Density means the mass of 1 cm3 of a substance. For example, 1 cm3 of water has a mass of 1 g – its density is 1 g/cm3. Lead, on the other hand is much denser than water. 1 cm3 of lead has a mass of 11.3 g ∴ lead has a density of 11.3 g/cm3.
The density of a substance depends on two factors; the mass of the particles from which it is made and how closely packed together those particles are. Gases, with their particles very much further apart than liquids or solids, have the lowest densities of all.
It is odd to think of gases as having mass, but if you burn some magnesium the magnesium oxide that forms is heavier than the piece of magnesium you started with. The difference is caused by the mass of the oxygen atoms that have combined with the magnesium. Different gases have different densities. The noble gases increase in density as they increase in atomic size.
Changes in the density of air as it warms or cools leads to convectional currents and wind. Warm air is less dense so it rises whilst cooler air is more dense and sinks.
We can calculate the density of a gas, such as carbon dioxide, in the lab if we can construct a way to know the mass and volume of the gas. We can collect carbon dioxide in a gas syringe to measure its volume and use a Berocca tablet as our source of gas (a Berocca tablet effervesces on contact with water releasing CO2).
You will need to do some initial experiments to see how much gas a single Berocca yields. The gas syringe only has a capacity of 100 cm3 so you need to use only as much Berocca as will release near to, but not more than, 100 cm3.
- Find the combined mass of the boiling tube, water and Berocca.
- Add the Berocca to the water and quickly seal the tube with the bung.
- When the fizzing is complete, reweigh the boiling tube to find the mass of carbon dioxide gas released by the reacting Berocca.
- Divide the mass of gas released by the volume of gas collected to work out the approximate density of CO2.
Questions…
- Which element on the Periodic Table has the lowest density?
- Which metal element has the highest density?
- Name a metal with a density less than 1 g/cm3.
- What is the chemical test for (a) hydrogen, (b) oxygen & (c) carbon dioxide.
- [Harder!] In the reaction at the top of the page, what job is the magnesium dioxide doing?
That was surprising and interesting, I never thought that gases could be so dense! Thank you.