Water freezes at 32 degrees Fahrenheit (0 degrees Celsius), but when a solute such as sugar is added, the freezing point changes. The sugar molecules prevent the water from making hydrogen bonds, which are required for solidity, and the water has to become even colder before it reaches its freezing point.
The temperature at which a liquid turns into a solid is known as its freezing point. In theory, the melting point of a solid should be the same as the freezing point of the liquid. For example, at 32 degrees Fahrenheit (0 degrees Celsius), there is an equilibrium between water freezing and ice melting. Molecules of ice are melting, and molecules of water are sticking to the ice and becoming frozen at the same time. The water looks frozen at this point.
A water molecule consists of one oxygen atom and two hydrogen atoms. Temperature measures how much energy is created by moving molecules. When water molecules are cold, they don’t have a lot of energy, so they don’t move around a lot. Instead, they move together and form hydrogen bonds to create a solid structure called ice.
Adding Sugar to Water
When you add sugar to water, the water (the solvent) becomes a solution (a solute dissolved in a solvent). Adding sugar disrupts the liquid state because sugar molecules move around aimlessly, making the liquid water molecules less organized. Sugar molecules don't pack together with water molecules, so when the water molecules start to freeze, the sugar molecules remain in the liquid water. When the water molecules create ice, the sugar molecules have a smaller volume of liquid in which to move.
Freezing Point Depression
Sugar particles are able to dissolve only in a liquid solvent and won't dissolve when the solvent is in a solid state. Therefore, adding sugar to water lowers the chemical potential of the solution, which also lowers its freezing point. In other words, a solution of sugar dissolved in water must be cooled to a lower temperature than the pure solvent in order for freezing to happen. When the freezing point of a liquid is lowered by the presence of an additive, freezing point depression occurs. The exact freezing point is determined by the quantity of solute particles dissolved in the solvent. The more solute particles there are in the water, the greater the freezing point depression of the solution.
- University of New South Wales: Freezing Point Depression and Boiling Point Elevation: The Effects of Solutes and of Pressure
- Smithsonian.com: At What Temperature Does Water Freeze?
- University of Arizona: The Chemistry of Water
- Georgia State University: Freezing Point Depression in Solutions
- University of Illinois at Urbana-Champaign: Q & A: Freezing Point of Sugar Water
- Purdue University: Melting Point, Freezing Point, Boiling Point
- Purdue University: Freezing Point Depression
About the Author
Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.