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I know the phase diagram shows negative slope for water which is an exceptional case... But how do you read that graph and get to the conclusion that density is increasing? It's the one with pressure plotted against temperature Thanks ..

sorry i couldn't attach the graph here...

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  • $\begingroup$ en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation $\endgroup$
    – orthocresol
    Commented Jun 9, 2016 at 17:53
  • $\begingroup$ I know this equation.. But i thought maybe a different way to explain.. Thanks orthocresol $\endgroup$
    – Felix_17
    Commented Jun 9, 2016 at 18:11
  • $\begingroup$ And water is not that exceptional at all. Amongst the elements at least Si, Ge, Sb, and Bi do as well (and at least one other that escapes me at the moment). $\endgroup$
    – Jon Custer
    Commented Jun 9, 2016 at 18:21
  • $\begingroup$ There is a nice phase diagram for water listed on physics.SE. $\endgroup$
    – jpaugh
    Commented Jun 9, 2016 at 20:11

1 Answer 1

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Draw an isotherm at a low enough temperature (< the "triple point temperature of 0.01°C), so that it passes through "all three" phases (ignoring the high-pressure ices that are not relevant to the question).

Note that as pressure goes up you go from solid to liquid. Increasing pressure always favors a denser phase, so liquid is denser than solid for water.

Water is not quite unique in this respect. Silicon and germanium are two elements that do the same thing.

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