Squeeze a gas — questions

1. identify / state

A sealed gas syringe holds a fixed amount of air. Disc weights are added to its plunger one at a time, and the water bath around it stays at 20 °C. Use the results to choose the statement that describes what happened to the volume of the air. Then state what was kept the same.

Circle one answer.

• The volume decreased by the same amount with each weight.
• The volume decreased with each weight, but by less each time.
• The volume stayed the same, because no air could get out.
• The volume increased, because the air was pushed harder.

What was kept the same while the weights were added?

Nudge (only if you are stuck): Work out how much the volume fell from each row to the next. Then reread the question: what did not change?

2. explain

One more weight is put on the plunger of the sealed syringe. The temperature stays the same. Put the steps in order to explain why the plunger moves in and then stops.

Write the number of each step in order.

1. The plunger moves in, so the same particles have less space.
2. The air pushes up harder on the plunger.
3. The plunger stops when the push of the air balances the push from outside.
4. The push from outside becomes bigger than the push of the air.
5. The particles hit the walls and the plunger more often.

• First: ______
• Second: ______
• Third: ______
• Fourth: ______
• Fifth: ______

Why doesn't the plunger keep moving all the way to the bottom?

Nudge (only if you are stuck): Start with what the new weight does. End with the moment the plunger stops moving.

3. identify / explain

Compare the trapped air with three weights on the plunger and with no weights. The temperature is 20 °C both times. Which statements are correct for the air with three weights?

Tick every correct statement.

• There are the same number of air particles in the syringe.
• The particles are closer together.
• Each particle is smaller.
• The particles hit the plunger more often.
• Some particles have been squeezed out through the cap.
• The particles move more slowly on average.

Choose one statement you rejected and explain what is wrong with it.

Nudge (only if you are stuck): Which of these statements would need a change in the amount of air, or a change in temperature?

4. explain / state

Another pupil does the squeeze experiment, but she leaves the hotplate on, so the water bath slowly warms while she adds the weights. (a) Explain why her results can't show the effect of pressure alone. (b) A third pupil's syringe has a split cap. He adds three weights at 20 °C, then takes them all off again. His plunger comes back to only 44 cm³, not the 50 cm³ he started with. What does this reading tell him, and why?

(a) Why her results can't show the effect of pressure alone

(b) What the 44 cm³ reading tells him, and why

Nudge (only if you are stuck): For (a), how many things were changing at the same time? For (b), what would the reading be if the air were the same as at the start?

5. evaluate / explain

A pupil writes: “When you squeeze the air in a sealed syringe, its particles get squashed and shrink, so the air takes up less space and pushes back harder.” Evaluate the pupil's explanation. Then give a correct particle explanation of why squeezed air pushes back harder.

Nudge (only if you are stuck): What happens to the size of the particles, and what happens to the spaces between them? How does that change the hits on the plunger?

6. describe / explain

A diver deep under water breathes out a bubble of air. The deeper you go, the more the water above pushes down on anything below it. As the bubble rises towards the surface, it gets bigger. The water is at the same temperature all the way up, and no air enters or leaves the bubble. (a) Describe how the pressure on the bubble changes as it rises, and how this changes its volume. (b) Explain, using particles, why the bubble gets bigger.

(a) The pressure on the bubble and its volume

(b) Why the bubble gets bigger, using particles

Nudge (only if you are stuck): Which part of the syringe experiment is this like: adding weights or taking them off?