As the temperature of liquids increases, the density decreases and the volume increases. The density of a liquid is found by dividing mass by volume.
Use the equation D=m/V, where D is density, m is mass and V is volume, to find the density of a liquid. In this case, mass remains constant, and only volume increases. As water is heated, energy is added to the molecules, causing them to move faster. This increase in velocity causes the molecules to take up more space, so the volume increases. Since the mass of the liquid remains constant and volume increases, the density must decrease.