There is a limit to how much solute you can dissolve in a solvent. If you keep adding sugar to a glass of water, eventually the sugar stops dissolving and starts piling up at the bottom. At that point, the solution is saturated — it is holding as much solute as it possibly can. Warming the water usually lets you dissolve more, and cooling it makes some solute come back out. The saturation limit depends on both the substance and the temperature.
Have students add sugar to water one teaspoon at a time, stirring after each spoonful, and record when the sugar stops dissolving. Repeat with warm water to show that more sugar dissolves at higher temperatures. Compare different solutes (salt vs. sugar) to show they have different saturation points.
You have learned that dissolving spreads a solute evenly through a solvent. But here is an important question: can you just keep adding more and more solute forever? The answer is no. Every solution has a limit, and that limit is called saturation.
Try this at home with sugar and water. Stir in one teaspoon of sugar — it dissolves. Add another — it dissolves too. Keep going. Eventually, you will add a teaspoon and no matter how long you stir, some sugar just sits at the bottom of the glass. The water has reached its saturation point. It is holding as much sugar as it can possibly hold at that temperature. The solution is now called a saturated solution.
Temperature makes a big difference. If you heat that same glass of saturated sugar water, something interesting happens — the undissolved sugar at the bottom starts dissolving again. Warm water can hold more dissolved sugar than cold water. This is why recipes for sweet drinks often say to dissolve the sugar in hot water first. You can pack in much more sweetness before the solution maxes out.
The opposite works too. If you make a hot saturated solution and then let it cool, the water can no longer hold all that solute. Some of it comes back out as solid crystals. This is actually how rock candy is made — you dissolve a huge amount of sugar in boiling water, hang a stick in it, and let it cool slowly. As the water cools, sugar crystals form on the stick because the cooling water is pushing out the extra sugar it can no longer hold.
Different substances have different saturation points. You can dissolve much more sugar in water than you can salt. And some substances barely dissolve in water at all — chalk, for example, has such a low saturation point that only a tiny trace dissolves. Scientists have measured the saturation limits for thousands of substances, and these measurements help them predict how solutions will behave in everything from cooking to medicine.