Heavy ice is the densest form of water. Regular water is denser than both regular ice and water vapor. Heavy water has both hydrogen atoms replaced with deuterium. Heavy-water ice sinks in normal water, whereas regular ice floats.
The hydrometer is the instrument used to measure the density of a liquid. It is a simple scientific measuring device. Density is the mass per unit volume of a substance. Water's density varies by its temperature. Regular ice is less dense than water because of the lattice arrangement of the water molecules. It allows the ice to become more spread out than the liquid.