**Temperature and salinity both affect the density of water.** As the temperature of the water decreases, the density of the water increases.

When water cools, it causes the molecules to move closer together as they slow down. This slowdown of the molecules causes the water to become denser. The opposite happens as water heats up. As the temperature of the water rises, the molecules speed up and begin moving farther away from one another. As the molecules begin spreading out, the water becomes less dense. On average, the less dense the water, the higher the level it floats within bodies of water. For instance, the bottom of the ocean contains water that is much denser than the water on the top.

Salinity also plays a part in the density of water. Water that contains salt is much denser than water that does not contain salt. The more salt in the water, the denser it becomes.

Scientists define density as the measure of mass per volume of liquid. The equation for calculating density is d = m / v. Rearranging the equation can help users determine the mass when given the density. This equation is mass = density x volume.