Getting back what dissolved
Show on a big screenA filter cannot catch what has dissolved. So how do you get a dissolved solid back?
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Boil the water away. Or cool a hot solution that holds all the solute it can. Such a solution is saturated.
The rule
For most solids, a hot saturated solution holds less when it cools. The extra solute comes out as crystals.
Solubility is how much solute a liquid can hold at one temperature. The solubility of most solids rises with temperature. The solubility of a gas in a liquid usually falls.
Take two saturated solutions, each made with 100 g of water. Cool both from 80 °C to 40 °C. Potassium nitrate gives about 106 g of crystals. Common salt gives about 2 g. The numbers are rounded.
| Salt | Holds at 80 °C | Holds at 40 °C | Crystals |
|---|---|---|---|
| Potassium nitrate | 170 g | 64 g | 170 − 64 = 106 g |
| Common salt | 38 g | 36 g | 38 − 36 = 2 g |
Three ways to use a solution
| Method | What you do | What you get |
|---|---|---|
| Evaporation | Let the water turn to vapour | The dissolved solid. The water is lost. |
| Crystallization | Cool a hot saturated solution | Crystals of the solid, and the water stays |
| Distillation | Boil, then cool the vapour | The liquid, collected in a flask |
Distillation separates two liquids that mix. Their boiling points should differ by about 25 °C or more. Methanol boils at about 65 °C and water at 100 °C. Methanol vapour leaves first. The condenser is a tube that cools the vapour back to liquid.
- A strip hangs above the solvent. Ink sits on the start line.
- The solvent rises up the strip.
- Dyes climb at different speeds. Bands appear.
Paper chromatography separates dyes and pigments. Each one climbs the wet paper at its own speed. The solvent level must start below the ink spot. Ink under the solvent would dissolve into the jar.
Try the lab to finish this lesson.