A phase describes the physical state of matter. The key word to note is physical. Matter only moves from one phase to another through a physical change. If energy is added (such as increasing the temperature) or if energy is taken away (such as decreasing the temperature) we have created a physical change.
The three phases of matter.
The three most common phases that matter takes on are solids, liquids, and gases. A compound or element can change from one phase to another, but still remains the same substance. Think about a nice refreshing glass of water, without the salt for now. At room temperature the water is a liquid, but if once the glass is put into the freezer the water turns into a solid. The content of the glass is still H2O, just H2O (liquid) versus H2O (solid).
The fundamental difference between phases is the distance between particles. In solids particles are extremely close together, essentially in a fixed, ordered arrangement. These particles would not be much fun at a party.
In liquids, there is a bit more disorder. The particles are free to move around, but they are still very close together. These particles want to party, but there's not room at the disco to move. The particles in gases, on the other hand, are far apart. They have complete freedom of motion and are in total disorder. They're going all Animal House on us.
Another related property is density, the mass per unit volume. Gases have the lowest density, followed by liquids, then solids that have the largest density.
What weighs more: a helium balloon, a water balloon, or a brick, assuming they are all roughly the same size? The balloon weighs the least because there is less stuff packed into the same amount of space—it has the smallest density. The brick weighs the most because it contains the most stuff in the same amount of space—it has the largest density.
Which object weighs the most? Density is the key.
The kinetic energy (the energy associated with motion) and intermolecular forces (IMF) amongst the particles in solids, liquids, and gases are different. When we talk about IMFs, we are referring to the strength of the attractions in between different molecules. A typical mistake is to think in terms of the actual bonds within the molecule. IMFs actually refer to attractions between one molecule and another. This usually depends on an electrical attraction. Positive charges are attracted to negative charges. Paula Abdul was right all along, opposites attract.
In the gaseous phase particles are in constant motion and have high energy. There are essentially no intermolecular forces of attraction between molecules because the particles are so far apart and rarely interact. They're like Kim Kardashian and your chemistry class, they're probably not likely to run into each other anytime soon.
Liquids, on the other hand, are made up of molecules that are very close to each other but still have some translational energy (they can move around). They're like the New Jersey Housewives, minus the table flipping. What keeps these liquid molecules close to one another? IMFs. This means IMFs for liquids must be stronger than the IMFs for gases.
Vaporization or boiling point (the temperature at which a liquid is converted into a gas) is the temperature at which we break all (or nearly all) the IMFs that keep the molecules close together in the liquid phase. Gases can turn into liquids by a process called condensation, while solids can turn into liquids by a process called melting.
What about the solid phase? It is the most ordered of the three phases. The IMFs are so crazy strong that the particles are effectively locked in place and are no longer able to move. The melting point is the temperature at which a solid is converted into a liquid. At this temperature some (but not all) of the IMFs are broken.
Solids sometimes go directly from the solid phase into the gas phase at a given temperature. We call this process sublimation. The opposite process, the conversion of a gas into a solid, is called deposition. The final process you should know is freezing, which is the process in which a liquid is turned into a solid. Can't get enough of this stuff? We'll go into more details about phase changes later.
Still confused about IMFs? Check out this video for an informative short review! Click me, please.
Picture a gymnasium filled with kids that each drank a gallon or so of go-go juice. The kids would be running around like crazy, bouncing off the walls and occasionally running into each other. These kids are like gas particles. Now let's tie the kids together with some rope. The kids are still moving like crazy, but their movement is limited and they hit each other more often. We've liquefied the kids. Finally, let's make the kids lock arms. The kids will stay in place because their movement is constricted. The kids are now in their solid form.
A fourth less common state of matter called plasma is found in fluorescent lights, neon signs, stars, and lightning. Intrigued? Check out more plasma facts here.