# What could cause a gaseous substance to liquify?

May 13, 2017

Temperatures at or below a critical temperature, ${T}_{c}$ for that gas, along with higher pressures.

#### Explanation:

Anytime a substance (solid or gas) is turned into a liquid, the process is called liquefaction. When turning gas into liquid, the distance between gas molecules is large compared to the distance between liquid molecules. So there is a lower density in the gaseous form and a higher density in liquid form. So, when a gas turns into a liquid, it's called condensation.

Ideal gases do not get converted into liquid form. So the gas must be a real gas. Three steps are required for liquefaction:

2. Vapor and liquid exist in equilibrium (vapor$r i g h t \le f t h a r p \infty n s$liquid)
If the temperature is too high, the molecules will be moving too quickly to allow the intermolecular forces of attraction to form the required gas$r i g h t \le f t h a r p \infty n s$liquid equilibrium. However, if the temperature is low enough, the molecules will also be moving slow enough for the intermolecular forces of attraction to liquefy.
Using a graph of isotherms for a gas of interest (e.g., ${\text{CO}}_{2}$), choosing a temperature below the critical temperature for that gas, and then looking in the high pressure region, you can identify a good Temperature / Pressure combo.