PADI Physics and Gas Laws 5 — Questions and Answers
Question 1: A diver fills a lift bag with 30 liters of air at 30 meters (4 ATA). When the bag reaches the surface, its volume will be approximately:
- 120 liters (Correct answer)
- 60 liters
- 30 liters
- 7.5 liters
Correct answer: 120 liters
Boyle's Law: V2 = (P1 × V1) / P2 = (4 × 30) / 1 = 120 liters at the surface.
Question 2: Which property of water makes it denser than air and affects a diver's buoyancy?
- Water molecules are more closely packed together (Correct answer)
- Water contains dissolved salts only
- Water has lower surface tension
- Water molecules are heavier because of hydrogen atoms alone
Correct answer: Water molecules are more closely packed together
Water's density (1000 kg/m³ vs ~1.2 kg/m³ for air) comes from the close packing of its molecules.
Question 3: The partial pressure of nitrogen at 40 meters (5 ATA) breathing air (79% N2) is:
- 3.95 ATA (Correct answer)
- 0.79 ATA
- 5.0 ATA
- 2.5 ATA
Correct answer: 3.95 ATA
ppN2 = fraction × total pressure = 0.79 × 5 = 3.95 ATA.
Question 4: Why do colors disappear at depth in the order red, orange, yellow, green, blue?
- Water absorbs longer wavelengths (red end) of light first (Correct answer)
- Eyes become less sensitive to red in cold water
- Pressure changes how the eye perceives color
- Nitrogen narcosis reduces color perception progressively
Correct answer: Water absorbs longer wavelengths (red end) of light first
Water absorbs red light most strongly, then progressively shorter wavelengths, causing colors to disappear in order of wavelength.
Question 5: An oxygen-enriched air (EANx) mix with 32% O2 reaches its maximum operating depth when ppO2 reaches 1.4 ATA. What is that depth?
- 111 feet (34 m) (Correct answer)
- 99 feet (30 m)
- 130 feet (40 m)
- 66 feet (20 m)
Correct answer: 111 feet (34 m)
Max ATA = 1.4 / 0.32 = 4.375 ATA → depth = (4.375 - 1) × 33 ≈ 111 feet (33.75 m, rounded to 34 m).
Question 6: The ideal gas law (PV = nRT) tells us that when a cylinder is filled rapidly, the air heats up. This is an application of:
- Gay-Lussac's Law and Charles's Law combined (Correct answer)
- Henry's Law
- Dalton's Law
- Archimedes' Principle
Correct answer: Gay-Lussac's Law and Charles's Law combined
Rapid compression raises temperature (Gay-Lussac's/Charles's relationship); as gas cools, pressure drops slightly — the hot fill effect.
Question 7: A diver is neutrally buoyant with a full tank. As they consume air during the dive, they will tend to become:
- More positively buoyant (Correct answer)
- More negatively buoyant
- Neutrally buoyant as before
- Alternately positive and negative
Correct answer: More positively buoyant
As air is consumed, the tank and diver system loses mass (the expelled gas) while displacing the same water volume, increasing net buoyancy.
PADI Physics and Gas Laws 5
A diver fills a lift bag with 30 liters of air at 30 meters (4 ATA).
When the bag reaches the surface, its volume will be approximately: