Liquids are one of the four phases or states of matter, along with solids, gases, and plasma. Liquids will always flow in order to fit the shape of the container they are poured into. This makes them like a gas. However, like a solid, liquids cannot be physically shrunk, or compressed, through an increase in pressure. Nor do they expand to fit the entire container they are placed into, as a gas does. In short, a liquid is fluid, like a gas, but condensed, like a solid.
Unlike a solid, in a liquid, molecules have obtained a high enough level of energy to be able to move relatively freely. For this reason, liquids do not have fixed shapes: when placed into a container, they will flow until they assume the shape of that container. At the same time, however, the molecules have not assumed the level of freedom of movement of those in a gas, which have higher energy and move more quickly. For this reason, a liquid will not expand to fill a space; given the same level of pressure, it will maintain the same volume. For this reason, when we pour a liquid into a container, gravity will hold the liquid down to the lower portion of the container, unless we physically pour in enough liquid to entirely fill the container.
Along the edges of the liquid, some molecules will be of higher energy than others; at any given moment, some of these will be bouncing away from other molecules and flying away from the liquid entirely. For this reason, liquids, unlike solids, seem to very gradually evaporate before our very eyes. How fast a liquid’s molecules move, and therefore how fast they tend to evaporate, depends largely on temperature: given the same liquid, its molecules will move more slowly when it is cool, but speed up as the sample is heated.
Eventually, liquids reach the so-called “boiling point,” at which they evaporate fully to become a gas. For example, water boils at 100 degrees Celsius, or 212 degrees Fahrenheit. Beyond this point, all of the water in the sample will evaporate, becoming water gas or what we know as “steam.” By the same token, liquids can be cooled to the point at which they freeze. At the freezing point, the molecules in the solid have slowed down to the point where they can no longer move freely; instead, they compress together into a solid and begin to vibrate rather than move freely.
Only two chemical elements are liquids in everyday temperatures, both of which are halogens: mercury and bromine. (Lighter halogens are gases, like chlorine and fluorine.) Most of the liquids which we encounter in everyday life are actually compounds, like water (H2O, or two hydrogen atoms and one oxygen atom). In virtually all cases, liquids are less dense than their associated solids, so solids sink into liquids: for example, if we were to heat aluminum until it melted and then we dropped an aluminum bar into the molten aluminum, the bar would drop. (And it would eventually heat up to the point where it melted, as well.)
Water, however, is an important exception: because of the way hydrogen chemical bonding works, water actually expands when it freezes, becoming less dense. This is why ice floats.
