Polystyrene floating on water. Image 1 of 2. Polystyrene floats as its density (mass in a given volume) is less than that of water. The amount of polystyrene below the water line has a volume equal to the volume of water with a mass equal to that of the entire polystyrene block. Objects with the exact same density as water displace the same volume. If they are denser, the volume of water displaced is greater than the object's volume, and it sinks. Image A350/197 shows the block pushed under the water's surface, showing it displacing a volume of water equal to its volume. Note the water level at just over 700 millilitres here.