Eli5: Why dose warm/hot water come out of the tap “milky” or foggy white when cold water comes out clear?



Eli5: Why dose warm/hot water come out of the tap “milky” or foggy white when cold water comes out clear?

In: Chemistry

cold water absorbs more air than hot. when it is no longer squeezed in the pipe hot water releases millions of tini tiny bubbles of air.

The tap water has gas dissolved in it, much like carbon dioxide in a fizzy drink.

The amount of gas that can be dissolved depends on the temperature and pressure. If you lower the pressure, less gas is soluble so it bubbles out, like opening a soda. Similarly, the lower the temperature, the more gas water can hold, like opening a warm soda vs a cold soda; the warm one will fizz a lot more because the gas is less soluble.

When water comes out of the tap, pressure is released. In cold water, the gases mainly stay dissolved. In hot water, they are less soluble and escape as small bubbles which make the water appear milky until it has settled for a while.

You can detect that the dissolved gas is present in cold water by freezing it. You will notice the center of the ice will have a milky region of small bubbles as the gases are forced to the center during freezing. If you freeze hot water, you will notice a lot less cloudiness in the ice because the heating forced the gas out.

Another reason may be that you have hard water and now have mineral/calcium build up in your water heater.

The closer you get to about 200F the closer you get to the minimum gas solubility in water. If you can somehow pressurize the water to keep it a liquid beyond its boiling point, then it can start holding more gas out beyond 200F.

I don’t know, but I did read on Reddit that humans can tell the difference from hot and cold running water by sound.

Fun trick, if it comes out really milky and clears after a few minutes (this means there is undissolved air in the form of countless miniature bubbles (the smaller the bubble the longer it will take to clear) then pour some into a ceramic mug, put a spoon in the mug and tap the bottom of the mug repeatedly with the spoon. You should get an awesome hollow sound. I believe the technical term fruit this is actually “the hot chocolate effect” as this is the best example. The sound will change slowly as the bubbles come out.

Water coming into your house has dissolved air in it. Unlike solids like salt or sugar, which are more soluble in liquid at higher temperatures, gases become less soluble at higher temperature. So when the cold water goes into your water heater and gets hot, all the dissolved gas wants to come out. But, since the water lines are under pressure and there’s nowhere for the gas to escape, the gas is forced to stay dissolved in the water. When you open the tap and that pressure goes away, the dissolved gas is free to come out of solution and creates that foggy appearance.

I think it has to do with minerals in the water, hot water has more minerals/sediment.. I could be wrong though, the people talking about gas in the water seem to now what they’re talking about.

Another alternate possibility:

If you live in north america and have a hot water tank then it’s possible there is something growing inside it

Whether it’s bacterial or some kind of algae I dont know, but it comes out as grey/white clumps in a runny slime when you drain the tank. As long as the clumps arent too big or the slime too thick to come out the drain

I’m not saying every tank has it. I’ve only seen it a couple times over the last ~10 years, but I also dont do that much work on hot water tanks