Question

Explain the statement ‘ a pure substance has characteristic properties ’ with reference to pure water and a solution of salt in water.

Answer 1

Pure substance

• A pure substance is one in which all the molecules are of the same kind.
• A pure substance is either an element or a compound.
• Example: Hydrogen, oxygen, sodium chloride in water.

Characteristics of a pure substance

1. A pure substance has a definite composition. Hence it can be represented by a chemical formula.
2. A pure substance has a specific melting and boiling point and characteristic colour, smell or taste.
3. A pure substance is homogeneous, i.e., its composition remains the same throughout.
4. A pure substance has fixed chemical properties.
5. The formation of a pure substance involves energy. Hence it cannot be separated by physical methods.

• Each pure substance has its characteristic properties such as melting and boiling point.
• The melting point of pure water at which ice changes to water is$0°\mathrm{C}$. Its boiling point at which water boils and changes to steam at sea level is$100°\mathrm{C}$.
• These values change if impurities are present in water. Hence we can use these characteristic properties to test whether a substance is pure or not.
• Water does not boil at $100°\mathrm{C}$ sea level and does not freeze at $0°\mathrm{C}$ is impure.
• A pure substance is homogeneous and has a definite composition. A solution of salt in water is homogeneous and it can be represented by a chemical formula.
• Hence we can use this characteristic property to test whether a substance is pure or not.