Solubility: how much can dissolve?

The salt test

Fill a glass tumbler half with water and add one spoon of salt, stirring until it disappears. Keep adding salt, a spoonful at a time, and note each time whether it dissolves (Activity 9.1). For a while every spoonful vanishes. Then a stage comes when stirring no longer helps and some salt lies at the bottom of the glass. The water has reached its limit for salt.

That limit has a name. The largest amount of a solute that can dissolve in a fixed amount of solvent at a given temperature is its solubility. The book’s summary takes 100 mL as that fixed amount, which makes it easy to compare different solutes.

Unsaturated and saturated

While the water can still take in more salt, the solution is unsaturated. Once no more will dissolve and the extra begins to settle, the solution is saturated at that temperature. The words “at that temperature” matter, because the limit moves when the temperature changes.

Sugar in tea shows the same thing. If you add too much, some sugar stays at the bottom of the cup: the tea is saturated with sugar. Warming the tea or adding more tea lets the extra sugar dissolve.

Dilute and concentrated

Concentration tells how much solute is present in a set amount of solution (or solvent). A solution with less solute is dilute; one with more is concentrated. These are relative words: the glass with one spoon of salt is dilute compared with the glass that has two or three.

To compare two solutions fairly, bring them to the same amount of water. Is 2 spoons of salt in 100 mL more concentrated than 4 spoons in 50 mL? Doubling the second gives 8 spoons in 100 mL, four times as much salt in the same water, so 4 spoons in 50 mL is more concentrated.

Heat helps solids dissolve

In Activity 9.2, a teacher stirs baking soda (sodium hydrogen carbonate) into about 50 mL of water at 20 °C until some stays undissolved. Heated to 50 °C, the leftover baking soda dissolves and more can be added. At 70 °C still more dissolves. For most substances, then, solubility increases as the temperature rises, and a solution that was saturated when cool behaves as unsaturated once it is heated.

This is why sugar dissolves so easily in hot milk or tea, and why syrups are made over a flame.

Gases dissolve less when warm

Gases such as oxygen also dissolve in water, only a little, and they spread evenly, so the mixture is uniform. That small amount of dissolved oxygen supports fish, water plants and other aquatic life. Here temperature works the other way: the solubility of gases generally falls as water warms. Cold water holds more oxygen, which is good for life in it, while warm water holds less.

Does the solvent matter?

Sugar and salt dissolve readily in water but hardly at all in oil. Water dissolves so many substances that it is called a good solvent, and traditional Indian medicine has long used it, along with oils, ghee and milk, to prepare medicines. Even so, water cannot dissolve everything: oil and fat stay separate. Learn about the other half of the chapter in density and how to measure mass and volume, or see the Chapter 9 notes and the NCERT solutions.