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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 0OC
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
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 0.5OC).
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
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.
Application of the Effect of
Dissolved Impurities in a Pure Substance
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.
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