Adding Salt to Ice in a Cooler

Why do people sometimes add salt to ice in a cooler for drinks or beer?  It’s a simple question.  There are tons of articles and videos online but I couldn’t find a comprehensive explanation of why it works.

Some Basics

It’s important to first understand what melting point and freezing point mean.

Melting point is a property of solids.  The melting point is simply the temperature at which the solid turns into a liquid.  For example, the melting point of normal ice is 0°C at standard pressure.

Freezing point is a property of liquids.  The freezing point is simply the temperature at which the liquid turns into a solid.  Thus the freezing point of normal freshwater is 0°C.

The value of the melting and freezing points is the same (ie. both are 0°C for water).

So as you can see, melting point and freezing point are kind of “mirrors” of each other.

It doesn’t make sense to talk about the freezing point of a solid (since it’s already solidified).

What Does Salt Do?

When salt is added to water it lowers the freezing point.  In other words the water needs to be chilled to a temperature lower than 0°C (say -2°C for example) in order to change to ice.  Another way to say the same thing is that salt allows water to exist as a liquid at a temperature lower than 0°C.  This is the important part that’s relevant to beer coolers.  We’ll come back to this later.

For completeness let’s look at this from the perspective of melting point.  When salt is added to ice it lowers the melting point.  In other words the ice begins melting at a temperature lower than 0°C.  This is why salt is added to ice on the roads in the winter.  It causes ice, that would have otherwise remained as a solid in sub-zero temperatures, to turn to water.  Note that the temperature of the water has not changed.  It’s still at a sub-zero temperature but, as mentioned above, the salt allows it to remain as a liquid at the lower temperature.  Don’t think that just because the salted-ice has become water the temperature has risen.

How Does this Apply to Beer Coolers?

In two ways: chilling beers fast and keeping beers cold.

Chilling Beers Fast

An important part of chilling beer fast is to maximize how much of the can/bottle’s surface area is in contact with the chilling agent, whether it’s ice, water, or cold air.

Salt-water lets you chill a warm beer really fast — much faster than a freezer [1].  Pour cold water, ice, and salt into the cooler to create a salt-water-ice bath.  Why does it work?  The ice will be at a temperature way below zero, usually -18°C for a household freezer [2].   The ice will cool the water down and the salt will allow the water temperature to drop below 0°C.  The beer will then be fully submersed in sub-zero water, maximizing the surface area in contact.

Without salt the water will remain at a temperature slightly higher than 0°C even though you have -18°C ice cubes floating in it.  The water will still get quite cold and will do a good job but not as cold as salt-water.

If you only put ice in the cooler then less of the beer’s surface is touching the chilling agent (ice) since ice cubes are irregularly-shaped.  It won’t chill the beer as fast as salt-water.

Keeping Beers Cold

If your goal is to keep the beer cold for a long time (ie. if you go camping or on a picnic) it’s still a good idea to add some water and salt to the ice because it will make your beers colder initially which means they’ll stay cold longer.  The salt will make the ice melt as well but the resulting water will still be very cold.  The specific heat capacity of water is double that of ice [3], which means, in theory, the sub-zero water will stay colder longer than plain ice will.  You could argue that the starting temperature of ice is much lower (-18°C) than sub-zero water (which might be just a few degrees below 0°C).  So it’s a bit of a tradeoff.  Also, adding salt to the water does lower its heat capacity but not by much [4].

Note: you need lots of salt (handfuls) for the above stuff to work.  Table salt is fine.