Things that flow

The idea

Liquids and gases both flow and keep no fixed shape, which distinguishes them from solids and classifies them as fluids.

Water and air

Water flows from a tap. Air flows as wind. A stone just stays where it is.

What is common to water and air?

• Both can flow
• Both have a fixed shape
• Both are solids

Open the tap

Put each one in the top jar and open the tap. Which ones flow along the bent pipe?

Fluids

Liquids and gases both flow and do not keep a fixed shape. This makes them different from solids, and so they are called fluids.

• Thing | Fluid?
• Water (a liquid) | Yes: it flows
• Smoke or air (a gas) | Yes: it flows
• Stone (a solid) | No: it keeps its fixed shape

Notes

Liquids and gases both flow and keep no fixed shape. This makes them different from solids: they are fluids.

Check yourself

Which of these are fluids?

Why is a stone not a fluid?

Dry sand can be poured from a bag. Is sand a fluid?

Which one of these is a fluid?

• Liquids and gases — correct. Yes. Both flow and keep no fixed shape.
• Solids and liquids. Solids do not flow. They keep a fixed shape.
• Only liquids. Gases flow too, so they are fluids as well.
• It is too heavy. A tiny pebble is not a fluid either. A solid keeps its fixed shape.
• It is not made of particles. A stone is made of particles too. They are held in fixed positions.
• It does not flow; it keeps a fixed shape — correct. Yes. Its particles are held in fixed positions.
• Yes, because it can be poured. Pouring moves the grains, but each grain keeps its own fixed shape. Sand is a solid.
• No. Each grain of sand is a solid with its own fixed shape — correct. Right. The grains tumble over each other, but each grain keeps its shape.
• Yes, because it is made of tiny grains. Small size does not make a fluid. Each grain is a solid with a fixed shape.
• An iron nail. An iron nail is a solid with a fixed shape.
• Milk — correct. Yes. Milk is a liquid, so it flows.
• A wooden block. A wooden block is a solid with a fixed shape.