9.2 Saturation & Solubility
Add too much sugar to your tea and some sits undissolved at the bottom. Water clearly can’t dissolve an unlimited amount — so how much can it hold?
Activity 9.1 — Keep adding salt Half-fill a glass with water and add salt one spoon at a time, stirring each time. At first every spoon dissolves. But after several spoons, a stage comes when the salt stops dissolving and settles at the bottom. The water has hit its limit — it can’t dissolve any more salt at that temperature.
Saturated vs Unsaturated - An unsaturated solution can still dissolve more solute at that temperature. - A saturated solution can dissolve no more — extra solute just settles at the bottom (at that temperature).
The maximum amount of solute that dissolves in a fixed quantity of solvent (at a given temperature) is its solubility.
Concentration: Dilute vs Concentrated The amount of solute in a fixed quantity of solution is its concentration. A solution with little solute is dilute; one with lots is concentrated. (These are relative terms — 1 spoon of salt is dilute compared to 3 spoons.) A tricky check: which is more concentrated, 2 spoons in 100 mL or 4 spoons in 50 mL? The second — twice the salt in half the water!
Heat helps solids dissolve
Activity 9.2 — Baking soda, hotter and hotter Make a saturated baking-soda solution at 20 °C (some solid left undissolved). Now heat it to 50 °C, stirring — the leftover solid dissolves! Add more until saturated again, then heat to 70 °C — and again it dissolves. Warmer water dissolves more baking soda.
Temperature and Solid Solubility For most solids, solubility increases as temperature rises. So a solution that is saturated at one temperature becomes unsaturated if you heat it (it can now hold more).
Try it: pick a temperature and add spoons of solute until the solution is saturated.
Be a Scientist: Asima Chatterjee Asima Chatterjee used solvents and solutions to extract and isolate important compounds from medicinal plants, developing anti-epileptic and anti-malarial drugs. She was the second Indian woman to earn a Doctorate of Science and the first woman to receive the Shanti Swarup Bhatnagar award. (In Ayurveda and Siddha, water — and sometimes oil, ghee or milk — has long been the solvent for medicines.)