Question

density of ice vs. liquid water

key knowledge
liquid water is denser than solid ice (at \\(0^\circ\text{c}\\), \\(\text{ice} = 0.917\text{ g/cm}^3\\), \\(\text{water} = 1.000\text{ g/cm}^3\\)).
when water freezes, molecules arrange into an open hexagonal lattice held by hydrogen bonds.
this lattice structure has more space between molecules than liquid water, so ice is less dense than liquid water.
this is anomalous - for most substances, the solid form is denser than the liquid form.
ecological consequence: ice floats on water, insulating aquatic life below during winter.

q7 explain at the molecular level why ice is less dense than liquid water. your answer must include reference to hydrogen bonding and the arrangement of molecules in each state.

Explanation:

Describe the molecular arrangement in ice

In solid ice, water molecules are held in a rigid, open hexagonal lattice structure by stable hydrogen bonds, which maximizes the distance between the molecules.

Compare with the molecular arrangement in liquid water

In liquid water, thermal energy causes hydrogen bonds to constantly break and reform, allowing the water molecules to pack more closely together.

Relate molecular spacing to density

Because the open lattice structure of ice contains more empty space between molecules than the disorganized arrangement of liquid water, ice has fewer molecules per unit volume, making it less dense.

Answer:

Ice is less dense than liquid water because when water freezes, the molecules arrange themselves into a rigid, open hexagonal lattice held together by stable hydrogen bonds. This crystalline structure keeps the water molecules further apart than they are in the liquid state, where hydrogen bonds are constantly breaking and reforming, allowing the molecules to pack more closely together. Consequently, ice contains more empty space and has a lower density than liquid water.