Select from the list the gas that matches the description given in each case: [ammonia, ethane, hydrogen chloride, hydrogen sulphide, ethyne]
This gas decolourizes bromine water,

Explanation

To determine which gas from the list decolorizes bromine water, we need to analyze the properties of each gas provided: ammonia, ethane, hydrogen chloride, hydrogen sulfide, and ethyne.

### Step-by-Step Solution:

1. **Understanding Bromine Water Decolorization**:
- Bromine water is a yellowish solution of bromine in water. The decolorization of bromine water indicates a reaction with a compound that has unsaturation (double or triple bonds), typically alkenes or alkynes.

2. **Analyzing Each Gas**:
...