States and changes of state
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What this model shows
One 50 g sample of pure water at normal atmospheric pressure, heated by an ideal 250 W hotplate that loses no heat to the room.
The melting point (0 °C) and boiling point (100 °C) of pure water are built in. How long each stage lasts is calculated from the energy water needs and the 250 W supply. The stopwatch shows this heating time; the animation runs about twelve times faster.
The thermometer’s scale is drawn for this model and read in the magnifier; a reading within 1 °C of the model’s temperature counts. The model waits at each half minute while you read it; in a real experiment you read the thermometer as the stopwatch reaches each half minute.
The white mist over the boiling water is tiny drops of liquid water that formed as the vapour cooled in the air. Water vapour itself is invisible.
The model does not cool the sample, so it shows no freezing or condensing, and it leaves out the slow evaporation before boiling.
Molecules are drawn enlarged, all the same size, and far fewer than real; a very thin layer of water shows no molecules. The drawing speeds them up as the temperature rises, more than real molecules do, so that the change is easy to see; during melting and boiling their speed stays the same. Their paths are illustrations, not a calculation.
Real heating needs a teacher-approved safety procedure. Never touch a hotplate that has been on.
With reduced motion, each press heats to the next reading or change of state. The molecules are drawn still, but real molecules never stop moving.
Select a piece of equipment, on the bench or here, to read about it in the card.