Energy and Heat · Grade 8

Why does boiling water stay at one hundred degrees while the hob is still on?

Because the incoming energy is not speeding the particles up — it is pulling them apart. At the boil, the bonds holding the molecules together in the liquid are being broken, and every joule goes into that job. Temperature measures average speed, and while the speed is not rising, neither is the reading, even after an hour.

Learning objectives

What the curve shows

A heating curve is a graph of temperature against time while a substance is heated at a steady rate. For ice taken from a freezer it has five sections: three that rise and two that are perfectly flat.

The rising sections are a substance warming in a single state: ice from minus twenty degrees up to zero, liquid water from zero to one hundred, and steam above one hundred.

The flat sections are the changes of state: melting at zero and boiling at one hundred. In both, the heating continues but the thermometer does not move at all.

Where the energy goes in the flat section

In a solid the particles are bonded in an ordered arrangement and vibrate in place. Turning a solid into a liquid means breaking some of those bonds, and breaking them is work that costs energy.

The energy arriving during the flat section does exactly that job. It does not increase the speed of the particles, it moves them apart — so the temperature, which is a measure of speed, does not change.

That energy is called latent heat. It has not been lost: when the steam condenses back to water, precisely the same amount is released again. This is what makes a steam burn far worse than a burn from boiling water.

Evaporation cools

In any liquid some particles are faster than average and some slower. The fastest are the ones that manage to escape from the surface and evaporate, and they do this far below the boiling point.

When the fastest leave, the average speed of those remaining falls. The temperature of the liquid left behind drops, even though nothing cooled it.

This is why sweating works: the evaporating sweat takes energy away from the skin. It is why a damp towel on the forehead cools, and why wind on wet skin feels much colder than the same wind on dry skin.

Worked examples

  1. A heating curve for water has a flat section at 0 degrees. What is happening there?

    1. The temperature is not rising although heating continues
    2. Zero degrees is the melting point of water
    3. The energy is breaking the bonds in the ice structure

    Answer: The ice is melting; the energy goes into latent heat, not into temperature

  2. Why is a steam burn worse than a burn from boiling water at the same temperature?

    1. The steam condenses on the skin
    2. Condensing releases the latent heat that was put in at the boil
    3. Only then does the hot water give up further heat

    Answer: Because steam delivers the latent heat as well as the heat of the water

  3. Why does wind on wet skin cool more than wind on dry skin?

    1. The wind clears away the vapour that had built up above the skin
    2. Evaporation can then continue at a faster rate
    3. Every particle that evaporates carries energy away

    Answer: Because it speeds up evaporation, and evaporation is what removes the energy

Common mistakes

Thinking a bigger flame will push water past one hundred degrees
A bigger flame delivers energy faster, so the water simply boils away faster. The temperature stays at one hundred until all the liquid has become vapour.
Assuming no energy enters during the flat section
Exactly as much energy enters as before. What differs is what it does: separating particles instead of accelerating them, so the thermometer does not respond.
Believing evaporation only happens at the boil
A puddle dries at room temperature. Fast particles escape the surface at any temperature; boiling is when the change happens throughout the liquid rather than only at the surface.

What to remember

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