ANS:C - Pressure

Among the options provided, the one that has the least (almost negligible) effect on the solubility of a solute in a solvent is pressure. Pressure typically has a minimal effect on the solubility of solids in liquids and gases in liquids. This is particularly true for systems that do not involve gases. In the case of solids dissolving in liquids, changes in pressure have negligible effects because solids are not significantly affected by changes in pressure. Temperature, the nature of the solute, and the nature of the solvent typically have more significant effects on solubility compared to pressure. Therefore, the correct answer is: (c) Pressure Pressure generally has a minimal effect on the solubility of solids in liquids and liquids in liquids. This is because changes in pressure have little influence on the interactions between the solute and solvent molecules. For solids dissolving in liquids:

• Increasing the pressure doesn't significantly affect the solubility of solids in liquids. This is because the volume occupied by the solid solute is usually negligible compared to the volume of the liquid solvent. Therefore, changes in pressure don't have a substantial effect on the solubility of the solid.
• Exceptions may occur in some rare cases, such as when the solid solute undergoes a change in volume upon dissolution, or when high pressures lead to changes in the crystalline structure of the solute. However, these cases are relatively uncommon.

For gases dissolving in liquids:

• Pressure does have a more significant effect on the solubility of gases in liquids, according to Henry's law, which states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. However, even in this case, the effect of pressure on the solubility is relatively small compared to the effect of temperature.
• At higher pressures, the solubility of gases in liquids generally increases, and at lower pressures, it decreases. This relationship is utilized in various processes like carbonation of beverages, where higher pressures are used to dissolve more gas (carbon dioxide) into the liquid (water).

In summary, while pressure does have an effect on the solubility of gases in liquids according to Henry's law, its effect on the solubility of solids in liquids is generally negligible. Temperature, the nature of the solute, and the nature of the solvent typically have more significant effects on solubility compared to pressure.