3.3 Working with solutions: concentration

In making a solution, the chemicals involved are dissolved in a liquid which is referred to as the solvent.

The substance that dissolves is called the solute.

The most common solvent is water, but in the laboratory a chemist may choose to work with any one of a number of others. Different substances dissolve more easily or less easily in a particular solvent i.e. they have different solubilities. Here the solubility of a substance is defined as the maximum amount of solute that will dissolve in a given amount of solvent.

For example, when common salt (sodium chloride, the solute) is dissolved in water (the solvent) this gives a solution of sodium chloride. The solubility of a substance varies with temperature and is usually expressed as the maximum amount of a substance that will dissolve in 100 g of water at a particular temperature. However, it is often more usual (and more useful) to express the amount of solute that is dissolved in a specific volume of solution at a particular temperature.

This is known as the concentration of a solution, which is expressed as the amount of solute in 1dm ³.

Thus, if 10.0 g of sodium chloride is dissolved in water to give 1 dm³ of solution, the concentration would be 10.0 g per dm³ and so we would write 10.0 g dm⁻³.

One important point: it is essential to define concentration as the amount of solute in exactly 1 dm ³, that is in 1 dm ³ of the final solution rather than the amount of solute dissolved in 1 dm³ of water. If you did dissolve 10.0 g of sodium chloride in 1 dm³ of water then the final volume would in effect be greater than 1 dm³. Therefore to make up a solution with a particular concentration of a solute, the required amount of solute is firstly dissolved fully in a suitable amount of water (less than 1 dm³) and then sufficient water is added to achieve the correct final volume of 1 dm³ .

Note 1dm³ = 1 litre

This is a label from a bottle of mineral water. It shows the concentration of each of the solutes expressed as mg/l.

Figure 8 Label from a bottle of mineral water

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This photograph shows a label from a bottle of mineral water, with a rundown of the contents' chemical analysis. The heading reads 'typical analysis', then the contents are measured in mg/1. The contents are: calcium, 55; magnesium, 19; potassium, 1; sodium, 24; bicarbonate, 248; chloride, 37; sulphate, 13; nitrate,

Figure 8 Label from a bottle of mineral water