Physics · 2 Thermal physics

2.1.1 States of matter

Before you start

Volume is the amount of space something takes up, measured in cm³ or m³.
Temperatures are measured in degrees Celsius (°C), and $- 30$ °C is colder than $- 10$ °C.
A force can change the size and shape of an object (1.5.1).
Energy is transferred to an object when it is heated, and away from it when it cools.

Syllabus objectives covered (2)

Know the distinguishing properties of solids, liquids and gases
Know the terms for the changes in state between solids, liquids and gases (gas to solid and solid to gas transfers are not required)

Part 1 of 5

Three states, four questions

Matter can exist in three states: solid, liquid and gas. The same substance can be in any of the three. Water is ice when it is a solid, water when it is a liquid, and steam when it is a gas.
The state is not the substance. Ice, liquid water and steam are all water. What is different from one to the next is the state, and the state decides how the substance behaves.
To tell the states apart, ask four questions about a sample. Does it keep its own shape? Does it keep its own volume? Can it be compressed easily? Can it flow?
Each state gives a different set of answers to those four questions. That is why they are called the distinguishing properties: they let you distinguish one state from another. The next two sections take the questions one at a time.

Part 2 of 5

Shape and volume

Volume is how much space a sample takes up. Shape is the form of that space. They are separate properties, and a state can keep one without keeping the other.
A solid has a fixed shape and a fixed volume. A stone keeps the same shape whether it sits on a table or in a jar, and it takes up the same space in both. It does not change shape to fit its container.
A force can still bend or squash a solid, as you saw with springs and rulers in 1.5.1. 'Fixed shape' means that a solid does not change shape by itself to fit whatever holds it.
A liquid has a fixed volume but no fixed shape. Pour $50$ cm³ of water from a tall glass into a wide bowl. It changes shape to fit the bottom of the bowl, and its volume is still $50$ cm³.
Because its volume is fixed, a liquid fills only as much of a container as its own volume. Pour $50$ cm³ of water into a $250$ cm³ flask and it sits in the bottom of the flask with a flat surface on top. The other $200$ cm³ of the flask holds air, not water.
A gas has no fixed shape and no fixed volume. It spreads out to fill the whole of any container it is in. Let $50$ cm³ of a gas into a sealed $250$ cm³ box from which all the air has been removed, and the gas fills all $250$ cm³ of the box.
apparatus diagram — not drawn yet
Three identical sealed glass flasks side by side, labelled solid, liquid and gas: the first holds a solid cube resting on the bottom; the second holds a liquid filling only the lower part of the flask, with a flat horizontal surface; the third holds a gas shown as light, even shading that fills the entire flask.
The solid keeps its own shape. The liquid takes the shape of the bottom of the flask but keeps its volume. The gas fills the whole flask.
So a gas does have a volume: its volume is the volume of its container. What a gas does not have is a volume of its own that stays the same when you move it.
One qualification. The volume of a solid or a liquid changes slightly when its temperature changes. That is thermal expansion, which comes later in this topic. 'Fixed volume' means the volume does not change when you move the sample into a different container.

Part 3 of 5

Compressing and flowing

To compress a sample is to squeeze it into a smaller volume. Gases are easily compressed. Solids and liquids are very difficult to compress: even a large force hardly changes their volume.
A syringe shows the difference. Block the nozzle and push the plunger. With air inside, the plunger moves in a long way. With water inside, it hardly moves at all, however hard you push.
apparatus diagram — not drawn yet
Two identical plastic syringes, each with its nozzle sealed by a cap: the left one contains air and its plunger is shown pushed well in, with an arrow showing the large distance moved; the right one contains water and its plunger is shown at almost its starting position, with an arrow showing a very small movement.
Same push, same syringe. The air is compressed into a much smaller volume; the water's volume hardly changes.
This fits the volume property. A gas fills whatever volume it is given, so a smaller space simply means a smaller volume of gas. A solid or a liquid keeps its own volume, so it resists being squeezed. The reason, in terms of particles, is in 2.1.2.
To flow is to keep moving and changing shape, so that a substance can be poured or can spread out. Liquids flow and gases flow. Solids do not flow.
Because liquids and gases can both flow, they are called fluids. A question about fluids is about liquids and gases together.
The awkward case is a powder such as sand, salt or flour. It can be poured, so it looks as if it flows. Look at one grain instead: it keeps its own shape and its own volume. The pile changes shape only because whole grains slide past one another. Each grain is a solid, so sand is a solid.
Put the four answers together and each state has its own set. Solid: fixed shape, fixed volume, very difficult to compress, does not flow. Liquid: no fixed shape, fixed volume, very difficult to compress, flows. Gas: no fixed shape, no fixed volume, easily compressed, flows.
Compare means identify similarities or differences. So when you compare two states, name both states in the same sentence. 'A liquid has a fixed volume but a gas does not' is a comparison. 'A gas has no fixed volume' on its own describes only one state.

