For one person, pure water may mean a beautiful lake to bathe in, for another it may be sterile water needed for a process to function optimally. For a third, pure water may be the difference between life and death.
What is considered to be “pure water” depends on the audience and the context in which the water is to be used. We impose different requirements on the water we drink than on that used in industry, for example. However, even within industry, different people and operations have completely different needs and perceptions regarding what is pure – water that may be classed as pure in one industry may be considered unusable in another.
In short, how we define pure water depends on the situation in which it is used.