Which of the following statements are correct?
I. Solubility of solids in liquids usually decreases with decrease in temperature and the extra amount of solute crystallises out.
II. The solubility of gases in liquids increases on increasing the temperature.
III. Solubility of gases in liquids increases on increasing the pressure.
IV. While expressing the solubility of a substance temperature is not specified.

Options

A. I and III only
B. II and III only
C. I, II and IV only
D. III and IV only

Answer

To determine which of the statements regarding solubility are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement I**: "Solubility of solids in liquids usually decreases with decrease in temperature and the extra amount of solute crystallizes out."
- **Analysis**: This statement is correct. Generally, the solubility of solids in liquids increases with an increase in temperature. Therefore, if the temperature decreases, the solubility decreases, leading to crystallization of the excess solute that cannot remain dissolved at the lower temperature.

2. **Statement II**: "The solubility of gases in liquids increases on increasing the temperature."
- **Analysis**: This statement is incorrect. The solubility of gases in liquids typically decreases with an increase in temperature. As temperature increases, the kinetic energy of gas molecules increases, leading to a higher tendency for gas molecules to escape from the liquid phase.

3. **Statement III**: "Solubility of gases in liquids increases on increasing the pressure."
- **Analysis**: This statement is correct. According to Henry's Law, the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. Therefore, increasing the pressure increases the solubility of the gas in the liquid.

4. **Statement IV**: "While expressing the solubility of a substance temperature is not specified."
- **Analysis**: This statement is incorrect. Temperature is a crucial factor that affects solubility. Without specifying the temperature, one cannot accurately determine the solubility of a substance.

### Conclusion:
- **Correct Statements**: I and III are correct.
- **Incorrect Statements**: II and IV are incorrect.

### Final Answer:
The correct statements are I and III.

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