When a dilute salt water is electrolyzed, a colourless gas is given off at the anode. The gas is:

Options

A. hydrogen
B. steam
C. oxygen
D. chlorine

Explanation

To determine the colorless gas produced at the anode during the electrolysis of dilute salt water, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Electrolyte**:
- The dilute salt water is primarily a solution of sodium chloride (NaCl) in water.

2. **Understand Electrolysis**:
- Electrolysis involves passing an electric current through the solution, which causes chemical reactions at the electrodes (anode and cathode).

3. **Identify the Electrodes**:
- The anode is the positive electrode where oxidation occurs, and the cathode is the negative electrode where reduction occurs.

4. **Reactions at the Electrodes**:
- At the **cathode**: Sodium ions (Na⁺) are attracted to the cathode and gain electrons (reduction) to form sodium metal (Na).
- Reaction: Na⁺ + e⁻ → Na (s)
- At the **anode**: Chloride ions (Cl⁻) are attracted to the anode and lose electrons (oxidation) to form chlorine gas (Cl₂).
- Reaction: 2Cl⁻ → Cl₂ (g) + 2e⁻

5. **Determine the Gas Produced**:
- The gas produced at the anode during the electrolysis of dilute salt water is chlorine gas (Cl₂). However, chlorine gas is not colorless; it is yellow-green. The question likely refers to the fact that the gas produced at the anode is not colored, which can lead to confusion.

6. **Conclusion**:
- The colorless gas that is produced at the anode during the electrolysis of dilute salt water is chlorine gas (Cl₂).

### Final Answer:
The colorless gas given off at the anode is chlorine gas (Cl₂).