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How do we judge whether milk, ghee, butter, salt, spices, mineral
water or juice that we buy from the market are pure? Have you ever
noticed the word ‘pure’ written on the packs of these consumables? For
a common person pure means having no adulteration. But, for a scientist
all these things are actually mixtures of different substances and hence
not pure. For example, milk is actually a mixture of water, fat, proteins
etc. When a scientist says that something is pure, it means that all the
constituent particles of that substance are the same in their chemical
nature. A pure substance consists of a single type of particles. In other
words, a substance is a pure single form of matter. As we look around,
we can see that most of the matter around us exist as mixtures of two or
more pure components. For example: sea water, minerals, soil etc. are all
mixtures.
Mixtures are constituted by more than one kind of pure form of
matter. We know that dissolved sodium chloride can be separated from
water by the physical process of evaporation. However, sodium chloride
is itself a pure substance and cannot be separated by physical process
into its chemical constituents. Similarly, sugar is a substance which
contains only one kind of pure matter and its composition is the same
throughout. Soft drink and soil are not single pure substances. Whatever
the source of a pure substance may be, it will always have the same
characteristic properties. Therefore, we can say that a mixture contains
more than one pure substance.
Based on the composition, mixture can be defined as
homogeneous mixture and heterogeneous mixture. A mixture in which
its constituents are distributed uniformly is called homogeneous mixture,
such as salt in water. A mixture in which its constituents are not
distributed uniformly is called heterogeneous mixture, such as sand in
water. Based on the particle size of the component, mixture are further
classified into solution, a suspension and a colloid. We will see these in
the next exercise.