Why does the ethanoic acid not conduct like most acids?

Here, pure ethanoic acid is used, CH₃COOH. The molecules are polar covalent – and not present as charged ions. Therefore, there are no ions (or very few) available to carry the charge.

Since pure ethanoic acid and pure water each show negligible conductivity, why does the conductivity of the acid increase when water is added?

Consider the equilibrium:

CH₃COOH_(l) + H₂O ∏  H₃O⁺_(aq) + CH₃COO^–_(aq)

When more water is added, the equilibrium shifts to the right hand side, favouring the formation of more ions. Adding more and more water makes this shift greater, increasing the number of ions further, and increasing the conductivity.

How does this fit with the theories of acidity?

Ethanoic acid donates a proton, H⁺, and is acting as an acid. Water accepts the H⁺ from the ethanoic acid and is acting as the base. This is an excellent example of the Bronsted Lowry theory of acidity, where a proton donor and a proton acceptor provides a more helpful description than the simple Arrhenius model of acids and bases. Theories of acidity are considered in greater depth in the topic, Proton donors and acceptors [future link].