Where did the sugar go?

The idea

When sugar dissolves in water it breaks up into its constituent particles, which occupy the spaces between water particles (interparticle spaces), so the solution's volume is less than the sum of the separate volumes.

Mark the level

Water is in a measuring cylinder. We mark its level. We add sugar and the level rises. Now we stir until all the sugar dissolves.

What will the level do as the sugar dissolves?

• Go down a little
• Go up even more
• Stay exactly where it is

Sugar, then sand

Add sugar and stir. Watch the level and the view inside the water. Then do the same with sand.

Interparticle spaces

When sugar dissolves, it breaks up into its constituent particles. They occupy the spaces between water particles, called interparticle spaces. So the volume of the solution is less than the sum of the volumes of the water and the sugar.

This shows that there is some space between water particles.

• What we added | The level
• Sugar (it dissolves) | Rises when added, then goes down a little as the sugar dissolves
• Sand (it does not dissolve) | Rises when added and stays up: the sand settles and occupies space

Notes

Dissolving sugar breaks up into constituent particles that occupy the spaces between water particles (interparticle spaces). So the solution's volume is less than the sum of the separate volumes.

Check yourself

What are interparticle spaces?

Just after adding sugar, the level is at mark B. After all the sugar dissolves, where is the level?

Why is the volume of a sugar solution less than the volumes of the water and the sugar added together?

Sand is added to water and stirred. What happens to the level?

• The air bubbles made when we stir. Bubbles are not interparticle spaces. Those spaces are between particles, far too small to be seen.
• The empty part of the cylinder above the water. That is space above the water. Interparticle spaces are between the particles themselves.
• The spaces between the particles of a substance — correct. Yes. Dissolved sugar particles occupy these spaces in water.
• Above mark B. Dissolving adds nothing new. The sugar particles fill spaces between water particles, so the level falls a little.
• A little below mark B — correct. Yes. The sugar particles moved into the spaces between water particles.
• Exactly at mark B. The sugar particles move into the spaces between water particles, so the level goes down a little.
• Some of the water is destroyed by the sugar. No water is destroyed. The sugar particles just fit into the spaces between water particles.
• Sugar particles occupy the spaces between water particles — correct. Yes. They do not need new room of their own.
• Stirring squeezes the water. Stirring only helps the sugar dissolve. The level falls because sugar particles fill the spaces between water particles.
• It does not change at all. The sand settles down and occupies some space, so the level rises.
• It rises and then goes down a little, as with sugar. Sand does not dissolve. Its particles do not go into the spaces between water particles.
• It rises and stays up, because sand does not dissolve — correct. Yes. The sand settles down and occupies space.