GCSE Chemistry Revision "Purity and Formulations"

[0:10]Hi, and welcome back to freesciencelessons.co.uk. By the end of this video, you should be able to describe how we can use melting and boiling point data to determine if a substance is pure. You should then be able to describe what's meant by a formulation. Now in chemistry, a pure substance can be a single element. So, for example, I'm showing you here a rod made out of pure silicon. A pure substance can also be a single compound, so this shows a sample of pure sodium chloride. So, the key point is that a pure substance is not mixed with any other substance. Now, there are lots of different ways to determine if a substance is pure, but one of the simplest ways is to measure its melting point and boiling point. A pure substance melts at a specific fixed temperature, and a pure substance also has a specific fixed boiling point. Now, impure substances melt and boil over a range of temperatures. So, if we take a substance and heat it, we can measure the melting point and the boiling point, and these will tell us if the substance is pure. I'm showing you here the results of that experiment using pure water. So, we're starting with ice and we're gradually increasing the temperature. At a certain point, the temperature stops rising. This is the melting point, and we can see that here. As you can see, the melting point in this case is fixed at 0 degrees Celsius. Once the water has melted, the temperature increases again, and again at a certain point, the temperature stops rising. This is the boiling point. As you can see, the boiling point in this case is fixed at 100 degrees Celsius. So, because both the melting and the boiling points are at specific fixed temperatures, we know that this water is pure. We get a similar shaped graph for any pure substance, although the melting and boiling points would not be the same as water. I'm showing you here the same graph, but this time for a mixture of water and an impurity. As you can see, in this case, the water melts and boils over a range of temperatures. This tells us that the water is not pure. And again, we'd get a similar shaped graph for any impure substance. Now, as we've said, a pure substance consists of only a single element or a single compound. However, scientists often produce mixtures. A formulation is a complex mixture that has been designed as a useful product. In a formulation, the quantity of each component is carefully measured so that the product has the properties we need. There are many examples of formulations, and I'm showing you some of these here. Remember, you'll find plenty of questions on pure and impure substances and on formulations in my revision workbook, and you can get that by clicking on the link above.