Correct statements about the hydrogen halides include that:

Options

A. they are all coloured
B. the thermal stability decreases with increasing atomic number of halogen
C. they all form soluble silver salts
D. they all donate protons to water

Answer

To solve the question regarding the correct statements about hydrogen halides (HF, HCl, HBr, HI), we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding Hydrogen Halides**:
- Hydrogen halides are compounds formed by hydrogen and halogens (F, Cl, Br, I). The common hydrogen halides are HF, HCl, HBr, and HI.

2. **Analyzing the Statements**:
- **Statement 1**: "They all are colored."
- **Analysis**: Hydrogen halides (HF, HCl, HBr, HI) are colorless gases or liquids. Therefore, this statement is **false**.

- **Statement 2**: "The thermal stability decreases with increasing atomic number."
- **Analysis**: As we move from HF to HI, the bond length increases due to the larger size of the halogen atoms, leading to weaker bonds. Thus, the thermal stability decreases. This statement is **true**.

- **Statement 3**: "They all form soluble silver salts."
- **Analysis**: When hydrogen halides react with silver, they form silver halides (AgF, AgCl, AgBr, AgI). While AgF is soluble, AgCl, AgBr, and AgI are not soluble in water. Therefore, this statement is **false**.

- **Statement 4**: "They all donate protons to water."
- **Analysis**: Hydrogen halides are strong acids and dissociate in water to donate protons (H⁺) to form hydronium ions (H₃O⁺) and halide ions (X⁻). This statement is **true**.

3. **Conclusion**:
- The correct statements about hydrogen halides are:
- Statement 2: "The thermal stability decreases with increasing atomic number." (True)
- Statement 4: "They all donate protons to water." (True)

### Final Answer:
The correct options are **Statement 2 and Statement 4**.