Salt (NaCl) dissolves into its ions in water, Na+ and Cl-. The ions diffuse throughout the water and block the water molecules from getting close enough together and in the right orientation to organize into the solid form (ice). Ice absorbs energy from its surroundings to undergo the phase transition from solid to liquid. This could cause pure water to re-freeze, but the salt in the water prevents it from turning into ice. However, the water gets colder than it was. The temperature can drop below the freezing point of pure water.

Adding any impurity to a liquid lowers its freezing point. The nature of the compound does not matter, but the number of particles it breaks into in the liquid is important. The more particles that are produced, the greater the freezing point depression. So, dissolving sugar in water also lowers the freezing point of water. Sugar simply dissolves into single sugar molecules, so its effect on freezing point is less than you would get adding an equal amount of salt, which breaks into two particles. Salts that break into more particles, like Magnesium Chloride (MgCl2) have an even greater effect on freezing point. Magnesium Chloride dissolves into three ions -- one Magnesium cation and two Chloride anions.

The video supports the Europlanet Mars Collection of Educational Resources - 4. Martian Chemistry, which looks at the chemistry of Mars and how this can affect its potential habitability.

Europlanet 2024 RI has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 871149.