Why Ice Stays at 0 °C While It Melts: Water's Three States of Matter

Water's three states are the most "everyday" topic in middle school science — and the easiest to think you already understand. Measure the temperature curve of melting ice yourself, and you meet a counter-intuitive fact: heat flows in continuously, yet the temperature stands still. That plateau is the key evidence for identifying a crystalline solid.

This article also gives the molecular-level explanation, plus an interactive simulation where you can drag the temperature slider yourself.

Safety first (read this before you start)

• This experiment involves hot and boiling water — an adult must be present throughout. Don't fill the beaker to the top, and never lean directly over the container while it heats; steam scalds.
• Use an alcohol thermometer (red liquid column). Never use a mercury thermometer — if it breaks, mercury vapor is toxic and very hard to clean up.
• The thermometer is glass: don't use it as a stirring rod and don't knock it against the bottom of the beaker. When measuring, keep the bulb immersed in the substance without touching the container's wall or bottom.
• Protect glassware from sudden temperature swings; after heating, set it on a heatproof mat to cool.
• A few bits of broken porcelain in the water prevent bumping (sudden violent boiling); turn off the heat before walking away.

Materials and equipment

• A supply of crushed ice (the finer the better — more contact area);
• 1 alcohol thermometer (range must cover −10 °C to 110 °C);
• 2 beakers (or heat-resistant glasses) and a larger pot (for a water bath);
• A heat source (hot plate, electric kettle or stove — adult operated);
• A timer (a phone stopwatch is fine), a data table and pencil;
• A glass rod or clean chopstick for stirring, heat-resistant gloves, a heatproof mat.

Step 1: Graph the melting of ice

1. Fill the small beaker with crushed ice and insert the thermometer so the bulb is buried in the middle of the ice (not touching the bottom).
2. Stand the small beaker inside the larger pot of warm water — a water bath. Heating this way is gentle and even, so the whole process is easy to watch.
3. Read and record the temperature every 30 seconds, noting each time whether you see "all ice / ice-water mix / all water."
4. When reading, keep your eye level even with the top of the liquid column — don't look down or up at it.
5. Stir the ice water gently to even out the temperature — but never stir with the thermometer.
6. Keep recording until the ice has fully melted and the temperature has been climbing again for a few minutes.

Plot the data as a temperature–time graph (time on the horizontal axis, temperature on the vertical) and you get a line that rises, runs flat in the middle, then rises again.

Step 2: Measure the boiling point of water

1. Switch to a beaker of plain water, insert the thermometer the same way, start heating, and record every 30 seconds.
2. Watch the bubbles: first small ones cling to the walls, then rising bubbles grow larger on the way up, and finally the water churns violently.
3. Once the water is boiling, keep heating for two or three more minutes — the reading barely moves. Write that temperature down.
4. That steady temperature is the boiling point at your location right now (about 100 °C at standard atmospheric pressure; lower at high altitude).

What you should observe

• The ice warms slowly from below zero up to 0 °C;
• At 0 °C water starts to appear, but the temperature parks at 0 °C even though heat keeps flowing in — the curve shows a flat plateau;
• Only after the last ice melts does the water temperature climb again;
• The water boils vigorously at about 100 °C, and after that the temperature holds steady;
• During boiling, bubbles grow as they rise and burst at the surface.

The science: where the heat goes while the temperature stands still

The key conclusion fits in one sentence: a crystalline solid absorbs heat while melting, yet its temperature stays constant. That constant temperature is the melting point — for ice, 0 °C.

So where does the absorbed heat go? The answer is at the molecular level. Ice is a crystal: its water molecules are locked in a tidy arrangement, able only to vibrate in place (exactly the "solid" picture in the simulation above). To break free of that arrangement and become a liquid whose molecules slide past one another, energy must be spent tearing the ordered structure apart. During the plateau, all the incoming heat is used for "demolition work" rather than for raising the molecules' average kinetic energy — and since temperature reflects average kinetic energy, the temperature doesn't move.

Once the demolition is complete (all the ice has become water), further heat goes back into speeding the molecules up, and the temperature resumes its climb.

Boiling works the same way: at the boiling point, a liquid keeps absorbing heat that goes into letting molecules escape the liquid entirely and become gas, so the temperature also holds constant during boiling. The boiling point shifts with air pressure — lower pressure means a lower boiling point. That is why water boils below 100 °C on a high plateau, and why pressure cookers exist.

Follow this thread and all six changes of state fall into place at once (three absorb heat, three release it):

• Absorb heat: melting (solid → liquid), vaporization (liquid → gas), sublimation (solid → gas);
• Release heat: freezing (liquid → solid), condensation (gas → liquid), deposition (gas → solid).

The trick for remembering the direction: any change that makes molecules freer absorbs heat; the reverse releases it. No need to memorize six names cold.

One more caveat: the word "crystal" matters. Ice, table salt and metals are crystals — they have a fixed melting point, so the curve shows a clear plateau. Wax, glass and rosin are amorphous (non-crystalline): they have no fixed melting point, just keep softening as the temperature keeps rising, and their curve has no flat section. The plateau itself is the evidence that you are looking at a crystal.

Notes for teachers and parents

• Time and grouping: 40 minutes (including waiting); groups of four taking turns reading the thermometer.
• Make clear before hands-on work: an adult handles the burner or hot water.

The step most often skipped. Students drift off during the plateau. Keep them engaged by rotating who calls out the reading every 30 seconds while everyone plots the point. The moment the curve flattens, have the whole class stop and look.

How to know they really got it. Students meet the bar when they can explain why the temperature refuses to rise despite continuous heating — and use that fact to conclude the substance is a crystal.

Take it further

• Run a comparison: measure the melting curve of candle wax the same way and plot it on the same axes as the ice curve. The difference between crystalline and amorphous jumps off the page.
• Watch sublimation: place a small piece of dry ice on a plate (only if it can be obtained safely — gloves always, never sealed in a container, never in the mouth). It goes straight from solid to gas with no liquid stage. A shrinking mothball demonstrates the same thing.
• Find evidence of condensation: a bottle straight from the fridge "sweats" — that is water vapor from the air condensing on the cold surface, not the bottle leaking. Then ask: why, in winter, do the droplets on a window always form on the indoor side?
• Connect to solutions: sprinkle salt onto ice and the mixture drops below 0 °C — the trick behind homemade ice cream. Why salt lowers the melting point is worth puzzling over together with our saturated solutions article.

When a student can explain why constant heating fails to raise the temperature — and knows that the flat section proves the substance is a crystal — they have captured the very core of changes of state.