How does sugar dissolve in water?

1. Drop a sugar cube into a glass of water and stir. The cube gets smaller and smaller until you cannot see it at all. But taste the water: it is sweet. The sugar is still there.
2. Zoom in on one grain. It is a crystal: countless tiny particles called sugar molecules, stacked in a neat pattern and clinging to each other.
3. Water is made of molecules too, and they never stop jiggling. They bump into the crystal, cling to the sugar molecules on its surface and tug them free, one at a time.
4. The freed sugar molecules drift off among the water molecules. Bumped this way and that, they spread until they are evenly mixed. This even mixture is called a solution.
5. Left alone, sugar dissolves slowly. Stirring sweeps away the sweet water around the cube and brings fresh water to it. In warm water the molecules move faster and hit harder. Both make it dissolve sooner.
6. Is the sugar really still there? Leave sugar water in a sunny dish. The water slowly evaporates into the air, and sugar crystals appear again. Dissolving can be undone.

Tap the parts of the glass, then the tiny particles in the magnifying glass.

• Sugar cube. A sugar cube is thousands of tiny sugar crystals pressed together. It shrinks from the outside in, because only its surface touches the water.
• Spoon. Stirring sweeps the sweet water away from the cube and brings fresh water to it. Fresh water can pull sugar away faster.
• Thermometer. Warmer water has faster-moving molecules. They bump the sugar more often and harder, so the cube dissolves sooner.
• Sugar water. Once the sugar has spread out, the mixture is called a solution. It looks clear, because the sugar particles are far too small to see or to scatter light.
• Sugar crystal. Inside the crystal, sugar molecules are stacked in a neat, repeating pattern and cling to their neighbors.
• Sugar molecule. One sugar molecule, pulled free of the crystal. It is still sugar, and still sweet. It is just alone now, among the water molecules.
• Water molecule. Water molecules are much smaller than sugar molecules, and there are huge numbers of them, always moving.
• The crystal's surface. All the action happens here, at the surface. Water molecules cling to the outermost sugar molecules and tug them loose, one by one.

Two glasses, two sugar cubes. The left one is left alone. Stir yours or change its temperature, and race them.

Experiment: You stir one glass and leave the other alone. Which sugar cube disappears first?

1. Press the button to drop a cube into each glass.
2. Stir yours: drag the spoon, or tap it again and again.
3. Start again and try warm water without stirring.

What we found: The stirred cube disappeared first, and warm water beat cold water too. Stirring brings fresh water to the cube. Warm water molecules move faster and pull harder. Either way, more sugar molecules are carried off every second.

Turn on JavaScript to try it yourself.

Thirsty customers want cold, sweet lemonade within 2 minutes. Dissolve all the sugar in time, and keep the drink cold.

When sugar dissolves, it does not disappear. A grain of sugar is a crystal: a huge number of tiny sugar molecules stacked in a neat pattern. Water is made of even smaller molecules that are always moving. They bump into the crystal, cling to the sugar molecules on the outside and pull them away, one by one.

The loose sugar molecules drift off between the water molecules and spread out until they are mixed evenly through the glass. A mixture like this is called a solution. It looks clear because each sugar molecule is far too small to see. But every sip is sweet, and the glass weighs just as much as the water and the sugar did before.

You can speed dissolving up in three ways. Stirring carries the sweet water away from the sugar and brings fresh water to it. Warm water has faster molecules that hit the crystal harder and more often. And small grains have more outside surface for the water to work on than one big lump.

There is a limit, though. Keep adding sugar and at last no more will dissolve. The water is full, or saturated, and extra sugar just sits at the bottom. Warm water can hold more. The huddle of children comparison leaves one thing out: the water molecules do not only pull the sugar away, they keep it surrounded afterward.

Key words

Dissolve

To mix into a liquid so completely that the pieces are too small to see.

Solution

A mixture that is the same all the way through, such as sugar in water.

Solute and solvent

The solute is the substance that dissolves (the sugar). The solvent is the liquid it dissolves in (the water).

Molecule

The smallest piece of a substance that is still that substance.

Diffusion

The way particles spread out by themselves from where there are many to where there are few.

Saturated

Full: holding as much dissolved material as it can.

Check your thinking

1. A sugar cube dissolves in a glass of water. What has happened to the sugar?

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   Its molecules are spread through the water. The sugar is still sugar. Its molecules are just separated and too small to see.

2. Why does stirring make sugar dissolve faster?

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   It brings fresh water to the sugar. The water next to the sugar fills up with sugar. Stirring replaces it with fresh water that can take more.

3. Which dissolves faster in the same water: a sugar cube or the same amount of loose sugar grains?

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   The loose grains: more of the sugar touches the water. Water can only pull sugar from the outside surface. Lots of small grains have much more outside than one lump.

4. You keep adding sugar to a glass of cold water. At last, some stays at the bottom however much you stir. Why?

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   The water is full: it can hold no more. Water can only hold so much dissolved sugar. A full solution is called saturated.

5. Sugar water is left in an open dish for a week and crystals appear. What does that show?

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   Dissolving is a physical change that can be reversed. The sugar molecules were never changed. When the water evaporates they stack back into crystals.

6. Why does sugar dissolve faster in warm water than in cold water?

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   Warm water molecules move faster and hit the sugar harder. Faster molecules bump the crystal more often and with more force, so more sugar molecules come loose every second.