Physics · Unit 5

States of Matter and Density

Solids, liquids, gases and more

Density is the idea that makes floating, sinking and pressure all one topic instead of three, and it is one of the most useful ratios in physics.

The unit covers the three states and the particle arrangements behind them, changes of state, density itself, floating and sinking, and pressure.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 2. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-2

What this unit covers

Floating and Sinking
Pressure
Changes of State
Density
Solids, Liquids and Gases

Where this fits

Sits alongside the early force units. Chemistry's States of Matter covers the same ground chemically.

Where people slip

Whether something floats depends on density relative to the fluid, not on weight or size. A steel ship floats and a steel nail does not, and both are made of the same steel.

How the unit is structured

States of Matter and Density 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 2 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.

Changes of State

Fact or fibLevel 1
1. Water freezes and turns to ice at 0 degrees C.
Answer: True
Water freezes into ice at 0 degrees C.
Fill the blankLevel 1
2. When a liquid gets very cold and turns into a solid, it is called ____.
- freezingcorrect
- melting
- boiling
- mixing
Freezing is when a liquid cools down and turns into a solid.
Guess the numberLevel 1
3. At what temperature does water boil?
Answer: 100 degrees C
Water boils and turns to steam at 100 degrees C at sea level.
Match the pairsLevel 1
4. Match each change of state to what happens.
Answer: Melting = Solid turns to liquid; Freezing = Liquid turns to solid; Boiling = Liquid turns to gas; Condensing = Gas turns to liquid
Each change of state has its own name for the way matter changes form.
Build the sentenceLevel 2
5. Build the sentence that describes sublimation.
Answer: Sublimation turns a solid directly into gas
Sublimation is the change from solid straight to gas with no liquid stage in between.
Choose all that applyLevel 2
6. Which statements correctly explain why evaporation cools the liquid left behind? Select all that apply.
- Only the most energetic particles escape from the surfacecorrect
- The average kinetic energy of the remaining liquid fallscorrect
- This cooling effect is why sweat cools your skincorrect
- The liquid left behind gets hotter
Evaporation removes the most energetic particles, lowering the average kinetic energy of what remains, which is why sweating cools skin.

Density

Choose all that applyLevel 1
7. Which of these are LESS dense than water? Choose all that are correct.
- Oilcorrect
- Corkcorrect
- Woodcorrect
- Iron nail
Oil, cork, and wood are all less dense than water, so they float, while an iron nail is denser and sinks.
Fill the blankLevel 1
8. Density is a measure of how much ____ is squashed into a space.
- masscorrect
- light
- sound
- colour
Density compares the mass of an object to the space it takes up.
Multiple choiceLevel 1
9. What does it mean if something is very dense?
- A lot of mass is packed into a small spacecorrect
- It is a bright colour
- It makes a loud sound
- It is very cold
Density means how much mass is packed into a certain amount of space.
Build the sentenceLevel 2
10. Build the equation that defines density.
Answer: density equals mass divided by volume
Density equals mass divided by volume.
Guess the numberLevel 2
11. A block has a mass of 240 g and a volume of 60 cm^3. What is its density in g/cm^3?
Answer: 4 g/cm^3
Density = mass / volume = 240 g / 60 cm^3 = 4 g/cm^3.
Match the pairsLevel 2
12. Match each quantity to a unit it is measured in.
Answer: Density = kg/m^3; Mass = kg; Volume = m^3; Length = m
Each physical quantity has its own SI unit, from kilograms for mass to kg/m^3 for density.

