Most liquids that are encountered in everyday life are not pure substances but solutions in which one substance is dissolved in another. For example, sea water is water with various salts dissolved in it, and perfume is alcohol with flower extracts dissolved in it. The phenomenon known as dissolving is a very important one in chemistry, without which many chemical reactions and processes would be impossible.
Solvents
There are two important parts of a solution. The liquid used to make the solution is called the solvent. The best known solvent of all is water. Because of its unique properties, water dissolves a remarkable number of other substances – gases, liquids and solids. Solutions that use water as a solvent are known as aqueous solutions, and include seawater, fizzy drinks and common laboratory liquids like hydrochloric acid and sodium hydroxide.
There are many other liquids that can be solvents, however. These are called non-aqueous solvents and include methylated spirits, nail-polish remover (mainly propanone) and paint stripper. Many glues, paints and varnishes have non-aqueous solvents which have a strong smell.
Solutes
The substance that is dissolved in a solvent is called the solute The solute is commonly a solid, but it can also be a gas or a liquid. Substances that dissolve in a particular solvent are designated as soluble, while substances that do not dissolve are insoluble, in that particular solvent. For example, salt is soluble in water but insoluble in petrol, while wax is insoluble in water but soluble in petrol.
Even when a substance is soluble in a solvent, only a certain amount will dissolve in a fixed quantity of that solvent. When the point is reached where no more solute will dissolve, the solution is saturated.
Dissolving Gases and Liquids
Examples of dissolved gases are fizzy drinks, swimming pool water and tap water. Swimming pool water and tap water have small quantities of chlorine gas, to help kill germs. Fizzy drinks have carbon dioxide dissolved in them, and are put under pressure to make as much as possible dissolve. When the cap is taken off a carbonated drinks bottle, the pressure is released and bubbles are formed in the drink – which are bubbles of carbons dioxide coming out of solution.
Liquids too, can be dissolved in other liquids. For example, adding detergent to water creates a solution of the detergent. Liquids which dissolve like this are called miscible liquids, liquids which do not mix, like oil and water are called immiscible.
Solubility and Crystallization
The solubility of a solute is the amount that will dissolve in a fixed amount of solvent. The solubilities of most solids in water and other solvents usually increase as the temperature is raised. The graph showing the way solubility changes with temperature is known as a solubility curve.
When a saturated solution cools, crystals will form as the solubility of the solute decreases. Crstals will also form when the solvent evaporates from a saturated solution, as there is not enough solvent to dissolve the remaining solute.
Source:
Colin Johnson, Chemistry for GCSE, Heinemann Educational Books, UK, 1987.
Simon Davies - Simon Davies has lived a varied life so far After completing a BSc in Chemistry at the University of Bristol, UK, he worked as a ...
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