A cool glass of water topped with huge chunks of ice is sure to quench your thirst on a hot summer day. But, before you guzzle the water, look at how the ice floats on the water surface. How do these large ice pieces manage to stay afloat?

Ice floats because it is less dense than water. (If you take a one-litre container with ice and weigh it, it will be lighter than a similar container with water.)

How Does Ice float?
How Does Ice float?

Density : How dense it is

All materials are made up of molecules. These molecules are either bound tightly to each other or are loosely bound. This packaging of molecules is called density. The tighter the molecules, the higher the density of the material.

Water flows because the molecules are loosely bound. When you heat water, the molecules go farther apart, converting water into a gas called water vapour.

When you freeze water, the water molecules come closer and increase the density. But, this process stops after a point. If you freeze water beyond four degrees Celsius, the water molecules stop contracting and start moving apart. This is called the anomalous expansion of water.

You can check this by filling a plastic jar with water and putting it in the freezer overnight. The next day, you will see that the jar has stretched (or even cracked) and the ice has extended beyond the point till which you had filled water.

This happens because at four degrees Celsius, water molecules are packed as tight as possible and any attempt to push them closer (by lowering the temperature) only causes strain – the water molecules begin repelling each other and move apart.

How Does Ice float?
How Does Ice float? [Illustration by Shinod AP]
At freezing point, the molecules form a type of structure that has a lot of air gaps – making the ice much less dense than liquid water.  In other words, the density of water reaches a maximum at four degrees Celsius and lowering of the temperature only decreases the density.

Since water freezes well below four degrees Celsius (at zero degrees Celsius), ice is less dense than water and so it floats.