In the reaction, `"Hydrogen (g)"+"Oxygen (g)"rarr"water vapour",` the ratio of volumes is `2:1:2`. This illustrates the law of

Options

A. Conservation of mass
B. Combining weights
C. Combining volumes
D. All the above

Answer

To solve the question regarding the reaction of hydrogen and oxygen to form water vapor, we will analyze the volume ratios and identify the law that this illustrates.

### Step-by-Step Solution:

1. **Write the Balanced Chemical Equation**:
The reaction between hydrogen gas and oxygen gas to form water vapor can be represented as:
\[ 2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(g) \]

2. **Identify the Stoichiometric Coefficients**:
From the balanced equation, we can see the stoichiometric coefficients:
- For hydrogen (H₂): 2
- For oxygen (O₂): 1
- For water (H₂O): 2

3. **Determine the Volume Ratio**:
According to the law of combining volumes, gases react in simple whole number ratios by volume. In this case, the volume ratio is:
\[ \text{Volume of H}_2 : \text{Volume of O}_2 : \text{Volume of H}_2\text{O} = 2 : 1 : 2 \]

4. **Identify the Relevant Law**:
The observed volume ratio of 2:1:2 illustrates the **Law of Combining Volumes**. This law states that when gases react together at constant temperature and pressure, the volumes of the reacting gases and the volumes of the products (if gaseous) are in simple whole number ratios.

5. **Conclusion**:
Therefore, the correct answer to the question is that the reaction illustrates the **Law of Combining Volumes**.