Most middle school science textbooks state that matter will be in a solid, liquid, or gas state depending on how fast its molecules are moving. According to these textbooks, since temperature is a measure of how fast the molecules in matter are moving, when matter is at a high temperature, its molecules are highly energetic and moving extremely fast. This pushes the molecules far apart, and the matter behaves like a gas. When matter is at a lower temperature, its molecules are less energetic and moving slower. This keeps the molecules closer together, and the matter behaves like a liquid. When matter is at a still lower temperature, its molecules are moving even slower, and when the molecules are moving so slow that they cannot move out of their positions, then the matter behaves like a solid.

Unfortunately, this is an oversimplification of how and why matter changes state, and it can create misconceptions about temperature and state instead of fully explaining the behavior of the molecules when they are in the solid, liquid, and gas states. Let’s take a look at what happens when the temperature changes if the molecules are simply behaving like hard spheres (bouncing balls).

Temperature
Cold
Hot
Temperature:

Lowering the temperature does slow the molecules, but the molecules never transition from a gas state to a liquid state. No matter how slowly the molecules are moving, the molecules remain spread out and the matter continues to occupy all of the available volume. Maybe the slow moving molecules will form a liquid in the presence of gravity?

Temperature
Cold
Hot
Temperature:

In the presence of gravity, the molecules at the coldest temperatures do behave more like they are in a liquid state. However, the molecules never enter a solid state where the molecules are in fixed positions and the matter maintains a fixed shape. There also is not a clear transition between the liquid state and the gas state in the simulation. When water molecules change from a liquid state to a gas state in the real world, the transition is very clear and sudden. In the simulation, the slow moving molecules behave less like a layer of liquid water with a distinct surface, and more like a heavy fog that is settling along the ground. It is also worth pointing out that water molecules may enter a liquid state even under weightless conditions. If you were to open a bottle of water on the International Space Station, the water molecules would form drops of water instead of spreading out like a gas. So, clearly molecules do not need gravity to form a liquid, and there is more to changes in states of matter than just temperature and the speed of the molecules.