let's discuss the differences between the heating curve and the cooling curve the heating curve of a substance shows the changes in temperature of the substance during a time period when heating occurs so we are adding heat or energy to a substance in this curve we're starting out at the solid phase we're adding heat we're adding energy and we are recording the temperature change over time so this axis over here below is heat absorbed or time because as time goes on heat is being absorbed and we are measuring temperature if we ever ask you to draw the curve or draw the graph it's important to remember your heading your axis labels and units right so let's take a look at how the heating curve what substance X looks like okay so we're going to start off with the substance in its solid phase it doesn't matter what the substance is we're calling it substance X and we're starting off by measuring the temperature of the solid and what we do is we add heat or energy to the substance it is heating that solid up so it starts off as a cold solid and then it gets warmer warmer warmer very very hot solid and then as you should know so if I start with an ice block for example if we pretend that this is water that we're dealing with which it's not um but let's just pretend then eventually if we have a solid ice block and we add heat to it eventually that solid is going to melt it will go through a phase change which is called melting so the next part of the graph will look as follows can you see that the graph is now flat it's horizontal when it's flat or horizontal like this the substance is going through a phase change it is changing its state it's going from a solid it was a solid in this initial inclined phase over here and it's changing it's busy changing into a liquid so when you see a flat part of the graph now you know that that is a phase change it corresponds to a phase change and the states that are present during the phase change would be both solid and liquid phase the solid and the liquid phase are both present at the same time because the substance doesn't change instantaneously into a liquid it takes place over a period of time this is the phase change over here it is called melting so the the solid is melting and turning into a liquid and we'll discuss the energy changes that takes place in these various parts of the graph afterwards but then what we're going to do is we're going to continue now it's melted completely and obviously after it's melted it's completely formed a liquid it's in its liquid phase and the liquid phase of the graph will look something like this notice how when it is in a singular state so either just a solid or just a liquid or just a gas we've got an inclined line going over here temperature is increasing so at this point over here we've got a colder liquid then we carry on adding heat heat is being absorbed we get a warmer liquid and then we get a hot liquid and as you should know eventually when we've added enough heat energy to a liquid it will reach its boiling point boiling or evaporation will occur another phase change and remember what we said about base changes the graph will be horizontal so this is when boiling or evaporation is occurring the substance is changing from a liquid to a gas so both liquid and gas phases are present at this stage liquid and gas then after it has gone through its pH change it becomes just totally a gas and we can continue heating that gas up so we get a cold gas a warmer gas and a hot gas okay so this was boiling or evaporation so a few important things to note about the graph is we are continuously adding heat energy if you look at this axis over here time is going on heat is being absorbed continuously by the substance however just because heat is being absorbed does not mean temperature is always increasing when we think of heat absorbed you need to think of energy so energy is always being taken up by my substance my substance whatever it is is always absorbing that energy it's always absorbing that heat however only at certain points in time is the temperature of the substance actually increasing so it increases here when it's in a solid phase then temperature stays constant then temperature increases again when it's in its liquid phase and then temperature stays constant and then temperature increases again when it's in its gaseous phase now why is this happening what happens is when a substance is in one phase like just a solid or liquid or gas when we add heat to it what happens over here is the kinetic energy the kinetic energy of the substance is busy increasing and as you should know if I increase kinetic energy I'm increasing average kinetic energy and that is a measure of the temperature of a substance the temperature of a substance is determined by the substance's average kinetic energy the particles begin moving faster and faster as the temperature increases however the substance is still in one phase it's still just in the solid phase here initially so nothing is happening to the potential energy okay the particles aren't moving further apart from each other there isn't a phase change taking place and that is the case so kinetic energy increasing which causes temperature to increase that happens in all of the inclined parts of the graph so kinetic energy increases average kinetic energy of the particles increase and that is why the temperature increases so the heat energy that I'm adding the heat that is being absorbed for the incline part of the graph it is causing this change to happen however when we reach the flat parts of the graph such as this part over here and this part over here we are still absorbing heat energy so heat energy is still being absorbed for this time period however it's not causing the kinetic energy of the particles to increase so the kinetic energy stays the same it doesn't change kinetic energy stays the same therefore average kinetic energy stays the same which means the temperature doesn't increase and you can see that if you read temperature of the graph for this entire time period the temperature stays constant let's and it's 30° or whatever it is it stays 30° C from here all the way to here but we added heat energy so something must happen and the thing that happens is that potential energy now increases and what that does is it allows the substance's intermolecular forces to be overcome the intermolecular forces are the forces that exist between the particles so we absorb heat energy we overcome the intermolecular forces we weak them it allows the particles to move further apart and that causes the phase change so it's basically like that heat energy is not actually going towards increasing the temperature it's not going towards increasing the kinetic energy rather it's going to increasing the potential energy causing a phase change so only one of the two energies will change at one time here's another visual representation of a heating curve and here is an explanation about what happens on the incline parts of the graph as we mentioned we're changing the average kinetic energy which is increasing the temperature and here's an explanation of what happens on the flat parts of the graph no kinetic energy is changing temperature doesn't change but there's a phase change and the potential energy will change the cooling curve is essentially the opposite of the heating curve so you obviously need to watch the video on the heating curve first in order to understand the cooling curve but it's basically the reverse so so think of the heating curve as we start with the solid we add heat we end up with the gas the cooling curve we start with the gas we remove heat or heat is released to the environment start with the gas end with the solid so this is basically how the cooling curve would look yeah we start off with gas at a certain temperature heat is being released to the environment we're cooling the gas heat is being removed eventually we're going to condense that gas into a liquid liid it's going to go from gas phase to liquid phase there we go so at this temperature over here whatever that temperature is on the graph that will be the temperature at which condensation occurs so condensation is our phase change we go from gas to liquid then we've got our liquid phase and we start off with what we can call a hot liquid and then as heat is released as time goes on remember this axis is time time is passing heat is being released to the environment or we're removing Heats and the temperature of the liquid decreases so hot liquid warm liquid cold liquid eventually if we cool down that liquid enough if enough heat is released it'll re it'll reach its freezing point think about when we put water in a tray in our freezer and the water the liquid water eventually turns into ice cubes so this temperature over here whatever that is when we read that off that temperature would be the freezing point of my substance X then remember when phase changes are occurring we have both phases present then after our phase Chang has happened we've gone from liquid to solid our graph ends up looking like that so hot solid warm solid cold cold colder solid colder solid and just like with our heating curve when we have an incline part of the graph kinetic energy is changing average kinetic energy of the particles are changing the particles are slowing down in this case that's why kinetic energy is changing and therefore temperature decreases so that's all the incline Parts when there's a single phase the horizontal Parts over here kinetic energy is constant but potential energy is decreasing the particles are moving closer together phase changes are happening and the intermolecular forces are actually getting a bit stronger and here is another representation of a cooling curve and remember you need to know how to interpret the curve how to draw it also you need to need you need to know how to read values off of the curves I hope this video has been helpful Please Subscribe for more chemistry physics and math videos I can't wait to see you in another video very soon check out links in the description box for more bye everyone