[katieHaylor (OP)]

Thomas asks:

Why does H₂O not conduct electricity but tap water and pool water do?

What do you think?

[chiralSPO]

Liquid water exists almost entirely as neutral H₂O molecules. These dissociate to a very small extent into hydronium (H₃O⁺) and hydroxide (OH^–) (pure water will have about a billion ions per liter, and this is a very small charge carrier density). In very pure water, this is the only source of ions, so it can have a resistivity of > 18MΩ•cm (conductivity <5.5µS/m), which is essentially an insulator.

However, water has a high dielectric constant, participates in hydrogen bonding, and is not particularly viscous, so any ions present have fairly high mobility. Just adding 100 mg of NaCl per liter of water increases the concentration of ions in solution by 12 orders of magnitude, and the conductivity similarly jumps by several orders of magnitude to be around 20 S/m. This is a pretty good conductor, edging out semiconductors like germanium (2 S/m) but not nearly as conductive as metals like copper (6×10⁷ S/m) or even glassy carbon (1500 S/m).

Concentrated solutions of hydrochloric acid can reach conductivities as high as 5000 S/m)

[jeffreyH]

Just in case the units of S/m are unclear.
https://en.m.wikipedia.org/wiki/Conductivity_(electrolytic)