Question

Why is pure water a very weak electric conductor?

Short answer

Pure water is a very weak conductor of electricity due to the extremely low concentration of ions present.

Step-by-step solution

Understanding Electrical Conductivity

Electrical conductivity refers to a substance's ability to conduct an electric current. This usually requires the presence of charged particles, such as ions, that can move freely.

Composition of Pure Water

Pure water (H₂O) is composed of molecules that are neutrally charged. It lacks the ions which are critical for conducting electricity.

Ionization of Water

Water does ionize to a very small extent, forming hydrogen ions (H⁺) and hydroxide ions (OH⁻), but the concentration of these ions is extremely low (about 1 ion per 10 million water molecules).

Conductivity Dependent on Ion Concentration

Electric conductivity is highly dependent on the concentration of ions in solution. Due to the very low ionization of water, the number of free ions present is insufficient to carry a significant electric current.

Conclusion

The very low concentration of ions in pure water makes it a very weak conductor of electricity. For significant electrical conduction, substances need higher ion concentrations.

Key concepts

pure water composition

When we talk about pure water, we refer to water that has no impurities or dissolved substances. Chemically, pure water is made up solely of H₂O molecules. These water molecules are held together by strong covalent bonds between two hydrogen (H) atoms and one oxygen (O) atom. In its pure form, water is a neutral substance with no free charges.
This neutrality is crucial because electrical conduction relies on the presence of charged particles called ions.
Since pure water lacks these charged ions, it doesn't readily conduct electricity. This absence of free-moving charged particles is why pure water cannot carry an electric current effectively.

ionization of water

Although pure water is mostly neutral, it does undergo a small degree of ionization. This means that a very tiny fraction of water molecules splits into ions.
Specifically, water dissociates to form hydrogen ions (H⁺) and hydroxide ions (OH⁻). However, this ionization happens at a very low level.
For instance, in 1 liter of water, only about 1 in 10 million water molecules ionizes. This limited ionization results in a very low concentration of ions.
Because the number of free ions is so minuscule, it significantly limits the ability of pure water to conduct electricity.
Simply put, the water's auto-ionization—while essential for various chemical reactions—does not provide enough ions to facilitate electric conductivity.

electric current

Electric current is the flow of electric charge. For this flow to occur, there must be charged particles that can move freely within the medium.
In conductors like metals, free electrons move through the material, enabling electric current. In solutions, ions serve this role.
In the case of pure water, the very few ions present due to minimal ionization of water (H₂O → H⁺ + OH⁻) are not sufficient to create a significant flow of electric charge.
This means that, while pure water can technically conduct electricity, it does so very poorly.
For good electric conductivity, a solution must have a higher concentration of ions, which is not the case with pure water. That is why pure water is considered a very weak electric conductor.