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Phase Change - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Phase changes are the transformations from one state of matter to another due to thermodynamics. The processes of phase change between solid and liquid are called melting and freezing. Phase changes between liquid and gas are vaporization and condensation. Phase changes between gas and solid are deposition and sublimation. Phase changes can be spontaneous or non-spontaneous.

We're going to talk about phase changes, going from different forms of matter; for the solid, liquid gases and how they interact with each other and how they change from one phase to another. So let's use this as an examp- as a good diagram to show us how these things interact with each other.

Alright, so if we're going to get from solid to a liquid we're actually call that melting which I know we've heard that word many times before and that actually requires energy, we need heat to melt something, so we're going to call that endothermic process meaning that it requires energy or requires heat for that reaction or that that phase that to occur. So if we're going to from liquid the opposite from liquid to solid, we're going to call that freezing which I know we've heard many times before too. That actually releases some sort of energy, it's going to be an exothermic process, meaning it releases energy. These guys are opposite of each other, melting and freezing, of the same thing.

Let's go over to liquid and gases. If we're going from liquid to gas, we're going to call that vaporization; we're going to vaporize that particular liquid. That actually, requires energy as well. We need heat or some sort of energy to make that happen. So we we're going to call that endothermic process. The opposite will be cond- condensation; when we're are condensing something from a gas down to liquid and that's an exothermic process meaning that's going also to release some sort of energy.

There are rare instances where substances will go straight from the gas phase to the solid phase. It doesn't happen as often as you probably know but they do happen with different substances so if we're going from the gas phase down to the solid phase, we're actually going to release that sort of energy because we know gas is in high has higher energy than solid phase so we're going to release that energy we're going to call that process deposition. The opposite would be sublimation going from solids to a gas we've seen this probably before when ice or solid CO2 maybe iodine crystals they go from this solid phase straight to the gas phase skipping over the liquid phase that actually it releases sort releases some sort of energy and that we call it endothermic process.

Alright so let's actually look at this in a different way this you might see more often in class. This is actually a different a graph describing all those things that we just talked about. Alright so on the x ax- sorry on the y axis we have temperature on the x axis we got x axis we're going to have energy okay so we know in this case we're going to talk about water the phase change of water and we know that below 0 degree Celsius that is in solid phase okay? So as we increase energy, our temperature of that solid is going to increase until we hit 0 degree Celsius which we know it is melting and freezing point s the increase if we increase energy it's going to melt and if we're going from liquid to solid it's going to start freezing but notice the temperature it's not changing even though we're increasing temperature why is that? Well that energy that we're pumping into the into this solid molecule this substance is actually being used to break up those intermolecular forces that are holding it together in a solid so here's the picture water and these blue dots are the hydrogen bonds that are holding it together in a solid so because solids have more hydrogen bonds than liquids that energy is going to be used to break up some of those bonding some of those forces that are holding it together. Then as you go from 0 degree Celsius to 100 degree Celsius we're going to be in a liquid phase all that and all the energy is going to be used to increase the temperature of that particular liquid in this case water. And here we have the same thing we have this plateau and 100 degree Celsius we know that is it's vaporization point or it's con- condensation point again it's flat and again that energy is being used to break apart more of these hydrogen bonds once at 100 degree Celsius these bonds are going to be pretty wear because their energy is being used to break them apart and have them flow around allover the place and then up at higher temperatures above it's always going be in a gaseous phase.

If we go straight from the solid to a gas which water doesn't do if it were to we would call that sublimation going to skipping this liquid phase completely, if we're going from gas to solid we're going to call it deposition we're going to complete again skipping that liquid phase so this actually cycle talks about the different phase changes that substances tends to undergo.