Which of the following statements about a compound is incorrect ?

1. A molecule of a compound has atoms of different elements.
2. A compound cannot be separated into its constituent elements by physical methods of separation.
3. A compound retains the physical properties of its constituent elements.
4. The ratio of atoms of different elements in a compound is fixed.

A compound retains the physical properties of its constituent elements.

Reason — When elements combine to form a compound, the resulting substance has completely different physical and chemical properties from the elements that make it up.
A compound is formed by the chemical combination of two or more different elements. The elements in a compound are chemically bonded, so you need chemical methods (like electrolysis) to separate them and not physical ones like filtration or evaporation. Compounds always have a definite composition.