It seems like a simple question, with a simple answer: “The water gets colder!” But why? And what do we mean by “colder?”
Water (like just about everything else) can exist as a solid, a liquid, or a gas. The difference is its level of energy. If you add thermal energy – also known as “heat” – to an ice cube, it will melt into a liquid and you can drink it. But if your thirst for knowledge exceeds your thirst for water, you could keep on adding energy to the water just to see what happens. Of course you know what will happen – after adding enough energy, the liquid water will boil and turn into a gas. These phase changes – melting and boiling – happen to pure water at 0°C/32°F and 100°C/212°F.
If you are a student like most of my readers, you may be wondering why the amount of energy in a substance determines its state of matter – solid, liquid, or gas. This is because energy makes the molecules in a substance vibrate. If they have less energy, they will not vibrate very much, and the substance will be in a solid state. More energy means more vibration; at some point the substance will be vibrating too strongly for it to keep its shape, and it will melt. Likewise, the boiling point is the amount of energy at which the vibration is too strong for the substance to stick together, and it becomes a gas.
We measure the amount of thermal energy in units called “degrees”. When we talk about cooking food at 350°, or about the temperature outside being 45°, we are describing the amount of thermal energy in the food, or in the air.
Now we can deal with the question of how an ice cube cools a glass of water. The water in the ice cube is a solid, so it has less energy than the liquid water in the glass. When you drop a few ice cubes into the glass of water, they absorb some of the energy in the water. The ice gets the same amount of energy that the liquid water loses. As the ice melts, the water cools. Putting a little ice in a large glass of water will melt the ice quickly without cooling the water much; but adding a cupful of ice to a small amount of water might freeze the water. It depends on how much or how little energy is in either substance.
If you perform this experiment at home, try putting a thermometer in the glass before you begin. Measure the water temperature before adding the ice, and again after the ice melts. The change in degrees is the amount of thermal energy absorbed by the ice!