A quick comparison of densities

Demonstration

This shows dramatic differences in the density of air, water and mercury.

Apparatus and Materials

• Similar bottles, 3 (about 150 to 200 ml volume)
• Domestic spring balance or a top pan balance (5 kg +/- 2 g)
• Mercury
• Tray to hold the bottle filled with mercury

Health & Safety and Technical Notes

Take the usual care when using mercury, but, as it is in a sealed container, it presents minimal risk.

Read our standard health & safety guidance

The bottles should be filled with air, water and mercury. All should be sealed securely and similarly. (Those sold as medical flats are suitable.)

Procedure

1. Find and record the masses of the three bottles.
2. Get the students to work out how much heavier the mercury is than the water, as a ratio.

Teaching Notes

• Before the weighings, discuss the purpose of the bottle filled with air. It leads to the assumption that the mass of air is negligible compared with the mass of the container. Hence the mass of water and mercury can be calculated.
• If possible, allow students to lift the bottle of mercury. (Insist that the bottle stays with its tray.) Their surprise at how heavy it feels will be memorable.
• The volumes of mercury and water are the same, so the ratio of their masses gives the ratio of their densities. The absolute density can be found if their volumes are measured.
• You could say: "Returning to the calculation of the velocity of air molecules at the base of the atmosphere, we now know that the density of mercury is about 13.5 times as dense as water. So, a water atmosphere would have to be 13.5 times as high as the mercury height, 13.5 x 75 cm: about 10 m. What about an atmosphere of air? Not air that gets thinner and thinner all the way up, but air that stays just as dense as it is here in this room?"
• Now we are going to need a comparison of the density of water and the density of air. See the experiment:

  Measuring the density of air 3

This experiment was safety-tested in July 2006