CG Board Class 8 Science Chapter 4 Solutions: Chemical Reactions – When and What Types (रासायनिक अभिक्रियाएँ – कब और कैसी) – 2026-27

What do you understand by chemical reaction?

A chemical reaction is a process in which one or more substances (reactants) undergo chemical changes to form new substances (products) possessing entirely different chemical composition and properties. During a chemical reaction, existing chemical bonds between atoms are broken and new bonds are formed.

Burning of candle is a chemical reaction. Give the properties connected with this.

When a candle burns, the wax reacts with atmospheric oxygen. The characteristics or properties indicating that a chemical reaction takes place are:

1. Change in State: Solid wax reacts and converts into gaseous carbon dioxide (CO₂) and water vapour (H_2O).
2. Evolution of Heat and Light: Heat and light energy are liberated during combustion, making it an exothermic reaction.
3. Formation of New Substances: Carbon dioxide and water vapour are produced as entirely new substances.

What are reactants and products? Explain with examples.

• Reactants: The chemical substances that take part in a chemical reaction are called reactants.
• Products: The new chemical substances formed as a result of a chemical reaction are called products.

Example: When magnesium ribbon is burned in the presence of oxygen, magnesium (Mg) and oxygen (O₂) are the reactants, while magnesium oxide (MgO) is the product.

Explain the following reactions with two examples each:

1. Combination
2. Decomposition
3. Displacement
4. Precipitation
5. Combination Reaction: A chemical reaction in which two or more chemical substances combine together to form a single new substance.

• Example (a): 2Mg + O₂ → 2MgO (Burning of magnesium)
• Example (b): NH₃ + HCl → NH_4Cl (Formation of ammonium chloride)

1. Decomposition Reaction: A chemical reaction in which a single reactant breaks down into two or more simpler products.

• Example (a): CaCO₃ → [Δ] CaO + CO₂↑ (Thermal decomposition of limestone)
• Example (b): 2H_2O → [Electricity] 2H₂ + O₂ (Electrolysis of water)

1. Displacement Reaction: A chemical reaction in which a more reactive element displaces a less reactive element from its compound.

• Example (a): Zn + 2HCl → ZnCl₂ + H₂↑ (Reaction of zinc with hydrochloric acid)
• Example (b): CuSO₄ + Zn → ZnSO₄ + Cu (Zinc displacing copper from copper sulphate हल)

1. Precipitation Reaction: A chemical reaction in which an insoluble solid product (called a precipitate) is formed when two solutions are mixed.

• Example (a): Ca(OH)₂ + CO₂ → CaCO₃↓ + H_2O (Passing CO₂ through limewater forms calcium carbonate precipitate)
• Example (b): AgNO₃ + NaCl → AgCl↓ + NaNO₃ (Formation of silver chloride white precipitate)

If you prepare lime water and leave it open, what changes will take place in this हल next day and why?

If transparent limewater (Ca(OH)₂) is left open to air overnight, it absorbs carbon dioxide gas (CO₂) present in the surrounding atmosphere. Carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate (CaCO₃), which forms a milky white film on the surface and settles down as a white precipitate.

Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s)↓ + H_2O (l)

What is neutralization reaction? Give one example.

A chemical reaction between an acid and an alkali (base) to form salt and water is known as a neutralization reaction. In this process, the acidic and basic properties neutralize each other.

Alkali + Acid → Salt + Water

Example: Reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl):

NaOH (aq) + HCl (aq) → NaCl (aq) + H_2O (l)

What is precipitate? Write a reaction in which a precipitate is formed.

An insoluble solid product formed during a chemical reaction in a liquid हल is called a precipitate.

Example: Passing carbon dioxide gas through clear limewater produces an insoluble white precipitate of calcium carbonate:

Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s)↓ + H_2O (l)

Explain oxidation, reduction and redox reactions.

1. Oxidation: A chemical reaction involving the addition of oxygen or the removal of hydrogen from a substance.

• Example: S + O₂ → SO₂ (Sulphur combines with oxygen)

1. Reduction: A chemical reaction involving the removal of oxygen or the addition of hydrogen to a substance.

• Example: 2HgO → [Δ] 2Hg + O₂ (Removal of oxygen from mercuric oxide)

1. Redox Reaction: A chemical reaction in which oxidation and reduction take place simultaneously.

• Example: Pb_3O₄ + 2C → 3Pb + 2CO₂ (Lead oxide is reduced, carbon is oxidized)

NaOH + HCl → NaCl + H_2O this is a:

(a) Decomposition reaction

(b) Combination reaction

(c) Neutralization reaction

(d) Precipitation reaction

(c) Neutralization reaction / (c) उदासीनीकरण अभिक्रिया

On ignition when a substance combines with oxygen, the process is called:

(a) reduction

(b) oxidation

(c) decomposition

(d) displacement

(b) oxidation / (b) ऑक्सीकरण

Which of these is not a characteristic of chemical reaction?

