Melting Snow & Ice with Salt

Colligative Properties and Freezing Point Depression

When you add a salt to ice with a little bit of water, the salt lowers the freezing point of the water, keeping it from re-freezing as easily and helping to melt the rest of the ice.
When you add a salt to ice with a little bit of water, the salt lowers the freezing point of the water, keeping it from re-freezing as easily and helping to melt the rest of the ice. Dave King / Getty Images

If you live in an area with a cold and icy winter, you have probably experienced salt on sidewalks and roads. This is because salt is used to melt the ice and snow and keep it from refreezing. Salt is also used to make homemade ice cream. In both cases, the salt works by lowering the melting or freezing point of water. The effect is termed 'freezing point depression'.

How Freezing Point Depression Works

When you add salt to water, you introduce dissolved foreign particles into the water.

The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving. For a solution of table salt (sodium chloride, NaCl) in water, this temperature is -21°C (-6°F) under controlled lab conditions. In the real world, on a real sidewalk, sodium chloride can melt ice only down to about -9°C (15°F).

Colligative Properties

Freezing point depression is a colligative property of water. A colligative property is one which depends on the number of particles in a substance. All liquid solvents with dissolved particles (solutes) demonstrate colligative properties. Other colligative properties include boiling point elevation, vapor pressure lowering, and osmotic pressure.

More Particles Mean More Melting Power

Sodium chloride isn't the only salt used for de-icing, nor is it necessarily the best choice. Sodium chloride dissolves into two types of particles: one sodium ion and one chloride ion per sodium chloride 'molecule'.

A compound that yields more ions into a water solution would lower the freezing point of water more than salt. For example, calcium chloride (CaCl2) dissolves into three ions (one of calcium and two of chloride) and lowers the freezing point of water more than sodium chloride.

Salts Used to Melt Ice

Here are some common de-icing compounds, as well as their chemical formulas, temperature range, advantages, and disadvantages:

NameFormulaLowest Practical TempProsCons
Ammonium sulfate(NH4)2SO4-7°C
(20°F)
FertilizerDamages concrete
Calcium chlorideCaCl2-29°C
(-20°F)
Melts ice faster than sodium chlorideAttracts moisture, surfaces slippery below -18°C (0°F)
Calcium magnesium acetate (CMA)Calcium carbonate CaCO3, magnesium carbonate MgCO3, and acetic acid CH3COOH-9°C
(15°F)
Safest for concrete & vegetationWorks better to prevent re-icing than as ice remover
Magnesium chlorideMgCl2-15°C
(5°F)
Melts ice faster than sodium chlorideAttracts moisture
Potassium acetateCH3COOK-9°C
(15°F)
BiodegradableCorrosive
Potassium chlorideKCl-7°C
(20°F)
FertilizerDamages concrete
Sodium chloride (rock salt, halite)NaCl-9°C
(15°F)
Keeps sidewalks dryCorrosive, damages concrete & vegetation
UreaNH2CONH2-7°C
(20°F)
FertilizerAgricultural grade is corrosive

Factors That Affect Which Salt to Choose

While some salts are more effective at melting ice than others, that doesn't necessarily make them the best choice for a certain application. Sodium chloride is used for ice cream makers because it's inexpensive, readily available, and non-toxic. Yet, sodium chloride (NaCl) is avoided for salting roads and sidewalks because the sodium can accumulate and upset the electrolyte balance in plants and wildlife, plus it can corrode automobiles.

Magnesium chloride melts ice more quickly than sodium chloride, but it attracts moisture, which can lead to slick conditions. Selecting a salt to melt ice depends on its cost, availability, environmental impact, toxicity, and reactivity, in addition to its optimal temperature.