Mass and volume during a change of state

When water changes state, two quantities behave very differently: mass and volume. Confusing the two is the most common mistake in this chapter.

Mass never changes

A change of state neither destroys nor creates matter: the molecules that were there before are still there afterwards, merely rearranged. If you melt 100 g of ice in a closed container, you end up with exactly 100 g of liquid water. This is a special case of the conservation of mass.

Volume, on the other hand, does vary

Volume depends on how tightly the molecules are packed, so it almost always changes from one state to another:

Phase change	Effect on volume (general case)
Solid → liquid	volume varies little (often a slight decrease)
Liquid → gas	volume increases enormously (hundreds of times)

The special case of water

Water is a well-known exception: unlike most substances, its volume increases when it freezes. This is why ice floats on liquid water, a bottle full of water can cause its cap to burst in the freezer, and pipes freeze in winter.

The mistake to avoid

‘Ice floats, so it weighs less than liquid water.’ This is a misleading simplification. Ice floats because, for the same mass, it occupies more volume than liquid water: its density is lower. The mass of an ice cube does not change as it melts — pour the water it produces into a glass, and it will weigh exactly the same as the original ice cube, whilst its volume will have decreased slightly.