How to explain phase changes and latent heat to children

Children meet melting, freezing, evaporation and condensation every day, in puddles, kettles, ice lollies and misted windows. Explaining why they happen is harder. The classic stumbling block is that heating something does not always make it hotter: ice at 0 degrees Celsius that is being warmed stays at 0 degrees until it has all melted. This guide gives a clear way to explain phase changes using the particle model, shows a worked example of how much energy it takes to turn a small amount of ice into steam, and lists the mix-ups that catch both children and adults.

Start with the particle model

All matter is made of tiny particles, and the three common states of matter differ in how the particles are arranged and how they move.

• In a solid the particles are packed close together and held in place. They vibrate but do not change places, which is why a solid keeps its shape.
• In a liquid the particles are still close, but they can slide past one another. A liquid takes the shape of its container but has a fixed volume.
• In a gas the particles are far apart and move freely and quickly. A gas fills whatever space it has.

Draw this before you do anything else. A particle picture is the key to every later explanation, and children can act it out: stand close together and wobble for a solid, shuffle past each other for a liquid, and spread across the room for a gas. Make clear that the diagrams are simplified, and that particles are not to scale or really drawn as little balls.

The six changes

Give each change its name and its direction, and say whether energy goes in or out.

The pattern is easy to remember: changes toward a more spread-out state take in energy, and changes toward a more compact state give it out. Sublimation and deposition are less familiar, and frost forming on a cold window is a good example of deposition, where vapour becomes ice without passing through a liquid.

Evaporation is not boiling

Children often treat these as the same, so separate them early. Evaporation happens at the surface of a liquid, at any temperature. The particles move at different speeds, and now and then a fast one near the surface escapes. That is why a puddle dries on a cool day. Boiling happens through the whole liquid, at a particular temperature, when bubbles of vapour form inside. For water at standard pressure, the NIST Chemistry WebBook lists the normal boiling point as 99.97 degrees Celsius, which we usually round to 100.

Pressure matters. Where the surrounding pressure is lower, as on a mountain, water boils at a lower temperature, and where it is higher, as in a pressure cooker, at a higher one. This is a good place to remind older children that the numbers in a table belong to stated conditions.

What latent heat means

If you add heat to ice at 0 degrees Celsius, it does not get warmer straight away. First it melts. The energy needed to change a substance from one state to another without changing its temperature is called latent heat. The latent heat of fusion is for melting and the latent heat of vaporisation is for boiling, and each is given per kilogram.

For a child, the simplest way to see it is as a flat stretch on a graph. Plot temperature against time as you heat a block of ice steadily. The line rises, then goes flat while the ice melts, rises again as the water warms, goes flat once more while the water boils, and rises again as steam warms. Explaining the flat parts is exactly the point of the heating curve. The energy is going into changing the arrangement of the particles and not into speeding them up.

Worked example: from ice to steam

Here is a calculation you can do in class, with values stated. Take 0.10 kg of ice at 0 degrees Celsius and ask how much energy turns it into steam at 100 degrees. We use three values:

• latent heat of fusion of ice: about 334 kJ per kg (a rounded textbook value, commonly given as 333.55; check your own reference);
• specific heat capacity of liquid water: about 4.19 kJ per kg per degree, a rounded value that agrees with the roughly 4.2 listed by NIST across this range;
• latent heat of vaporisation at about one atmosphere: 2,257 kJ per kg. From the NIST saturation tables, the enthalpy of the vapour at about 100 degrees is 2,675 kJ per kg and of the liquid is 418 kJ per kg, a difference of about 2,257.

The steps:

1. Melt the ice: 0.10 x 334 = 33.4 kJ.
2. Warm the water from 0 to 100 degrees: 0.10 x 4.19 x 100 = 41.9 kJ.
3. Boil it to steam: 0.10 x 2,257 = 225.7 kJ.

The total is 33.4 + 41.9 + 225.7 = 301.0 kJ. Look at the shares. Boiling takes 225.7 kJ, which is about 5.4 times the 41.9 kJ needed to warm the water all the way from 0 to 100 degrees (225.7 / 41.9 = 5.4), and about three quarters of the whole (225.7 / 301.0 = 75 per cent). That is a surprising result for many children, and it explains why a pan can boil for a long time before it is empty.

Notes on honesty. This ignores heat lost to the room, the container, and the energy to warm the steam. The values depend on temperature: NIST shows the energy to vaporise water is larger at 0.01 degrees than at 100. A real experiment will give a different answer, and discussing why is a good lesson in itself.

The triple point and why it matters less than it sounds

You may meet the phrase "triple point" in a table. NIST lists the triple-point pressure of water as about 0.0061 bar, where solid, liquid and gas can exist together at 0.01 degrees Celsius. For children aged 8 to 12 it is enough to know that the states of matter depend on both temperature and pressure, and that everyday conditions are only one place on a larger map.

Heating curves and honest data

When you run the experiment, expect your plateau to be imperfect. A beaker of ice water is not at a perfectly even temperature, and the thermometer may lag. Teach children to read a curve for its shape, to mark where the flat parts are, and to say why a real graph differs from a clean one in a book. This is also an opportunity to link to fair testing: change one variable at a time, and repeat.

Common misconceptions

• "Steam is the white cloud you see." The visible cloud is tiny droplets of liquid water that have condensed. Water vapour itself is invisible.
• "Cold flows into a warm thing." Heat flows from warmer to cooler.
• "Temperature always stays constant during any change." For a pure substance at constant pressure it stays level during a phase change, but real samples are mixtures and are not perfectly uniform.
• "A bigger flame makes water boil hotter." Once boiling, the temperature stays at the boiling point for the pressure. A bigger flame only boils it faster.

Tool that fits

Matter Made Visible (EUR 12, paid once) is a set of 12 illustrated charts: solid, liquid, gas, particle spacing, melting, freezing, evaporation, condensation, sublimation, deposition, heating curves and everyday examples. They use one colour for each state, show whether energy goes in or out in every state-change chart, and the heating curve uses energy values computed from standard reference data for water. You get print-ready PDFs in A4 and US Letter (24 files), 12 PNG images at 3508 x 2480 pixels, 12 editable SVG files and a reference PDF with vocabulary, suggested answers and a source register. Diagrams are simplified and marked not to scale. Digital download only.

Questions people ask

Sources

1. NIST Chemistry WebBook, Thermophysical properties of fluid systems: Water (saturation properties, enthalpy and boiling point)
2. U.S. Geological Survey, Water Science School: Water cycle (water changing between solid, liquid and gas)
3. HiddenWay worked example: our own arithmetic, using NIST values and a rounded value for melting

We checked each source on 17 October 2026. If a fact here is out of date, tell us at hello@hiddenway.info.

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