ICSE Class 8 Answered

Gas: A substance that remains in the gaseous state under normal conditions of room temperature and atmospheric pressure is called gas. It is a substance which exists in the gaseous state at a temperature equal to or more than the boiling point of its liquid state. Examples: Oxygen, hydrogen, nitrogen, etc. 

Vapour: A substance that is solid ir liquid under normal conditions of room temperature and pressure but changes into gaseous state on heating is called vapour. It is a substance which exists in the gaseous state at a temperature which is lower than that of boiling point of its liquid state. Examples: Water vapour, iodine vapour, etc.