Welcome! Scuba divers are taught one golden rule: never hold your breath on the way up. In this lesson you’ll find out why, using the physics of pressure in gases and liquids, and then write a dive safety briefing. It takes about one hour. (Separate Physics only; the p = hρg calculations are Higher.)
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Pressure in gases and liquids – Deep Sea Diver.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. explain gas pressure using particles
2. use pV = constant for a gas at constant temperature
3. explain why pressure increases with depth in a liquid
4. (Higher) use p = hρg.
Step 1: Starter (5 minutes)
If you swim to the bottom of a deep swimming pool, your ears start to hurt. A bubble breathed out by a diver gets bigger as it rises to the surface. Can you explain either of these? Write your ideas.
Show answer
The deeper you go, the more water is above you pushing down, so the pressure increases – that’s what you feel in your ears. As a bubble rises, the pressure on it decreases, so the gas inside can expand.
📝 Copy into your book
pressure = force normal to a surface ÷ area of that surface p = F ÷ A
p in pascals (Pa), F in N, A in m²
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What causes the pressure of a gas?
- What happens to the volume of a gas if the pressure on it decreases (at constant temperature)?
- Why does pressure increase as you go deeper in water?
📖 Read: Pressure under the sea
The particles in a gas move constantly in random directions. When they hit the walls of their container, they exert a force on them. The total force per unit area is the pressure of the gas. If a gas is heated at constant volume, its particles move faster, so they hit the walls harder and more often, and the pressure increases.
For a fixed mass of gas at a constant temperature, if the volume is made smaller, the particles hit the walls more often, so the pressure increases. Pressure and volume are linked by pV = constant. So if the pressure on a gas is halved, its volume doubles.
Liquids also exert pressure. The deeper you go, the greater the weight of liquid above you, so the pressure increases with depth. Denser liquids exert more pressure at the same depth. The pressure caused by a column of liquid is given by p = hρg, where h is the depth, ρ is the density of the liquid and g is the gravitational field strength. Every 10 m of seawater adds roughly the same pressure again as the whole atmosphere at the surface (about 100 000 Pa).
Show answers
(a) Gas particles colliding with the walls of the container, exerting a force.
(b) The volume increases (pV = constant).
(c) There is a greater weight of water above you.
📝 Copy into your book
Gas pressure is caused by particles colliding with the walls.
Fixed mass of gas at constant temperature: p V = constant, so p1V1 = p2V2
(Higher) Pressure due to a column of liquid: p = h ρ g
Step 3: Calculations (10 minutes)
Use: density of seawater = 1030 kg/m³; g = 9.8 N/kg; atmospheric pressure = 100 000 Pa (100 kPa).
Q1 A diver’s mask has an area of 0.02 m². The water pushes on it with a force of 6000 N. Calculate the pressure.
Show answer
p = F ÷ A = 6000 ÷ 0.02 = 300 000 Pa (300 kPa)
Q2 (Higher) Calculate the pressure caused by 20 m of seawater.
Show answer
p = h ρ g = 20 × 1030 × 9.8 = 201 880 Pa (about 200 kPa). Add the 100 kPa of the atmosphere and the total pressure on the diver is about 300 kPa – three times the pressure at the surface.
Q3 At 20 m deep (300 kPa), a diver breathes out a 2.0 cm³ bubble. What is its volume when it reaches the surface (100 kPa)? Assume the temperature stays the same.
Show answer
p1V1 = p2V2
300 × 2.0 = 100 × V2
V2 = 6.0 cm³ – three times bigger!
✅ Score out of 3.
Step 4: Main task – Dive Safety Briefing (15 minutes)
You’re a dive instructor. Before their first deep dive, your students need to understand the science behind three safety rules. For each rule, explain why it matters using physics.
- “Never hold your breath while swimming up.” Use pV = constant and your answer to Q3.
- “Your air tank empties much faster on deep dives.” Each breath has to fill your lungs at the pressure around you. Use the word pressure and your answer to Q2.
- “Don’t leave your air tank in the sun on the boat.” Use the idea of particles and collisions.
Stuck? For each rule, think: what happens to the particles? What happens to the pressure or volume?
Show a model briefing
1. At 20 m your lungs are full of air at about 300 kPa. As you rise, the pressure falls to 100 kPa. Since pV = constant, the air would try to expand to three times its volume. If you hold your breath, it can’t escape and could seriously damage your lungs. Breathe out steadily as you go up.
2. At 20 m the pressure is about three times the pressure at the surface. The air you breathe is squashed to the same pressure, so each lungful contains about three times as much air. Your tank runs out about three times faster.
3. Heating the tank makes the gas particles move faster. They hit the walls of the tank harder and more often, so the pressure rises. The tank’s volume can’t change, so the pressure could get dangerously high.
✅ Check: did you explain the particles or the pressure change for each rule?
Step 5: Exam practice (14 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (3 marks) Explain, in terms of particles, why the pressure of a gas in a sealed container increases when it is heated.
Show model answer
Heating increases the kinetic energy of the particles, so they move faster (1). They collide with the walls more often (1) and with more force (1), so the pressure increases.
Q2 (3 marks) A gas has a volume of 0.60 m³ at a pressure of 150 kPa. It is compressed at constant temperature until its pressure is 450 kPa. Calculate its new volume.
Show model answer
p1V1 = p2V2 (1)
150 × 0.60 = 450 × V2 (1)
V2 = 0.20 m³ (1)
Q3 (3 marks – Higher) Calculate the pressure due to the water at the bottom of a 3.0 m deep swimming pool. Density of water = 1000 kg/m³; g = 9.8 N/kg.
Show model answer
p = h ρ g (1)
p = 3.0 × 1000 × 9.8 (1)
p = 29 400 Pa (1)
✅ Add up your marks out of 9 (or out of 6 if you skipped the Higher question). Write the score in your book.
Step 6: Exit ticket (8 minutes)
- A bubble gets bigger as it rises because…
- Gas pressure is caused by…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Particle model mixed equation practice.
