Chemistry · Unit 14

Gases and Gas Laws

The behaviour of gases

Gases are the one state where a simple model predicts behaviour accurately, which is why gas laws were discovered long before anyone knew what a molecule was.

The unit covers gas properties, the individual gas laws and the ideal gas equation that combines them, molar volume, kinetic theory, and calculations.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-3

What this unit covers

Properties of Gases
The Gas Laws
Molar Volume
Kinetic Theory of Gases
Gas Calculations

Where this fits

Needs States of Matter and Moles and Calculations.

Where people slip

Gas law calculations need absolute temperature. Using Celsius instead of Kelvin produces answers that are wrong in a way that looks reasonable.

How the unit is structured

Gases and Gas Laws runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Gas Calculations

Build the sentenceLevel 2
1. Arrange the tokens to give the equation for Boyle's law.
Answer: P1 V1 = P2 V2
Boyle's law is written as P1 V1 = P2 V2 for a fixed mass of gas at constant temperature.
Fill the blankLevel 2
2. When using pV = nRT with R = 8.31 J K-1 mol-1, pressure must be measured in ____ and volume in cubic metres.
- pascals (Pa)correct
- kilopascals (kPa)
- atmospheres (atm)
- cubic decimetres (dm3)
The SI value of R = 8.31 requires pressure in pascals, volume in m3 and temperature in kelvin.
Guess the numberLevel 2
3. A gas has a volume of 200 cm3 at a pressure of 100 kPa. At constant temperature it is compressed to 100 cm3. What is the new pressure, in kPa?
Answer: 200 kPa
Using P1V1 = P2V2: (100 x 200) / 100 = 200 kPa.
Multiple choiceLevel 2
4. Using Boyle's law, if the volume of a gas is halved at constant temperature, its pressure will...
- doublecorrect
- halve
- stay the same
- fall to zero
Pressure and volume are inversely proportional, so halving the volume doubles the pressure.
Guess the numberLevel 2
5. A gas has a volume of 200 cm3 at 300 K. At constant pressure it is heated to 600 K. What is its new volume, in cm3?
Answer: 400 cm3
Using V1/T1 = V2/T2: (200 x 600) / 300 = 400 cm3, since volume is proportional to kelvin temperature.
Multiple choiceLevel 3
6. Which equation is the combined gas law for a fixed mass of gas?
- P1V1/T1 = P2V2/T2correct
- P1T1/V1 = P2T2/V2
- V1T1/P1 = V2T2/P2
- P1V1T1 = P2V2T2
The combined gas law is P1V1/T1 = P2V2/T2, joining Boyle's, Charles's and the pressure law into one relationship.

Kinetic Theory of Gases

Multiple choiceLevel 1
7. In the kinetic model, how do the particles of a gas move?
- Quickly and randomly in all directionscorrect
- Slowly along fixed straight lines
- They only vibrate about fixed positions
- They do not move at all
Gas particles move quickly and randomly in all directions, colliding with each other and the container walls.
Build the sentenceLevel 2
8. Build the sentence that describes how the particles in an ideal gas move.
Answer: particles move in constant random motion
The kinetic model assumes gas particles are in constant, random motion in all directions.
Fact or fibLevel 2
9. Ideal gas particles are assumed to have no forces of attraction or repulsion between them.
Answer: True
Assuming zero intermolecular forces is one of the central assumptions of the ideal gas model.
Choose all that applyLevel 3
10. The kinetic model of an ideal gas makes several assumptions. Select ALL of the statements that are assumptions of this model.
- Gas particles have negligible volume compared with the containercorrect
- Collisions between particles are perfectly elasticcorrect
- There are strong attractive forces between the particles
- The particles are held in fixed positions
An ideal gas is modelled as tiny particles of negligible volume with negligible forces between them, whose collisions are perfectly elastic.
Fill the blankLevel 3
11. The average kinetic energy of the particles in an ideal gas is directly proportional to the absolute ____ measured in kelvin.
- temperaturecorrect
- pressure
- volume
- density
Doubling the kelvin temperature doubles the average kinetic energy of the gas particles.
Odd one outLevel 3
12. Which statement is NOT an assumption of the kinetic theory of an ideal gas?
- Particles occupy most of the container's volumecorrect
- Particles are in constant random motion
- Collisions between particles are perfectly elastic
- Particles have negligible volume of their own
The model assumes the particles' own volume is negligible, not that they occupy most of the container.

