Final answer:
Water molecules move faster at high temperatures due to increased kinetic energy, leading to less structured liquid water. At low temperatures, molecules slow down, form stronger hydrogen bonds, and create a dense, lattice structure, resulting in the less dense ice floating on water.
Step-by-step explanation:
When the temperature is high, the water molecules move more rapidly due to an increase in thermal energy which leads to a higher average kinetic energy (KE). The greater motion allows these molecules to overcome some of the hydrogen bonds between them, making the water less structured and more fluid. In contrast, at low temperatures, the movement of water molecules slows down considerably. The decreased kinetic energy results in stronger hydrogen bonding which begins to form a rigid, lattice-like structure as seen in ice. Consequently, ice has a lower density and can float on the surface of liquid water, providing insulation to aquatic life below in cold environments.