States of Matter: Gases and Liquids — Definition
Definition
Imagine you have a substance, say water. Depending on its temperature and pressure, it can exist as ice (solid), liquid water, or steam (gas). These are the three common states of matter. This chapter focuses on two of these: gases and liquids.
Gases: Think about the air around you. You can't see it, but it fills any container it's in, whether it's a balloon or a room. This is because gas particles (molecules or atoms) are very far apart from each other and move around randomly and rapidly.
They have very weak attractive forces between them, almost negligible. This means gases don't have a fixed shape or a fixed volume; they simply expand to fill whatever container they are placed in. Because there's so much empty space between particles, gases are highly compressible – you can squeeze a lot of gas into a small space, like in a gas cylinder.
They also have very low densities compared to solids and liquids. The behavior of gases is largely governed by simple relationships between pressure, volume, temperature, and the amount of gas, known as gas laws.
Liquids: Now, think about liquid water. If you pour it into a glass, it takes the shape of the glass, but its volume remains the same. If you pour it into a bowl, it takes the shape of the bowl, but the volume is still the same.
This is the defining characteristic of a liquid: it has a definite volume but no definite shape. The particles in a liquid are much closer together than in a gas, but they are still free to move past one another.
They experience stronger attractive forces (intermolecular forces) than gas particles, which keeps them relatively close but not rigidly fixed like in a solid. These forces are strong enough to prevent the liquid from expanding indefinitely but not strong enough to hold them in fixed positions.
Liquids are much less compressible than gases because there's less empty space between their particles. They also have higher densities than gases, often comparable to solids. Important properties of liquids include vapor pressure, surface tension, and viscosity, all of which are direct consequences of the intermolecular forces present.