The Boiling Point Is Increased by Dissolved Sugar and Salt

Appears in
On Food and Cooking

By Harold McGee

Published 2004

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When salt, sugar, or any other water-soluble substance is added to pure water, the boiling point of the resulting solution becomes higher than the boiling point of water, and the freezing point lower than water’s freezing point. Both effects are due to the fact that the water molecules are diluted by the dissolved particles, which interfere with the water molecules as they change phase from liquid to gas or liquid to solid. In the case of the boiling point, the solution contains sugar molecules or salt ions that also absorb heat energy, but cannot themselves turn into a gas. So at water’s normal boiling point, there is a smaller proportion of molecules with enough energy to escape from the liquid and form a bubble of vapor, and the cook has to add more energy than usual in order to get those bubbles to form. The boiling point and freezing point rise and fall predictably as the concentration of dissolved sugar or salt increases, a fact that is handy for making both sugar candies and ice creams.