The Heat That Doesn't Warm

Put crushed ice in a beaker with a thermometer in it, and start heating. Write down the temperature every half minute.

At first the reading climbs, exactly as you'd expect. Then it reaches 0°C, and it stops.

Not slows. Stops. You are still heating. The flame is still on. And for the next several minutes the thermometer sits at 0°C and does not move, while the ice steadily disappears and water takes its place. Only when the very last piece of ice has gone does the reading start climbing again.

You met this fact in lesson 7 and you were told, correctly, that it happens. But a fact you cannot explain is a fact you will forget. So here is the question this lesson exists to answer:

You put heat in for minutes on end. The temperature did not change. Where did that heat go?

It did not vanish — you settled that in lesson 6, where energy never disappears. It went somewhere and did something. And the answer only makes sense now that you know matter is made of particles, which is why this lesson had to wait until the one before it was finished.

By the end you'll be able to:

• name all the changes of state and say what each does to the particles
• explain why the temperature stops during melting and boiling
• use melting point and boiling point to identify a substance and test it for purity
• tell evaporation from boiling — two words most people use as if they meant the same thing