Question: how does ice float in water

Sarah Thomas answered on 17 Jun 2011:

There are two parts to the answer of this question.

Firstly a substance will float if it is less dense than the other components of the mixture. Density means the amount of mass per unit of volume. This is the reason that if you chucked a bunch of rocks into a river, they sink. basically this means that the water floats on top of the rocks!

As Derek said, normally when you cool a substance down it becomes more dense. However as water cools and freezes it becomes less dense. The reason for this is “Hydrogen Bonding”. I don’t know if you have learned about his in school so I will explain it below:

A water molecule is made from one oxygen atom and two hydrogen atoms, strongly joined to each other with covalent bonds. Individual water molecules are also attracted to each other by extremely weak bonds called hydrogen bonds. The oxygen atom has pairs of electrons in it’s outer shell that give out a netative charge. Hydrogen atoms are attracts to this. In the liquid, water molecules are constantly moving around and making short-lived hydrogen bonds with each other. As water freezes, the molecules stop moving and the hydrogen bonds between them all line up and form a crystal lattice.

The crystal lattice is what makes the ice less dense than the water.

You can stop ice from being able to make these hydrogen bonds by adding lots of food colouring to the water before you freeze it. Then your colourful ice cubes will sink! This will also happen if you add detergent to the water.

The other way to make ice sink is to add something to the water than is less dense than ice, for example, alcohol. This is why the adults reading this may have noticed the ice sinking in their rum and cokes! 😀

Hydrogen bonding is responsible for a lot of surprising things. Like the formation of droplets from a dripping tap, and how plants manage to suck water up from their roots all the way to their leaves i.e. against gravity!