The Sugar That Refuses
Saturated solutions and cooling crystals
What this lesson covers
Why it matters
Stir sugar into iced tea and a stubborn heap gathers at the bottom, no matter how hard you stir. Pour the same sugar into steaming hot tea and spoon after spoon vanishes. Same sugar, same tea — the only difference is temperature.
Predict first
You keep stirring the iced tea, but the heap of sugar refuses to shrink. What is true of the tea just above the heap?
A saturated solution cannot dissolve any more solute at that temperature — however long or vigorously you stir. Warm it, though, and it finds new room: the solubility of most solids rises with temperature.
- It is saturated — it already holds all the sugar it can at that temperature — correct
- Stirring longer and harder would eventually dissolve the heap
- That leftover sugar must be a different, insoluble kind
What you do
Grow crystals the way a chemist does: put the five steps of the cooling-crystal story in order.
Check yourself
A saturated solution is one that…
Saturation is about the limit at a temperature, not the amount. Even a weak solution of a sparingly soluble salt like calcium hydroxide can be saturated.
Potassium nitrate dissolves 31.6 g per 100 g of water at 20°C, but 108 g at 60°C. What does this show?
Most solids, like KNO₃ and NaNO₃, dissolve far more in hot water. Gases do the opposite — their solubility always decreases with rising temperature, and a few solids like calcium sulphate are exceptions too.
A hot, saturated solution of nitre cools to room temperature. Why do crystals appear?
The cooler water can hold less solute, so the extra comes out of hiding as crystals. The liquid left behind is still saturated — but at the new, lower temperature.
- cannot dissolve any more of the solute at a given temperature — correct
- contains a very large amount of solute
- has equal masses of solute and solvent
- The solubility of most solids rises with temperature — correct
- Heating destroys part of the salt
- Every substance dissolves more on heating
- Solubility falls on cooling, so the excess solute separates out as crystals — correct
- The water is used up in building the crystals
- Cold turns the whole solution solid