Mention any four important uses of hydrogen.

Explanation

### Step-by-Step Solution:

1. **Synthesis of Industrial Chemicals**: Hydrogen is widely used in the production of various industrial chemicals. It is a key component in the synthesis of ammonia through the Haber process, which is essential for fertilizers.

2. **Manufacture of Fuel Gases**: Hydrogen is used in the production of fuel gases, such as hydrogen gas itself, which can be used as a clean fuel alternative in various applications, including fuel cells.

3. **Extraction of Metals**: Hydrogen acts as a reducing agent in the extraction of certain metals from their oxides. For example, it can reduce metal oxides to pure metals, making it valuable in metallurgy.

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