Does Distilled Water Conduct Electricity? Explained

Does distilled water conduct electricity? Only very poorly. Electricity moves through water when it contains ions, electrically charged atoms or groups of atoms. Distillation removes almost all the dissolved ions, so distilled water is a far weaker conductor than tap water or salt water. It is still not a perfect insulator, because a small number of ions are always present.

To answer what is distilled water first: it is water boiled into steam and condensed back into liquid, so most of its minerals and salts stay behind.

Why does water conduct electricity at all?

Pure water is a very poor conductor, but it is not zero. Water molecules split a tiny amount into hydrogen ions (H⁺) and hydroxide ions (OH⁻):

H₂O ⇌ H⁺ + OH⁻

At 25 °C this happens to about one molecule in every 550 million, which gives [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol/dm³ and a pH of 7. The ions are so few that the water barely conducts. The theoretical limit for pure water at 25 °C is about 0.055 µS/cm (microsiemens per centimetre), which is the same as a resistivity of 18.2 MΩ·cm.

Electric current works because charge can move. In a metal wire, the charges are electrons. In water, the charges are ions, and they carry the current only when they can move freely. This is why electrolytes (solutions of ionic compounds) conduct and pure, non-ionic liquids do not.

Why does tap water conduct?

Tap water contains dissolved minerals and salts, which split into ions as they dissolve:

• Sodium chloride (table salt): NaCl → Na⁺ + Cl⁻
• Calcium and magnesium (the cause of hard water): Ca²⁺ and Mg²⁺
• Hydrogencarbonate and sulfate ions from the rocks the water flows through

The more of these ions there are, the higher the conductivity. Tap water is often in the range of several hundred µS/cm, while sea water is around 50 mS/cm (50 000 µS/cm), about a hundred times more than tap water.

Why does distilled water conduct so poorly?

Distillation boils water into steam and condenses the steam back into liquid. Salts and minerals stay behind because they do not evaporate at 100 °C, so the condensed liquid contains very few ions. This is why distilled water does not conduct electricity well: its conductivity is far lower than tap water's.

There are two sources of ions that remain:

1. Dissolved gases. Carbon dioxide from the air dissolves in water and forms ions: CO₂ + H₂O ⇌ H⁺ + HCO₃⁻. This is why distilled water left open to the air becomes slightly acidic, usually around pH 5.5 to 6, and conducts a little more.
2. Self-ionisation. The H⁺ and OH⁻ from H₂O ⇌ H⁺ + OH⁻ are always there, as described above.

In a laboratory, distilled water typically conducts at a few µS/cm. Deionised water, which is treated by ion exchange to remove ions specifically, is cleaner still and is close to the 0.055 µS/cm limit.

How is conductivity measured?

Distilled water conductivity is measured with a conductivity meter, which has two electrodes that sit in the liquid. A voltage is applied, and the meter measures how much current flows, then reports the conductivity in µS/cm. Because the readings are fast and simple, they are used to check water purity in labs, aquariums, boilers and drinking-water plants.

You can also see the effect with a simple circuit: a bulb, a battery and two electrodes. Put the electrodes in salt water and the bulb lights. Put them in distilled water and it stays dark or glows only faintly, because the circuit has very few ions to carry the current.

Common mistakes

• "Distilled water is a perfect insulator." It is not. It conducts a little, and the amount depends on how much CO₂ and other ions it has picked up.
• "Electricity travels through the water itself." The current is carried by the ions, which move through the liquid. Pure water molecules do not carry charge.
• "Distilled water is safe near electrical appliances." Water with dissolved ions conducts, and mains electricity is dangerous. Keep electrical equipment away from water whatever it is made of.

Frequently asked questions

Does distilled water conduct electricity?

Distilled water conducts electricity only very weakly, because distillation removes most dissolved salts and minerals, which are the ions that carry the current. It is not a perfect insulator. A typical value is a few µS/cm, compared with several hundred µS/cm for tap water.

Why does distilled water not conduct electricity well?

Electrical current in water is carried by ions. Distillation boils off water and leaves salts and minerals behind, so few ions remain. The only ions left are from water's own self-ionisation (H⁺ and OH⁻ at about 1.0 × 10⁻⁷ mol/dm³) and from carbon dioxide dissolved from the air.

Is distilled water a good conductor?

No. Good conductors such as copper have free electrons that carry current easily. Distilled water has only a tiny number of ions, so it is a very poor conductor. Salt water is a much better one because the dissolved sodium and chloride ions carry the current.

Does deionised water conduct electricity?

Less than tap water, and usually less than distilled water. Deionised water has had its ions removed by ion exchange, so its conductivity can be close to the theoretical limit of about 0.055 µS/cm for pure water at 25 °C.

Try it yourself

Open the Electrolysis tool and read what it says about the ions in the electrolyte. Electrolysis only works when the ions are free to move, which is the same reason a solution conducts. Then compare with the Chemistry Lab: think about which of the substances you can add would make the water conduct, and why adding a salt changes the result. For the ideas behind ions and charge, read how do ions form? and ionic vs covalent bonds.