 So now we're going to look at a little bit of terminology. You may have heard of saturated and unsaturated fats who would probably come across it in the context of fats. Fats are in fact also hydrocarbons of a particular kind, but there is a particular chemical meaning to this saturated and unsaturated. The reasoning behind these terms is this, when a hydrocarbon has all single bonds as in an alkane, that means it's bonding to the maximum number of hydrogen atoms that it's possible for that molecule to carry. Let me draw out butane, C4H10. Now there's no way to attach more hydrogen atoms to this molecule. All possible bonds have been used. So we say the molecule is saturated with hydrogen. However if I now draw butene with a double bond between two of the carbon atoms, in order to form this double bond we need to lose a hydrogen from each of the carbons in order to free up the two electrons to form the new double bond. So the molecular formula for this compound is C4H8, that's two hydrogens less than butane. It's therefore called unsaturated because if you broke that second bond in the double bond it would be possible to attach more hydrogens to this molecule. So it's not saturated with hydrogen. Now let's have a look at how alkanes and alkenes compare in terms of properties. For their physical properties they're pretty similar. Both are insoluble in water, both are less dense than water and both are colourless. However their chemical properties are quite distinct. They have in common that they're both good fuels. They combust in air to give carbon dioxide and water. However they're the similarities ent. Alkanes are generally unreactive which has to do with the fact that the carbon-carbon and carbon-hydrogen bonds are quite stable. Alkenes on the other hand have this carbon-carbon double bond and the second bond in the double bond is not as strong as the first which means that alkanes, sorry alkenes are able to undergo a number of reactions that involve breaking that second bond and adding something new onto the molecule. And for this reason these reactions are called addition reactions. Alkenes are also polymerisable. It means they can be turned into polymers. Again it's because of that less stable second bond they can be joined together in long strings to give polymers.