Describe two variables could be manipulated that would change a substance from a liquid to a gas?

Answer 1

The short and sweet is that you can cause the liquid to gas transition by increasing the temperature or decreasing the pressure.

The liquid phase of a substance exists within a certain range of pressures and temperatures. If you hold other factors (pressure, volume) constant and increase temperature you can encourage a phase transition towards the gaseous form. Think of temperature as a measure of thermal energy. Raising the thermal energy of the system raises the (kinetic energy) of the particles in the system. What do you like to do when you have a lot of energy? Are you more likely to sit still or move around and be active? The particles are similar. If you raise their energy level they move faster and faster and spread out until they are far apart. Eventually they spread so far apart that we can't see them, and we call that their gaseous phase.

You can also encourage the shift to gas by lowering the pressure. Think of pressure as a type of "molecular crowding". Imagine yourself and a friend entering a room. If the room is vacant (low pressure), the two of you have the space to spread out and position as you wish. Alternatively, if you are entering a crowded room full of people (high pressure), you would likely be forced to remain closer to each other than you would in the empty room. As pressure decreases, molecules are likely to spread farther apart, and when they spread so far that intermolecular forces become negligible, the gas phase is reached!