Particles and their behaviour · Process
Changes of state
Put a beaker of ice on a hot ring and the temperature climbs. Then, for six full minutes, it stops climbing — while the flame is still on. Where is that energy going?
Start here
Seal it in the bag first.
An ice cube, 50 g, sealed inside a bag that nothing can get in or out of. Weigh it. Leave it on the bench until it is a puddle of water, then weigh the bag again.
What does the balance read the second time?
50 g. Exactly 50 g, and it will read 50 g again when the water evaporates inside the sealed bag, and again if you freeze it back. A change of state moves particles around and never removes a single one. Keep the mass readout in the corner of your eye for the rest of this lesson — it will not move once.
Melting, freezing, boiling, condensing, evaporating. Five words for the same short list of events: particles gaining enough energy to break away from their neighbours, or losing enough to be captured again. Nothing else happens. Nothing is made and nothing is lost.
The heating bench · scrub through it
Heat it steadily and watch the temperature refuse to rise.
0 of 2 plateaus visited
50 g of ice at −20 °C in a sealed flask, on a ring that delivers the same energy every second from start to finish. Drag through the run.
Commit first. The energy goes in at a steady rate. What will the temperature do?
Temperature
−20 °C
What is in the flask
IceMeltingWaterBoilingSteam
Mass in the flask
50.0 g
Solid ice, warming. The energy is going straight into making the particles vibrate harder, and the thermometer reports it faithfully.
The thermometer has stopped at 0 °C and it will not move until the last of the ice has gone — but the heater has not stopped. Every joule going in now is being spent breaking particles out of the rigid arrangement. Watch the flask: order is being destroyed, and the mass has not shifted by a milligram.
All liquid now, and the temperature climbs again. Notice this stretch is much longer than the ice stretch — water takes more energy per degree than almost anything else, which is why the sea moderates the climate.
The second plateau, at 100 °C, and it is far longer than the melting one. Melting only loosens the particles; boiling has to separate them completely, and that costs about seven times as much energy.
Steam, above 100 °C, warming quickly — there is almost nothing to slow it down now. Same 50 g of water that started as ice. Not one particle has been created or destroyed in the whole run.
Key fact
During a change of state the temperature does not change: the energy goes into breaking the particles apart from each other, not into speeding them up. Mass stays the same throughout.
The bubble · a small question with a large answer
What is inside a bubble in boiling water?
A pan of water at a rolling boil. Bubbles form at the bottom, rise, and burst at the surface. Commit to what is in one.
Not this one. Follow the consequence of your answer through.
Not this one. Follow the consequence of your answer through.
Correct — and it is worth knowing why the other three fail.
No. Splitting water into hydrogen and oxygen is a chemical change and takes far more energy than a kettle has; boiling is a change of state and the water is still water.
Steam. Water that has boiled into a gas, right there at the bottom of the pan where it is hottest. If a bubble were air, the pan would run out of air after a minute and stop bubbling — and boiling water does not stop bubbling. If a bubble were empty, the water pressure would crush it instantly.
And the steam you can see above the pan is not steam. Steam is invisible. What you see is the steam having already condensed back into tiny drops of liquid water in the cold air — a change of state happening in front of you, twice, in the space of a few centimetres.
Think again
“Sugar melts in tea.”
Everyone says it and it is the wrong word. Sort each of these into the right column before you read on — the sorting is the point, not the score.
A sugar cube stirred into hot tea
Dissolving. Two substances, and the sugar particles spread out among the water particles.
Not melting — tea is nowhere near sugar’s melting point of about 186 °C. Two substances are involved, so this is dissolving.
Butter left in a pan over a low flame
Melting. One substance, heated until its particles break out of their fixed positions.
Nothing is being spread through a liquid here. One substance, heated — that is melting.
Salt stirred into cold water
Dissolving. Cold water, so no heating is involved at all — and it still happens.
The water is cold, so nothing has been heated past a melting point. Salt melts at 801 °C.
A chocolate bar left on a sunny windowsill
Melting. One substance, warmed above its melting point, and it sets again when it cools.
There is no second substance for it to spread into. Warmed above its melting point, it melts — and re-solidifies on cooling.
Melting needs one substance and heat: the particles of that substance break out of their fixed positions, and if you cool it the solid comes straight back. Dissolving needs two substances and no heat at all: the particles of the solid separate and slot in among the particles of the liquid, and to get the solid back you have to remove the liquid.
Sugar in tea is dissolving. Sugar in a dry pan over a flame really does melt, at about 186 °C. It is an awkward example, because sugar starts browning and breaking down at around its melting point — and that browning is a chemical change, not a melt. Same substance, two entirely different events, and the word you choose says which one you mean.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is the name of the change from a gas to a liquid?
Rung 2 · The one that catches people
A beaker of ice and water is heated steadily. For six minutes the thermometer stays at 0 °C. What is the energy doing during those six minutes?
Rung 3 · Explain
Explain why the temperature stops rising while ice is melting, even though the heater is still supplying energy at the same rate.
Rung 4 · Take it somewhere new
A wet towel dries on a washing line on a cold day, at 8 °C. Water boils at 100 °C. Explain how the water can become a gas at a temperature far below its boiling point.
Key note
Changes of state are reversible, cost or release energy, and never change the mass. Temperature holds still at the melting and boiling points because the energy is buying separation, not speed.
Going further
The plateau has a practical consequence you have felt. Water at 100 °C will scald you; steam at 100 °C — the same temperature — will do far worse, because every gram of it has to give back the whole boiling plateau's worth of energy as it condenses on your skin, and that is about seven times the energy released by the same water cooling from 100 °C to room temperature. Nothing about the temperature reading warns you. This is also how a sweating body cools itself, how a fridge works, and why a puddle can evaporate on a cold day without ever reaching 100 °C.
Before this lesson
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