HOW COME? Salt speeds melting, freezing

If salt melts ice on roads in winter, why do we use it in ice cream makers to freeze homemade ice cream? asks a reader.If you've ever made ice cream with a hand-cranked ice cream maker, you know that resting the tub of ice cream mix in a layer of salted ice is the final step in the process. While it seems like adding salt should short-circuit ice-cream freezing, it actually speeds it up.

How come? Even during an August heat wave, try to imagine a snowy, icy road in January. In a puddle, water molecules slide past each other, their chemical bonds holding the liquid loosely together. Take away enough heat energy, and H2O molecules begin to cling stiffly together, in a kind of lattice. Presto: solid ice.

But freezing isn't a smooth process. Some molecules glom onto forming ice; others break free. At 32 degrees Fahrenheit, a parcel of water is balanced between freezing and melting. Below 32 degrees F., as molecules lose more and more energy, more and more water freezes.

Salt dumped onto an icy road disrupts the freezing process. The water molecules' tiny electric fields pull the salt molecules apart into sodium and chlorine atoms. Sodium atoms (ions) have a positive electrical charge; chlorine ions are negative. The charges attract H2O molecules, causing them to cluster around the ions rather than attaching to forming ice.

As salt on the road slows the rate of freezing, melting goes on as usual, with some H2O molecules leaving the ice, returning to liquid. The end result: Ice gradually disappears, and the road becomes safer for driving.

For ice to re-form, the temperature must drop enough that freezing outpaces melting. In effect, salt lowers the freezing point of water. Which is why the Great Salt Lake in Utah begins to freeze at about 12 degrees F.

An ice cream mixture's freezing point is below 32 degrees F to begin with, since dissolved sugars interfere with forming ice. Salt to the rescue! Experiments show that adding salt to a blender of crushed ice and water can lower the temperature to minus 2 degrees F. in just a few seconds. By packing a "brine" of chopped ice and salt around a tub of ice cream mix, you've created an instant deep freezer.

Heat energy flows from a warmer region into a colder region. As the ice cream mix loses heat to the extra-cold brine, it begins to freeze. Stirring the ice cream keeps its constantly forming ice crystals small rather than chunky. Churning also whips air into the mix, creating a fluffier texture.