Why do distilled water and deionized water have essentially the same purity?

Answer 1

The problem with using tap water for making solutions is that tap water may contain ions which react with the chemicals in your solutions. Try mixing AgNO3 with tap water and you will see a cloudy precipitate form.

Distilled water is great for making stock solutions since it is pure water with no ions. I use it all the time in preparing labs for my classes. I have a distillation unit to purify the school tap water.

Deionized water has been filtered through a resin which removes ions from the water. This filter may not remove ALL impurities - it removes those which carry a charge (ions). In other words, deionized water is perfectly suited for making stock solutions, too. It is the ions in the water that can cause a reaction with the chemicals you are mixing into solution.

The only difference is that distilled water has no molecular impurities (non-ion impurities) while deionized water may contain a few molecular impurities. They are both well-suited for a chemistry laboratory. My school happens to use a distillation unit, but others use a resin column to deionize water for chemistry labs.

Answer 2

Deionized water (DI water or de-ionized water; also spelled deionised water, see spelling differences) is water that lacks ions, such as cations from sodium, calcium, iron, copper and anions such as chloride and bromide. This means it has been purified from all other ions except H3O+ and OH−, but it may still contain other non-ionic types of impurities such as organic compounds. This type of water is produced using an ion exchange process. Deionized water is similar to distilled water, in that it is useful for scientific experiments where the presence of impurities may be undesirable...