What's in tap water?

11 September 2018

Water Droplet

Water Droplet



Is the water that comes out of the tap actually 100% H20? If not, what else is in it and why?


We received this question from Kit. Chris Smith put it to Chemist Peter Wothers to pour out an answer...

Peter - If it were a hundred percent H2O, my job would be a lot easier in many ways because when you're carrying out chemistry experiments you need to try and control all the variables, things that could change, and you don't know quite how they're changing. And one of the things of course is there anything that might be in your solvent, which could well be water. And we go to great lengths to try to make a pure water and this is incredibly difficult. So the purest sort of water that you can commonly come across would be distilled water and this is where you are heating it up and then turning it into steam and then condensing this. But what you're trying to remove here is what would be naturally occurring, certainly in any bottled mineral water that you buy, and also wouldn't really be removed unless there were extreme levels from any tap water and these would be certain dissolved ions such as sodium ions, potassium ions, magnesium, calcium...

Chris - Calcium, I know from where I live!

Peter - Exactly. So these are the things that wouldn't naturally be in water and they're not really worth removing from tap water. So they're definitely present there and it's very difficult to remove some of these things are not worth the effort generally. But of course some other things are added and perhaps one of those controversial things is fluoride ions. There's absolutely no doubt that this has helped to tackle tooth decay in this country because it protects the teeth makes, them much harder and more resistant to decay. So that is something that is deliberately added in very very small concentrations is highly monitored. But this can be very good for us. So it's definitely not pure 100 percent water.

Chris - Thank you very much Peter.


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