Thermal expansion — answer key

1. GQ19 · Practice

Model answer. The ball, the water and the air all expand, and all three contract when they cool. Solids, liquids and gases all expand when they are warmed and contract when they cool. The particles themselves do not get bigger, and no matter is added, so the mass stays the same. Warmer particles vibrate more (in the solid) or move faster (in the liquid and the gas), so they spread a little further apart and the volume grows.

A full answer includes

• I chose the four about expanding and contracting: the ball, the water and the air expand, and all three contract again when they cool.
• I explained that the particles move faster or vibrate more and spread a little further apart; they do not get bigger, and the mass does not change.

Also accept

• a, b, c, d

Watch for

• “The particles in each one get bigger.” — The particles stay the same size. They move faster, or vibrate more, and spread further apart.
• “Only the water and the air expand; a solid metal ball cannot get bigger.” — Solids expand too, only much less: a hot metal ball no longer fits through a ring that it passed through cold.

2. GQ20 · Practice

Model answer. (1) expands: warming makes it bigger. (2) vibrate: particles in a solid stay in fixed positions and vibrate more when warmed. (3) apart: they push each other a little further apart, so the ball takes up more space. (4) contracts: as it cools it gets smaller again.

A full answer includes

• (1) expands
• (2) vibrate
• (3) apart
• (4) contracts

Also accept

• expands; vibrate; apart; contracts.

Watch for

• “(1) melts” — The ball stays solid; it only gets a little bigger. Melting would need a much higher temperature.
• “(2) grow” — The particles keep their size. They vibrate more and push each other further apart.

3. GQ21 · Practice

Model answer. Metal cables expand when they warm, so on a hot day they are a little longer and sag lower between the pylons. On a cold day they contract and are pulled tighter. They are put up with some slack so that, when they contract in winter, they do not pull too hard on the pylons or snap.

A full answer includes

• I chose “The cables expand in the heat, so they are longer and sag more”.
• I explained that in cold weather the cables contract and pull tighter; if they were put up tight, they could pull too hard on the pylons or snap.

Also accept

• a; slack lets the cables contract in the cold without snapping.

Watch for

• “The cables get heavier when they are hot.” — Warming does not add any matter, so the mass and the weight stay the same. The cables are longer, so they sag.
• “The cables contract in the heat.” — Metals expand when warmed and contract when cooled. Longer cables hang lower.

4. GQ22 · Practice

Options as numbered on the pupil sheet: 1 — Iron (a solid); 2 — Air (a gas); 3 — Water (a liquid).

Model answer. Air, then water, then iron. In air, a gas, the particles are far apart and the forces between them are very weak, so when they move faster they spread out a long way. In water, a liquid, the particles are close together with forces between them, but they can move past each other, so they spread out more than in a solid but far less than in a gas. In iron, a solid, strong forces hold the particles close together in fixed positions; warming only makes them vibrate more, so it expands least.

A full answer includes

• Air expands most, then water, then iron.
• I explained that the particles of a gas are far apart with very weak forces between them, a liquid has close particles that can move past each other, and a solid has particles held in fixed positions by strong forces.

Also accept

• a, b, c (air, water, iron)

Watch for

• “Iron expands most, because metals are good at getting hot.” — How much something expands depends on how strongly its particles are held together. Strong forces in a solid keep the expansion small.
• “Water expands most, because liquids can flow.” — Liquids expand more than solids, but a gas, with its particles far apart and almost no forces between them, expands far more.

5. GQ23 · Practice

Model answer. Y, the lower layer. When the strip is heated, the brass gets longer than the steel, so the strip curls with the brass on the outside of the curve. If the brass is underneath (layer Y), the outside of the curve is the bottom, so the free end bends up and touches the contact: the circuit is complete and the bell rings. With the brass on top, the strip would bend down, away from the contact.

A full answer includes

• I chose “Y, the lower layer”.
• I explained that the brass expands more, so it ends up on the outside of the curve; for the free end to bend up, the outside of the curve must be underneath, so the lower layer Y must be brass.

Also accept

• b; the brass, which expands more, must be underneath so the strip curls upwards.

Watch for

• “Either layer will do: a heated strip always bends up.” — A heated strip bends towards the metal that expands less. Swap the layers and it bends the other way.
• “The brass must be on top, because it expands upwards.” — The brass gets longer, not taller. With the longer brass on top, the strip curls downwards.

6. GQ24 · Practice

Model answer. The hot water warms the lid, so the metal lid expands. Metal expands more than glass, and the glass is also slower to warm up, so the lid gets slightly bigger compared with the neck of the jar. It no longer grips the glass so tightly, and it turns easily.

