10°C (about 50°F) and ice at 0°C

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Under normal conditions, ordinary water freezes at 0°C, or 32°F. However, if you add salt to water, its freezing point becomes lower. Let's look at why a salt water solution has a freezing point below zero, and how you can use this fact to make ice cream!

At the right is a container of water with several ice cubes in it (we'll just show one ice cube for simplicity). We've started with cold tap water at 10°C (about 50°F) and ice at 0°C, and we've placed the mixture in a nice, insulated cup that prevents the flow of heat into the cup from the outside world.

Water molecules are constantly escaping from the solid ice into the liquid water (melting). At the same time, water molecules are being captured on the surface of the ice (freezing). But because the water molecules in the liquid are moving quickly (they are at elevated temperature compared to the ice), they can't easily be captured by the surface of the ice, so not very many of them freeze.

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