 So we saw in the previous video that substances are either pure, meaning that they contain particles of only one kind, or they're mixtures. Now we have to step back and ask, how do we distinguish between substances? For many substances that's easy, they look different. But consider metals. There are many metals that are all a kind of silvery color. If you had a number of different metals, how could you determine what each one was? The answer is that you need to look at their properties. What do our senses tell us about them? What can we measure about them? How do they react chemically? All these things are properties. We can divide properties into physical properties, those that are intrinsic to the substance as it is without a chemical reaction having to occur, and chemical properties, those that show up when the substance undergoes a chemical reaction. Examples of physical properties are mass, volume, density, melting and boiling point, brittleness and heat capacity. That's the amount of heat energy that it takes to raise the temperature of one gram of substance by one degree Celsius. We're going to explore that particular property later on this unit. Chemical properties are things like reactivity, acidity, flammability, half-life for things that are radioactive and toxicity. Now it turns out that we can classify the physical properties further. How we make this classification depends on the amount of material that we have. Watch the video and see if you can work out what the classification is and how the properties that I've got listed here might be divided up. Did you figure it out? Physical properties can be further classified depending on whether they are affected by how much of the material there is. For example, a small block of lead may have dimensions of 2 cm by 2 cm by 2 cm. It would have a side length of 2 cm, a volume of 2 by 2 by 2, which is 8 cm cubed. And imagine that we determine its mass to be 90.7 grams. Okay, now imagine another block of lead, so they're both lead, but this one has dimensions of 5 by 5 by 5 cm. That means its volume is 5 times 5 times 5, which equals 125 cubic cm. And imagine that we weigh this block and we find that its mass is 1418 grams, that's 1.418 kilos. Okay, so these blocks are made of the same material, they're both made of lead, but their side lengths and their volumes and their masses are different. This kind of property, where it depends how much of the substance is present, is called an extensive property. That's a property that's external to the substance itself, that's imposed on it by its form, if you like. But what our two blocks of lead do have in common though is density. Now you can work this out for yourself. Density is calculated as mass divided by volume. Pause the video and calculate the density for each block. What do you get? You get the same value. No matter the size of a piece of lead, as long as it's actually lead, its density is a fixed value. This has to be the case because lead is always made of the same kind of atoms, lead atoms. And the size and mass of a lead atom is fixed, which means that the density of each individual lead atom is a fixed value also. So if all the particles that a substance is made of are identical, and in this case they're all lead atoms, and if they're always arranged in the same way, then the density of the substance can never vary. This kind of property is called an intensive property. It is internal or intrinsic to the substance, since it's determined by the type of atom or molecule that makes the substance up, and not how many of them there are. Okay, your task for this video is to classify the following physical properties as intensive or extensive. Do some research on the internet if you like, and try and justify your classification for each one. The key thing to think about is whether the property depends on how many atoms or molecules are present, or just on the kind of atoms or molecules that are present.