Part 4 of 5

The names for the changes of state

A change of state is a substance changing from one state to another. The substance stays the same; only its state changes. Ice that melts is still water.
Solid to liquid is melting. An ice cube turning to water in a drink is melting.
Liquid to solid is freezing. The syllabus also calls it solidification; the two words name the same change. Water turning to ice in a freezer is freezing.
Freezing does not mean 'getting cold'. Molten metal poured into a mould and turning solid is freezing too, even though the metal is still very hot. The word names the change of state, not the temperature.
Liquid to gas is boiling or evaporation. Both are names for a change from liquid to gas. Water boils in a kettle; a puddle dries up by evaporation. How boiling and evaporation differ is part of 2.2.3.
Gas to liquid is condensation, and the verb is to condense. The drops of water that form on a cold window or on a cold can of drink are water vapour from the air condensing on the cold surface.
The white cloud above a boiling kettle is not the gas. Steam itself cannot be seen. The cloud is tiny drops of liquid water, made when the steam condenses as it meets the cooler air.
The changes come in pairs that go opposite ways. Melting and freezing go between solid and liquid. Boiling or evaporation, and condensation, go between liquid and gas.
$solid \overset{\overset{}{melting}}{\to} liquid \overset{\overset{}{boiling or evaporation}}{\to} gas$
Read the same chain backwards for the reverse changes.
$gas \overset{\overset{}{condensation}}{\to} liquid \overset{\overset{}{freezing (solidification)}}{\to} solid$
To get the direction right, link each change to heating or cooling. Melting, boiling and evaporation need energy to be transferred to the substance. Freezing and condensation happen when energy is transferred away from it. So 'heated' points you to melting or boiling, and 'cooled' points you to freezing or condensation.
Changes straight from solid to gas, or from gas to solid, do happen. The syllabus says you do not need them. The four changes above, each to or from the liquid state, are what this objective requires.
Do not call dissolving 'melting'. Sugar stirred into hot tea seems to disappear, but it has not become liquid sugar. It has dissolved in the water, and dissolving is not a change of state.

Part 5 of 5

Melting point, boiling point and deducing a state

A pure substance melts at one particular temperature, its melting point. It boils at one particular temperature, its boiling point.
The melting point and the freezing point of a pure substance are the same temperature. Ice melts at $0$ °C, and liquid water freezes at $0$ °C. There is one temperature for the change between solid and liquid, whichever way it goes.
A boiling point depends on the pressure of the air above the liquid, so it is quoted at standard atmospheric pressure. At standard atmospheric pressure, water boils at $100$ °C. You learn these two values for water in 2.2.3.
If you know a substance's melting point and boiling point, you can deduce its state at another temperature. Below the melting point it is a solid. Between the melting point and the boiling point it is a liquid. Above the boiling point it is a gas.
Watch the negative numbers. A substance that melts at $- 40$ °C is already a liquid at $- 10$ °C, because $- 10$ °C is warmer than $- 40$ °C. Put the temperatures on a number line in your head before you decide.
At exactly the melting point, the temperature alone does not tell you the state. While the substance melts or freezes it can be solid, liquid or a mixture of the two. The same is true at exactly the boiling point, for liquid and gas.
A liquid below its boiling point can still evaporate slowly from its surface; that is how a puddle dries. The rule tells you the state of the sample, not that no gas ever leaves it.
The temperature range over which a substance is a liquid runs from its melting point up to its boiling point. Its size is the boiling point minus the melting point. For water that is $100 - 0 = 100$ °C.
$range = boiling point - melting point$
When the melting point is negative, subtracting it means adding. A substance that melts at $- 20$ °C and boils at $85$ °C is a liquid over $85 - (- 20) = 105$ °C, not $65$ °C.

