Why do gases liquify under pressure?
Think of it simply as spheres in a box. The spheres are your molecules. The higher the temperature, the higher the energy the molecules have and the more they move around. So when they have high enough energy for the molecules to move around a lot and stay away from each other they are a gas. (This is a simplified answer). If you decrease the energy they have (cool the system down), they move about less and there's a certain point the molecules aren't moving far away enough from each other to stay as a gas, so they liquify. As Raditz_35 said, look at a phase diagram. So when you put a gas under pressure, the molecules are getting "squished" together and they don't have a lot of room to move around, so they can't move away from each other enough and thus liquify. This is how they store Hydrogen as fuel. It's too dangerous as a gas and they put the Hydrogen under pressure to store it as a liquid. Hope this helps.