Due to what is ice less dense than water?

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Answer 1 · 2018-02-16T18:25:24

Due to hydrogen-bonding!

Hydrogen-bonding leads to water forming a bulk interaction that leaves it more dense than the crystal structure of ice.

As we know,

#rho_"ice" = "0.9167 g/cm"^3# at #0^@ "C"#
#rho_"water" = "0.9998425 g/cm"^3# at #0^@ "C"#

so water is denser by a factor of about #1.091#.

That leads to a solid-liquid coexistence curve with a left-tilt, i.e. increasing the pressure a lot forms water, not ice.

The slope of that curve is approximately given by the Clapeyron equation for the solid-liquid phase equilibrium:

#(dP)/(dT) = (DeltabarH_(fus))/(T_fDeltabarV)#

Since #barV prop 1/rho#, and the density decreases going from water to ice, #DeltabarV# is positive
#T_f# (being in #"K"#) is positive.
Freezing removes heat, so #DeltabarH_(fus)# is negative.

#(dP)/(dT) = ((-))/((+)(+)) = (-)#

Thus, #(dP)/(dT) < 0#, and the slope of this solid-liquid coexistence curve is negative.