Chapter 21, Problem 45R

To determine

The cause of cooking of potatoes more quickly in salt water than in pure water.

Explanation of Solution

Introduction:

The salt particles (solute) interfere with the water molecules (solvent) as they change from liquid to gas at the surface of solution (salt water).

The vapor pressure of water lowers. As a result, more energy is needed for the water molecules to escape from the liquid surface. Hence, the boiling point of the solution (salt water) will be higher than the boiling point of pure water.The potatoes boils at a higher temperature in salt water and hence cook more quickly. 

Conclusion:

Thus, potatoes boils at a higher temperature in salt water because the boiling point of salt water is more than the boiling point of pure water and hence cook more quickly.