Why is water sometimes white?

Question

such as from the tap (at times) , or when it's hot

Answer 1

It is often from air that got into the water lines. Also if there is high mineral content an agitated state of movment or heat can cause the water to whiten.

Answer 2

It's full if tiny air bubbles that appear white or cloudy because they reflect light. Most faucets even have airators on the end of the tap to give a more even, less splashy stream of water.  

If you let the glass sit on the counter for an hour or so (not sure exactly how long), you'll see that it eventually clears.

Answer 3

Air in your pipes. when they fix something down the road it happens, just air.