Why should distilled water be tested for conductivity?

The conductivity of the aqueous solution water/salt is proportional to the ions concentration in the solution. When you try to measure the conductivity of tap water you can verify that it has higher conductivity in comparison with distilled water because this contains ions (less pure).

Explanation:

sodium (na), for example, has one electron in its valence shell. this is an unstable state because that valence shell needs eight electrons to be full. in order to fill its valence shell, find a way to add seven electrons to its valence shell