Very pure hydrogen `(99.9%)` can be made by which of the following processes ?

Options

A. Reaction of methane with steam
B. Mixing natural hydrocarbons of high molecular weight
C. Electrolysis of water
D. Reaction of salt like hydrides with water

Answer

To determine which process can produce very pure hydrogen (99.9% purity), we can analyze the various methods of hydrogen production.

### Step-by-Step Solution:

1. **Identify the Processes**: The question mentions several processes for hydrogen production. Common methods include:
- Reaction of metals with acids (e.g., zinc with hydrochloric acid)
- Reaction of metals with alkalis
- Steam reforming of methane
- Electrolysis of water

2. **Evaluate Each Process**:
- **Reaction of Metals with Acids**: This method typically produces hydrogen gas, but the purity can vary depending on the metal and acid used. Impurities from the metal or acid can contaminate the hydrogen.
- **Reaction of Metals with Alkalis**: Similar to acids, this method can produce hydrogen but may also introduce impurities.
- **Steam Reforming of Methane**: This is a common industrial method for hydrogen production, but it often results in hydrogen that is not pure enough (usually around 95% purity) due to the presence of carbon monoxide and other by-products.
- **Electrolysis of Water**: This method involves passing an electric current through water (often acidified with sulfuric acid) to produce hydrogen and oxygen. The reactions at the electrodes are:
- At the anode: 4 OH⁻ → O₂ + 2 H₂O + 4 e⁻ (oxidation)
- At the cathode: 2 H₂O + 2 e⁻ → H₂ + 2 OH⁻ (reduction)
- Electrolysis, especially when using pure water and a suitable electrolyte, can yield hydrogen gas with very high purity, often reaching 99.9%.

3. **Conclusion**: Among the listed processes, the **electrolysis of water** is the method that can produce hydrogen with a purity of 99.9%.

### Final Answer:
The process that can produce very pure hydrogen (99.9% purity) is **Electrolysis of Water**.

---