Why a Bag of Ice and Salt Can Turn Milk Into Ice Cream

Here is a favourite kitchen experiment. Pour some milk, sugar and a drop of vanilla into a small sealed bag. Put that bag inside a bigger one filled with ice cubes and a few generous spoonfuls of salt. Seal it, wrap it in a tea towel because it is about to get very cold, and shake it for about five minutes. When you open the little bag, there is soft ice cream inside.

The odd thing is that it would not work without the salt. Ice on its own sits at about 0 degrees Celsius, the temperature at which water freezes and ice melts. Ice cream needs to be colder than that to set, because the sugar in it lowers the temperature at which it freezes. Plain ice, however much of it you use, simply is not cold enough.

The salt changes that. When salt dissolves in water, it lowers the temperature at which the water will freeze. This is called freezing-point depression. Pure water freezes at 0 degrees; salty water has to get colder before it will turn to ice, and the more salt there is, the colder it has to get.

Now think about what is happening in the bag. The surface of every ice cube is always slightly wet, and at that surface, water molecules are constantly leaving the ice and others are joining it. At 0 degrees, the two balance out. Salt dissolves in that thin wet layer, and the dissolved salt gets in the way of water molecules trying to lock back into the orderly pattern of ice. Molecules keep leaving, fewer manage to rejoin, and the ice melts, even though it is already very cold.

Melting takes energy. To turn from solid to liquid, ice has to absorb heat, and it takes that heat from whatever is nearest: the salty water around it, the bag, and the ice cream mixture in the middle. So the temperature keeps falling, well below zero. A mixture of ice and salt can get many degrees colder than ice alone. In one classroom version of the experiment, published in the Journal of Chemical Education, frozen salt solution reached about minus 21 degrees, colder than an ordinary home freezer.

The salt never touches the ice cream; it stays in the outer bag, doing its work through the plastic. Shaking keeps the freezing mixture moving against the cold, so that it chills evenly instead of forming one hard lump against the side. And it does not have to be table salt. What matters is how many dissolved particles get in the way of the water, not which ones, which is why chemists call freezing-point depression a colligative property.

The same idea is at work when salt is spread on an icy path in winter, and in the old-fashioned ice cream makers with a crank handle and a bucket of salted ice. Salt does not make things hot. It helps ice melt, and melting, as the ice cream proves, is a very good way to make things cold.