States of matter (solid, liquid, gas)
The idea
Water can be an ice cube, a drink, or the invisible steam rising from soup — the same stuff in three different states. The three states of matter are solid, liquid, and gas. A solid keeps its own shape, like a rock or a crayon. A liquid flows and takes the shape of whatever holds it, like juice in a cup. A gas spreads out to fill all the space it can reach, like the air inside a balloon.
A good way to sort something is to ask two questions. Does it keep its shape when you move it to a new container? If so, it is a solid. If it flows and settles into the shape of the container, it is a liquid. If it spreads out everywhere and you cannot even see it, it is probably a gas. One common mix-up: people think a gas is nothing at all because it is invisible. But gas is real material — blow up a balloon and you can see the space the air takes up, and feel it push back when you squeeze.
Worked example
You have an ice cube, half a cup of water, and an empty balloon. Using a square box, a round bowl, and your own breath, sort the three things into solid, liquid, and gas.
- Put the ice cube in the square box, then move it into the round bowl. It stays a cube in both — it keeps its own shape, so ice is a solid.
- Pour the half cup of water into the round bowl. The water flows and spreads into a round puddle shape, but when you pour it back you still have about half a cup. A liquid changes shape, not amount.
- Blow your breath into the balloon. The air spreads out and fills the whole balloon evenly, with no empty corner left over. Stuff that spreads to fill all the space it can is a gas.
- Squeeze the balloon gently as a check. You cannot see the air, but you can feel it pushing back, which shows the gas is real material taking up space.
Answer. The ice cube is a solid, the water is a liquid, and the air in the balloon is a gas.
Check your understanding
- What clues would you look for to decide whether sand is a solid or a liquid, since it pours like a drink but each grain keeps its shape?
- How could you show a friend that air is real material even though you cannot see it?
- What happens to the shape and the amount of juice when you pour it from a tall glass into a wide bowl, and why?
- Where in your home can you find all three states of matter in the same room at the same time?