Same water, three states — answer key

1. SQ1 · Practice

Model answer. A liquid. It takes the shape of its container but keeps a fixed volume. It can hardly be squashed, so it isn't a gas.

A full answer includes

• I chose liquid.
• I used the changing shape with an unchanged volume as evidence.
• I used “hardly squashes” to rule out a gas.

Also accept

• Liquid: it takes the shape of the container, keeps the same volume and cannot be compressed.

Watch for

• “It changes shape, so it must be a gas.” — Liquids and gases both change shape. A gas would also change its volume: it would fill a closed container and squash in a syringe.

2. SQ2 · Practice

Options as numbered on the pupil sheet: 1 — Solid; 2 — Liquid; 3 — Gas.

Model answer. Solid: particles vibrate about fixed positions in a regular pattern. Liquid: particles are close together and move around each other. Gas: particles are far apart and move quickly in all directions.

A full answer includes

• Regular pattern, vibrating about fixed positions: solid.
• Close together, moving around each other: liquid.
• Far apart, moving quickly in all directions: gas.

Also accept

• 1 solid, 2 liquid, 3 gas.

Watch for

• “Particles in a solid don't move.” — They vibrate all the time about fixed positions; they just can't move past each other.

3. SQ3 · Practice

Model answer. The sponge is a solid with lots of holes full of air. On its own it keeps its shape and doesn't spread out to fill a container, so it isn't a gas. When you squeeze it, its thin, bendy walls bend, the holes close up and most of the air is pushed out of them. The particles of the solid sponge stay close together and touching, so the solid parts themselves still take up almost the same space. The sponge gets smaller because its holes close and the air leaves. Let go, and the walls spring back and air flows back in.

A full answer includes

• I said the sponge is a solid: on its own it keeps its shape, and it does not flow or spread out to fill a container.
• I said what squeezing does: the holes close up and most of the air in them is pushed out (the solid parts of the sponge only bend).
• I explained with particles: the particles of the solid sponge stay close together and touching, so the solid parts themselves still take up almost the same space. The sponge gets smaller because the holes close and the air leaves.

Also accept

• The sponge is a solid: it keeps its own shape and doesn't flow or fill a container. Squeezing bends its solid parts, so the holes close and most of the air is pushed out. The particles of the solid sponge stay touching, so the solid parts themselves take up almost the same space.

Watch for

• “Anything that can be squashed is a gas.” — Check all the properties. A sponge keeps its own shape and doesn't fill its container, so it's a solid. Squeezing bends its solid parts and pushes most of the air out of its holes; the solid parts themselves still take up almost the same space.
• “Squeezing pushes the sponge's own particles closer together, or makes them smaller.” — The particles of a solid already touch, and squeezing doesn't change their size. The solid parts only bend. The space that disappears is the holes, and most of their air has been pushed out.

4. SQ4 · Practice

Model answer. The particles are close together, arranged randomly with no regular pattern, and move around each other.

A full answer includes

• Separation: close together, touching.
• Arrangement: random, with no regular pattern.
• Motion: they move around and slide past each other.

Also accept

• close together; random; move around each other

Watch for

• “Liquid particles are far apart because liquids flow.” — A liquid flows because its particles can slide past each other, not because they are far apart. They stay close together, which is why a liquid keeps its volume.

5. ST1 · Apply

Model answer. X is a solid, Y is a gas and Z is a liquid. Gas particles move quickly in all directions until they hit the walls, so Y fills the whole container. They are far apart, with lots of space between them, so the plunger can push them closer together.

A full answer includes

• I identified X as a solid, Y as a gas and Z as a liquid.
• I said Y's particles move quickly in all directions, so they spread through the whole container.
• I said Y's particles are far apart, so pushing moves them closer together.

Also accept

• X solid, Y gas, Z liquid. Y's particles move randomly in all directions and are far apart, so it fills the container and can be compressed.

Watch for

• “Z must be a solid because it can't be squashed.” — Solids and liquids both hardly squash. Z settles at the bottom with a level surface, taking the container's shape: it is a liquid.