How Salt Melts Ice and Snow

How Salt Melts Ice and Snow 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 isnt 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: Name Formula Lowest Practical Temp Pros Cons Ammonium sulfate (NH4)2SO4 -7 C(20 F) Fertilizer Damages concrete Calcium chloride CaCl2 -29 C(-20 F) Melts ice faster than sodium chloride Attracts moisture, surfaces slippery below -18Â °C (0Â °F) Calcium magnesium acetate (CMA) Calcium carbonate CaCO3, magnesium carbonate MgCO3, and acetic acid CH3COOH -9 C(15 F) Safest for concrete vegetation Works better to prevent re-icing than as ice remover Magnesium chloride MgCl2 -15 C(5 F) Melts ice faster than sodium chloride Attracts moisture Potassium acetate CH3COOK -9 C(15 F) Biodegradable Corrosive Potassium chloride KCl -7 C(20 F) Fertilizer Damages concrete Sodium chloride (rock salt, halite) NaCl -9 C(15 F) Keeps sidewalks dry Corrosive, damages concrete vegetation Urea NH2CONH2 -7 C(20 F) Fertilizer Agricultural grade is corrosive Factors That Affect Which Salt to Choose While some salts are more effective at melting ice than others, that doesnt necessarily make them the best choice for a certain application. Sodium chloride is used for ice cream makers because its 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.