[Music] hi welcome back the free size lessons kool UK by the end of this video you should be able to describe the difference between potable and pure water you should then be able to describe the steps used to produce potable water from fresh water an from salty water now drinking waters essential for human life but humans cannot drink any water they find drinking water has have sufficiently low levels of dissolved salts such as sodium chloride and drinking water cannot have high levels of microbes such as bacteria now scientists call water that's safe to drink potable water and we're going to look at the steps needed to make that in this video the first key idea is a total water is not the same as pure water that's because pure water in the chemical sense contains no dissolved substances at all however potable water does contain dissolved substances although in quite small amounts in the UK rain water provides most of our potable water but because rainwater contains low levels of dissolved substances rainwater collects in the ground in aquifers and also in lakes rivers and in reservoirs so all of these are good sources of fresh water so to produce potable water we first use a good source of fresh water for example of liver like this one we then pass the water through filter beds like these that's to remove material such as leaves and suspended particles finally the water sterilized to kill microbes in the UK we use chlorine to sterilize potable water however in some parts of the world ozone or ultraviolet light are used instead now in the UK we've got access to huge amounts of fresh water for example in rivers fresh water already has very low levels of dissolved minerals however in many places fresh water scarce in these countries the only available water may be too salty to drink for example seawater seawater has very high levels of dissolved minerals so in this case potable water is produced by these nation desalination reduces the levels of dissolved minerals down to an acceptable level for potable water one way of carrying out desalination is to use distillation which we looked at in a previous topic another way is to pass the water through membranes this is called reverse osmosis both distillation and reverse osmosis would use the levels of dissolved minerals however both processes require very large amounts of energy which makes them very expensive remember you'll find plenty of questions on potable water in my revision workbook and you can get that by clicking on the link above [Music]