Properties of Gases: 40 flashcards
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1. What is a gas?
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A state of matter whose particles are very far apart and move freely, so it has neither a fixed shape nor a fixed volume.
2. Why do gases have no fixed shape or volume?
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Because gas particles are loosely packed and move randomly in all directions, so they spread out to fill any container.
3. What does the kinetic theory of gases state about particle motion?
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Gas particles are in constant, random motion, moving in straight lines until they collide, and all collisions are perfectly elastic.
4. What is meant by an elastic collision in a gas?
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A collision in which no kinetic energy is lost when particles bounce off each other or off the container walls.
5. What causes gas pressure?
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The collisions of gas particles with the walls of their container — total force per unit area.
6. What does temperature measure in a gas?
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The average kinetic energy of the gas particles. Hotter gas means faster-moving particles.
7. Why are gases highly compressible?
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Because there is a large amount of empty space between gas particles, so they can be pushed closer together under pressure.
8. Why do gases have much lower density than solids and liquids?
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Because gas particles are spread far apart, so there is very little mass in a given volume.
9. What is diffusion in gases?
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The spontaneous mixing of gas particles due to their random motion, causing gases to spread and mix uniformly.
10. Which gases diffuse faster — lighter or heavier ones?
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Lighter gases diffuse faster than heavier gases.
11. What is expansibility of a gas?
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The property by which a gas spreads out instantly to fill the entire volume available, even in a vacuum.
12. State Boyle's Law.
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At constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume: $P \propto \frac{1}{V}$.
13. State Charles's Law.
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At constant pressure, the volume of a gas is directly proportional to its absolute temperature in kelvin: $V \propto T$.
14. State Gay-Lussac's Law.
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At constant volume, the pressure of a gas is directly proportional to its absolute temperature: $P \propto T$.
15. Write the combined gas law.
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$$\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$$
16. Write the ideal gas law.
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$$PV = nRT$$ where $P$ is pressure, $V$ is volume, $n$ is moles, $R$ is the gas constant, and $T$ is absolute temperature.
17. When do real gases deviate from ideal behaviour?
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At high pressure and low temperature, where particles are close together and attractive forces between them become significant.
18. What happens to pressure when you heat a gas in a rigid sealed container?
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Pressure increases, because particles move faster and hit the walls harder and more often. Volume stays fixed.
19. What happens to pressure when you add more gas to a rigid container?
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Pressure increases, because more particles collide with the walls. The volume does not change.
20. Why is LPG stored as a liquid under pressure?
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Because gases are easily compressible, so a large amount of gas can be stored in a small volume as a liquid under pressure.
21. Why does a helium balloon float in air?
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Because helium is less dense than air, so the buoyant force of the displaced air is greater than the balloon's weight.
22. Are gases fluids?
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Yes. Gases are fluids — they flow and take the shape of their container, just like liquids, but they also expand to fill the entire volume.
23. Why does a pressure cooker cook food faster?
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Because the higher pressure inside raises the boiling point of water, so food cooks at a higher temperature.
24. Why must scuba divers ascend slowly?
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Because dissolved gases in the blood expand as pressure decreases during ascent, which can be dangerous if the change is too rapid.
25. Why do aerosol cans carry warning labels about heat?
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Because heating the gas inside a sealed can increases its pressure, which can cause the can to burst.
26. What is the relationship between pressure and volume in Boyle's Law?
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They are inversely proportional at constant temperature — as one increases, the other decreases.
27. What is the SI unit of pressure?
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The pascal (Pa), which equals one newton per square metre.
28. What is the SI unit of temperature used in gas laws?
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The kelvin (K), which is the absolute temperature scale. Gas law calculations must use kelvin, not Celsius.
29. What is the value of the universal gas constant $R$?
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$R \approx 8.314\ \text{J mol}^{-1}\text{K}^{-1}$ (or $0.0821\ \text{L atm mol}^{-1}\text{K}^{-1}$).
30. Why does a small gas leak quickly fill an entire room?
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Because of expansibility and diffusion — gas particles spread out rapidly to fill all available space.
31. What is the main assumption of the ideal gas law about particle volume?
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It assumes gas particles have negligible volume and no attractive forces between them.
32. How does the speed of gas particles change with temperature?
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As temperature increases, particles gain kinetic energy and move faster, increasing both pressure and volume effects.
33. Give one everyday example of gas compressibility.
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A syringe can trap and squeeze air, or a gas cylinder holds a large amount of gas in a small volume.
34. Why is natural gas odorized before being supplied to homes?
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Because natural gas is odourless and leaks are invisible; adding an odour makes leaks detectable for safety.
35. What is the relationship between gas density and temperature?
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As temperature increases, gas expands and density decreases (at constant pressure).
36. What happens to gas volume if pressure is doubled at constant temperature?
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The volume is halved, because pressure and volume are inversely proportional (Boyle's Law).
37. What happens to gas volume if absolute temperature is doubled at constant pressure?
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The volume doubles, because volume is directly proportional to absolute temperature (Charles's Law).
38. Why do gases mix completely and uniformly in a container?
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Because of diffusion — random particle motion causes spontaneous and uniform mixing.
39. What is the difference between a gas and a vapour?
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A gas is a substance in the gaseous state at room temperature, while a vapour is the gaseous form of a substance that is normally a liquid or solid at room temperature.
40. Why does the pressure of a gas increase when its volume is reduced at constant temperature?
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Because particles hit the walls more frequently in a smaller volume, increasing the total force per unit area.
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