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Why does hot water come out of a faucet cloudy white?

The cold water comes out clear, but if you turn the hot water tap on, it'll be clear until it gets hot and then turns a cloudy white colour.

I've noticed this happens with several faucets, not sure if it always happens or not though.

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This phenomenon is due to the presence of air bubbles in the water.

First note that the solubility of air in water decreases as temperature increases.

Therefore when water is heated, less air can be dissolved in the water and when the water leaves the highly pressurized pipes, the air within the water is able to form bubbles and escape into the surrounding cooler air.

The presence of these escaping air bubbles gives the water a cloudy appearance and the water is perfectly safe to drink.

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  • $\begingroup$ "Perfectly safe to drink" as long as there are no other factors involved. Which often do exist. It's generally as safe to drink as the cold tap, though. $\endgroup$ – Emilio Pisanty Jun 30 '15 at 9:24

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