Does pure water conduct electricity?

We all learn early that water and electricity are a terrible combination, and that lesson is absolutely right. So it comes as a surprise to discover that water itself, the pure stuff, is actually lousy at carrying an electric current. The danger comes from what is hiding in it.

The short answer

Pure water conducts electricity very poorly. The US Geological Survey goes as far as calling pure water an excellent insulator. What makes ordinary water conductive is the stuff dissolved in it, mainly salts and minerals that break apart into electrically charged particles called ions. Even a small amount of those ions is enough to let current flow.

The real story

For electricity to travel through something, there need to be charged particles that can move. In a metal wire, those are electrons that are free to wander. In water, the job is done by ions.

A water molecule on its own is electrically neutral. It has no overall charge, so a glass of perfectly pure water has very little to carry a current. But real-world water is almost never pure. Tap water, rainwater, river water and seawater all contain dissolved substances. When a salt such as table salt dissolves, it splits into positively charged ions, called cations, and negatively charged ions, called anions. Common examples in everyday water include sodium, calcium and magnesium. Those ions are free to drift through the liquid, and when a voltage is applied, they move and carry the current with them.

That is why scientists measure a water sample’s conductivity to estimate how much is dissolved in it. The more dissolved salts, the more ions, and the better it conducts. Seawater, packed with salt, conducts very well. Water that has been carefully distilled or deionised conducts far less, though the USGS points out that even very pure water can still contain some ions.

Why people get it wrong

The myth is born from a safety message that is completely correct for real life. Every bit of water you will ever meet outside a laboratory contains dissolved minerals, so in practice it does conduct, and electrical appliances near baths, sinks and puddles are genuinely dangerous. Over time, the useful rule of thumb that water conducts electricity got shortened into the idea that water itself is a great conductor.

It is a bit like blaming the bus for being late when it is actually the traffic. The water molecules are mostly innocent bystanders; the dissolved ions are doing the work. But since you can never be sure the water in front of you is pure, and it almost certainly is not, the old advice still stands completely. Keep water and electricity apart.

The fun part is that this is a genuinely useful piece of science. Measuring conductivity is a quick, cheap way for scientists to check water quality in rivers and lakes, because a sudden change can reveal that something new has been dissolved into it.

“Water’s been copping the blame for years when it’s the salt doing the dirty work. I know the feeling, mate. Still wouldn’t take a toaster in the bath, mind.”