_____ is a colourless gas which is produced by the reaction of active metal with dilute hydrochloric acid.

The gas you are looking for is **hydrogen gas (H₂)**.

### Explanation:

1. **Chemical Reaction**: When an active metal (like zinc, magnesium, or aluminum) reacts with dilute hydrochloric acid (HCl), the following reaction occurs:

\[
\text{Metal} + \text{Dilute HCl} \rightarrow \text{Metal Chloride} + \text{Hydrogen Gas}
\]

For example, if we take zinc (Zn) as the active metal, the reaction can be written as:

\[
\text{Zn (s)} + 2\text{HCl (aq)} \rightarrow \text{ZnCl}_2\text{ (aq)} + \text{H}_2\text{ (g)}
\]

2. **Properties of Hydrogen Gas**:
- **Colourless**: Hydrogen gas is colourless and odourless.
- **Flammable**: It is highly flammable and can produce a loud pop sound when ignited in the presence of air.

3. **Active Metals**: Active metals are those that readily lose electrons and form cations. Examples include sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), and zinc (Zn).

Therefore, the answer to the question is **hydrogen gas (H₂)**.