(a) What is a chemical reaction? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place. OR (b) (i) Define a decomposition reaction. How can we say that (I) electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions? Mention the type of energy involved in each case. (ii) "The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other." Justify this statement with the help of chemical equations.

Option (a)

1. Definition of a Chemical Reaction:

Whenever a chemical change occurs, the nature and identity of the initial substances change, and new substances with entirely different properties are formed. This process is called a chemical reaction. It involves the breaking and making of bonds between atoms to produce new substances.

2. Activity showing Change of Colour:

• Activity (based on Activity 1.5): Take about $2\text{ g}$ of green ferrous sulphate crystals ($\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$) in a dry boiling tube. Heat the boiling tube over the flame of a burner.
• Observation: The green colour of the ferrous sulphate crystals changes to a reddish-brown solid (ferric oxide, $\text{Fe}_2\text{O}_3$), and a characteristic suffocating smell of burning sulphur (due to $\text{SO}_2$ and $\text{SO}_3$ gases) is observed.
• Chemical Equation:

$$2\text{FeSO}_4(s) \xrightarrow{\text{Heat}} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)$$

3. Activity showing Change in Temperature:

• Activity (based on Activity 1.4): Take a small amount of calcium oxide (quick lime, $\text{CaO}$) in a beaker. Slowly add water to this.
• Observation: Calcium oxide reacts vigorously with water, producing a hissing sound and forming slaked lime (calcium hydroxide). On touching the beaker, it feels extremely hot, indicating a large amount of heat is released (exothermic reaction).
• Chemical Equation:

$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$$

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Option (b)

(i) Decomposition Reaction:

A chemical reaction in which a single reactant breaks down to give two or more simpler products is called a decomposition reaction.

• (I) Electrolysis of water:

When electric current is passed through acidified water, it decomposes into hydrogen and oxygen gases. Since a single reactant (water) breaks down into simpler substances, it is a decomposition reaction.

• Chemical Equation:

$$2\text{H}_2\text{O}(l) \xrightarrow{\text{Electricity}} 2\text{H}_2(g) + \text{O}_2(g)$$

• Type of energy involved: Electrical energy.

• (II) Blackening of silver bromide in sunlight:

When pale yellow silver bromide is exposed to sunlight, it decomposes into grey silver metal and bromine gas. Since a single reactant (silver bromide) decomposes into simpler substances under light, it is a decomposition reaction.

• Chemical Equation:

$$2\text{AgBr}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Br}_2(g)$$

• Type of energy involved: Light energy (photochemical energy).

(ii) Justification of the Statement:

• Reaction (I) - Formation of Calcium Oxide:

Calcium oxide is formed by the thermal decomposition of calcium carbonate (limestone). Here, a single reactant breaks down into multiple products on heating:

$$\text{CaCO}_3(s) \xrightarrow{\text{Heat}} \text{CaO}(s) + \text{CO}_2(g)$$

This is a decomposition reaction.

• Reaction (II) - Formation of Calcium Hydroxide:

Calcium hydroxide is formed when calcium oxide reacts vigorously with water. Here, two reactants combine to form a single product:

$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq)$$

This is a combination reaction.

Conclusion:

Since Reaction (I) involves the breaking down of a single compound into simpler substances (decomposition), and Reaction (II) involves the combining of two substances to form a single compound (combination), these two types of reactions are opposite to each other.