Question

Which of the following is not the property of a pure substance?
(A) Homogeneous
(B) Definite composition
(C) Constituents can be separated by physical means
(D) Constituents have same chemical nature

Answer 1

Hint: A pure substance is one which has all its particles of the same type. The constituents of a pure substance have the same chemical nature. The composition of such substances is fixed and does not change with time.

Complete step by step solution:
-A chemical substance is nothing but a form of matte. The only difference is that it has a fixed chemical composition and this gives it certain characteristic properties which are possessed only by such substances only.
-These substances are called pure substances and are different from the mixtures because the mixtures do not have a fixed composition or any certain characteristics. The attributes of a mixture vary with time.
-A good example of a pure substance is water. It has a fixed composition of hydrogen and oxygen. Not only that, but the ratio of the mixture of both the gases to form water is also fixed and is 2:1. 2 moles of hydrogen combine with 1 mole of oxygen to give water molecules. This composition is fixed both with respect to time and place.
-Also, the physical and chemical properties of water are always the same like its shape, bond length, type of bond, colour, odour, boiling point, melting point and all of its characteristic reactions.
-Pure substances can be in the form of elements or compounds. Mixtures, on the other hand, are subdivided into homogeneous and heterogeneous mixtures.
-Some properties of pure substances are
1. They are homogeneous. It means that they are made up of only 1 kind of particle.
2. They have a constant composition throughout their volume.
3. They have fixed melting point and boiling point.
4. The constituents of a pure substance cannot be separated by physical means. They can be separated but by chemical means only.

Therefore the correct option is C. Constituents can be separated by physical means.

Note: Do not confuse a pure substance with a homogeneous mixture. Though the composition of a homogeneous mixture is constant yet it is not a pure substance as it is not made up of single kinds of particles. It is made up of 2 or more types of particles and has different chemical properties.