Final answer:
Water's density is different from most other substances because it reaches its highest density at about 4°C. As water cools, its density increases, but when it freezes into ice, it becomes less dense due to the arrangement of its molecules.
Step-by-step explanation:
The density of water is different from the densities of most other substances due to its unique property of reaching its highest density at about 4°C. As water cools, its molecular motion slows down, causing the water molecules to move closer together and increasing its density. However, when water freezes into ice, it expands and becomes less dense than liquid water. This is because the hydrogen bonds between water molecules cause them to line up less efficiently in ice, leading to more space between the molecules and lower density.