Solubility and saturation

2.1 How much solute can dissolve

Take a clean glass tumbler, fill it half with water and stir in one spoon of salt until it dissolves. Add another spoonful, and another, stirring each time. At some point the added salt stops dissolving and settles at the bottom of the glass. The water has reached the limit of what it can dissolve at that temperature.

2.2 Saturated and unsaturated solutions

An unsaturated solution can take more solute at that temperature. A solution in which no more solute dissolves and the added solute settles at the bottom is called a saturated solution at that temperature. The table below records what the activity would show.

The temperature matters. Heating a saturated solution often lets it dissolve more solute, which means a solution that was saturated at the lower temperature is unsaturated after heating. On cooling, the extra solute may crystallise out again.

2.3 Concentration and solubility

Concentration tells how much solute a fixed amount of solution or solvent contains. A solution with less solute is dilute, and one with more solute is concentrated. Dilute and concentrated are relative terms: a solution made with one spoon of salt is dilute compared with one made with three spoons.

Concentration is not the same as solubility. Solubility measures the most solute that a fixed quantity of solvent can hold at a given temperature. A solution can be concentrated and still unsaturated if it holds less than the maximum. To compare concentrations fairly, the amount of solvent must be the same: a solution of two spoons in 100 mL is more dilute than one of four spoons in 50 mL.

2.4 Temperature and the solubility of solids

The baking soda activity makes the effect of temperature visible. Water at 20 °C dissolves a certain amount of baking soda, and some solid is left undissolved. When the beaker is heated to 50 °C while stirring, the leftover solid dissolves. Heating further to 70 °C lets even more baking soda dissolve.

For most solids, solubility increases as the temperature rises. This is why a saturated solution made at room temperature can often take more solute once it is warmed.

2.5 Temperature and the solubility of gases

Gases behave differently. The solubility of most gases in liquids decreases as the temperature rises. Only a little oxygen dissolves in water, yet that little keeps aquatic plants, fish and other organisms alive. Cold water holds more dissolved oxygen than warm water, so a rise in water temperature reduces the oxygen available to aquatic life.

2.6 Water as a solvent in Indian medicine

In Ayurveda, Siddha and other Indian systems of medicine, water served as the solvent for preparing formulations. Medicinal plants were also extracted with hydro-alcoholic mixtures, and oils, ghee and milk were used as solvents to carry the active substances of herbs and make their benefits easier to achieve.

Asima Chatterjee used solvents and solutions extensively in her work on anti-epileptic and anti-malarial drugs. She extracted and isolated compounds from medicinal plants and earned a Doctorate of Science, the second Indian woman after Janaki Ammal to do so. She became the first woman to win the Shanti Swarup Bhatnagar Award in chemical science, and she also received the Padma Bhushan.

2.7 Comparing concentrations

The activity's comparison question can be answered with numbers. Two spoons in 100 mL works out to 0.02 spoon per mL, while four spoons in 50 mL gives 0.08 spoon per mL. The second solution is four times as concentrated, because it has more salt in less water. Comparing concentrations without fixing the amount of solvent can be misleading.

A useful habit is to state both quantities: the amount of solute and the amount of solvent or solution. A dilute solution of a strong-tasting substance and a concentrated solution of a mild one can look very different even when their concentrations are similar.

2.8 Crystallisation

When a saturated solution cools, some of the dissolved solid may separate as crystals. This is called crystallisation, and it is a useful way to obtain a pure solid from a solution. The solid that comes out has a regular shape because its particles arrange themselves in an ordered pattern.

Salt is obtained from seawater by evaporation and crystallisation. In Ningel village in Manipur's Thoubal district, salt water is drawn from traditional wells and boiled in large metal pans over firewood kilns. As the water evaporates, salt crystals form, and the families shape them into round salt cakes wrapped in banana leaves and a traditional cloth called the phanek. The practice is part of the region's living heritage.