Adding salt to water raises the temperature at which the water boils. The more salt that is dissolved in the water, the higher the boiling point. This phenomenon is a result of certain properties of solutions called colligative properties. The effect is important for chefs and home cooks to understand because it can change cooking times of foods.
The boiling point of a substance is the temperature at which it changes from a liquid to a gas. Many factors affect the boiling point, such as air pressure and presence of dissolved solids. The boiling point corresponds to the amount of energy needed to set molecules "free" into gaseous form. When the amount of energy needed to set the molecules free is higher, the boiling point is also higher.
When Salt Dissolves
Table salt, or sodium chloride, is a crystal made of two ions: sodium and chloride. The ions are held together by chemical bonds in a geometric pattern. These bonds are called ionic bonds. Their pattern results in the crystal shape and structure. When salt dissolves in water, the chemical bonds are broken, the crystal structure disappears, and the sodium and chloride ions float freely in the water.
Effect of Salt on Boiling Point
A liquid substance boils when the pressure at its surface is greater than the air pressure. The pressure of the liquid comes from the energy of everything in the liquid (including salt), but in order to boil, the pressure of only the pure substance must equal the air pressure. When salt is present, it "stores" some of the energy so that it is unavailable to the water molecules. Therefore, in order for the water to boil, more energy, and therefore a higher temperature, is needed.
Size of the Effect
The number of degrees by which salt raises the boiling point corresponds to the salt's concentration. Adding more salt ions to a given quantity of water results in a higher boiling point. Adding one teaspoon of salt per quart raises the boiling point -17.2 to -16.7 degrees Celsius.
Importance in Cooking
The higher boiling point of salt water has two important implications in cooking. First, water takes longer to come to a boil if it contains salt. Second, the temperature of the boiling water is higher. Adding salt to pasta water means it takes longer to bring it to a boil, but the pasta will cook more quickly due to the higher temperature.