I have seen the various posts regarding the comparison of weight of heated and cold water.But is there any contradiction?I live in really hot conditions and as such tap water literally 'boils' here.What we do is to let the water flow to release the initial hot water and later obtain the cold water. Now , as water storage tanks have the supply ducts at the bottom and the hot water at the top,how is it that one gets hot water first? Could someone please ,explain this?

  • $\begingroup$ Tap water literally boils there? $\endgroup$ – BMS May 19 '14 at 5:23
  • $\begingroup$ Yeah,'LITERALLY' $\endgroup$ – Abhinav May 19 '14 at 5:26
  • $\begingroup$ Anyways what could could be the cause of this? $\endgroup$ – Abhinav May 19 '14 at 5:26
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    $\begingroup$ Could be the pipes between the storage tank and your faucet are warmed by your house. So running the faucet flushes the house-warmed water from the pipes. $\endgroup$ – BMS May 19 '14 at 5:28
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    $\begingroup$ This question appears to be off-topic because it is about plumbing (which is far harder than general relativity - trust me I've attempted both) $\endgroup$ – John Rennie May 19 '14 at 5:56

I'm no plumber, but one explanation might be that one of your water pipes is in physical contact with something that gets hot during the day.

For example if the pipe that supplies water to your kitchen faucet were to run directly beneath a blacktop or driveway or something, the water that sat in the pipe while the sink wasn't running would be basically getting baked by heat from the sun. Turning your faucet on for a brief time would let this hot water flow through the pipe and after a moment the water would feel normal again.

Oh and regarding what you said about the "weight" of hot vs. cold water:

Changing the temperature of water does not make it weigh any more or less than it did before. The same amount of matter is still there. An increase in temperature, however, will allow the particles that compose the substance to move much faster. Have you ever taken a balloon outside on a cold day and watched it shrivel up? That's because the particles that make up the gas in the balloon get cold, slow down a lot and begin to not press as hard on the sides of the balloon as they did before because they aren't moving as fast when they bounce around inside there.

It's a similar story with liquid water, except liquid water is a condensed phase so the changes in volume that accompany changes in temperature are MUCH less noticeable. Condensed phases generally have a small coefficient of thermal expansion, which is defined as:

Coefficient of Thermal Expansion

Just for the sake of being thorough :]

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