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Name: Shannon
Status: educator
Grade: K-3
Location: ME
Country: N/A
Date: October 2006

Question:
Why is it when I get a glass of water from my tap, it appears cloudy,(but it is just a lot of, what looks like air bubbles). And in a few seconds the water comes clear?

What causes this? And why?



Replies:
Shannon,

Water pipes are not completely filled with water. Since air is much more compressible than water, having pipes being partially occupied by air, allows for much easier changes in volume of water without too much pressure changes in the pipes.

Depending on the turbulence of the flow out of your tap, some of the air can get physically trapped within the water. Since water has a limit on how much air may be dissolved in at any particular temperature, the trapped air does not dissolve and remain as air bubbles which eventually rise to the surface and escape.

The water will appear turbid for a while because the air bubbles tend to reflect light and this multiple reflections tend to make the water appear turbid.

Greg (Roberto Gregorius)


One possibility:

Solubility of gasses in water _decrease_ with temperature.

Water at the supply plant is deliberately well aerated in open air. So this water is saturated with respect to 0.8 atmosphere N2 and 0.2 atmosphere O2 when it is at room temperature. Then when it is pumped into the pipes to be sent to you, the pressure around the water increases to roughly 100psi, ~ 5-10 atmospheres. So the gas is locked into the water while at pressure; no tiny precipitation bubbles can form in the pipe even if the temperature rises a little. When the water enters your home water-heater and heats up a lot, the gas-solubility decreases by maybe a factor of 3. So now it tries to precipitate bubbles at up to 3 atmospheres, but inside your house pipes the pressure is still >5 atmospheres, so the bubble still cannot form. Finally when the water squeaks past your faucet valve the pressure around it drops to 1 atmosphere, and finally the bubbles can form.

Because that pressure drop is very sudden and the oversaturation is now ~200%, the gas molecules have no time to wander about and find the best big bubble to join. Instead they must precipitate in a large number of separate tiny bubbles.

Being bubbles, they slowly rise to the top and escape into the surroundings. Or, if you fill a clear bottle and seal it, then wait until the hot water cools, the cooled water may re-absorb the gas causing the bubbles to disappear, even the ones that stuck to the walls and never floated to the top.

This is a round trip. Now the water is at room temperature and 1 atmosphere, and if you leave the bottle un-capped for a week, eventually the water is 100% air-saturated, all the same parameters it had before entering the pipes.

Jim Swenson



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