Here's why:

* Vapor refers to the gaseous state of a substance that is normally a liquid or solid at room temperature.

* Gas is one of the fundamental states of matter, characterized by particles that are far apart and move freely.

Think of water:

* Liquid water is the familiar form we drink.

* Water vapor is the gaseous form of water, invisible in the air.

* Ice is the solid form of water.

So, while vapor is a gaseous form, it's often used to emphasize that the substance is normally a liquid or solid.