Salt helps melt ice and prevent it from re-freezing on sidewalks and roads, yet adding salt to ice makes it colder so you can freeze ice cream. Melting ice but making it colder seems contradictory, but makes sense once you understand freezing point depression. How does this work? How cold does salt make ice? Here’s the simple explanation.
Salt Lowers the Temperature of Ice Water
Adding salt to ice water lowers the temperature from the normal freezing point of water (0 °C or 32 °F) to as low as -21 °C or -5 °F. You get the same effect if you add salt to ice cubes because the outer surface of ice always contains a thin film of water. Eventually, all the ice melts, leaving very cold salt water. You can use this information to make ice cream without a freezer. Simply put the ingredients into a bowl and rest this bowl in a container of salted ice!
How Salt Melts Ice
When you salt a road, once again, there is a thin film of water on the ice. Salt dissolves in this water, lowers the temperature at which it can re-freeze, and prevents it from solidifying back into ice. But, there is a lower temperature limit for this process. If the temperature is cold enough, the salt water can freeze. In practice, ordinary road salt isn’t that effective below -7 °C or 20 °F.
This is why road salt isn’t necessarily the same as table salt. Different types of salt lower the freezing point of the resulting salt water different amounts. Potassium chloride (KCl), magnesium chloride (MgCl2), and calcium chloride (CaCl2) all lower the freezing point of water more than table salt (NaCl). But, they each have different environmental effects and other properties, so choosing the best de-icer isn’t always a simple matter. Sometimes its best to just add sand and let heat from friction melt ice.
How Freezing Point Depression Works
To understand freezing point depression, it helps to first understand how melting and freezing work. Ice must absorb energy (heat) from the environment to break the hydrogen bonds that make it a solid, allowing it to turn into a liquid. In contrast, water releases energy (heat) when it solidifies into ice. At the freezing point, these two processes are at equilibrium, so the energy absorbed by water freezing equals the energy released by ice melting. But, when you add salt, you lower the freezing point of the water. Ice melts into water, absorbing heat, but doesn’t re-freeze into ice and release heat. So, the temperature drops.
How does salt lower the freezing point? Salt (NaCl) dissolves in water to form its ions (Na+ and Cl–). The ion particles get between the water molecules and make it harder for them to align to crystallize into ice. This means the temperature has to be colder for the liquid to freeze. Table salt dissolves to form two particles, but some salts dissolve to form more particles and lower the freezing point even more. For example, magnesium chloride (MgCl2) dissolves to form three particles (Mg2+, Cl–, Cl–).
Freezing point depression is an example of a colligative property. A colligative property does not depend on the chemical identity of a substance, but on the number of particles that are formed. Other colligative properties include boiling point elevation and osmotic pressure.
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