Energy changes in reactions · Model
Energy and changes of state
Keep heating a beaker of melting ice and the thermometer refuses to move. The energy is still going in — so where is it going?
Start here
A beaker of ice on a Bunsen. The thermometer climbs to 0 °C and then stops for four minutes.
The flame has not changed. Energy is going in at exactly the same rate it was a minute ago. The ice is visibly melting. And the thermometer sits at 0 °C and will not move until the last piece of ice has gone — then it climbs again.
Where is the energy going during those four minutes?
Into pulling the particles apart. In ice the particles are held in a fixed arrangement by forces of attraction, and melting means breaking out of that arrangement. That takes energy — a great deal of it — and while it is being spent, none is left over to make the particles move faster. Temperature is a measure of the average kinetic energy of the particles — the energy they have because they are moving. During a change of state they are not speeding up; they are being separated.
Changing state always involves energy, and the direction is fixed. Melting, evaporating and boiling take energy in — the particles have to be pulled apart against the forces holding them together. Freezing and condensing give energy out — the particles fall back together and release it.
Nothing new is made and nothing is destroyed. A change of state is a physical change: the same particles, rearranged, and reversible by putting the energy back or taking it away again.
Your turn · heat it a minute at a time
Ice at −20 °C, a steady flame, and a thermometer.
The flame is steady, so every minute delivers the same energy. Step through and watch for the minutes where the thermometer refuses to move.
-20 °C
Solid — ice
Minute 0
Below freezing. The particles are locked in a fixed arrangement and can only vibrate. Energy going in makes them vibrate faster, so the temperature climbs steadily.
-10 °C
Solid — ice
Minute 1
Still solid, still climbing. Every joule from the flame is going into making the particles vibrate harder.
0 °C
Solid — at its melting point
Minute 2
The ice has reached 0 °C. The next joule of energy will not raise the temperature — it will start breaking particles out of the arrangement instead.
0 °C
Melting — solid and liquid together
Minute 3 · temperature unchanged
The thermometer has stopped. The flame has not. Energy is being spent pulling particles out of the fixed arrangement, and separating particles does not make the remaining ones move any faster.
0 °C
Melting — solid and liquid together
Minute 4 · temperature unchanged
Still 0 °C, still melting. This is the most energy-hungry part of the run so far, and the thermometer shows nothing at all.
0 °C
Just melted — all liquid
Minute 5 · temperature unchanged
The last of the ice has gone. From here the energy has nothing to separate, so it goes back into making particles move faster.
30 °C
Liquid — water
Minute 6
Climbing again, and the thermometer is working normally once more. The three flat minutes did not pause the flame — they were the energy that melted the ice, and it went somewhere a thermometer cannot look.
60 °C
Liquid — water
Minute 7
Still climbing. Some particles are already escaping from the surface as vapour — evaporation happens at any temperature — but the bulk of the liquid is simply warming.
100 °C
Liquid — at its boiling point
Minute 8
At 100 °C the liquid is about to change state throughout, not just at the surface. Bubbles of vapour can now form inside the liquid.
100 °C
Boiling — liquid and gas together
Minute 9 · temperature unchanged
Flat again. Energy is now pulling particles completely away from each other, which takes far more than merely loosening them did at 0 °C.
100 °C
Boiling — liquid and gas together
Minute 10 · temperature unchanged
Still 100 °C, and the flat run is going to be much longer this time. Loosening particles so they can slide takes a certain amount of energy; tearing them right away from each other takes several times as much.
100 °C
Boiling — liquid and gas together
Minute 11 · temperature unchanged
Four minutes at the same reading. Nothing is stuck and nothing is broken: the beaker is emptying, one particle at a time, into the air above it.
100 °C
Boiling — liquid and gas together
Minute 12 · temperature unchanged
The bubbles are not air and they are not nothing. They are water in the gas state, forming inside the liquid because every part of it now has enough energy to break away.
100 °C
Boiling — liquid and gas together
Minute 13 · temperature unchanged
This is the part of the run that a kettle spends most of its electricity on. Bringing the water to 100 °C is quick; turning it into steam is what takes the time and the energy.
100 °C
Boiling — liquid and gas together
Minute 14 · temperature unchanged
Turn the flame up and the reading still says 100 °C. A bigger flame boils the water away sooner; it does not make boiling water hotter.
100 °C
Boiling — liquid and gas together
Minute 15 · temperature unchanged
The steam leaving the beaker is carrying away every joule that has gone in since the reading stopped moving. That is where the energy is: not in the beaker, and not lost.
100 °C
Just boiled — all gas
Minute 16 · temperature unchanged
The last of the liquid has gone. Everything in the beaker is now steam at 100 °C, carrying all the energy that boiling put into it.
