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I have come across this Quora Answer where it was written that cold water is heavier than hot water.

Are there any proofs which support it?

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  • $\begingroup$ You understand what mass and density are? Colder objects usually have higher density. $\endgroup$ – Mithoron Feb 10 '16 at 11:23
  • $\begingroup$ @Mithoron Yeah, I understand that. However, I have seen a counter argument against that which confused me. $\endgroup$ – Dawny33 Feb 10 '16 at 11:25
  • $\begingroup$ So about mass see chemistry.stackexchange.com/a/20103/9961 Yes higher energy means higher mass, but this effect is usually negligible. $\endgroup$ – Mithoron Feb 10 '16 at 12:11
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    $\begingroup$ It's confusing because you are confusing mass and density. One molecule of hot water is slightly heavier than one molecule of cold water because of the extra energy. However, one liter of cold water is heavier than one liter of hot water because many more cold molecules will fit in the liter. Therefore if you take a liter of cold water and heat it the mass will increase, but so will the volume, for a net decrease in density. $\endgroup$ – James Feb 10 '16 at 12:58
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    $\begingroup$ I'm voting to close this question as off-topic because it is about physics, not chemistry. $\endgroup$ – Jan Feb 10 '16 at 20:20
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The "counter argument" you link to cites E = mc^2 and the higher energy content of hot water.

First off, note the "c^2" in that formula. That's the speed of light, squared. It is a huge number, meaning that you need huge differences in energy to get tiny changes of mass.

The other way around, you only need little mass to generate lots of energy, something nuclear physicists have been exploiting for quite some time. ;-)

So, while that effect is "true" to some extend, it is tiny. But yes, one molecule of H2O has more mass when hot than the same molecule when cold.

But when we talk about "weight", we usually mean "density", i.e. weight per volume. And here, a different, much more pronounced effect comes into play: Most substances expand when heated, reducing their density. Each molecule might be a little bit heavier, but the molecules are much farther apart, more than offsetting the E = mc^2 effect.

Cooling down increases the density -- the same amount of matter takes up less volume. An effect that home preservers have exploited for generations when canning things: You put hot produce into a container, and close it. The contents cool down and contract, producing underpressure in the container, sealing it. That's the "pop" you hear when opening a glass of marmelade the first time.

So, per molecule hot H2O is (a bit) heavier, but per volume hot H2O is (much) less dense.

At least down to about 4 degrees celsius, where a peculiar quirk to the H2O molecule makes it become less dense again with further cooling. That's why a pond starts freezing at the surface, not the bottom, where the 4 degree water is up to the point where the whole pond is frozen solid. An effect that fish have been exploiting for quite some time now. ;-)

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  • $\begingroup$ This was the complex answer I was hoping for when Googling a similar question, but doesn't quite answer the question I had. I supposed I should ask it as a separate question: Where is the coldest water in a glass of ice water? Is cooling fairly uniform? $\endgroup$ – ND Geek Jun 24 '17 at 17:21
  • $\begingroup$ @NDGeek: The coldest water in a glass of ice water would be the ice in the ice cube, in solid state. ;-) But you're right, that's better asked as a separate question (as you are actually asking about the coldest liquid water, I presume). $\endgroup$ – DevSolar Jun 24 '17 at 18:03
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Relativistic effects (which is what that answer talks about) are tiny compared to chemical effects (that's why it took us so long to understand them). Hot water has a lower density than cold water like most other substances.

Note that there is also an anomaly concerning cold water and ice that causes ice to float, but for the most part of the temperature range, water behaves like any other substance that expands with heat.

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Water below 4 degrees is lighter than water at 4 degrees. So that means within that range that cold water is lighter than hotter. However when you compare water above 4 degrees, it gets less dense as it gets hotter.

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A proof will be the fact that water is most dense at $4^o$C and therefore sinks to the bottom and supports aquatic life even when a water body is frozen.

State of a matter or the state of aggregation as the name suggests gives us that state in which matter wants to collect or aggregate. As the temperature increases the atoms gain energy and start to move around and occupy more space as compared to when their temperature was lower hence their density decrease and hence becomes lighter.

This is a true but not completely since this is would be true if water was just what we imagine it to be. Which it's not. Due to presence of hydrogen bonding in water we get to see many amazing phenomenons.

While $\ce{H2S}$ is a gas $\ce{H2O}$ is liquid all because of H-Bonding.

Water is most dense at $4^o$C. This contradicts my original explanation but is supported by the fact that the explanation is very broad and not completely correct. The anomaly can be explained by presence of H-Bonding.

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