AP BIOLOGY

Detailed explanation-1: -The “stuff” (molecules) in water is more tightly packed than in ice, so water has greater density than ice. Don’t let the fact that ice is a solid fool you! As water freezes it expands. So, ice has more volume (it takes up more space, but has less density) than water.

Detailed explanation-2: -The extra hydrogen bonds that occur when water freezes increase the space between molecules, causing a decrease in overall density. In fact, each water ice molecule forms hydrogen bonds with four other molecules, while water molecules only form hydrogen bonds with an average of 3.4 other molecules.

Detailed explanation-3: -The orderly, crystalline way in which the hydrogen-bonds form causes density to decrease because each water molecule is held away from its neighbors at a distance equal to the length of the hydrogen bonds. Thus water expands as it freezes, and ice floats atop water. This property is crucial to life as we know it.