Worked examples

Example 1. A student tests a substance S in three ways. Test 1: she pours $40$ cm³ of S from a measuring cylinder into a wide dish. It spreads over the bottom of the dish, and when she measures it again its volume is still $40$ cm³. Test 2: she puts S in a syringe, blocks the nozzle and pushes hard on the plunger, and the plunger hardly moves. Test 3: S can be poured. Deduce the state of S, giving a reason from each test.

Show the working

Take each test in turn and ask which states it allows. Using all the evidence is safer than guessing from one clue.
Test 1: S changes shape to fit the dish, so it has no fixed shape. That rules out a solid. Its volume stays $40$ cm³, so it has a fixed volume. That rules out a gas.
Test 2: S is very difficult to compress. That fits a solid or a liquid, and rules out a gas again.
Test 3: S flows. That fits a liquid or a gas, and rules out a solid again.
The only state that every test allows is liquid. Give the answer with one reason per test, because the question asks for a reason from each.

Answer: S is a liquid: it takes the shape of the dish but keeps its volume of $40$ cm³, it is very difficult to compress, and it flows.

Example 2. $120$ cm³ of oil is poured into an empty $500$ cm³ beaker. In a separate experiment, $120$ cm³ of a gas is released into a sealed $500$ cm³ container from which all the air has been removed. Determine the volume occupied by (a) the oil and (b) the gas.

Show the working

Start by naming the state, because the state decides what happens to the volume. The oil is a liquid. The gas is a gas.
(a) A liquid has a fixed volume. The oil takes the shape of the bottom of the beaker but still occupies $120$ cm³.
The rest of the beaker is filled with air.
$500 - 120 = 380 {cm}^{3}$
(b) A gas has no fixed volume. It spreads out to fill the whole container, so it occupies $500$ cm³.
Notice that the volume of the gas went from $120$ cm³ to $500$ cm³ with no gas added. That is what 'no fixed volume' means.

Answer: (a) $120$ cm³, with $380$ cm³ of air above it. (b) $500$ cm³, the whole container.

Example 3. At standard atmospheric pressure, substance P melts at $- 110$ °C and boils at $- 35$ °C. Substance Q melts at $- 20$ °C and boils at $85$ °C. Substance R melts at $60$ °C and boils at $240$ °C. (a) Deduce the state of each substance at $25$ °C. (b) Q is cooled from $25$ °C to $- 50$ °C. Name the change of state that takes place. (c) Calculate the range of temperature over which Q is a liquid.

Show the working

(a) For each substance, compare $25$ °C with both its melting point and its boiling point.
P: $25$ °C is above its boiling point of $- 35$ °C, so P is a gas.
Q: $25$ °C is above its melting point of $- 20$ °C and below its boiling point of $85$ °C, so Q is a liquid.
R: $25$ °C is below its melting point of $60$ °C, so R is a solid.
(b) At $25$ °C, Q is a liquid. At $- 50$ °C it is below its melting point of $- 20$ °C, so it is a solid. Liquid to solid is freezing. This fits the cooling: energy is transferred away from Q.
(c) The range is the boiling point minus the melting point. The melting point is negative, so take care with the signs.
$85 - (- 20) = 85 + 20 = 105^{\circ} C$

Answer: (a) P is a gas, Q is a liquid, R is a solid. (b) Freezing (solidification). (c) $105$ °C.

Where people go wrong

Mistake: Saying a gas has no volume.

Instead: A gas has a volume: the volume of the container it fills. What it does not have is a fixed volume of its own.