Molar Volume

Guess the numberLevel 1
13. What is the molar gas volume at rtp, in dm3 per mole?
Answer: 24 dm3
One mole of any gas occupies 24 dm3 at room temperature and pressure.
Multiple choiceLevel 1
14. At room temperature and pressure (rtp), what volume does one mole of any gas occupy?
- 24 dm3correct
- 1 dm3
- 12 dm3
- 100 dm3
The molar gas volume at rtp is 24 dm3 (24000 cm3) per mole, whatever the gas.
Fill the blankLevel 2
15. At room temperature and pressure (rtp), one mole of any gas occupies about ____ dm3.
- 24correct
- 22.4
- 2
- 1000
The molar volume at rtp (about 25 degrees Celsius and 100 kPa) is taken as 24 dm3 per mole.
Odd one outLevel 2
16. Three of these describe exactly one mole of a gas at rtp. Which is the odd one out?
- 12 dm3 of nitrogencorrect
- 24 dm3 of hydrogen
- 24 dm3 of oxygen
- 24 dm3 of carbon dioxide
One mole of any gas occupies 24 dm3 at rtp regardless of the gas, so 12 dm3 of nitrogen is only half a mole.
Sequence recallLevel 2
17. Recall the volumes of gas at rtp in order of increasing number of moles.
Answer: 1 mole = 24 dm3 -> 2 moles = 48 dm3 -> 3 moles = 72 dm3 -> 4 moles = 96 dm3
Each mole of gas adds 24 dm3 at rtp: 1 mole = 24 dm3, 2 moles = 48 dm3, 3 moles = 72 dm3, 4 moles = 96 dm3.
Tap the pairsLevel 2
18. Match each amount of gas to the volume it occupies at rtp (molar volume 24 dm3/mol).
Answer: 1 mol = 24 dm3; 2 mol = 48 dm3; 0.5 mol = 12 dm3; 0.25 mol = 6 dm3
Multiply the number of moles by 24 dm3/mol to get the volume at rtp.

Properties of Gases

Fill the blankLevel 1
19. Gas particles constantly collide with the walls of their container, and this is what creates gas ____.
- pressurecorrect
- mass
- colour
- weight
The force of countless particle collisions on the container walls is what we measure as gas pressure.
Multiple choiceLevel 1
20. Which statement best describes a gas?
- It spreads out to completely fill any container it is incorrect
- It has a fixed shape and a fixed volume
- It keeps a fixed volume but changes shape
- It cannot be squashed at all
A gas has no fixed shape or volume and spreads out to completely fill any container it is put in.
Picture questionLevel 1
21. 🎈 A helium balloon floats upward in air. This is mainly because helium gas has a very...
- low densitycorrect
- high density
- strong smell
- bright colour
Helium is much less dense than air, so a helium-filled balloon is pushed upward and rises.
Fact or fibLevel 2
22. Hydrogen is less dense than air, so it is collected by upward delivery into an inverted (upside-down) gas jar.
Answer: True
A gas lighter than air rises, so it is collected by upward delivery into an inverted jar where it gathers at the top.
Choose all that applyLevel 3
23. Select ALL of the reasons why a real gas deviates from ideal behaviour at very high pressure.
- The volume of the molecules themselves becomes significant compared with the container volumecorrect
- Molecules are forced so close together that intermolecular forces become importantcorrect
- The gas molecules gain mass as the pressure rises
- Collisions with the container walls stop happening altogether
At high pressure both the finite volume of the molecules and the closeness of the molecules make the gas depart from ideal behaviour.
Match the pairsLevel 3
24. Match each real-gas term to its correct meaning.
Answer: Critical temperature = Highest temperature at which the substance can exist as a liquid; Critical pressure = Pressure needed to liquefy a gas at its critical temperature; Compressibility factor = Ratio PV/nRT that measures deviation from ideal behaviour; van der Waals constant b = Correction for the finite volume of the molecules
These four terms describe how and why real gases depart from the ideal gas model.

The Gas Laws

Fill the blankLevel 2
25. Charles's law states that at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute ____.
- temperaturecorrect
- pressure
- mass
- density
Volume is proportional to temperature in kelvin, so V/T stays constant at fixed pressure.
Guess the numberLevel 2
26. Convert 25 degrees Celsius into kelvin. Enter the temperature in K.
Answer: 298 K
Add 273 to a Celsius temperature to get kelvin: 25 + 273 = 298 K.
Match the pairsLevel 2
27. Match each gas law or term to the relationship it describes.
Answer: Boyle's law = P inversely proportional to V; Charles's law = V proportional to T in kelvin; Pressure law = P proportional to T in kelvin; Absolute zero = equal to 0 kelvin
Each gas law links two of pressure, volume and kelvin temperature, and absolute zero is the bottom of the kelvin scale.
Multiple choiceLevel 2
28. Boyle's law states that, at constant temperature, the pressure of a fixed mass of gas is...
- inversely proportional to its volumecorrect
- directly proportional to its volume
- exactly equal to its volume
- completely independent of its volume
Boyle's law: at constant temperature P is inversely proportional to V, so squeezing a gas into a smaller volume raises its pressure.
Spell itLevel 2
29. Spell the surname of the scientist whose gas law states that pressure is inversely proportional to volume at constant temperature.
Answer: Boyle
Robert Boyle described how the pressure and volume of a gas are inversely proportional at constant temperature.
True or falseLevel 2
30. At constant volume, the pressure of a fixed mass of gas increases as its temperature rises.
Answer: True
This is the pressure law: at constant volume, pressure is proportional to the kelvin temperature, so hotter gas pushes harder on the walls.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Gases and Gas Laws

120 questions across 20 steps. Start with step one and crawl at your own pace.

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