The characteristics of some gases are given. Explain how they cause pollution.
A colourless, odourless gas which turns lime water milky

Explanation

### Step-by-Step Solution:

1. **Identify the Gas**: The question describes a gas that is colorless and odorless and can turn lime water milky. Lime water is a solution of calcium hydroxide (Ca(OH)₂), which turns milky when carbon dioxide (CO₂) is bubbled through it. This indicates that the gas in question is carbon dioxide.

2. **Explain the Reaction**: When carbon dioxide (CO₂) is dissolved in lime water, it reacts to form calcium carbonate (CaCO₃), which is insoluble in water and causes the lime water to turn milky. The reaction can be represented as:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O} \]
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