Copyright © 2007 Dorling Kindersley
As water flows along a river, it constantly changes its shape to fit the space available. This is because water is a liquid, and liquids flow and do not have a fixed shape. Instead, they take on the shape of whatever container they are in. If you pour a liquid from a glass onto a plate, the volume of liquid (the space it takes up) stays the same, but its shape changes.
Mercury is a liquid metal that is poisonous. When mercury is dropped onto a surface, it rolls off in little balls. This is because the forces between the mercury particles are very strong, so the particles clump together. This force between particles of the same type is called cohesion. Water particles do not have such strong cohesion, so they wet surfaces.
A measure of how fast or slowly a liquid can flow is its viscosity. Crude oil, for example, is a liquid that does not flow very easily. It is said to have high viscosity. Heating crude oil lowers its viscosity and enables it to flow more freely through pipes. Other liquids, such as water, flow easily without being heated. Water has low viscosity.
Although they look very different, these two containers contain the same volume of liquid. The volume of a liquid is the amount of space it takes up. Although liquids change their shape when moved from one container to another, their volume always stays the same. For this reason, liquids are usually measured by their volume, in litres or gallons.