When you stir sugar into water and it seems to disappear, it has dissolved — the sugar broke into pieces too tiny to see and spread evenly throughout the water. The result is called a solution. In a solution, the substance that dissolves (like sugar) is called the solute, and the substance it dissolves in (like water) is called the solvent. The solute is still there even though you cannot see it — you can taste the sweetness, proving the sugar did not vanish.
Have students dissolve sugar, salt, and sand in separate cups of water, stir, and compare results. Sugar and salt dissolve (clear solution); sand does not (settles to the bottom). Taste the sugar water to prove the sugar is still present. Try dissolving in cold water versus warm water to see the temperature effect.
You already know that mixtures combine different materials without changing them. A solution is a special kind of mixture — one where the materials are so thoroughly mixed that you cannot tell them apart, even with a magnifying glass. When you stir sugar into water and the sugar vanishes from sight, you have made a solution. The sugar dissolved.
Let's get the vocabulary straight. The substance that dissolves is called the solute. The substance it dissolves in is called the solvent. In sugar water, sugar is the solute and water is the solvent. Water is so good at dissolving things that scientists sometimes call it the "universal solvent," although that is a bit of an exaggeration — there are plenty of things water cannot dissolve, like oil or sand.
When sugar dissolves, the sugar particles break apart into pieces far too tiny for your eyes to see. These tiny pieces spread out evenly through the water. That is why every sip of sugar water tastes equally sweet — the sugar is distributed throughout, not clumped in one spot. The sugar has not been destroyed or turned into water. It is still sugar, just spread out. You can prove this by letting the water evaporate: the sugar crystals will appear again right there on the dish.
Several things affect how fast dissolving happens. Stirring speeds it up because it moves fresh solvent over the solute's surface. Heating the solvent helps too — warm water dissolves sugar faster than cold water because the water particles move faster and pull the sugar apart more quickly. Crushing the solute into smaller pieces also speeds things up because smaller pieces have more surface area touching the solvent.
Not everything dissolves, though. Drop sand into water and stir all day — the sand will still sit on the bottom. Drop oil into water and it floats in blobs instead of mixing in. Whether or not a substance dissolves depends on the properties of both the solute and the solvent. This is useful to know: if you spill oil on your shirt, water alone will not dissolve it, but soap (which can interact with both water and oil) will help pull the oil away.