Floating and Sinking

Build the sentenceLevel 1
13. Arrange the words into a rule about floating.
Answer: objects that are less dense than water will float
Anything less dense than water floats, and anything denser sinks.
Fact or fibLevel 1
14. A small stone will sink when you drop it in water.
Answer: True
A stone is denser than water, so it sinks.
Fill the blankLevel 1
15. Ice is less dense than water, so ice ____ on top of water.
- floatscorrect
- sinks
- melts
- boils
Ice is less dense than liquid water, which is why ice cubes and icebergs float.
Multiple choiceLevel 1
16. Why does a piece of wood float on water?
- It is less dense than watercorrect
- It is warmer than water
- It is a bright colour
- It is very heavy
Wood floats because it is less dense than water.
Guess the numberLevel 2
17. A wooden block has a mass of 600 g and a volume of 800 cm^3. What is its density in g/cm^3?
Answer: 0.75 g/cm^3
Density = 600 g / 800 cm^3 = 0.75 g/cm^3, which is less than water's 1 g/cm^3, so the block floats.
Match the pairsLevel 2
18. Match each object or force to what it does in water.
Answer: A cork = Floats on water; A steel nail = Sinks in water; Upthrust = Pushes objects upwards; Weight = Pulls objects downwards
Whether something floats or sinks depends on the balance between the downward weight and the upward upthrust.

Pressure

Choose all that applyLevel 1
19. Which of these make the pressure on the ground HIGHER? Choose all that are correct.
- Standing on one foot instead of twocorrect
- Wearing thin high heelscorrect
- Wearing wide flat snowshoes
- Lying flat on the ground
Pressing your weight onto a smaller area, like one foot or thin heels, raises the pressure.
Fill the blankLevel 1
20. A sharp knife cuts well because its force is pressed onto a very ____ area.
- smallcorrect
- large
- cold
- bright
A thin sharp edge squeezes the force onto a tiny area, which makes a high pressure that cuts.
Multiple choiceLevel 1
21. What does pressure mean?
- How hard a force pushes on an areacorrect
- How bright a light is
- How loud a sound is
- How cold something is
Pressure is how much a force pushes on each bit of an area.
Picture questionLevel 1
22. 📌 A pin has a very sharp point. What does the sharp point do?
- Makes a high pressure to push into thingscorrect
- Makes the pin lighter
- Makes the pin colder
- Makes the pin glow
The sharp point puts the force onto a tiny area, giving a high pressure that pushes into the board.
Guess the numberLevel 2
23. A force of 200 N pushes on an area of 4 m^2. What pressure does it produce in pascals?
Answer: 50 Pa
Pressure = force / area = 200 N / 4 m^2 = 50 Pa.
Match the pairsLevel 2
24. Match each quantity to its correct unit.
Answer: Pressure = pascal; Force = newton; Area = square metre; Volume = cubic metre
Pressure is measured in pascals, force in newtons, area in square metres and volume in cubic metres.

Solids, Liquids and Gases

Build the sentenceLevel 1
25. Arrange the words to describe a solid.
Answer: particles in a solid are packed close together
In a solid the particles are packed closely together, which gives it a fixed shape.
Fill the blankLevel 1
26. A gas spreads out to completely fill its ____.
- containercorrect
- shadow
- weight
- colour
Gases spread out in every direction to fill whatever container they are in.
Match the pairsLevel 1
27. Match each material to the way it behaves.
Answer: Brick = Keeps its own shape; Water = Takes the shape of its cup; Air in a balloon = Fills the whole balloon; Ice cube = A solid you can melt
Each material behaves in the way that matches its state of matter.
Choose all that applyLevel 2
28. Which of these are true about the particles in a solid?
- They vibrate about fixed positionscorrect
- They are held in a regular arrangementcorrect
- They are far apart with large gaps
- They can flow past each other freely
Solid particles vibrate about fixed positions in a regular arrangement; they are not far apart and cannot flow freely.
Fact or fibLevel 2
29. Fact or fib: Brownian motion, the random jittering of smoke specks seen under a microscope, is evidence that air is made of tiny moving particles.
Answer: True
The specks are knocked about by unseen, fast-moving air molecules, so Brownian motion is strong evidence for the particle model.
Guess the numberLevel 2
30. Roughly how fast do air molecules move on average at room temperature?
Answer: 500 m/s
Air molecules zip about at hundreds of metres per second, with an average speed near 500 m/s at room temperature.

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 15 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
Sort into groups
Spell it
True or false
Type the answer

Practise States of Matter and Density

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

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Read about States of Matter and Density

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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