Salt and ice: why melting can make the mix colder

Measure the apparent contradiction

Salt makes ice melt, yet the mixture can get colder rather than warmer. You can compare the temperature changes yourself with two small portions of ice. This is a bowl experiment, not general advice for de-icing paths.

Set up a simple comparison

You need two matching small bowls, ice, table salt, a spoon and two suitable probe thermometers that can measure below 0 °C. Put everything on a tray that will not be damaged by salt water.

1. Divide equal portions of ice from the same batch between the bowls. Add the same small amount of water to each so the probes make good contact with the mixture.
2. Place the probes at the same depth without touching the bottom. Write down both starting temperatures.
3. Add about a teaspoon of salt to just one bowl. Gently stir both portions with a spoon, without transferring salt into the comparison bowl.
4. Record the temperatures roughly every minute for several minutes. Also watch how the amount of ice changes.

Why the temperature can drop

Dissolved salt lowers the freezing point of water. More ice can then melt even when the mixture is already at or below 0 °C. Melting needs energy, supplied partly as heat from the mixture and its surroundings. This can cool the salt-and-ice mixture. Salt does not simply provide heat that warms the ice.

Know the limits

The temperature drop depends on the amount of salt, the starting temperature, the balance of ice and water, and heat entering from the room. No particular reading or temperature difference is guaranteed. Compare the trend, not just one measurement.

Do not touch the cold salt-and-ice mixture with bare skin: it can cause cold burns. Do not taste it. Keep salt water away from sensitive surfaces, wipe up splashes promptly and rinse the bowls and spoon after the experiment.