Density is the mass of a substance for a specific volume, the mass per unit volume. Water is roughly 1 gram per milliliter. A substance that is less dense than the liquid around it floats, and one that is more dense sinks. Ice floats because freezing spreads water molecules out, so ice is less dense than liquid water. Dissolved salt makes water denser, and warming most materials makes them less dense. Density is a property that helps identify a substance.