120 °C
Gas — steam
Minute 17
Climbing once more. Steam above 100 °C is called superheated, and it is the state that does the work in a power station turbine.
Temperature against time
0 min · · · 17 min
Two flat steps in a graph that should be a straight climb.
The flame delivered energy at the same rate for all seventeen minutes. For six of them the temperature rose. For eleven of them it did not move at all — three minutes at 0 °C and eight minutes at 100 °C.
Those flat steps are the changes of state, and they are where the energy went into separating particles rather than speeding them up. The boiling step is much longer than the melting step, because pulling particles completely apart takes far more energy than merely letting them slide past each other.
This curve is a schematic and not a measurement. Boiling a given mass of water really does take about seven times the energy that melting the same mass takes, and the second flat step is drawn longer for that reason — but no part of this graph is to scale, and no number of minutes should be read off it.
Key fact
Melting and boiling take energy in; freezing and condensing give energy out. During a change of state the temperature does not change, because the energy is separating particles rather than speeding them up.
Three judgements
Where this shows up outside a beaker
0 of 3 decided
Commit to each before reading. All three are the same idea wearing different clothes.
Why does sweating cool you down?
Evaporation takes energy in, and it takes it from the nearest available source — your skin. The sweat leaves as vapour carrying that energy with it, and what is left behind is cooler. This is why a humid day feels so much worse: if the air is already saturated the sweat cannot evaporate, so the cooling mechanism stops working.
Steam at 100 °C and water at 100 °C touch your hand. Which does more damage?
The steam, by a long way. Both are at the same temperature, so a thermometer cannot separate them — but the steam also carries all the energy that went into boiling it, and it releases that energy into your skin as it condenses. Same temperature, very different quantity of energy.
Orange growers spray their trees with water when a frost is forecast. Does that help?
It genuinely helps, and it looks like madness. Freezing gives energy out, so as the sprayed water turns to ice it releases energy into the fruit and holds the temperature at 0 °C — which is cold, but not cold enough to destroy the crop. The ice coating is doing the protecting.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“While ice is melting it has stopped absorbing heat.”
The thermometer genuinely does not move. Commit before you read on.
It is absorbing heat faster than at any other point in the experiment. Melting a beaker of ice takes several times more energy than warming the same water by a single degree — the flat step is not a pause, it is the most energy-hungry part of the whole run.
The confusion comes from treating the thermometer as an energy meter. It is not. Temperature is a measure of the average kinetic energy of the particles — the energy they have because they are moving. During melting, that average is not rising — the particles are being pulled apart, not sped up. Energy in, no temperature change, and nothing contradictory about it.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these changes of state gives energy out?
Rung 2 · The one that catches people
A beaker of ice and water is heated steadily. For four minutes the temperature stays at 0 °C. What is happening to the energy?
Rung 3 · Explain
Sketch in words the shape of a heating curve for ice heated from −20 °C to 120 °C, and explain what is happening to the particles in each part.
Rung 4 · Take it somewhere new
A cool box is packed with either 1 kg of ice at 0 °C or 1 kg of water at 0 °C. Predict which keeps the food cold for longer and explain why, using the ideas from this lesson.
Key note
Melting, evaporating and boiling take energy in, because the particles must be separated against the forces attracting them. Freezing and condensing give the same energy back out. While a substance is changing state its temperature stays constant, which produces the flat steps on a heating curve — the energy is going into separating particles rather than making them move faster.
Going further
The energy taken in during a change of state has a name: latent heat, from the Latin for hidden. It is hidden in the sense that a thermometer cannot see it. Joseph Black worked it out in Glasgow in the 1760s by noticing exactly what you just plotted — that ice in a warm room takes hours to melt while staying at 0 °C the whole time — and the idea went straight into James Watt's improvements to the steam engine.
It is also why steam at 100 °C burns far worse than water at 100 °C. Both are at the same temperature, so a thermometer cannot tell them apart, but the steam carries all the energy that went into boiling it and dumps that energy into your skin as it condenses. The same arithmetic keeps a cold drink cold: the ice holds the drink at 0 °C not by being cold but by absorbing energy while it melts, and the moment the last cube disappears the drink starts warming.
Before this lesson
Next in this unit
At GCSE this becomes
- Specific heat capacity and specific latent heat with real calculations, and energy level diagrams for changes of state.
Where to next
- Next: Exothermic reactions
- Previous: Catalysts
Acids and alkalis
Ask Mr Badmus AI
Still not sure why the thermometer stops?
Steam burns worse than boiling water, which is the point of use 2 and the reason a boiling beaker is watched from the side and never from above. Keep hands and face out of the plume, and let glassware cool before moving it.
Lesson content © MrBadmusAI.