The seemingly foamy appearance of water issuing from hot water faucet is due to the air that is dissolving in the water. The heat tends to make the air come out of any solution and form bubbles. This is true in case of water also. But the pressure of the water within the pipe prevents air’s escape until the faucet is opened. Once the faucet is opened, it reduces the pressure and thus permits the bubbles to form which give the water its milky-white appearance. In a few minutes the bubbles escape and the water is again clear.
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