Tap water in the UK is 'wholesome' but not pure. That is the definition that water supply companies must work to according to the water supply regulations. What comes out of the tap varies widely in total dissolved solids (TDS) and hardness across the country, and reflects largely where your water comes from
It is your statutory right to request an analysis of the water coming out of your tap. Just contact your local water company.
What you call pure water very much depends upon your industry, below you can see that there are a number of different definitions
Semi-conductor fabrication demands the highest purity of water available, so-called ultra-pure water. To produce water of this quality requires considerable investment not just to make it, but to maintain it at that purity. It is so pure that to expose it to air would render it 'out of spec'. The conductivity of this water is approximately 0.06 uS/cm2 (or its resistivity is '18 mega ohm'). This water is 10-100 times purer than 'pure water' used by window cleaners
Water for injection must be 'pyrogen free' and prepared by distillation technique.
This is the type of water used by window cleaners and by laboratories. It is commonly used also for boiler feed water in industry and for battery top up and also various specialist uses. Definitions vary widely but generally we work to a specification that deionised water has less than 1 part-per-milion total dissolved solids, or a conductivity of less than 2 uS/cm2 (microsiemens per centrimetre squared)
This is water that has been stripped of its bicarbonate alkalinity
This is water with the calcium and magnesium content removed
Effectively these are all the same thing, as all of the above signify a water that has substantially reduced levels of total dissolved solids. Of all of the above definitions distilled is the only one that defines the process of manufacture
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