Water density

Water density changes with temperature and salinity. Density is measured as mass (g) per unit of volume (cm³). Water is densest at 3.98°C and is least dense at 0°C (freezing point).

Why does ice float in water?

• Each water molecule is made up of two hydrogen (H) atoms and one oxygen (O) atom. The bonds between water molecules are called hydrogen bonds.
• As water cools to 3.98°C, its mass stays the same but volume decreases – the same mass fits into a smaller space so it is more compact.
• When water freezes at 0°C, the mass stays the same but its volume expands by 9 percent.
• In liquid water, molecules are attracted to each other and temporarily held together by hydrogen bonds.
• When water freezes at 0°C, a rigid open lattice (like a web) of hydrogen-bonded molecules is formed. It is this open structure that makes ice less dense than liquid water. This is why icebergs float.

Density at different temperatures

Water at:

• 30°C is 0.9957 g cm-³
• 4°C is 1.0000 g cm-³
• 0°C is 0.9998 g cm-³

How does salinity affect water density?

• Salt is made up of many sodium and chlorine ions stacked together in a lattice. For every sodium ion you will find one chlorine ion (1:1 ratio).
• Salt dissolves in water because the attraction between the water molecules and the sodium ions or chlorine ions is stronger than the attraction between the sodium ions and chloride ions in the lattice. This allows the sodium and chlorine ions to be pulled apart by the water molecules. The hydrogen of the water molecule is attracted to chlorine ions and the oxygen to the sodium ions.
• The addition of salt to water makes a solution that is denser than fresh water – it freezes at a lower temperature.
• The salinity of seawater is about 3.5 percent and it freezes at about -1.9°C.
• As ice forms in the sea, the salt cannot form part of the ice crystal so the ice is almost pure water. The salt that is rejected forms brine beneath the ice and becomes more and more salty until it becomes so dense that it sinks, displaces less dense seawater that moves to the surface. This is why seawater at Antarctica is very salty.

The article Ocean density further explores how salinity, temperature and depth all affect the density of seawater.