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I have basic knowledge of chemistry and how it all works (will be taking the class next year) but I understand density and mass. Since the hotter a liquid, in this case, the molecules are more everywhere and not contained so to say. In a way, a $\ce{200 cm^3}$ cup will hold less water if say brought up to a temp of 90°C (194°F) and will hold more water if brought to a temp of 2°C(35.6°F) due to the molecules being more contained and not bouncing everywhere. Does this make sense or did I underestimate my understanding of this whole thing?

*I am talking about mass not volume.

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Water reaches its maximum density at about $\pu{4 ^\circ C}$. As its cooled from higher temperatures the density decreases as you would expect as the molecules move more slowly and they can thus "settle" in closer together.

But, as it approaches the freezing point, the molecules are moving slowly enough that they start combining together into a more crystalline, ice-like conformation. It's still completely liquid, but is starting to organize into the ice structure, and ice is about $\pu{7\%}$ less dense than liquid water.

So in the case of water, you can "fit" the most water into a given volume at about $\pu{4 ^\circ C}$.

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    $\begingroup$ The solid is less dense than the liquid for (at least) Si, Ge, Sb, and Bi (and perhaps more I've forgotten). Bismuth is technologically important as an additive in welding and brazing alloys for just this reason. Not a bad percentage for the elements. Any good synthetic chemist can do similar things with more complex molecules. $\endgroup$ – Jon Custer Jan 16 '17 at 22:50
  • $\begingroup$ Very good point @JonCuster. My statement should have been more clear and I will edit it. I meant to say something like "it is unusual for liquids found in nature". Since it was an aside not really relevant to the question I think I will just remove the statement. I appreciate the clarification and interesting examples. $\endgroup$ – airhuff Jan 16 '17 at 22:59
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    $\begingroup$ No problem! There is a very persistent meme that water is unique in having the liquid denser than the solid. I try to fight the good fight... $\endgroup$ – Jon Custer Jan 17 '17 at 0:33
  • $\begingroup$ Ah. I have got it now. Thank you very much @airhuff ! $\endgroup$ – The Animator Jan 17 '17 at 0:43
  • $\begingroup$ @Jon_Custer also Gallium, which is fun. It melts in the palm of your hand (low toxicity, but stains your skin black), and will then crack a glass bottle if you pour it into one. $\endgroup$ – nigel222 Jan 17 '17 at 10:16
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The volume is the amount of space in the container, and no matter what temperature, a liter of water at that temperature occupies one liter.

What I think you mean to say is, "Does a liter of cold water have more mass than a liter of hot water?" The answer is yes, it does.

BTW, an open container (e.g. a beaker or drinking glass), can hold a larger volume of cold water than of hot water because the cold water has higher surface tension and therefore a higher meniscus... it's over the top.

meniscus in test tube

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  • $\begingroup$ Ah yes. This does make sense. I didn't think about the surface tension of the water. That is what I mean too. I am not the best at wording questions. Thank you for your explanation! $\endgroup$ – The Animator Jan 17 '17 at 0:41
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Yes this does make some sense, though probably less than you expect. Look up water density at different temperatures. Indeed, water at 90°C is about 4% less dense than water at 2°C. Whether or not this counts as significant is up to you.

Also, welcome to Chem.SE.

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Depends on your unit of measure; volume of water, or count of water molecules. As you've noted, as water heats up, the amount of space per molecule increases. Thus, 1) the same amount of molecules will either occupy more space, or 2) the same volume will hold fewer molecules of water.

When chemists talk about water, they talk about the chemical compound H2O, and can be a bit unclear on whether they mean gaseous, liquid, or solid water. (Solid water being ice). H2O is a bit strange as far as molecules go; almost all of them take up LESS space as they get colder. Water, as noted elsewhere, takes up more space when it freezes. (Which is why ice floats). The reason which is somewhat complicated and has to do with the strength of electron bonds and crystalline structures.

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  • $\begingroup$ Yes, the water would expand due to it's crystalized structure when frozen. Above, it was stated that 4°C was the ideal temp. for holding the most water in a given volume. $\endgroup$ – The Animator Jan 17 '17 at 3:28

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