 Now, normally, when you add heat to an object, its temperature goes up. As you add the heat, say, put a flame under it, the atoms will jiggle faster, or the chemical bonds will be more stretched, the atoms will spin around more. But there is one particular special situation where you can add heat, say, put a fire under something, and it will not change the temperature. Energy is still going into whatever it is you're heating up, but the energy is actually doing something different, it's breaking chemical bonds. This is the concept of latent heat. Now, latent heat is the energy needed to change one kilogram of a substance from one state of matter to another. Now, what do I mean by state of matter? Well, state of matter is like solid, liquid, or gas, or in fact, plasma, or Bose-Einstein condensate. So, for example, this is the energy needed to turn one kilogram of ice at zero degrees centigrade into one kilogram of water at zero degrees centigrade. And it turns out that even though the atoms are going at the same speed in the water as they were in the ice beforehand, a lot of energy is needed because it's breaking all the chemical bonds that hold the crystals close together. Likewise, if you take one kilogram of water at 100 degrees and you boil it dry, so it turns into one kilogram of steam at 100 degrees, the temperature hasn't changed, the atoms are going at the same speed at the end as they were in the beginning. However, you've had to break all the bonds that are holding them together in liquid, making them all free as it is in a gas. So that is called the latent heat. So let's do an example of this. Let's imagine that we had some ice and we can plot the temperature of the ice against time. And we start up with ice, I don't know, say minus 20 degrees centigrade, and we put a fire under it. So what happens is the temperature will rise until it gets to zero degrees centigrade. At that point, even though there's still a fire underneath it, the temperature will not rise. Instead, it will steadily turn from 100% ice to 100% water, and that will take a while. So we begin with the most ice, a little bit of water that's melted, and as time goes on, more and more water melts to eventually, by here, all the ice is turned to water, and then the temperature will start rising again, and it will rise until we get to 100 degrees. At that point, the temperature will stop rising. Now what's going to happen is here, you're going to start off with 100% water and 0% steam, and as it goes along here, more and more of the water will turn into steam, until eventually it will be 100% steam, and then the temperature can keep on rising as you make the steam hotter and hotter. And this is what happens for any substance, not just water. The energy here, it's the latent heat of fusion, the energy needed to turn something from solid to a liquid is actually quite large, it's 334 kilojoules, that's 334,000 joules per kilogram. But then you appear, the latent heat of vaporization to turn a liquid into a gas is even more immense, it's actually 2,265 kilojoules, 2,265,000 joules per kilogram. So these are very large numbers indeed. So let's do a specific work example. Let's say we had water at room temperature, say 20 degrees centigrade, and we want to put it in a frying pan, and we want to boil it dry, so it's all turned into steam. How much energy will it require? Well first of all you've got water at 20 degrees C, and you're going to need to increase it to 100 degrees C. Now that's specific heat capacity, so the energy you need to do that is equal to the normal equation of specific heat capacity, which is the mass times the specific heat capacity times the change in temperature, which in this case, specific heat capacity of water is 4186 joules per kilogram per Kelvin. Let's say we have 2 kilograms of water, that would be a nice big saucepan. This is 2. Change in temperature, we're going from 20 to 100, so that'll be 80 Kelvin change in temperature. But then once it's got to water at 100 degrees, you have to turn it into steam at 100 degrees, and that's going to need the latent heat. So we now need to add the mass times the latent heat, which is this 226500 joules per kilogram. So this equals the mass, 2 kilograms, times the latent heat, a specific heat capacity 4186 times the change in temperature, plus the mass, 2 kilograms, times the latent heat of vaporization, 226500, which comes out as a whopping 5199760 joules, so about 5 megajoules. So that's why it takes so long to boil a kettle, it takes to boil it dry, a lot of energy is required.