Let $50$ cm³ of a gas into an empty $250$ cm³ container and its volume becomes $250$ cm³. The volume is there; it is just not fixed.

Mistake: Saying a liquid fills its container.

Instead: A liquid takes the shape of the part of the container it is in. It fills the container only if its own volume is the same as the container's.

A liquid's volume is fixed, so $50$ cm³ of water in a $250$ cm³ flask stays $50$ cm³. Filling the whole container is what a gas does.

Mistake: Calling sand, salt or flour a liquid because it can be poured.

Instead: A powder is a solid. Each grain keeps its own shape and its own volume.

The pile changes shape only because whole grains slide past one another. The material of each grain does not flow.

Mistake: Saying sugar 'melts' in hot tea.

Instead: The sugar dissolves. Melting is the change of a substance from solid to liquid.

The sugar does not become liquid sugar. It spreads out through the water to form a solution, which is not a change of state.

Mistake: Thinking freezing can only happen at a cold temperature.

Instead: Freezing is any change from liquid to solid. It happens at the substance's own melting point, whatever that temperature is.

Molten metal turning solid in a mould is freezing, even though the metal is still very hot. The word names the change of state, not the temperature.

Check yourself

State the two properties that solids and liquids share.
Answer
Both have a fixed volume, and both are very difficult to compress.
Which two states can flow, and what word covers both of them?
Answer
Liquids and gases. Together they are called fluids.
$30$ cm³ of a gas is pushed out of a syringe into a sealed $90$ cm³ bottle from which all the air has been removed. What volume does the gas occupy now?
Answer
$90$ cm³. A gas has no fixed volume, so it spreads out to fill the whole bottle.
Salt can be poured from a salt shaker. Explain why salt is still a solid.
Answer
Each grain of salt keeps its own shape and its own volume. The salt changes shape as it pours only because whole grains move past one another; the grains themselves do not flow.
Name the change of state from (a) liquid to solid, (b) gas to liquid, (c) solid to liquid.
Answer
(a) Freezing, also called solidification. (b) Condensation. (c) Melting.
A bathroom mirror mists over during a hot shower. Name the change of state, and say which state changes into which.
Answer
Condensation: water vapour (a gas) in the air changes into liquid water on the cold mirror.
A substance melts at $- 15$ °C and boils at $70$ °C. State its state at (a) $- 30$ °C, (b) $0$ °C, (c) $95$ °C.
Answer
(a) Solid, because $- 30$ °C is below the melting point. (b) Liquid, because $0$ °C is between the melting point and the boiling point. (c) Gas, because $95$ °C is above the boiling point.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. This question is about the properties of solids, liquids and gases.

(a) For each of solid, liquid and gas, write yes or no for each property: (i) has a fixed shape, (ii) has a fixed volume, (iii) can flow. 3 marks No calculator

Mark scheme

B1 (i) fixed shape: solid yes, liquid no, gas no; all three needed
B1 (ii) fixed volume: solid yes, liquid yes, gas no; all three needed
B1 (iii) can flow: solid no, liquid yes, gas yes; all three needed

(b) The outlet of a bicycle pump is blocked. Explain why the handle can be pushed in a long way when the pump is full of air, but hardly moves when the pump is full of water. 2 marks No calculator

Mark scheme

B1 air is a gas and a gas is easily compressed / its volume can be made smaller / its particles are far apart and can be pushed closer together, owtte
B1 water is a liquid and a liquid is very difficult to compress / has a fixed volume / its particles are already close together, owtte; accept 'cannot be compressed'

(c) Dry sand can be poured from a bucket. Suggest why sand is still described as a solid. 1 mark No calculator

Mark scheme

B1 each grain keeps its own (fixed) shape and volume / only whole grains move past one another, the grains themselves do not flow, owtte

Question 2. Water is heated in a pan until it boils, and steam (water as a gas) rises from the pan. Some of the steam reaches a cold window, where drops of water form on the glass. Later, a tray of water is put in a freezer and becomes ice.

