Water is unusual in that its maximum density occurs as a liquid, rather than as a solid. This means ice floats on water. Density is the mass per unit volume of a material. For all substances, density changes with temperature. The mass of material does not change, but the volume or space that it occupies either increases or decreases with temperature. The vibration of molecules increases as temperature rises and they absorb more energy. For most substances, this increases the space between molecules, making warmer liquids less dense than cooler solids. However, this effect is offset in water by hydrogen bonding. In liquid water, hydrogen bonds connect each water molecule to approximately 3.4 other water molecules. When water freezes into ice, it crystallizes into a rigid lattice that increases the space between molecules, with each molecule hydrogen bonded to 4 other molecules.