Air has a density of 0.0012 g/cm3. Any gas that has a smaller density than that of air will float in air. If a balloon is filled with helium gas, the balloon will float in air, because the density of helium is 0.00017g/cm3, which is less than that of air.

Weather balloons, which are used to gather information for weather forecasting, use hydrogen which has a density of 0.0000899 g/cm3. (Note that this is less than the density of air.) The density of carbon dioxide is 0. 00198 g/cm3 which is larger than the density of air, and therefore a balloon filled with carbon dioxide will sink to the ground and not float. Carbon dioxide is used in fire extinguishers. Because carbon dioxide is more dense than the oxygen in the air, it will displace the oxygen and ‘starve the fire’ of oxygen.

Any liquid or solid that has a density less than that of water (Density of 1 g/cm 3) will Boat on water. Oil will float on water because it has a density of between 0.8g/cm3 and 0.96g/cm3.

Although polystyrene has a density of 1.05g/cm3, which is slightly higher than that of water, polystyrene will float on water. Polystyrene has a large number of air spaces between its particles. This makes its overall density smaller than that of water. Hence, it floats on water. Similarly, ships, which are made mostly of iron (with a density of 7.8g/cm 3) float on water. A large portion of the volume of the ship is made up of air, making the average density of the ship (air plus iron) less than the density of the water on which it floats.