This goes for all matter.
The solid form of matter refers to its cold state, while the gas form is its hot.
If you heat up a rock far enough, it eventually "melts" into a liquid. If you go to a volcano and happen to see some magma, really all you are seeing is liquid rock. On the surface, it will quickly cool, and turn back into its solid form. But, if you could heat it up further still, it would eventually turn into a gas. It would really have to be hot! We know of one such thing that could technically just be called a gasseous rock... the sun. It's just a ball of gas. A very hot ball of gas.
Let's think of a different example.
If you cool down water to 32 degrees (0 degrees celcius), it freezes. In scientific terms, it solidifies. Just like a rock. Go the other way, heat it up to 212 degrees (100 celcius), and it starts to boil. Heat it further still, and it becomes a gas. Allow it to cool and condense into one place, and you start to see water again.
One more example.
We breathe air. Air is a gas.
At room temperature, the air we breathe is hot enough to remain as a gas. But if we cool it down a couple of hundred degrees, it starts to solidify into a liquid. You've seen those tanks of liquid hydrogen and liquid oxygen, haven't you? Someone had to cool down air to a liquid state, and then put it in a container (tank). If opened improperly, that liquid, when exposed to the temperatures we consider normal, will boil, steam, and turn into a gas almost instantaneously. This would cause the tank that held it to be turned into a missile, or even explode like a bomb (theoretically).
So, each type of matter has a different temperature range that determines when it is solid, liquid, or gas. Here on Earth, we have all three states of matter. On the sun, it is so hot that you only have the gasseous state of all matter. Travel to the distance of pluto, and it's so cold that there's almost no form of gas, and probably no form of liquid (though I do not know the temperature at that distance, so maybe even some material from the sun could still be in liquid form at that distance).
As I mentioned, air is the gas form of matter that we are able to use it. It is clearly several hundred degrees hotter than required to make it a liquid. But that's still within our normal range to survive.
Whether a material is solid, liquid, or gas all really depends on how fast the atoms that make up that material are bouncing around.
If the atoms are lazy, and don't interact with other atoms very often (bounce off one another), then the cooler that object is, and the more solid it is. If the atoms are playing happily and are having a lot of fun, then the object is warmer, and turns to liquid form. If they are bouncing around in a rage and going berserk, then the matter is really hot, and probably in gas form.
Typically, the hotter something is, the more space it needs. When the atoms are bouncing off of each other with that kind of force, they tend to spread out more.
But! But, there are other ways. If you put pressure on an object/material, you will cause the atoms to bounce off each other more quickly as well, thus heating it up. One example is a pressure cooker, where, instead of using flame or heating elements, you just add pressure to the stew, and it starts to heat up, and eventually cook. Another example is the diesel engine, where the piston keeps compressing the fuel until it gets so hot that it explodes (as opposed to the combustible engine that uses a spark to ignite the fuel). In each case, the fuel turns from a liquid into a gas, which expands the chamber which was holding it, and causes a chain reaction which eventually turns a drive shaft and allows the vehicle to move. (this is also why a liquid oxygen/hydrogen cylinder can turn into a missile, the expansion of the gas is so fast that the force of energy coming out of it actually causes it to move... if it doesn't explode like a balloon, which it usually won't.)