Question

Why is it necessary to use deionized water when testing the conductivity of aqueous solutions?

Answer 1

Deionized Water (We call it "DI water" in the chemistry labs) is just what it sounds like: Water that has the ions removed. Tap water is usually full of ions from the soil (Na⁺, Ca²⁺), from the pipes (Fe²⁺, Cu²⁺), and other sources. Water is usually deionized by using an ion exchange process.
Often, when you are doing chemistry experiments, the ions in water will be an interference. They can switch places with other ions you may be interested in experimenting on. You may also be interested in finding out what elements are in a small sample of material. For example, a farmer may want to know what's in his soil, or the Environmental Protection Agency wants to know what a factory's emitting into the air. Dissolving the sample in water and doing tests on the result is a common technique, and contaminants in the water will make the whole test give the wrong answers. Water with ions in it is also quite a lot more electrically conductive than water without ions in it. If you boil water with lots of ions in it until all the water's gone, you'll have a crusty salt residue in your pot.

We guess de-ionized water isn't necessarily pure water, given the usual de-ionization procedure. Non-ionic contaminants may persist. Electrically polar molecules dissolve easily in water, and some complicated molecules have polar ends and non-polar ends, which can help non-polar stuff (like oils) mix in water. Soap is an example. Soapy water may count as deionized, but most people would insist that their de-ionized water doesn't have (much) other stuff in it.Special note for Reddit users. I noticed that somebody who linked to this answer got the impression that deionized water really has no ions. Of course it still has about 10^-7 molar each of H+ and OH- ions, at room temperature. You can't stop that chemical equilibrium from occurring just by pulling out the other ions. / Mike W.
Even if you start with deionized water, it can pick up a few ions from a container. What the sign is of the charge depends on whether that particular surface mainly