15.1. Properties of Water http://www.ck12.org
Structure of Ice
Liquid water is a fluid. The hydrogen bonds in liquid water constantly break and reform as the water molecules
tumble past one another. As water cools, its molecular motion slows, and the molecules move gradually closer to
one another. The density of any liquid increases as its temperature decreases. For most liquids, this continues as
the liquid freezes, and the solid state is denser than the liquid state. However, water behaves differently. It actually
reaches its highest density at about 4°C (Table15.1).
TABLE15.1: Densities of Water and Ice
Temperature (°C) Density (g/cm^3 )
100 (liquid) 0.9584
50 0.9881
25 0.9971
10 0.9997
4 1.000
0 (liquid) 0.9998
0 (solid) 0.9168Between 4°C and 0°C, the density of water gradually decreases as the hydrogen bonds begin to form a network
characterized by a generally hexagonal structure with open spaces in the middle of the hexagons (Figure15.6).
FIGURE 15.6
The structure of liquid water (left) con-
sists of molecules connected by hydrogen
bonds that are short-lived, since water
is a fluid. In ice (right), the hydrogen
bonds become rigid, resulting in an inter-
connected, hexagonally shaped molecu-
lar framework.Ice is less dense than liquid water, so it floats. Ponds or lakes begin to freeze at the surface, closer to the cold air. A
layer of ice forms, but it does not sink as it would if water did not freeze in this unique structure, which is dictated by
its shape, polarity, and hydrogen bonding. If the ice were to sink as it froze, entire lakes would freeze solid. Since the
ice does not sink, liquid water remains under the ice all winter long. This is important, as fish and other organisms
are capable of surviving through winter (Figure15.7). Ice is one of only very few solids that is less dense than its
liquid form.
Properties of Water
Compared to other molecular compounds of relatively low molar mass, ice melts at a very high temperature. A
great deal of energy is required to break apart the hydrogen-bonded network of ice and return it to the liquid state.
Likewise, the boiling point of water is very high. Most molecular compounds of similar molar mass are gases at
room temperature.