[Music] I'm welcome back to 3 times less Nicole UK by the end of this video you should be able to describe what's meant by specific latent heat you should then be with a calculated energy change taking place when a substance changes state now I need to be honest with you some of this is pretty tricky you might need to watch this video a couple of times to get the idea in the last video we looked at heat in glass such as this one we saw that when we heat a substance we increase the temperature as we increase the energy the particles however when the substance changes state for example melts the temperature stops increasing and it stays constant that's because now the energy we're putting in is weakening or breaking the force of attraction between the particles scientists call the energy needed for a substance to change state the latent heat and we're looking at that in this video so here's a key definition the specific latent heat of a substance is the amount of energy required to change the state of one kilogram of substance with no change in temperature I'm sure you hear a block of ice now imagine that I wanted to melt 1 kilogram of ice how much energy would that take well scientists have worked this out it takes 334,000 joules of energy to melt 1 kilogram of ice scientists call this energy the specific latent heat of fusion so this is the energy required to change 1 kilogram of a substance from a solid to a liquid with no change in temperature now what if we wanted to change a substance from a liquid to a gas in other words boil it how much energy would that take well scientists call this the specific latent heat of vaporization and here's the definition the specific latent heat of vaporization is the energy required to change one kilogram of a substance from a liquid to a vapor with no change in temperature now we can use the idea of specific latent heat to calculate the energy change when a change of state takes place we use this equation the energy for a change of state in joules equals the mass in kilograms multiplied by the specific latent heat and you will be given this equation in the exam you'll also be given any values for specific latent heat that you need here's the triangle for this equation but remember that you're not given triangle like this in the exam ok so here's a question for you calculate the energy required to convert nor point five kilograms of ice to liquid water and the specific latent heat of fusion of water is three hundred and thirty-four thousand joules per kilogram pause the video now and try this yourself okay so the energy for a change of state equals the mass multiplied by the specific latent heat in this case the mass is not point five kilograms and the specific latent heat of fusion of water is three hundred and thirty-four thousand joules per kilogram multiplying these numbers it is an energy of one hundred and sixty seven thousand joules here's another question for you calculate the energy required to convert no point one five kilograms of ethanol from a liquid to a vapor and the specific latent heat of vaporization of ethanol is eight hundred and forty six thousand joules per kilogram so pause the video now and try this yourself okay so the energy for a change of state is the mass multiplied by the specific latent heat the mass of ethanol is nour point one five kilograms and the specific latent heat of vaporization of ethanol is eight hundred and forty six thousand joules per kilogram I don't need numbers into the equation give us an energy of 126 thousand nine hundred joules we know that you'll find plenty of questions on specific latent heat in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe what's meant by specific latent heat you should then be able to calculate the energy change taking place when a substance change your state Oh [Music]