Rise of temperature on mixing with water: Sodium chloride

When certain substances are mixed with water, they can either absorb heat energy from the surroundings (endothermic reaction) or release heat energy into the surroundings (exothermic reaction). In this case, we are looking for the substance that will result in a rise of temperature when mixed with water.

Explanation:

- Sodium chloride (NaCl): When sodium chloride is mixed with water, it undergoes a highly exothermic reaction. This means that it releases heat energy into the surroundings, causing a rise in temperature. The dissolution of sodium chloride in water is an exothermic process because it involves the formation of new bonds between the sodium and chloride ions and the water molecules. The heat released during this process raises the temperature of the solution.

- Sodium hydroxide (NaOH): When sodium hydroxide is mixed with water, it also undergoes an exothermic reaction. However, the heat released is not as significant as in the case of sodium chloride. The dissolution of sodium hydroxide in water is also exothermic, but the reaction is less vigorous compared to sodium chloride.

- Potassium nitrate (KNO3): When potassium nitrate is mixed with water, it undergoes an endothermic reaction. This means that it absorbs heat energy from the surroundings, causing a decrease in temperature. The dissolution of potassium nitrate in water requires energy to break the bonds between the potassium and nitrate ions, resulting in a cooling effect.

- CuSO4·5H2O: Copper sulfate pentahydrate (CuSO4·5H2O) is a hydrated compound, which means it contains water molecules within its crystal structure. When it is mixed with water, the water molecules already present in the crystal structure are released, but this process does not result in a significant rise in temperature.

Based on the above explanations, the correct answer is option 'A' - Sodium chloride.