How do ice cubes cool a drink?

Matthew Koss, College of the Holy Cross

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How do ice cubes cool a drink? – Helen, age 12, Boston, Massachusetts

I like to make my own iced tea. First, I steep about a quart of terrifically strong hot tea and pour it into a pitcher loosely filled with ice cubes straight from the freezer. The result is about two quarts of delicious, cold tea, with a few ice cubes floating on top.

What’s the science behind my perfect pitcher of iced tea? A simple answer is that the near-boiling tea transferred its hotness to the below-freezing ice cubes. This transfer raised the temperature of the ice cubes so that much of the ice melted. That both cooled the tea to a refreshing drinking temperature and diluted it for a proper taste.

My iced tea process reminds me of physics experiments I worked on that flew on the space shuttle Columbia in the 1990s. I was interested in how liquids turn into solids in the absence of gravity.

Both those space experiments and the making of iced tea depend on liquids or solids getting hotter or colder in just the right combination. The explanation for how it all works depends on two related concepts: temperature and energy.

What are temperature and energy?

The concept of temperature grew out of the sense of what feels hot and cold to human touch. Boiling water is too hot to touch safely, and a candle flame is even hotter. Ice cubes feel cold. A thermometer is a special hotness meter that more reliably and repeatably determines the degree of hotness or coldness of substances than our sense of touch.

Eighteenth-century physicist Daniel Gabriel Fahrenheit defined the point when ice melts or water freezes as 32 degrees of temperature – what we we now call 32 degrees Fahrenheit in his honor. He also defined the temperature at which water boils and turns into steam to be 212 degrees F.

Energy is a more complicated concept. One way to think about energy is as a way to track what it costs nature to makes changes from one set of conditions to another. There are many types of energy. Thermal energy is related to temperature. Kinetic energy is based on how fast stuff is moving. Other forms of energy include, to name only a few, electromagnetic energy, chemical energy and nuclear energy.

Another type of energy is called potential energy and it accounts for the energy stored by the forces acting between or within objects. It is those forces that control the transfers from one type of energy to another. Examples of potential energy that you can see are a rubber band stretched and about to snap back or a boulder sitting at the top of an incline, ready to roll. On a microscopic level, potential energy is composed of all the interacting forces between the atoms and molecules that make matter.

Once a boulder begins to roll down the hill, its potential energy from the force of gravity first transforms to kinetic energy and then maybe to thermal energy or to the work of knocking down a tree. Although energy can transform from one type to another, the total amount of energy always remains the same. Whatever amount of energy is gained in one place must be lost from somewhere else. Scientists call this rule of nature conservation of energy, or in cases involving thermal energy, the first law of thermodynamics.

On a molecular level

As an example of the distinction between energy and temperature, compare a cup of hot tea and an iceberg. The hot tea is clearly at a much higher temperature than the iceberg, but the large iceberg has vastly more energy than the tiny cup of tea.

The tea is hotter than the iceberg because the molecules that make up the tea are moving faster than the molecules that make up the iceberg. The temperature is essentially the average energy of the molecular movements.

The speed and energy of a typical single liquid molecule are greater than for a typical single iceberg molecule. But the iceberg has many, many more molecules, all moving around, interacting with each other and transferring vast amounts of energy between their kinetic and potential energies.

The trick is energy transfer

Back to the question of making iced tea. When the hot tea encounters the ice cubes, the molecules of tea-flavored water collide with the molecules on the surface of the ice. The liquid’s molecules slow down. The ice’s molecules speed up, and some even leave the solid and mix in with the liquid tea.

The tea loses thermal energy and lowers its temperature. That energy transfers to the ice, which gains that energy and increases its temperature.

The process of energy moving from one place to another due to differences in temperature is called heat transfer. Left to itself, nature transfers thermal energy only from higher to lower temperatures. That observation is called the second law of thermodynamics. It’s why you will never see a cup of cold tea turn into a cup of ice cubes and hot tea.

In my pitcher, this process continues until the ice eventually reaches a temperature of 32 degrees F, its melting temperature. At that point, the ice cannot take any more energy and stay solid, even though it is still absorbing thermal energy from the hotter tea. Instead, any additional thermal energy absorbed by the 32 degrees F ice transforms it into 32 degrees F water.

Heat transfer cooling means that energy leaves the tea. Conservation of energy means a matching amount of energy raises the temperature of the ice to 32 degrees F and then turns some of that ice into 32 degrees F water. Thus the thermal energy of the tea turns into the potential energy of the water molecules that used to be part of the ice, lowering the temperature of the liquid in the pitcher. The process continues until eventually all the tea and whatever ice that survived the melting are at the same refreshing 32 degrees F.

Keep all this in mind as you enjoy your ice-cold drink on a hot summer day. Don’t dawdle, though, as the air temperature in the room or your warm hand on the glass can transfer their thermal energy to melt the rest of the ice and then begin to raise the temperature of your drink!

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