Final answer:
The gas phase of a substance that normally exists as a liquid or solid under ordinary conditions is called a vapor. This state occurs when intermolecular forces are overwhelmed by the energy of the particles, allowing them to move freely and occupy the volume of their container.
Step-by-step explanation:
When a substance is commonly found as a liquid or solid under ordinary conditions, and it transitions to the gas phase, it is typically referred to as a vapor. Matter can exist in various states, including solid, liquid, and gas. The phase a substance exhibits at a given temperature and pressure is determined by a balance between the energy of its particles and the intermolecular forces acting between them. The gas phase occurs when these forces are weak and particles have enough energy to move apart from each other, effectively filling the space of their container and having neither a definite volume nor shape.
For example, water usually exists as a liquid at room temperature. When heated to boiling or under certain conditions, water transitions from liquid to its gas phase, which in this context we would refer to as water vapor. Hence, gases like steam arising from water are called vapors when they are in conditions where the liquid or solid phase is more common.