To discover the difference in density between hot and cold water.
Fill one balloon with hot water from the tap.
Fill a second balloon with cold water
Fill tank with hot water from tap
The balloon filled with hot water will float and the balloon filled with cold water will sink! This happens because cold water is denser than hot water. As water is heated, it molecules move faster. This causes the molecules to spread apart from one another, causing the density of the water to decrease. as water cools, its molecules move slower. This causes the molecules to move closer together, causing the density of the water to increase.