Hi and welcome back to freesciencelessons.co.uk. At the end of this video you should be able to describe how the properties of hydrocarbons depend on the size of their molecules. Now this is a relatively easy topic, but it's very important that you learn the details as they often come up in exams. In the last video we started looking at hydrocarbons. And we saw that one family of hydrocarbons is called the alkanes.
The first four alkanes are methane, ethane, propane and butane. And remember that you could be asked to draw any of these in your exam. Now there are three key properties of hydrocarbons that you need to be able to describe.
And the first is viscosity. Viscosity tells us the thickness of a fluid. Fluids with a high viscosity flow slowly. And a good example is honey.
Now as the size of the hydrocarbon molecules increases, the molecules get more viscous. So very long chain hydrocarbons flow very slowly indeed. In other words, long chain hydrocarbons are extremely viscous. The second property we're going to look at is flammability. Flammability tells us how easily a hydrocarbon combusts, in other words burns.
And we'll be looking at how hydrocarbons combust in the next video. Now short chain hydrocarbons are extremely flammable. In fact methane is the gas that we use in burns and burners. As the size of the hydrocarbon molecules increases the molecules get less flammable. And in fact long chain hydrocarbons are very difficult to burn.
The final property we're going to look at is boiling point. The boiling point is the temperature at which a liquid turns to a gas. Short chain hydrocarbons have low boiling points.
In fact all of the hydrocarbons I'm showing you here are gases at room temperature. And that's because they all have boiling points which are lower than room temperature. And I'm showing you the boiling point of these hydrocarbons here.
As you can see as the size of the hydrocarbon molecules increases, the boiling point also increases. So very long chain hydrocarbons have very high boiling points. Remember you'll find plenty of questions on the properties of hydrocarbons in my vision workbook and you can get that by clicking on the link above.