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What is a Pure Substance?

A pure substance, whether element or compound, is a substance that does not contain atoms of other elements, or elements of other compounds, but only those of its kind. For instance, pure hydrogen is composed of only hydrogen atoms, while pure water is composed of atoms of hydrogen and oxygen only.

A pure substance, under certain condition of temperature, pressure and surroundings shows a set of recognizable properties which are unique to it, and which can be reproduced at anytime. Some of these properties are: melting point and density of a solid; boiling point, density and refractive index of a liquid; and vapor pressure of a gas.

These properties can be used to identify, and to determine the degree of purity of a substance. A change in these properties for a particular substance is an indication of the presence of impurities in the substance. For example, the melting point of pure water is 0^OC and the boiling point of its liquid is 100⁰C. If there is a change in these values, then it’s an indication of the presence of impurities in the water.

Please note: 

• The melting point of a solid (e.g. ice) is the same as the freezing point of its liquid.

• Pure solid will melt sharply at it recognizable melting point, and will melt completely over a very narrow temperature range (less than 0.5^OC).

• An impure solid will not melt sharply at it recognizable melting point, and will melt completely over a wide range of temperatures.

• The presence of dissolved impurities (i.e., involatile solute) in a pure liquid will 1. Raise its boiling point; 2. Lower or depress its freezing point; 3. Depress its vapor pressure; 4. Cause it to exert osmotic pressure. These four properties are known as colligative properties.

• The degree of the above colligative properties depends on how much of the impurities are present, and not on the nature of the impurities. Therefore, a mole of sodium chloride would elevate the boiling point of pure water to the same extent as a mole of calcium sulphate, or a mole of glucose or a mole of magnesium oxide, e.t.c. would.

• The mixture of water and common salt freezes at temperature lower than the freezing point of water. This mixture, called freezing mixture can be used in cooling other substances.

• In cooler climates, ices formed on roads are melted quickly by spreading salt on them. Also, seas (containing dissolved salts) will not freeze even when fresh water rivers are frozen during extreme low temperature conditions.