The Surprising Science of Water Density It may seem counterintuitive, but the statement that hot water is heavier than cold water actually requires a bit of clarification regarding the principles of density and buoyancy in fluids. In reality, cold water is denser than hot water. This is because as water is heated, its molecules gain energy, move more vigorously, and spread apart, leading to a decrease in density. The density of water reaches its maximum at around 4 degrees Celsius. Beyond this t