______ volume(s) of hydrogen and ______ volume(s) of oxygen under similar conditions of temperature and pressure combine to give two volumes of water.

Explanation

To solve the question regarding the volumes of hydrogen and oxygen that combine to form water, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Chemical Reaction**: The formation of water from hydrogen and oxygen can be represented by the balanced chemical equation:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]
This equation indicates that 2 volumes of hydrogen gas (H₂) react with 1 volume of oxygen gas (O₂) to produce 2 volumes of water (H₂O).

2. **Identify the Volumes**: From the balanced equation, we can see that:
- For every 2 volumes of hydrogen gas, 1 volume of oxygen gas is needed.
- This means that the ratio of hydrogen to oxygen is 2:1.

3. **Fill in the Blanks**: Based on the above information, we can fill in the blanks in the question:
- The first blank (for hydrogen) will be "2".
- The second blank (for oxygen) will be "1".

4. **Final Answer**: Therefore, the complete answer to the question is:
- **2 volumes of hydrogen and 1 volume of oxygen combine to give 2 volumes of water.**