Vapor (or vapour) and gases are often used synonymously. Informally there isn’t much difference between them with gas often being used as a term that encompasses vapours, and vapours being considered a particular type of gas. However, there is a technical difference that arises from the physics of matter with gases being one of the permanent states, or phases, of a substance and vapours being a transient condition.
State or Phases of Matter
Gases are one of the four main states, or phases, of matter along with solids and liquids and plasmas. Vapours are not a state of matter but rather are substances in transition, typically containing both gas a liquid particles, as a substance changes from its liquid state to a gas state. Vapours are not permanent states of matter, while gases are permanent states of matter.
Changes between the different states of matter occur because of changes in temperature or pressure or both. Heat is normally important because it reduces the molecular forces that hold the atoms in rigid structure and allows the particles to move with ever greater freedom from their rigid arrangement in solids, to the freely flowing particles of liquids, to the randomly moving particles in gases and finally the plasma particles found in everyday plasma phenomenon such as the flame of a candle or a fire.
Change of State or Phase of Matter
A pressure-temperature phase diagram can usefully show how a substance changes state according to the temperature and pressure. A typical phase diagram has pressure on the y-axis and temperature on the x-axis. The diagram shows the ranges of values for which substances are solid, liquid or gas. It also includes curves showing the combinations of pressures and temperatures at which two phases can exist in equilibrium, that is, where a transition occurs.
In particular the pressure-temperature phase diagram will include a vaporization (or condensation) curve showing the temperature/pressure range for which substances can be vapours, and hence in transition between gaseous and liquid states. Vapours are not permanent states of matter but rather substances transitioning between these other permanent states. Vapours are part liquid and part gas.
Vapours, Boiling Points and the Change from Liquid to Gas
Vapours are interesting because they are phenomenon that occur on the boundary between states of matter. The vapour phase occurs when a substance is present as a gas, under the conditions when the substance would normally be a solid or a liquid (e.g. below the boiling point of the substance).
Vapours actually help explain the change of state that occurs, for example when water is boiled on a stove and its temperature is gradually increased. As the temperature is increased the vapour pressure within the liquid also increases. Eventually the vapour pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapour to form inside the bulk of the liquid.
Thus the boiling point of a substance can be understood in terms of the internal vapour pressure of a liquid and the surrounding ambient pressure. When vapour pressure and ambient pressure are equal a change of state occurs. At sea level the boiling point is called atmospheric boiling point. Boiling points generally increase as the ambient pressure increases and so vapour pressure would also have to increase within a liquid at higher pressures.
Gases and vapours differ slightly in their physical properties. As a substance in transition a vapour shares some but not all of the properties of a gas. Gases are easy to compress, they expand to fill their containers, and they occupy far more space than the liquids or solids from which they form because their particles are unstructured moving randomly and freely.
These properties mean that gases are generally invisible unless viewed under certain conditions while vapours are typically visible, for example water vapour is visible as a cloud. Also when viewed under a microscope gas particles do not have a definite shape and will be a collection of atoms, ions, electrons and molecules (see Difference between vapor and Gas) freely moving in random motion. In contrast vapour particles may have a definite shape when observed under a microscope.
Colloquially there is little distinction between gases and vapours although in technical terms, a gas is a permanent state of matter, while a vapour is not.