(a) In a solution the relative proportion of the solute and solvent can be varied. At a given temperature, when no more solute can be dissolved in a solution, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility. For example, In order to obtain a saturated solution of sodium chloride and water at 20°C, a maximum of 36 grams of sodium chloride can be dissolved in 100 grams of water.

(b) A pure substance is made up of only one kind of particles. All the constituent particles of a pure substance are the same in their chemical nature. These particles may be atoms or molecules. For example, sulphur is made up of only one kind of particles called sulphur atoms, therefore, sulphur is a pure substance. Similarly, water is made up of only one kind of particles called water molecules. All the elements and compounds are pure substances because they contain only one kind of particles. A pure substance cannot be separated into other kinds of matter by any physical process. A pure substance has a fixed composition as well as a fixed melting point and boiling point.

(c) A colloid is a kind of solution in which the size of solute particles is intermediate between those in true solutions and suspensions. Due to the relatively smaller size of particles, as compared to that of a suspension, the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture, for example, milk. Because of the small size of colloidal particles, we cannot see them with naked eyes. But, these particles can easily scatter a beam of visible light.

(d) A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension can be seen with naked eye. The particles of a suspension are able to scatter a beam of light passing through them and make its path visible. When left undisturbed, the solute particles are able to settle down. This shows that a suspension is unstable. They can be separated from the mixture by the process of filtration.