Changing State
Heat something and its particles get more energy. Cool it and they lose it. Every change of state is that one sentence, playing out four different ways.
Heat gives particles energy
Heating matter makes its particles move more. Cooling it makes them move less.
Give the particles in a solid enough energy and they eventually have too much of it to stay in the pattern, so they break out and start sliding over one another. That is melting, and there is nothing more to it than that. Take energy away from a gas instead and its particles slow down until they can no longer escape each other's pull, and they clump together into a liquid. That is condensing.
The four changes below are that one idea, pointed four ways.
The four changes
The four names
Melting is solid to liquid. Heat in.
Freezing is liquid to solid. Heat out.
Evaporating is liquid to gas. Heat in.
Condensing is gas to liquid. Heat out.
The two temperatures for water
Water changes state at two temperatures, and a child who knows both by heart has half of most questions on this topic already done, because the numbers turn up again and again in the wording.
Both numbers work in reverse as well, which surprises people who have learned them only in the heating direction. Water freezes at the same 0 degrees at which ice melts, and steam condenses at the same 100 degrees at which water boils. Melting point and freezing point are not two facts to learn. They are one number, read from either side.
Evaporating and boiling are not the same
Both turn a liquid into a gas, both involve heat, and in ordinary conversation nobody minds which word you use. Examiners mind a great deal, and the distinction is easy once you see where each one happens.
- Boiling happens at one exact temperature, 100 degrees for water, and it happens throughout the liquid. That is what the bubbles are.
- Evaporating happens at any temperature and only at the surface. It is why a puddle dries up on a cool day without ever boiling.
What makes washing dry faster
Anything that makes it easier for a particle at the surface to break away and leave: warmth, wind, more surface spread out to the air, and air that is not already full of water vapour. A windy day dries washing faster than a still warm one for exactly that reason, and the same four factors are what an exam question is fishing for.
Nothing new is made
A change of state is a physical change, which is a phrase children repeat long before it means anything to them. Here is what it actually promises: the particles are the same before and after, so you can always go back.
Melt an ice cube, freeze the puddle, and you have ice again with nothing gained and nothing lost. Now burn a sheet of paper. The ash and smoke are new substances made out of the old one, and no amount of cooling will hand the paper back. That is the difference between a physical change and a chemical one, and it is worth naming while the ice cube is still in view.
Common mistakes
1. Saying evaporating when you mean boiling
Using the two words as if they were the same, which they nearly are in everyday speech. Boiling happens at one fixed temperature and throughout the liquid, which is what the bubbles rising from the bottom of a pan actually are. Evaporating happens at any temperature and only at the surface. A puddle drying on a cold morning has not boiled.
2. Thinking the water on a cold glass leaked through
Assuming the water on the outside seeped through, which is a sensible guess if you have never been told that the air is carrying water around all day. What happened is that water vapour already in the room touched the cold glass, lost energy, and condensed. Mark the level of the drink before and after if the argument continues. It does not go down.
3. Thinking melting and dissolving are the same
Saying the sugar melted in the tea. Melting needs heat and turns a solid into a liquid version of itself, so melted sugar would still be sugar. Dissolving needs a second substance to disappear into, and what you end up with is a mixture of sugar and tea. The sugar is still there, which is why the tea tastes sweet.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
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For parents
The cold glass is the one demonstration worth doing at home. Fill a glass with iced water, leave it on the table for ten minutes, then ask your child where the drops on the outside came from. Most say the glass leaked, and that is a good enough answer to be worth arguing with rather than correcting. Mark the drink level before and after. When it has not moved, the word condensation lands and stays.
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