate is formed.

Explanation:

This is a double displacement reaction.
A white insoluble substance, barium sulphate (BaSO₄), is formed as a precipitate.
This type of reaction is also called a precipitation reaction.

Chemical Equation:

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

Activity 1.11

Oxidation of Copper to Copper oxide

Observation:

On heating, the reddish-brown copper powder turns black.

Explanation:

This is a combination reaction where copper reacts with oxygen in the air to form copper(II) oxide (CuO).
The black coating is due to the formation of copper oxide

Chemical Equation:

2Cu (s) + O₂ (g) → 2CuO (s)

Intext Pg.10

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

The substance ‘X’ used for whitewashing is calcium oxide.
Formula: CaO

(ii) Reaction of calcium oxide with water:

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

This reaction forms calcium hydroxide (slaked lime), which is used in whitewashing.
The solution reacts with carbon dioxide in the air to form a shiny white coating of calcium carbonate on walls.

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.

In Activity 1.7 (Electrolysis of water), water decomposes into hydrogen and oxygen gases.
The volume of hydrogen gas is double that of oxygen gas because each water molecule (H₂O) has two hydrogen atoms and one oxygen atom.
Hence, twice the amount of hydrogen is released compared to oxygen.
Name of the gas with double volume: Hydrogen gas (H₂)

Intext 1:Pg 6

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

Magnesium ribbon is cleaned before burning to remove the layer of magnesium oxide (MgO) that forms on its surface due to reaction with atmospheric oxygen. This ensures that magnesium burns efficiently in air.

2. Balanced Chemical Equations

(i) Hydrogen + Chlorine → Hydrogen chloride

H₂+Cl₂→2HCl

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

3BaCl₂+Al₂(SO4)₃→3BaSO₄+2AlCl₃

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

2Na+2H2O→2NaOH+H2↑2Na + 2H₂O → 2NaOH + H₂ ↑

3. Balanced Chemical Equations with State Symbols

(i) Barium chloride and sodium sulphate in water react to form barium sulphate (insoluble) and sodium chloride (soluble):

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

(ii) Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water:

NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)

Intext Pg 13

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

Iron is more reactive than copper.
When an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution.
As a result:
- Copper gets deposited on the iron nail (reddish-brown layer), and
- The blue colour of copper sulphate fades due to the formation of colourless iron sulphate.

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

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Example:
When sodium carbonate reacts with calcium chloride:

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

A white precipitate of calcium carbonate is formed.
This is a double displacement and precipitation reaction.

3. Identify the substances that are oxidised and reduced:

(i) 4Na(s) + O₂(g) → 2Na₂O(s)

Sodium (Na) is oxidised to sodium oxide (Na₂O).
Oxygen (O₂) is reduced to oxide ion (O²⁻).

Oxidised: Sodium (Na)

Reduced: Oxygen (O₂)

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Hydrogen (H₂) is oxidised to water (H₂O).
Copper oxide (CuO) is reduced to copper (Cu).

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

Book back answers: pg 14

Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) → 2Pb(s) + CO₂(g)

Statements:

(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.

Correct Answer: (i) (a) and (b)

(a) Lead is getting reduced – Correct
(b) Carbon dioxide is getting oxidised – Incorrect (CO₂ is the product, it is not being oxidised)
Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a

combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.

Correct Answer: (d) displacement reaction

Book back exercises:

Which of the statements about the reaction below are incorrect?

2PbO(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

Ans: (b) Carbon dioxide is getting oxidised

CO₂ is a product and already fully oxidised (carbon = +4).
It is not undergoing oxidation in this reaction.

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

Here, aluminium displaces iron from iron oxide.

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.

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

4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer:
A balanced chemical equation has equal numbers of atoms of each element on both sides of the equation.
Chemical equations should be balanced to obey the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.

5. Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.
Answer: N₂ + 3H₂ → 2NH₃

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
Answer: 2H₂S + 3O₂ → 2H₂O + 2SO₂

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Answer: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer: 2K + 2H₂O → 2KOH + H₂

6. Balance the following chemical equations.

(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
Answer: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
Answer: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃
Answer: Balanced as it is: NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Answer: BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

7. Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Answer: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(b) Zinc + Silver nitrate → Zinc nitrate + Silver
Answer: Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

(c) Aluminium + Copper chloride → Aluminium chloride + Copper
Answer: 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer: BaCl₂ + K₂SO₄ → BaSO₄ + 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)
Answer: 2KBr + BaI₂ → 2KI + BaBr₂
Type of Reaction: Double Displacement

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Answer: ZnCO₃ → ZnO + CO₂
Type of Reaction: Decomposition

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Answer: H₂ + Cl₂ → 2HCl
Type of Reaction: Combination

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Answer: Mg + 2HCl → MgCl₂ + H₂
Type of Reaction: Displacement

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

Exothermic reactions are chemical reactions that release heat.
Example: C + O₂ → CO₂ + Heat
Endothermic reactions are chemical reactions that absorb heat.
Example: CaCO₃ → CaO + CO₂

10. Why is respiration considered an exothermic reaction? Explain.
Answer:
Respiration is considered an exothermic reaction because it releases energy.
Glucose reacts with oxygen to produce carbon dioxide, water, and heat energy.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer:
In a decomposition reaction, a compound breaks down into simpler substances, while in a combination reaction, two or more substances combine to form a single compound. Thus, they are opposites.

Decomposition reaction: 2H₂O → 2H₂ + O₂
Combination reaction: 2H₂ + O₂ → 2H₂O

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:

Heat: CaCO₃ → CaO + CO₂
Light: 2AgCl → 2Ag + Cl₂ (in presence of sunlight)
Electricity: 2H₂O → 2H₂ + O₂ (electrolysis of water)

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer:

Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
Example: Zn + CuSO₄ → ZnSO₄ + Cu
Double Displacement Reaction: Two compounds exchange ions to form two new compounds.
Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

14. In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write down the reaction involved.
Answer:
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag

15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer:
A precipitation reaction is a type of double displacement reaction in which an insoluble solid (precipitate) is formed.
Example: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl (BaSO₄ is a white precipitate)

16. Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation: Gain of oxygen by a substance.
Examples:

2Mg + O₂ → 2MgO
C + O₂ → CO₂

(b) Reduction: Loss of oxygen by a substance.
Examples:

CuO + H₂ → Cu + H₂O
ZnO + C → Zn + CO

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.
Answer:
Element ‘X’ is Copper (Cu).
The black coloured compound is Copper(II) oxide (CuO).
Reaction: 2Cu + O₂ → 2CuO

18. Why do we apply paint on iron articles?
Answer:
To prevent rusting of iron by protecting it from moisture and air (oxygen) which cause corrosion.

19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer:
To prevent rancidity. Nitrogen displaces oxygen and prevents the oxidation of fats and oils.

20. Explain the following terms with one example each.

(a) Corrosion:
It is the slow conversion of metals into undesirable compounds due to the reaction with moisture, air, or chemicals.
Example: Iron reacts with water and oxygen to form rust (Fe₂O₃·xH₂O).

(b) Rancidity:
It is the spoilage of oils or fats due to oxidation, causing a foul smell and taste.
Example: Chips packed with nitrogen to avoid rancidity of oil in them.

class 10 chemical reactions complete pdf

Class 10 Chemical Reactions and Equations Book back intext

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