Why do people spread salt on icy roads and footpaths in winter?
Salt lowers the freezing point of water — a colligative effect.
Salt lowers the freezing point of water — a colligative effect. Dissolved salt particles get in the way of water molecules trying to lock into an ice crystal, so the water has to be cooled further before it can freeze, melting the ice already there. NaCl works especially well because it splits into two ions (Na⁺ and Cl⁻), giving roughly twice the particle count and twice the depression of a non-ionic solute.
Key point
Dissolved particles depress the freezing point; NaCl (i ≈ 2) depresses it about twice as much as glucose.
Common mistake
thinking the salt chemically 'burns' or heats the ice.
Memory tip
more dissolved particles → lower freezing point (salt beats sugar because it splits into ions).
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