At very high pressures and low temperatures, the gas particles will be closer together and their kinetic energy will be insufficient to completely overcome the attractive forces. The more polar the molecules of a gas are, the greater the attractive forces between them and the more the gas will not be an ideal gas.

Gas is a state of matter that has no fixed shape and no fixed volume. When more gas particles enter a container, there is less space for the particles to spread out, and they become compressed. The particles exert more force on the interior volume of the container. This force is called pressure.

Adding or removing energy from matter causes a physical change as matter moves from one state to another. For example, adding thermal energy (heat) to liquid water causes it to become steam or vapor (a gas). And removing energy from liquid water causes it to become ice (a solid).

Bose–Einstein condensate

Originally discovered by Sir William Crookes in 1879, plasma is created through a process called ionization, which adds energy to a gas so that some of its electrons leave its atoms.

The solid-to-gas change is called sublimation, while the reverse process is called deposition. Sublimation is isothermal, like the other phase changes. There is a measurable energy change during sublimation; this energy change is called the enthalpy of sublimation, represented as ΔH sub.

Hydrogen is a gas at STP and platinum is solid. Hence, Hydrogen gas adsorbed on the surface of platinum is an example of gas in solid solution.

Examples of Liquid to Gas (Vaporization)

• Water to steam – Water is vaporized when it is boiled on the stove to cook some pasta, and much of it forms into a thick steam.
• Water evaporates – Water evaporates from a puddle or a pool during a hot summer’s day.

Deposition is the phase transition in which gas transforms into solid without passing through the liquid phase. Deposition is a thermodynamic process. The reverse of deposition is sublimation and hence sometimes deposition is called desublimation.

We can change a solid into a liquid or gas by changing its temperature. Water is a liquid at room temperature, but becomes a solid (called ice) if it is cooled down. The same water turns into a gas (called water vapor) if it is heated up. The changes only happen when the substance reaches a particular temperature.

Condensation is when a gas becomes a liquid. It happens when a gas, like water vapor, cools down. Photo from: Wikimedia Commons. Evaporation and condensation are two processes through which matter changes from one state to another.

Heat energy is absorbed to vaporize a liquid because molecules which are held together by intermolecular forces in the liquid are separated as the gas is formed. Such a separation requires energy. As with melting, the amount of energy needed to vaporize a substance is proportional to the amount of substance present.

When a gas loses enough thermal energy, the gas changes to a liquid, or condenses. The change of state from a gas to a liquid is called condensation. Overnight, water vapor often condenses on blades of grass and forms dew. A solid can become a gas without turning into a liquid.

We can observe a gas system by looking at a sealed container of water vapor. We can reduce the temperature by cooling the walls of the container; as gas molecules collide with the walls, some of their energy is transferred to the wall and then removed by the cooling system.