Not necessarily - if you have a combi heating system, the chances are that all your water is coming straight from the mains, if not your cold water in the kitchen will always come from the mains but the cold water in your bathroom may or may not come from the mains, it may come from a tank in the loft which as ratter pointed out could have all sorts in it - when you brush your teeth, you use a minimal amount of water and it is unlikely to do you any harm, but would not really be good for drinking. Your hot water will pass throuigh a hot water tank fed either from the mains or your cold water tank in the loft. If you have a combi system you will not have a hot water tank.