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I read this on the wikipedia page for tap (valve):
At intermediate flow settings the pressure at the valve restriction drops nearly to zero from the Venturi effect; in water taps, this causes the water to boil momentarily at room temperature as it passes through the restriction. Bubbles of cool water vapor form and collapse at the restriction, causing the familiar hissing sound.
I don't understand why the water boils when going through the tap. Could someone explain it in more detail?
When the water goes through the tap, there is the so-called Venturi effect. In this effect, a fluid going through a constricted section experience a reduction in his pressure.
The boiling point of water is dependent from its pressure. At normal atmospheric pressure ($101325$ Pa) water boils at 100 grades Celsius, but if you reduce somehow its pressure (like it happens in a valve) its boiling temperature get lower, as you can see from this table.
Eventually, the boiling temperature will result lower than the room temperature, and this will cause water to boil. When pressure get bigger again, the boiling temperature will increase and you won't have boiling any more.
As the water travels into the constriction it speeds up, the same volume is trying to squeeze down a smaller tube.
This speed is kinetic energy that comes from the water pressure. Thus it takes energy out of the water pressure, reducing it.
The molecules speed up as they enter the constriction and the gaps between them and the molecules behind them become larger.
If the pressure drops low enough the water molecules escape their hydrogen bonds with their neigbours becoming gaseous.