A full answer includes

• I said the hot water warms the metal lid, and the lid expands more than the glass neck of the jar.
• I said the lid becomes slightly bigger compared with the neck of the jar, so it grips the glass less tightly and turns easily.

Also accept

• The metal lid expands more than the glass jar, so it grips less tightly.

Watch for

• “The hot water melts the jam that glues the lid on.” — Warm jam may soften a little, but the main reason is that the metal lid expands more than the glass, so it loosens.
• “The glass jar contracts in the hot water.” — Warming makes things expand, not contract. Both expand, but the metal lid expands more than the glass.

7. GQ25 · Practice

Model answer. In the steel, a solid, strong forces hold the particles close together in fixed positions. Warming makes them vibrate more about those positions, but the strong forces keep them close, so the steel expands only a little. In the air, a gas, the particles are far apart and move fast in all directions, and the forces between them are very weak. When they move faster they hit the balloon more often and harder and push it out, spreading out until the pressure matches the air outside again, so the air expands far more.

A full answer includes

• I said that in the steel, a solid, the particles are close together in fixed positions, held by strong forces; warming makes them vibrate more, but they move only slightly further apart.
• I said that in the air, a gas, the particles are far apart and move fast, with very weak forces between them; nothing holds them together, so when they move faster they spread out a long way.

Also accept

• Solid: close, fixed positions, strong forces, only vibrate more. Gas: far apart, very weak forces, spread out a lot.

Watch for

• “The air particles get bigger than the steel particles when heated.” — No particles grow. The difference is in how strongly they are held together and how far apart they are.
• “The steel does not expand at all.” — The steel expands too, just very little, because strong forces hold its particles in fixed positions.

8. GQ26 · Practice

Model answer. The liquid in the bulb expands more than the glass around it, so it needs more room, and the only place it can go is up the tube. The tube is very narrow, so even a small increase in volume pushes the thread a long way up: a small change in temperature makes a movement big enough to read.

A full answer includes

• I chose “The liquid expands more than the glass, so it has to go up the tube”.
• I explained that a narrow tube turns a small increase in the volume of the liquid into a long movement of the thread, so small changes in temperature are easy to see.

Also accept

• a; a narrow tube turns a small increase in volume into a long movement.

Watch for

• “The glass contracts in the warm water and squeezes the liquid up.” — Warm glass expands; it does not contract. The liquid rises because it expands more than the glass.
• “The particles of the liquid get bigger and fill the tube.” — The particles keep their size. They move faster and spread a little further apart, so the liquid takes up more space.

9. GT3 · Apply

Model answer. (a) As the car warmed in the sun, both the fuel and the steel tank expanded. A liquid expands much more than a solid warmed by the same amount, so the fuel needed more extra room than the tank gained. With the tank full to the brim, the extra fuel had nowhere to go but out of the overflow. (b) In the steel the particles are held close together in fixed positions by strong forces; warming only makes them vibrate more, so they move very slightly further apart. In the fuel the particles are close together but not in fixed positions, and they are held less tightly, so warming moves them further apart and the fuel expands more. (c) Leave a gap at the top of the tank, so that the fuel has room to expand without overflowing. (d) The air in the can warms, so its particles move faster. A gas would normally expand, but the rigid can keeps the volume fixed. So the faster particles hit the walls more often and harder, and the pressure of the air rises instead.

A full answer includes

• (a) Both expand as they warm, but the fuel, a liquid, expands much more than the steel, a solid, so the tank has no room left for it.
• (b) Steel: particles close together in fixed positions, held by strong forces, which only vibrate more. Fuel: particles close together but able to move past each other, held less tightly, so they spread further apart.
• (c) Leave a space at the top of the tank (do not fill it to the brim), so the fuel has room to expand.
• (d) The particles move faster, but the volume cannot change, so they hit the walls more often and harder: the pressure rises.

Also accept

• (a) the fuel expands more than the steel tank; (b) solid: fixed positions, strong forces; liquid: can move past each other, held less tightly; (c) leave a space at the top; (d) the volume cannot change, so the pressure rises

Watch for

• “(a) The steel tank contracts in the heat and squeezes the fuel out.” — The steel tank expands too as it warms, but much less than the fuel inside it.
• “(d) The air in the can expands anyway, by pushing the particles closer together.” — In a rigid can the volume cannot change. The particles speed up and hit the walls more often and harder, so the pressure rises.