KS3 · Physics

Cooling

Leave a cup of tea and it's cooler within minutes, yet lukewarm tea takes ages to finish cooling. That isn't your imagination, and there's a neat reason for it.

A hot drink cooling in a room

0102030400255075100Time (minutes)Temperature (°C)(20, 29.5)

Time (minutes): 20. Temperature (°C): 29.5

Slide the point along the curve. Where is it steepest? Where is it nearly flat?

Watch out: A flatter curve doesn't mean the drink has stopped cooling. It is still cooling, just more slowly.

Inside the cooling drink

Very hot

Drink temperature: 90 °C. State: Very hot. static

Drink temperature90 °C

At 90 °C the drink's particles are moving fast. Every time one hits a slower particle in the air, the drink's particle slows down and the air particle speeds up.

Slide the drink's temperature down and watch its particles slow.

Watch out: Only the drink's particles are drawn here. The slower air particles they collide with are all around the outside of the drink.

Why the curve flattens

?

Reason it through

Why does a hot drink cool quickly at first, then more and more slowly?

Link 1 of 4

First link · your turn

The drink starts at 90 °C in a room at 20 °C. Is the temperature difference big or small?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Energy check

Where does the drink's energy go?

A mug of hot chocolate is left on the kitchen table. An hour later it has cooled to nearly room temperature.

Which is closest to what you think happened to its energy?
How sure are you?

Which cools faster?

Problem

Two cups of water are cooling (these are made-up readings for practice). Cup A falls from 60 °C to 48 °C in 4 minutes. Cup B falls from 85 °C to 78 °C in 1 minute. Which cup is cooling faster?

Investigating how very hot water cools

Step through in order. The first step comes first for a reason.

  2. Pour in the very hot waterOnly once everything is set up, pour in the very hot water.
  3. Measure the starting temperatureRead the thermometer to get the starting temperature of the water.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Where a hot drink's energy goes, and why the last few degrees take so long

What you need to know

  • A temperature difference makes energy transfer from the hotter object to the cooler one. For a hot drink, that means from the drink to the surroundings.
  • The higher the temperature of a drink, the faster its particles are moving.
  • A hot drink cools when its fast particles collide with slower particles in the air: in each collision the drink's particles slow down and the air's particles speed up.
  • The bigger the temperature difference between a hot drink and the air, the faster it cools. As it cools the difference shrinks, so the cooling rate falls, and it is lowest close to room temperature.
  • Cooling rate is the fall in temperature over a time interval, such as the fall in one minute. To compare two cooling rates you need both the fall and the time.
  • A cooling curve plots temperature (y-axis) against time (x-axis). The steeper the slope, the higher the cooling rate, and a hot drink's curve gets less steep over time.
  • The energy isn't lost. It is transferred to the surroundings and spread out; the room gains what the drink loses, but its temperature rise is often too small to measure.

The big picture

When a hot drink cools, energy is transferred from the drink to its cooler surroundings. The drink's fast-moving particles collide with slower particles in the air: the drink's particles slow down and the air's particles speed up. The bigger the temperature difference between the drink and the air, the faster the drink cools, so as the drink cools and that difference shrinks, the cooling rate falls. That's why a cooling curve starts steep and flattens out near room temperature. The energy isn't lost: the surroundings gain what the drink loses, but there are so many more particles in a room that its temperature rise is often too small to measure.

Key points

1Energy always moves from hotter to cooler: out of the hot drink, into the air.
2Hot drink = fast particles. Collisions with slower air particles slow them down.
3Big temperature difference = fast cooling. Small difference = slow cooling.
4Cooling curve: steep at the start, flatter near room temperature.
5Cooling rate = temperature fall ÷ time. Never compare falls over different times.
6Energy isn't lost when something cools. It's transferred and spread out.

Worked example

Problem

Describe and explain what happens to a beaker of very hot water as it is left to cool in a room.

⚠ Watch out

Saying the drink's energy is 'lost' or 'used up'. It isn't: it is transferred to the surroundings. The other easy slip is thinking a drink cools at a steady rate. It doesn't: it cools fastest at the start, when the temperature difference is biggest.

🧠

Memory hook

Big gap, fast drop. Small gap, slow drop. The gap is the temperature difference between the drink and the room, and cooling keeps shrinking it.

✓

Check yourself

Sketch the cooling curve for a cup of hot water left on a desk. Mark where it cools fastest and slowest, then explain the shape using the words 'temperature difference'.

Flashcards

(14)
Which way does energy transfer between a hot drink and the cooler air around it?
From the hotter drink to the cooler air.
How does a drink's temperature link to its particles?
The higher the temperature, the faster its particles are moving.
A fast particle of a hot drink hits a slower air particle. What happens to each?
The drink's particle slows down; the air particle speeds up.
What makes a hot drink cool faster?
A bigger temperature difference between the drink and the air.
What happens to the cooling rate as a drink gets closer to room temperature?
It falls. The cooling rate is lowest close to room temperature.
How do you find a cooling rate?
Measure the fall in temperature over a time interval, e.g. °C fallen in one minute.
Why can't you compare two cooling rates using the temperature falls alone?
The falls may be over different lengths of time. You need the fall and the time.
What is a cooling curve?
A line graph of temperature (y-axis) against time (x-axis) for something that is cooling.
On a cooling curve, what does a steeper slope mean?
A higher cooling rate: the object is cooling faster.
How does the slope of a hot drink's cooling curve change over time?
It gets less steep: the curve flattens out.
Is energy lost when a hot drink cools?
No. It is transferred to the surroundings and dissipated; the surroundings gain what the drink loses.
Why does a room's temperature hardly rise when a drink cools in it?
The room has many more particles than the drink, so the rise is often too small to measure.
In the cooling investigation, why set up before pouring in the very hot water?
To reduce the risk of burns.
What is the smallest temperature change a thermometer can measure?
The smallest division on its scale.

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.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

More KS3 Physics topics

See the full KS3 Physics curriculum →

How this lesson was checked. This KS3 Physicslesson 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 30 September 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.