The point of no return

On the bench: electric frypan, 2 tablespoons of white sugar in a small dish, a metal spoon, two saucers labelled stage 3 and stage 5, a heat-resistant mat, a wooden spoon for stirring, and the fire blanket within reach. Students seated behind the one-metre line.

1. Before you switch on — tip the sugar into the cold frypan and spread it thin, no deeper than a few millimetres. Deep sugar burns at the bottom before the top has melted and you lose the middle stages. Point out the grainy white crystals: that is stage 1, and they record it now.
2. Switch on, medium. Say out loud that nothing goes in or out of the pan from here. The whole thing takes about eight minutes, so keep talking and keep them recording.
3. Call each stage as it arrives — "edges going clear, that's two", and so on. Give them fifteen seconds to write before you move on. Do not rush from 3 to 5; that stretch is the lesson.
4. At stage 3, all liquid and still colourless — lift a spoonful onto the saucer marked stage 3 and set it aside to cool. Say what you are doing and why.
5. At stage 5, definite amber — lift a second spoonful onto the saucer marked stage 5. The caramel smell should be obvious across the room by now. Ask them to write down what they can smell, because that is the evidence that matters.
6. Take it to stage 6 — dark going black, sharp bitter smell, first wisps of smoke — then switch off and slide the pan off the element. Do not chase a blacker result. Once smoke appears you have what you need.
7. While it cools, the two saucers go round the room on a tray, not in hands. Five minutes is enough for them to set hard. Nobody tastes anything.
8. Leave the pan to cool completely before it goes near water. Hot sugar and water spits.

Thermometer stays out of the sugar. Molten sugar is over 180 °C, it sticks to the probe and drags hot sugar with it when you lift it. The temperatures in this lesson come from the chocolate and the egg, not the sugar.

1. Hard, clear or slightly cloudy, glassy. It looks different from the grainy powder it started as, but it is still sugar — it tastes of sugar and nothing new has been made.

2. Hard, brown, and it smells of caramel rather than of nothing. It is not sugar any more.

3. Somewhere between stage 4 and stage 5. The evidence to look for is more than one thing at once: the colour changed and the smell changed and it does not go back.

4. At stage 3 the sugar changed state — same substance, different form, and cooling reverses it. By stage 5 the heat has broken the sugar apart and built new substances, which is why it smells different. A change of state can be undone. A new substance cannot be un-made.

5. No. It keeps decomposing until what is left is mostly carbon — the black stuff. Every step takes it further away.

On the bench: the frypan on low, two small metal dishes, one digital thermometer per dish if you have two, a few pieces of broken chocolate, one egg white in a cup, and a spoon. Same exclusion line, students seated.

Chocolate first — about 5 minutes

1. Three or four small pieces in the metal dish, thermometer probe resting in among them, dish into the frypan on low. Low matters: chocolate scorches and then you are demonstrating the wrong change.
2. Read the temperature aloud every thirty seconds and have them write the numbers as you go.
3. The moment the pieces lose their edges and slump, call the temperature. Expect somewhere between 34 and 38 °C. That is the number that goes on their sheet.
4. Dish out of the pan, onto the mat. Say you will come back to it — it needs a few minutes to set again, and it will.

Egg white second — about 6 minutes

1. A tablespoon of egg white into the second dish, probe in the middle of it, into the frypan on low.
2. Read the temperature aloud as it climbs. It will sit around 50-something for a while and look like nothing is happening. Let that silence run — the contrast with what comes next is the point.
3. The instant it turns opaque white, call the temperature. Expect about 63 °C. Write it on the board beside 100 °C for boiling water, and leave both numbers there through CP1.
4. Out of the pan, onto the mat, next to the chocolate.

Then show both — the chocolate has hardened again, and the egg has not gone anywhere. Carry them round on the tray. That side-by-side is what makes the point, so do not skip it for time.

If a thermometer reads oddly, say so rather than adjusting the number. An honest "that reading does not look right, let's take it again" is worth more than a tidy figure, and Week 5 is built on trusting your instrument.

1. The chocolate, somewhere around 34–38 °C. Body temperature is 37 °C, so it melts at roughly the temperature of your own mouth — which is exactly why it melts in your hand and on your tongue.

2. The egg white, at around 63 °C. Still well below boiling.

3. Because boiling only changes water's state — it is still water, whether liquid or steam, and cooling brings it straight back. Cooking the egg builds something new out of what was there. Cooling cannot un-build it.

4. No. Not on its own. Temperature tells you how much energy went in. What happens depends on the substance and on which of its thresholds that energy crosses.

Heat on its own is not evidence — ice melts when it is heated and nothing new is made. What today showed is subtler and more useful:

Adding heat can cause either kind of change. What decides it is the substance, and how much energy goes in.