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Lowering the freezing point

Why Does Salt Melt Ice?

Salt makes it harder for water to freeze, so ice at that temperature turns to liquid.

1 Ice at −5°C,solid2 Salt is added3 It can nolonger stayfrozen
No heat added. The rules changed.

  1. Step 1 of 5

    Freezing is a two-way race.

    At 0°CMoleculesjoining theiceMoleculesleaving it

    At the surface, molecules are constantly joining the ice and leaving it. At 0°C the two rates match exactly.

  2. Step 2 of 5

    Salt gets in the way of joining.

    Pure water Molecules attach freelySalt water Salt blocks attachmentsites

    Dissolved salt particles occupy space at the surface, so fewer water molecules can attach. Leaving continues unchanged.

  3. Step 3 of 5

    So melting wins below zero.

    Pure water freezes0°CSalty water freezesas low as −20°C

    With joining slowed and leaving unchanged, ice loses the race. It keeps melting until the temperature drops far enough to rebalance.

  4. Step 4 of 5

    It gets colder as it melts.

    Melting ice Draws heat fromaround it

    Melting absorbs energy, and that energy is taken from the surroundings. Salted ice is genuinely colder than plain ice.

  5. Step 5 of 5

    Which is why roads are salted.

    Down to about −10°C Salt keeps roads wetMuch colder Salt stops working

    Salt keeps a thin liquid film on the road down to about −10°C. Below that it stops working and grit is used for traction instead.

    Old ice cream makers used salt and ice around the churn to reach temperatures below freezing without a fridge.

The short version

Salt blocks water molecules from rejoining the ice, lowering the freezing point so ice melts below zero.

Try it yourself

Put ice in two cups, salt one, and take both temperatures after ten minutes. The salted one is colder.

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