in today's video we're going to look at the concept of specific latent heat and see how to use it in calculations we saw in the last video that as we heat up a substance the particles gain energy in their kinetic energy store and this increases their internal energy we also saw that temperature is just a measure of the average internal energy of all the particles in a substance so as we heat objects and the internal energy of their particles increases the object's temperature increases as well the whole point of this video though is that there is an exception to this rule and that's when there's a change in states if you take a look at this graph which shows how the temperature of a substance changes with time as we heat it you can see that it's not a straight line like you might expect this is because when the substance is changing state so at the melting point or the boiling point the energy that we're providing is being used to weaken or break the forces holding the particles together rather than increasing the particle's internal energy and remember that the temperature is just a measure of the average internal energy of the particles so in this case the overall temperature won't change in these regions until all of the substance has changed states the same principle applies if we look at a similar graph for cooling a substance this time though the state change involves the formation of new bonds between particles which releases energy and so counteracts the cooling and this is what keeps the temperature constant during the changing state now the exact amount of energy that's required to change the state of the substance while the temperature remains the same is known as the latent heat and it depends on two things the type of substance that we have and the amount of that substance to standardize these values though we use the term specific latent heat or slh which we can define as the energy required to change one kilo of a particular substance from one state to another without changing its temperature and for cooling the specific latent heat would be the amount of energy released by changing states now there are actually two types of specific latent heat one is the specific latent heat of vaporization which is the energy change when a substance changes between a liquid and a gas so either evaporates or condenses the other one is the specific latent heat of fusion which is the energy change when a substance changes between a solid and a liquid so either melts or freezes to see how all this works let's imagine that we had one kilo of water and that we heated it all the way from minus 50 degrees celsius where it was solid ice to 150 degrees celsius where it was gaseous water vapor as we heat our one kilo of ice as particles all gain internal energy and so its temperature increases however once we reach zero degrees the ice starts to melt and because all the heat energy that we're supplying is being used to break the forces between the particles the overall temperature stays the same the energy required for this stage is the specific latent heat of fusion and in the case of water this is 334 000 joules per kilo once this much energy has been provided though and the ice has all melted into liquid water then the temperature will continue to increase all the way to 100 degrees where the same process happens again this time though the energy required is the specific latent heat of vaporization which is two million two hundred sixty thousand and once the water has all boiled into water vapor its temperature continues to increase like normal bear in mind that you don't need to memorize these figures they'll be given to you an exam if you needed them as we said earlier if we were to cool our water the same concepts would apply but the amount of energy that we mentioned for the specific latent heat of vaporization and the specific latent heat of fusion would be the amounts of energy released rather than required at each stage the last thing we need to look at is the equation for specific latent heat which says that the energy that's required or released is equal to the mass of the substance times the specific latent heat you can also use this formula triangular that's easier so a typical question could be something like how much energy is required to completely boil 2.5 kilos of water at 100 degrees celsius use the relevant data from the table well as we're boiling the water we're going to need to use the specific latent heat for vaporization value as that's the one that refers to boiling so we just do two point five times two million two hundred sixty thousand which gives us five million six hundred and fifty thousand joules or 5 650 kilojoules anyway that's everything for today so if you enjoyed it then do give us a like and subscribe and we'll see you next time