Where Does the Sugar Go?

Kofi stirs a spoonful of sugar into a glass of water. For a moment he can see the white grains swirling around. Then they are gone, and the water looks as clear as before. But when he takes a sip, it tastes sweet. So where did the sugar go?

The sugar is still there

The sweet taste tells Kofi the sugar has not vanished. There is a second way to check. Put the glass of water and the spoonful of sugar together on a kitchen scale, and write down the number. Then stir the sugar in until it is gone, and weigh again. The scale shows the same number. The sugar is still in the glass. You just can't see it any more.

Three steps, one after another: a spoon of white sugar held over a glass of water, the sugar swirling as it is stirred, and the glass looking clear again with the spoon resting in it.

When something mixes into a liquid like this and seems to disappear, we say it dissolves. The sugar dissolved in the water. The mixture you get, sweet and see-through, is called a solution. In the Pure or Mixed? lesson we met mixtures. A solution is a special kind of mixture: one where the parts are mixed so finely that it looks like one thing.

What the particles are doing

In the Tiny Moving Particles lesson we saw that everything is made of tiny particles that are always moving. A grain of sugar is a huge crowd of sugar particles stuck together. When the grain is in water, the moving water particles bump into it and pull sugar particles away from the grain, one by one. The sugar particles spread out between the water particles, all through the glass.

Each sugar particle is far too small to see. That is why the water looks clear, and why it tastes sweet everywhere, at the top and at the bottom.

Two boxes of tiny particles. In the first, a tidy block of amber sugar particles sits among blue water particles. In the second, the amber sugar particles are spread out one by one between the blue water particles.

Warm water dissolves sugar faster than cold water, because its particles move faster and bump harder. Stirring helps too. But a glass of water can only dissolve so much: keep adding sugar, and at some point the extra sugar just sits at the bottom.

Not everything dissolves

Salt dissolves in water, like sugar. We say salt and sugar are soluble in water. But sand does not. Stir sand into water and it swirls around, then sinks to the bottom, still sand. Pepper floats or sinks, and cooking oil floats on top in a layer. These are insoluble in water: they don't dissolve.

Three glasses of water after stirring. The glass with salt looks clear. The glass with sand has a layer of sand at the bottom. The glass with cooking oil has a yellow layer floating on top.

Try it at home

Fill four glasses with the same amount of warm tap water.
Add one teaspoon of something different to each: sugar, salt, sand (or soil) and flour.
Stir each one for a minute, then wait a minute and look.

You should see that the sugar and the salt disappear and the water stays clear: they are soluble. The sand sinks and the flour makes the water cloudy and slowly settles: they don't dissolve.

Careful

Don't drink from the glasses, and only ever taste things that are food.

Words to know

dissolve - to mix into a liquid so finely that you can't see it any more, like sugar in water
solution - the clear mixture you get when something dissolves in a liquid
soluble - able to dissolve in a liquid, like salt and sugar in water
insoluble - not able to dissolve in a liquid, like sand in water

Big idea

When sugar dissolves in water it doesn't disappear. Its particles spread out between the water particles, too small to see. Some substances dissolve in water and some don't.