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%.
- 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% 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.
Related content
The article Ocean density further explores how salinity, temperature and depth all affect the density of seawater.
Learn more about density and its effect on floating and sinking in the article Building Science Concepts: Floating and sinking.
A sinking feeling is a level 4 Connected article that introduces the concepts underpinning floating and sinking in the context of a boat race. The teacher support materials provide curriculum links and include suggestions on how the article can be used to grow science capability in critiquing evidence.
Activity ideas
Density – students are introduced to the concept of density. They calculate the volume of various materials to determine and then rank their densities.
Floating eggs – students investigate water density by floating an egg in fresh water and salt water.
Investigating sea water – students investigate some of the properties of seawater.
Water temperature – students look at what happens when hot and cold water meet.
Useful links
Video from Khan Academy showing the hydrogen bonding for liquid water.
In this TedEd video find out Why does ice float in water?
Water density calculator from the Omni Calculator website.
There are many fish and invertebrates that survive in the oceans and beneath the sea ice around Antarctica. Scientists have found that they have various strategies that allow them to survive in this environment, explore Antarctic animal adaptions on the Cool Antarctica website.
