Question

Which of the following is an impure substance?
a.) salt
b.) sugar
c.) milk
d.) sulphuric acid

Answer 1

Hint: Let us recall what we learnt about matter in our previous classes. Anything around us which has some mass and occupies some space is known as matter. It is made up of very small particles called atoms and molecules.

Complete step by step solution:

Now, matter can be classified into two fundamental types:
Pure substances
Pure substances are made up of only one kind of particle and have a fixed or constant structure. They also have definite chemical and physical properties. Pure substances are further classified into two types, Elements and Compounds.

Example:
Salt: All the particles present in the salt are of the same kind. They even look and taste the same. This indicates that salt is a pure substance.
Sugar: Same as salt, sugar also doesn’t contain any other particle in it. So, it is also a pure substance.

Impure substances or Mixtures
Impure substances are made up of different kinds of elements combined together or more pure substances mixed together in any proportion. Impure substances are also called mixtures. Impure substances or Mixtures are further classified into two types, homogeneous or heterogeneous.

Example:
Sand and Water
Petroleum
So, the correct answer is (c) milk as it is a heterogeneous mixture of fatty acids, proteins, calcium etc.

Note: We should not get confused between compounds and mixtures as compounds are pure substances which contain atoms of different elements chemically combined together in a fixed ratio while mixtures are impure substances, made up of two or more physically mixed substances.