Why is tap water sometimes milky white, then clear?
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A glass of fresh tap water, white as milk. On a winter morning, plenty of kitchen taps pour a glass like this. Don't pour it out yet. Set it on the table and watch it for a minute.
Something odd happens: the bottom clears first. The clear part climbs, bit by bit, from the bottom up. A minute or two later, the whole glass is clear, with nothing left behind.
If it were dirt, it would sink, and the bottom would be the cloudiest. This glass does the opposite. So the white went up and out. What goes up? Air.
The white is thousands upon thousands of tiny bubbles, each too small to see. But every bubble bounces the light that hits it off to the side. Thousands bouncing at once, and light can't get through. The whole glass looks white.
That's the same reason clouds and milk are white. So how did the air get into the water? Think of a water pipe as a subway car at rush hour.
The water in the pipe is always being pushed hard, packed in tight. The harder the push, the more people fit in the car. The harder water is squeezed, the more air it can hide. That air is dissolved, like sugar in water: you can't see it.
Opening the tap is like the doors opening at a station. The squeeze is gone, and the water can't hold all that air. The extra pours out as tiny bubbles. Passengers head upstairs to the exit, and bubbles float up to the surface.
The bottom empties first, so it clears first. So why is it worse in winter? Here the subway stops working as a picture. A train holds the same crowd hot or cold. But the colder water is, the more air it can hold.
Water near freezing can hold more oxygen. About one point six times as much as at twenty degrees. Icy water warms up in your home's pipes. The extra air is held in by the pressure, until you open the tap. Right after pipe repairs, when the water comes back on, air has gotten into the line, and the same thing happens.
Water utilities say this white is harmless. Let it sit, and use it as usual. But if it clears from the top down, or stays white after five minutes, or smells odd or has a color, it isn't air. Ask your water company. You've seen this before: crack open a soda, hear the hiss, and watch it fizz.
That's gas squeezed in too, and when the door opens, out it all comes. Tap water, soda, and what else fizzes the moment you open it? You fill in the blank.
Don't pour it out. Wait a minute
On a winter morning you turn on the tap and fill a glass. The water comes out cloudy white, like milk.
Set it on the table and watch. The bottom clears first, and the clear part slowly climbs. In a minute or two, the whole glass is clear.
Clearing from the bottom up is the biggest clue.
Dirt sinks. This goes up
If there were dirt in the water, it would sink, and the bottom would be the murkiest. White water does the opposite: the bottom clears first, so the white is leaving upward.
What's going up is air.
Millions of tiny bubbles bouncing light
The white is millions of tiny bubbles, each too small to see. But every bubble bounces the light that hits it off to the side.
With so many bouncing at once, light can't get through, so the whole glass looks white. Clouds and milk are white for the same reason. (Light bouncing off things can also turn the sky blue: see why the sky is blue.)
The water pipe is a rush-hour subway car
Water in the pipes is always being pushed hard, packed in tight. The harder a subway car is packed, the more people fit inside. The harder water is squeezed, the more air it can hide. That air dissolves in the water, like sugar does, so you can't see it.
Turning on the tap is the doors opening at a station. The squeeze is suddenly gone, the water can't hold all that air, and the extra comes out as tiny bubbles. Passengers head upstairs to the exit; bubbles float up to the surface. The bottom empties first, so the bottom of the glass clears first.
Where the comparison breaks: how many people fit in a car has nothing to do with temperature, but colder water can hold more air.
Why it happens more in winter
Water near freezing can dissolve about 1.6 times as much oxygen as water at 20°C (14.6 vs 9.1 milligrams per liter).
In winter, ice-cold water flows into the pipes in your home and slowly warms up. The air it can no longer hold is kept in by the water pressure. Open the tap and it all comes out. The same thing happens after pipe repairs, or when the water comes back after a shutoff, because air got into the pipes.
When it's not air
from the bottom up is fine Water utilities say it's just air, harmless to your health; let it sit and use it as usual. But if you see any of these, it isn't air. Call your water company:
- It clears from the top down, or leaves grit at the bottom
- It's still white after five minutes
- It smells odd, or looks yellow or brown
It's air squeezed into it,
rushing out when the door opens.
Besides tap water and soda,
what else bubbles the moment you open it?
Fill in the blank.
Quick check
A glass of white tap water starts clearing from the bottom up. What does that tell you?
Like passengers heading upstairs when the subway doors open, the bubbles float up, so the bottom empties first.
