How Salt Melts Ice

Salt Prevents Water From Freezing

Salt prevents water from freezing into ice. As more ice melts, the salt water gets colder but doesn't freeze.
Salt prevents water from freezing into ice. As more ice melts, the salt water gets colder but doesn't freeze. Paul Taylor / Getty Images

Salt melts ice essentially because adding salt lowers the freezing point of the water. How does this melt ice? Well, it doesn't, unless there is a little water available with the ice. The good news is you don't need a pool of water to achieve the effect. Ice typically is coated with a thin film of liquid water, which is all it takes.

Pure water freezes at 32°F (0°C). Water with salt (or any other substance in it) will freeze at some lower temperature. Just how low this temperature will be depends on the de-icing agent. If you put salt on ice in a situation where the temperature will never get up to the new freezing point of the salt-water solution, you won't see any benefit. For example, tossing table salt (sodium chloride) onto ice when it's 0°F won't do anything more than coat the ice with a layer of salt. On the other hand, if you put the same salt on ice at 15°F, the salt will be able to prevent melting ice from re-freezing. Magnesium chloride works down to 5°F while calcium chloride works down to -20°F.

How It Works

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.

On the flip side, adding a tiny amount of insoluble particulates can actually help water freeze at a higher temperature. While there is a bit of freezing point depression, it's localized near the particles. The particles act as nucleation sites that allow for ice formation. This is the premise behind the formation of snowflakes in clouds and how ski resorts make snow when it's slightly warming than freezing.

Use Salt to Melt Ice - Activities

  • You can demonstrate the effect of freezing point depression yourself, even if you don't have an icy sidewalk handy. One way is to make your own ice cream in a baggie, where adding salt to water produces a mixture so cold it can freeze your treat.
  • If you just want to see an example of how cold ice plus salt can get, mix 33 ounces of salt with 100 ounces of crushed ice or snow. Be careful! The mixture will be about -6°F (-21°C), which is cold enough to give you frostbite if you hold it too long.
  • Gain a better understanding of freezing point depression by examining the effect of dissolving different substances in water and noting the temperature required to freeze it. Good examples of substances to compare are table salt (sodium chloride), calcium chloride, and sugar. See if you can dissolve equal masses of each substance in the water in order to get a fair comparison. Sodium chloride breaks into two ions in water. Calcium chloride forms three ions in water. Sugar dissolves in water, but it doesn't break into any ions. All of these substances will lower the freezing point of water.
  • Take the experiment a step further by exploring boiling point elevation, another colligative property of matter. Adding sugar, salt, or calcium chloride will change the temperature at which water boils. Is the effect measurable?