Squeezing a gas — questions

1. identify / explain

A sealed syringe of air is joined to a pressure gauge. It is held nozzle up, then nozzle down, then on its side, and each time the gauge reads 100 kPa. What causes the pressure of the air on the walls of the syringe? Then explain why the reading does not depend on which way up the syringe is held.

Circle one answer.

• The weight of the air pressing down on the bottom of the syringe
• Particles moving at random hit the walls and bounce off
• The particles are packed tightly and push outwards on the walls
• The particles stick to the walls and pull on them

Why does the reading not depend on which way up the syringe is held?

Nudge (only if you are stuck): Think about what the particles of a gas are doing all the time, and in which directions they move.

2. complete

Complete the sentences. The particles of a gas move fast in ____ (1) directions. When a particle hits a wall it bounces off and exerts a small ____ (2) on the wall. The gas pressure is the force of all these hits on each unit of ____ (3) of the wall. If the particles hit each square centimetre of the wall more ____ (4), the pressure rises.

Gap 1

Gap 2

Gap 3

Gap 4

Nudge (only if you are stuck): Pressure is about a surface: what is spread over it, and over what?

3. identify / explain

A sealed box of air has a large lid of area 200 cm² and a small window of area 20 cm². The pressure of the air inside is 120 kPa. Which statement is correct? Then explain your choice in terms of the particles hitting the lid and the window.

Circle one answer.

• The pressure on the lid is 10 times the pressure on the window
• The force on the lid is bigger, but the pressure on both is the same
• The force and the pressure are the same on both
• The pressure on the window is bigger, because its area is smaller

Why are the forces different while the pressures are the same?

Nudge (only if you are stuck): Pressure is the force on each unit of area. Does one square centimetre of the lid get more hits each second than one square centimetre of the window?

4. explain

The nozzle of a syringe of air is sealed, and the plunger is pushed in slowly, so that the temperature of the air stays the same. Put the steps of the explanation for the rise in pressure in order. The steps are listed in a mixed order.

Write the number of each step in order.

1. So the force on each unit of area of the walls increases: the pressure rises
2. The plunger moves in, so the same particles have less space
3. So each square centimetre of the walls is hit more often each second
4. The particles are more crowded, but they move at the same speed

• Step 1: ______
• Step 2: ______
• Step 3: ______
• Step 4: ______

Nudge (only if you are stuck): Start with what you do to the plunger, and end with what the gauge shows.

5. calculate

A sealed syringe holds 60 cm³ of air at 100 kPa. The plunger is pushed in slowly, so that the temperature stays the same, until the volume is 20 cm³. Calculate the new pressure of the air.

New pressure of the air: ______________ unit ________

Nudge (only if you are stuck): pV stays the same for a fixed mass of gas at a constant temperature. Work out p × V at the start.

6. identify / calculate

A pupil squeezes a fixed mass of air slowly, at a constant temperature, and records five readings. One pressure was written down wrongly. Use pV = constant to identify the wrong row: give its volume, and calculate the pressure that should have been recorded.

Volume of the wrong row: ______________ unit ________

Pressure that should have been recorded: ______________ unit ________

Nudge (only if you are stuck): Multiply p by V in every row. Which row is the odd one out?

7. calculate / explain

A sealed packet of crisps holds 400 cm³ of air at 100 kPa. On a plane the pressure of the air in the cabin falls to 80 kPa, and the temperature stays the same. The packet is soft, so the air inside it ends up at the same pressure as the cabin. Calculate the volume of the air in the packet on the plane.

Volume of the air on the plane: ______________ unit ________

Nudge (only if you are stuck): Which stays the same: p, V or p × V? At a lower pressure, should the volume be bigger or smaller?

8. identify

A fixed mass of gas is kept at a constant temperature while its volume is changed. Match each graph of its readings to the shape it has.

Write the number of the matching option.

1. A curve that falls steeply at small volumes and gets flatter
2. A straight line through the origin
3. A horizontal straight line
4. A straight line sloping down to the volume axis
5. A curve that rises more and more steeply

• p against V: ______
• p against 1/V: ______
• p × V against V: ______

Nudge (only if you are stuck): Use the syringe readings: 50 cm³ at 100 kPa, 25 cm³ at 200 kPa, 20 cm³ at 250 kPa. Work out 1/V and p × V for each, and sketch the three graphs roughly.

9. explain

A pupil says: “When you push in the plunger of a sealed syringe, the gas particles get squashed smaller, and that is why the pressure goes up.” The temperature stays the same. Explain what is wrong with this, and give the correct explanation in terms of particles.

Nudge (only if you are stuck): What is the same about the particles before and after the squeeze, and what is different?

10. explain

A pump takes most of the air out of a sealed, empty plastic bottle, and the bottle is crushed inwards. The air outside is at 100 kPa. Explain, in terms of particles and of the force on each unit of area, why the bottle is crushed.

Nudge (only if you are stuck): Compare the hits on one square centimetre of the bottle wall from the inside and from the outside.

11. explain / calculate / state

A walker on a mountain top drinks from a plastic bottle and screws the cap back on. The air in the bottle is now 1500 cm³ at 70 kPa, the pressure of the air on the mountain. Back at the beach, where the air pressure is 100 kPa, the bottle is crushed. The temperature is the same in both places. (a) Explain, using the force on each unit of area, why the bottle is crushed at the beach. (b) The soft bottle is squeezed until the air inside is at 100 kPa. Calculate the volume of the air inside it at the beach. (c) Explain, in terms of particles, why the pressure of the air inside rose as it was squeezed. (d) State the two conditions that must hold for your calculation in (b) to be right.

Why the bottle is crushed

Volume at the beach, with working

Why the pressure inside rose

The two conditions

Nudge (only if you are stuck): Compare the hits on each square centimetre of the bottle from inside and from outside. For (b), keep p × V the same.