(a) State the name of the change of state when (i) the water in the pan turns to steam, (ii) the steam turns to drops of water on the window, (iii) the water in the freezer turns to ice. 3 marks No calculator

Mark scheme

B1 (i) boiling; accept evaporation
B1 (ii) condensation / condensing
B1 (iii) freezing / solidification

(b) Identify which one of the three changes in (a) needs energy to be transferred to the water. 1 mark No calculator

Mark scheme

B1 (i) / boiling (of the water in the pan); CON if a second change is also given

(c) Compare the liquid water in the pan with the steam above it, in terms of shape and volume. 2 marks No calculator

Mark scheme

B1 similarity: neither has a fixed shape / both take the shape of their container, owtte
B1 difference: the liquid water has a fixed volume but the steam does not / the steam spreads out to fill the space available, owtte

Question 3. At standard atmospheric pressure, substance W melts at $- 200$ °C and boils at $- 180$ °C. Substance X melts at $- 45$ °C and boils at $70$ °C. Substance Y melts at $50$ °C and boils at $300$ °C.

(a) Deduce the state of each of W, X and Y at $20$ °C. 2 marks No calculator

Mark scheme

B2 W gas, X liquid, Y solid; B1 for any two correct

(b) Calculate the range of temperature over which X is a liquid. 2 marks

Mark scheme

C1 range = boiling point − melting point, or 70 − (−45) seen
A1 115 °C; unit required

(c) Y is heated from $20$ °C to $320$ °C. State, in order, the two changes of state that take place. 2 marks No calculator

Mark scheme

B1 melting first
B1 boiling second; accept evaporation

How this is examined

Matching each state to its properties: fixed shape, fixed volume, how easily it compresses, whether it flows: a table to complete with yes or no for solid, liquid and gas, or a short Describe or Compare worth one mark per property One correct property per mark, written in property words: 'fixed shape', 'fixed volume', 'takes the shape of its container', 'fills the whole container', 'easily compressed', 'very difficult to compress', 'can flow'. In a Compare, both states are named in the same sentence: 'a liquid has a fixed volume but a gas does not'.
Naming a change of state in the right direction: a State question set in an everyday situation such as a kettle, a freezer or a cold window, one mark for each change named The exact term for that direction: melting (solid to liquid), freezing or solidification (liquid to solid), boiling or evaporation (liquid to gas), condensation (gas to liquid). A description such as 'it turns into water' is not a name and is insufficient.
Explaining a syringe or pump observation with compressibility: a two-mark Explain about a sealed syringe or pump that is filled first with air and then with water Each mark needs the state and its property together: 'air is a gas, and a gas is easily compressed' for one mark; 'water is a liquid, which is very difficult to compress' for the other. An answer about only one of the two substances can earn only one mark.
Deducing a state from a melting point and a boiling point: a table of melting and boiling points followed by a Deduce (the state at a stated temperature) or a Calculate (the range over which a substance is liquid) Comparing the temperature with both values: below the melting point means solid, between the two means liquid, above the boiling point means gas. For a range, boiling point minus melting point, with the unit °C; subtracting a negative melting point means adding it.

Words to know

Solid: A state with a fixed shape and a fixed volume. It is very difficult to compress and does not flow.
Liquid: A state with a fixed volume but no fixed shape. It takes the shape of its container, is very difficult to compress and can flow.
Gas: A state with no fixed shape and no fixed volume. It spreads out to fill its whole container, is easily compressed and can flow.
Compress: To squeeze a sample into a smaller volume.
Fluid: A liquid or a gas: a substance that can flow.
Melting: The change of state from solid to liquid.
Freezing (solidification): The change of state from liquid to solid.
Boiling: A change of state from liquid to gas that takes place at the boiling point, as when water boils in a kettle.
Evaporation: A change of state from liquid to gas at the surface of a liquid. It can happen below the boiling point, as when a puddle dries.
Condensation: The change of state from gas to liquid.
Melting point: The temperature at which a pure substance melts. It is the same temperature as its freezing point.
Boiling point: The temperature at which a liquid boils. It depends on pressure, so it is quoted at standard atmospheric pressure.

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