 In the previous video we looked at the properties of substances and found that we could classify them into intensive and extensive properties. Intensive properties are those that are characteristic of a substance and don't depend on how much there is of it. Density and melting or boiling points are all examples of intensive properties. They depend only on the atoms or molecules that make the substance up, not how many of them there are. When you're measuring properties, however, it's important to know the conditions under which you're measuring them. And temperature and pressure are the most important conditions to pay attention to. So although we've said the intensive properties of a pure substance don't change, we have to add the caveat that they don't change as long as you measure them under constant conditions. For instance, the density of a substance can change with temperature and pressure. So if you're quoting the density of a substance, you also need to state the conditions under which the density was measured. And note that if the density changes, the volume of the object is probably changing too. So this also affects extensive properties. For instance, a cup of water ready for tea might have a temperature of 90 degrees C. And it's sitting at normal air pressure, which is about one atmosphere. An atmosphere is a unit of pressure. And if we were to measure its density, we'd find it was about 0.972 grams per centimeter cubed. That is, every centimeter cubed of water or every milliliter, if you like, would weigh 0.972 grams. If I now looked at a glass of iced water with a temperature of 4 degrees Celsius straight from the fridge and also at one atmosphere pressure, I'd find its density was one gram per centimeter cubed. The two containers hold the same pure substance, but because they're at different temperatures, their densities are different. This is why it's possible to float hot water on cold water. But the upshot of it all is that if you want to be able to compare the properties of different samples of a substance, you must make the measurements under the same conditions. There's a good reason for all this. All these properties have to do with how the atoms or molecules in the substance interact with each other. And if you heat them up or squish them together with pressure, it changes how they interact. And that means that the properties of the substance change as well.