in today's video we're going to take a look at potable water which is what we call water that's safe to drink we'll also cover how we can get potable water by either treating fresh water sources or desalinating seawater now although potable water is by definition safe to drink that doesn't mean that it's pure in chemistry pure water would have to contain only h2o molecules whereas portable water often contains other dissolved substances which makes it impure this by itself isn't a problem but there are three criteria for water to meet before it can be considered possible one is that the levels of dissolved substances needs to be fairly low another is that it has to have a ph of between 6.5 and 8.5 so that it's not too acidic or alkaline and lastly we have to make sure that there are no microorganisms like bacteria or fungi swimming around in it next we need to consider where you actually get potable water from generally this depends on where in the world you live most countries have a pretty good supply of fresh water which is just water that doesn't have much dissolved in it for example in the uk we get a fair bit of rain which is a type of fresh water and once it's hit the ground it generally collects as either surface water or groundwater surface water sources are things like lakes rivers and reservoirs literally bodies of water that are exposed at their surface groundwater sources on the other hand are those found under the ground like aquifers which are areas of permeable rock under the ground that trap water the benefit of using surface water is that it's easy to access and gets replaced frequently by the rain however if it's hot and sunny it can dry up because it's exposed to the sun so in warmer parts of the uk we often rely on groundwater instead whichever source we get our fresh water from though we still need to treat it to make sure that it's safe to drink the exact technique we use varies but it generally involves three steps first we take our fresh water and pass it through a wire mesh to filter out any big things like twix next we pass it through a bed of sand and gravel which filters out any other smaller solid bits and finally we have to sterilize it to kill any harmful microbes like bacteria which we can do in three ways either bubbling chlorine gas to it or exposing it to ozone or ultraviolet light in some countries though like those in the middle east there isn't very much rain and so they don't have very good supplies of fresh water instead some of them rely on desalination which is a technique used to extract potable water from seawater this sounds like a great idea because there's tons of seawater so basically a limitless supply unfortunately though both of the techniques that we currently use namely distillation and reverse osmosis require loads of energy ants are really expensive which makes them kind of impractical for producing large quantities of water in order to desalinate water by distillation we basically just do the same thing as in simple distillation like you've seen in school except we're boiling much larger quantities of salty water and the apparatus will look different regardless though we still collect the water vapor and condense it to give us pure distilled water on the other hand in the reverse osmosis technique the salty water is passed through a membrane which only allows the water molecules to pass through this means that all of the ions and larger molecules get trapped and so separated from the water so again we end up with pure distilled water anyway that's all for this video so if you enjoyed it then please do give us a like and subscribe and we'll see you next time