(a) production of heat

(b) change in colour

(c) liberation of gas

(d) formation of no new substance

(d) formation of no new substance / (d) कोई नया पदार्थ न बनना

2KClO₃ → [Δ] 2KCl + 3O₂ is a reaction:

(a) combination

(b) decomposition

(c) displacement

(d) neutralization

(b) decomposition / (b) अपघटन

Identify the type of reaction for each equation:

1. NH₃ + HCl → NH_4Cl
2. NaOH + HCl → NaCl + H_2O
3. 2NH₃ → N₂ + 3H₂
4. CuSO₄ + Zn → ZnSO₄ + Cu
5. Combination Reaction
6. Neutralization Reaction
7. Decomposition Reaction
8. Displacement Reaction

When calcium carbonate is heated we get calcium oxide and carbon dioxide.

CaCO₃ → [Δ] CaO + CO₂

What type of reaction is this? What are the reactants and products? Write it.

1. Type of Reaction: Thermal Decomposition Reaction.
2. Reactant: Calcium carbonate (CaCO₃).
3. Products: Calcium oxide (CaO) and Carbon dioxide gas (CO₂).

Give two examples for each of the following reactions in which:

1. Precipitate is formed
2. Heat is liberated
3. Colour is changed
4. Precipitate is formed:

• (a) Ca(OH)₂ + CO₂ → CaCO₃↓ + H_2O (White precipitate of calcium carbonate)
• (b) AgNO₃ + NaCl → AgCl↓ + NaNO₃ (White precipitate of silver chloride)

1. Heat is liberated (Exothermic reactions):

• (a) CaO + H_2O → Ca(OH)₂ + Heat (Slaking of quicklime)
• (b) NaOH + HCl → NaCl + H_2O + Heat (Neutralization)

1. Colour is changed:

• (a) Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂↑ (brown gas) + 2H_2O
• (b) Pb_3O₄ + 2C → 3Pb (grey liquid/solid) + 2CO₂ (Red lead turns to yellow and then grey)

Identify whether each process is a physical change or a chemical reaction/change, along with the reasons:

1. Melting of ice
2. Dissolution of sugar in water
3. Burning of wood
4. Blackening of pieces of potatoes, brinjals and apples
5. Melting of wax
6. Blackening of silver ornaments
7. Souring of milk
8. Synthesis of food by green plants in presence of sunlight

Point out the difference between:

1. Oxidation and reduction
2. Combination and decomposition

Write the characteristics of a chemical reaction.

A chemical reaction can be identified by the occurrence of one or more of the following properties/characteristics:

1. Formation of New Substances: New compounds with distinct physical and chemical properties are formed.
2. Liberation of Gas: Gas bubbles are released during the reaction.
3. Change in Colour: The colour of reactants changes as products are formed.
4. Formation of Precipitate: An insoluble solid residue is formed in the हल.
5. Heat/Temperature Change: Liberation of heat (exothermic) or absorption of heat (endothermic).
6. Change in Physical State: Transformation between solid, liquid, or gas states.

Give an example of a reaction which shows more than one of these characteristics.

Example: Reaction between calcium carbonate (CaCO₃) and dilute hydrochloric acid (HCl).

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g)↑ + H_2O (l)

Characteristics shown by this reaction:

1. Liberation of Gas: Carbon dioxide gas (CO₂) is liberated with effervescence.
2. Change in State: Insoluble solid calcium carbonate dissolves into a liquid हल (CaCl₂) and gas (CO₂).
3. Heat Change: Heat is released, causing a temperature change in the test tube.

Enlist various chemical changes that you come across in your daily life and also classify or categorize them.

When Seema tasted glucose, she felt cold in tongue. Why did she feel so? What is this reaction called?

When glucose (C_6H₁₂O₆) crystals are placed on the tongue, they dissolve in the saliva present on the tongue. The dissolution of glucose absorbs heat energy from the tongue. Due to the absorption of thermal energy from the tongue’s surface, the local temperature drops, causing Seema to feel a cooling sensation.

A chemical reaction or process that absorbs heat from its surroundings is called an Endothermic process / Endothermic reaction.