KS3 · Chemistry

Boiling and condensing

Watch a kettle boil and bubbles race up through the water. What is actually inside one of them? Air? Nothing? The answer surprises most people.

Water particles on either side of 100 °C

Liquid water

Temperature of the water: 20 °C. State: Liquid water. static

Temperature of the water20 °C

20 °C is below water's boiling point of 100 °C, so this is a liquid. The particles are touching, in a random arrangement, and they slide past each other.

Slide the temperature up past 100 °C, then back down again. These are the same water particles each time. Watch how far apart they are and how they move.

Same change, opposite directions

BoilingvsCondensing

Read across each row. Notice how every row flips.

Focus

Direction of the change

Boiling

Liquid state to gas state

Condensing

Gas state to liquid state

The insight

Condensing is the reverse of boiling. Run the same change backwards and you get the other one.

What you do to the substance

Boiling

Heat the liquid

Condensing

Cool the gas

What you can see

Boiling

Gas bubbles form in the liquid

Condensing

Liquid forms, for example on windows and mirrors

Name for its temperature

Boiling

Boiling point

Condensing

Condensing point

Which substances do it

Boiling

Water boils, and so does ethanol

Condensing

Every substance in the gas state can condense

Chemistry · Boiling

What is inside a bubble?

A pan of water is boiling. Bubbles keep forming in the water, rising to the surface and bursting.

Which idea is closest to what you think is going on?
How sure are you?

Chemistry · Boiling points

Where does each substance switch?

Each of these substances has one temperature where it boils on the way up and condenses on the way down. Drag each one to where you think its boiling and condensing point sits, then check.

Chemistry · Measuring a boiling point

Measuring the temperature of boiling water

Put the steps in the order you would do them

1 · First step4 · Last step

  1. Read the temperature on the thermometer, with your eye level with the top of the coloured alcohol

  2. Wait until you see the water boiling, with bubbles being produced

  3. Set up the equipment: heat resistant mat, tripod, Bunsen burner, gauze, beaker of water and thermometer

  4. Turn the Bunsen burner onto the blue flame to heat the water

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One change, run forwards and backwards

What you need to know

  • Boiling is when a liquid is heated and gas bubbles form; condensing is when a gas is cooled and becomes a liquid.
  • Boiling and condensing are reverses of each other, and for any one substance they happen at the same temperature.
  • In a gas the particles are far apart with only space between them, moving very quickly in random directions. In a liquid they are touching and sliding past each other.

The big picture

Boiling and condensing are the same change of state run in opposite directions. Heat a liquid to its boiling point and it turns into a gas: its particles break away from their touching neighbours, spread far apart and move very quickly in random directions. Cool the gas back to the same temperature and it condenses into a liquid again. Every substance has its own temperature for this switch, and the bubbles in a boiling liquid are made of that liquid itself in the gas state, never air.

Key points

1Boiling is a change from the liquid state to the gas state when a liquid is heated. The boiling point is the temperature at which it happens.
2Condensing is a change from the gas state to the liquid state when a gas is cooled. The condensing point is the temperature at which it happens. It is the same temperature as the boiling point.
3Different substances have different boiling points: ethanol 78 °C, water 100 °C, mercury 357 °C and nitrogen −196 °C.
4A substance is a liquid below its condensing point and a gas above its boiling point.
5The bubbles in a boiling liquid are always made of that liquid in the gas state: water particles for water, ethanol particles for ethanol. Never air.
6Steam is water in the gas state. It is not a word for every hot gas.
7On boiling, particles move further apart and move very quickly in random directions, so the substance expands to many times its liquid volume. On condensing the reverse happens. The particles themselves stay the same size.
8To measure the boiling point of water, set up, heat on the blue flame, wait for bubbles, then read the thermometer with your eye level with the top of the alcohol.

Worked example

Problem

Mercury boils at 357 °C. Is mercury a liquid or a gas at 300 °C? And at 400 °C?

⚠ Watch out

Thinking the bubbles in boiling water are air, or that a gas is 'just air'. The bubbles are the boiling substance itself in the gas state. For water that is steam, made of water particles with only empty space between them.

🧠

Memory hook

One doorway, two directions. Each substance has one doorway temperature. Go through it heating and a liquid becomes a gas (boiling); come back cooling and the gas becomes a liquid (condensing). It's only a picture: the real change is particles spreading out or gathering together.

✓

Check yourself

Someone says, 'Those bubbles in the boiling pan are full of air.' What two things would you say to change their mind?

Flashcards

(12)
What is boiling, and what is the boiling point?
Boiling is when a liquid is heated and gas bubbles form. The boiling point is the temperature at which a substance changes from a liquid to a gas.
What is condensing, and what is the condensing point?
Condensing is when a gas is cooled and changes to a liquid. The condensing point is the temperature at which a substance changes from a gas to a liquid.
Can a substance have a boiling point that differs from its condensing point?
No. They are one temperature, because condensing is boiling in reverse.
What are the boiling points of water and ethanol?
Water boils at 100 °C and ethanol at 78 °C.
What are the boiling and condensing points of nitrogen and mercury?
Nitrogen: −196 °C. Mercury: 357 °C. Each is the same temperature for boiling and for condensing.
Is a substance a liquid or a gas below its condensing point? And above its boiling point?
Below its condensing point it is a liquid. Above its boiling point it is a gas.
What are the bubbles in boiling water made of?
Water in the gas state (steam): only water particles, not air.
What does the word 'steam' mean?
Water in the gas state. It is not used for every hot gas, just water.
What happens to the particles' spacing and movement when a substance boils?
They move further apart. They go from sliding past each other to moving very quickly in random directions.
What happens to a substance's volume when it boils, and when it condenses?
Boiling: the volume increases, to many times the liquid volume, because the particles gain space between them. Condensing: the volume decreases because the particles lose space between them.
What is between the particles in a gas, and how big are the particles?
Only space, and nothing else. The particles stay the same size, and the particles are the substance.
Where should your eye be when reading a thermometer, and why?
Level with the top of the coloured alcohol. Too high or too low and the alcohol won't line up with the scale, so you might read the wrong temperature.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Chemistrylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 9 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.