Properties of fluids

Matter comes in different states: as solids, liquids, and gases. Liquids and gases have something in common that makes them different from solids. The particles that make up liquids and gases can move around. Because of that, liquids and gases don’t have a fixed shape like solids do. Instead, they adjust to the space they are in.

The movement of particles allows liquids and gases to flow. Because of this characteristic, we can group them together as fluids. While all fluids flow, each also has its own unique properties. Let’s take a look at some of them! Here is a spoonful of milk and a spoonful of honey.

What happens if we tip them? The milk is thin and runny, so it flows off the spoon easily. Honey, meanwhile, is thick and flows off the spoon very slowly. These two fluids have a different viscosity. Viscosity tells us how thick or thin a fluid is.

Fluids with low viscosity, such as milk, are thin and flow easily. Fluids with high viscosity, like honey, are thick and resist flowing. So, viscosity can also be described as the resistance of a fluid to flow. This resistance is due to the forces of attraction among the particles in the fluid. This is called cohesion.

The stronger the cohesion, the more resistant the particles are to flowing past one another, and the thicker, or more viscous, the substance is. There is another property that affects how quickly a fluid flows. Have you ever tried to get the last drops out of a ketchup bottle? Tough, isn’t it! That’s because the particles of the ketchup are attracted to the particles of the bottle.

This property is called adhesion. The particles of different fluids are attracted more or less strongly to other substances. Some fluids have strong adhesion, and others weaker adhesion. Adhesion can also slow down the flow of a fluid through tubes or pipes. Particles of fluid stick to the walls of pipes, meaning the fluid typically travels faster near the centre of the pipe than at the edges.

So, when you take the lid off that ketchup bottle and tip it, you might get a big blob of ketchup on your plate, while lots of ketchup is still stuck to the sides of the bottle! Another thing about ketchup bottles… Have you noticed how hard it is to squeeze a full bottle of ketchup? This is because ketchup is a liquid, and in liquids the particles are quite close together. This means they can’t be squeezed — or compressed — much more. But when you’ve used up the ketchup, the bottle contains another fluid instead — air.

Air is a gas, and particles in gases are spread out. This means there is still space for them to be moved closer together, so it is much easier to squeeze the bottle. This property of fluids is compressibility. Compressibility is a measure of how much a fluid decreases in volume when placed under pressure. Gases are more easily compressed than liquids.

Let’s sum up. Liquids and gases are fluids. Fluids have no definite shape and have the ability to flow. Some properties of fluids include: viscosity — a measure of how thick a fluid is and its resistance to flow; cohesion — the forces of attraction among particles of the fluid; adhesion — the tendency of fluids to cling to other substances; and compressibility — a measure of how much a fluid decreases in volume when squeezed.