Which of the following is a suitable example for illustrating the law of conservation of mass? (Atomic mass of O = 16, H = 1)

Options

A. 18 g of water is formed by the combination of 16 g of oxygen with 2 g of hydrogen.
B. 18 g of water in liquid state obtained by heating 18 g of ice.
C. 18 g of water is completely converted into vapour state on heating.
D. 18 g of water freezes at 4°C to give same mass of ice.

Explanation

To determine which of the given options illustrates the law of conservation of mass, we need to analyze each option based on the principle that the total mass of reactants must equal the total mass of products in a chemical reaction.

### Step-by-Step Solution:

1. **Understanding the Law of Conservation of Mass**:
The law states that in a closed system, the mass of the reactants before a chemical reaction must equal the mass of the products after the reaction.

2. **Analyzing Option A**:
- **Statement**: 18 gm of water is formed by the combination of 16 gm of oxygen with 2 gm of hydrogen.
- **Chemical Reaction**:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]
- **Calculation**:
- Mass of reactants = Mass of Oxygen + Mass of Hydrogen = 16 gm + 2 gm = 18 gm
- Mass of products = Mass of Water = 18 gm
- **Conclusion**: This option illustrates the law of conservation of mass because the mass of reactants equals the mass of products.

3. **Analyzing Option B**:
- **Statement**: 18 gm of water in liquid state obtained by heating 18 gm of ice.
- **Analysis**: This is a physical change (state change from solid to liquid) and does not involve a chemical reaction. Therefore, the law of conservation of mass does not apply in the context of chemical reactions.

4. **Analyzing Option C**:
- **Statement**: 18 gm of water is completely converted into vapor state on heating.
- **Analysis**: This is also a physical change (state change from liquid to gas). Again, it does not involve a chemical reaction, so the law of conservation of mass is not applicable here.

5. **Analyzing Option D**:
- **Statement**: 18 gm of water freezes to give the same mass of ice.
- **Analysis**: This is another physical change (state change from liquid to solid). As with the previous options, it does not involve a chemical reaction, and therefore, the law of conservation of mass is not relevant.

### Final Conclusion:
The only suitable example that illustrates the law of conservation of mass is **Option A**: 18 gm of water is formed by the combination of 16 gm of oxygen with 2 gm of hydrogen.