Why Does Tap Water Sometimes Come Out Milky?

Gas dissolves better when the pressure is higher and the water is colder. Inside water pipes the pressure is high, so the water holds more dissolved air than usual. The moment that water passes the faucet and returns to the pressure of the surrounding air, the dissolved air escapes all at once as tiny bubbles.

It is the same principle as the fizz that rises when you open a soda. The many bubbles scatter light in every direction, so the water looks milky white. The U.S. Geological Survey (USGS) explains that this often happens when it is very cold outside. Air that was dissolved in the cold water escapes more easily once the water warms up a little on its way through the pipes.

When water looks cloudy, it is easy to think too much disinfectant went in or that dirt got mixed in. But local water-supply guides in Korea say this is caused by air in the water, not by foreign matter or chemicals. It can also appear when water flows back in after a supply outage.

Watch a glass and you can see a clue. Bubbles are light and rise, so the water clears from the bottom of the glass upward. Once the bubbles have escaped into the air, it is over.

If the water stays cloudy after several minutes, or you see sediment settling at the bottom or a yellowish tint, bubbles may not be the whole story. In that case, let the water run for a while, and if it is still the same, it is a good idea to contact your local water service.

Rusty water is often said to come from old pipes or from pressure changes after an outage. White bubbles rise and vanish as time passes, while colored water keeps its color even if left alone, and that is a clue for telling them apart.