Question

In most compounds, the solid phase is denser than the liquid phase. Why isn't this true for water?

Short answer

Water is an exception to the general rule that the solid phase is denser than the liquid phase due to the hydrogen bonds in its solid form, ice. In ice, water molecules form a stable, hexagonal lattice structure that forces the molecules to be spread out further. This increases the volume per mass, resulting in a lower density for ice compared to liquid water. In contrast, the liquid phase has weak hydrogen bonds that constantly form and break, leading to a relatively large volume and higher density.

Step-by-step solution

Density is a measure of mass per unit volume. It can be calculated using the formula: \[ Density = \frac{Mass}{Volume} \] The higher the mass of the particles and the closer the particles are together, the denser the substance. We will now discuss how the arrangement of water molecules differs between the solid and liquid phases, and how this affects density. #Step 2: The structure of water molecules in liquid form#

In the liquid phase, water molecules are in continuous motion and have weak hydrogen bonds between them. These hydrogen bonds are constantly forming and breaking as the molecules interact. This results in a relatively large volume, which affects the overall density. #Step 3: The structure of water molecules in solid form#

When water molecules are cooled to solid phase (ice), they form a stable, hexagonal lattice structure due to the hydrogen bonding between the molecules. This structure forces the water molecules to be spread out further, resulting in a larger volume per mass. Therefore, although the mass of the water molecules remains the same, the volume increases, leading to a decrease in the density of ice compared to liquid water. #Step 4: Conclusion#

In most compounds, the solid phase is denser than the liquid phase because the particles are more closely packed together. However, water is an exception because the hydrogen bonds in its solid form (ice) create a hexagonal lattice structure that results in a larger volume per mass. This means that ice is less dense than liquid water, causing it to float when placed in water.