Liquid is one of the three basic states in which matter exists. The other two states are gaseous and solid. A liquid is similar to a gas because its molecules are not fixed to each other in any particular way. Liquids and gases are both called fluids because they can flow to fit the shape of any container in which they are put. A liquid is unlike a gas and similar to a solid because it has a definite volume, and its molecules are only slightly compressible. A liquid always seeks its own level. If a liquid is put in a container with several arms, it will rise to the same level in all the arms.
A thin layer on the surface of a liquid has a tension caused by molecular action, and acts like a skin. This is called surface tension. Because of surface tension, a greased needle will rest on the surface of water without sinking.
The molecules of a liquid often have a greater attraction for other substances than they have for each other. For this reason, they will rise in narrow tubes above their own level. This action is called capillary action. Plants draw water by capillary action.
If liquids are heated beyond a certain point, they vaporize (change into gas). Water changes into steam when it boils. If liquids are cooled below a certain point, they change into solids. Water freezes into ice. Different liquids have different freezing and boiling points. Substances that are normally gases can be cooled and compressed into a liquid state. Some normally solid substances can be heated until they turn into liquids. For more information, see the articles on Gas , Solid , and Water .
