Colourless gas formed by treating phosphorus with NaOH is phosphine.

Answer

### Step-by-Step Solution:

1. **Understanding the Reaction**: The question states that a colorless gas is formed when phosphorus is treated with sodium hydroxide (NaOH). We need to identify this gas.

2. **Identifying the Gas**: When phosphorus (P) reacts with sodium hydroxide (NaOH), it produces phosphine (PH₃) as one of the products. The other product formed is sodium hypophosphate (NaH₂PO₂).

3. **Verifying the Statement**: The statement claims that the colorless gas formed is phosphine. Since we have established that phosphine is indeed produced in this reaction, we can conclude that the statement is true.

4. **Characteristics of Phosphine**: Phosphine (PH₃) is a colorless gas with a distinct smell resembling rotten fish. It is also known to be poisonous and is slightly soluble in water. Additionally, phosphine solutions are unstable and can decompose in the presence of light.

5. **Conclusion**: Based on the information gathered, we can confidently state that the original statement is true.

### Final Answer:
The statement is **True**.

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