This picture neatly captures the idea of liquids having different densities. The lowest density liquid floats on the top and all the other layers stay as they are because of their relative densities. This must have been done very carefully because some of these liquids will mix with one another (miscible). Immiscible liquids will not mix so the oil and water at the top were probably the easiest to put together.
The density of a material depends upon how tightly packed together its particles are. It can be defined as…
The mass of 1 cm3 of a substance.
This gives us a means to calculate density by dividing something’s mass by its volume. Mass is usually measured in grams (g) but it could be measured in kilograms (kg). Volume is usually measured in cm3 but you might see it as litres (1000 cm3) or m3 (1,000,000 cm3).
Water has a density of 1 g/cm3 because the gram was originally defined as the mass of 1 cm3 of pure water. Objects with a density higher than 1 g/cm3 will sink in water and objects with a lower density will float. The densest naturally occurring material on Earth is the metal osmium at 22.6 g/cm3, closely followed by iridium (22.5 g/cm3). To put that in perspective, lead is 11.3 g/cm3, gold 19.3 g/cm3 and mercury 13.5 g/cm3. The densest material in the Universe is probably the stuff that neutron stars are made from although they did make a quark-gluon plasma at the LHC that was 100 times hotter than the Sun’s core and denser than a neutron star – they probably didn’t make very much of it!
If you have two of density, mass or volume then you can calculate the missing value.
Density = Mass ÷Volume
Mass = Density x Volume
Volume = Mass ÷ Density
If you do not know enough of the variables then there are a couple of ways to find them out. Mass is measured with a balance. That is pretty much it. Volume is a little more subtle. It can be calculated by base area times height or discovered by dunking it in water and measuring the displacement.
If you have a regularly shaped object like a cube, cylinder or Toblerone tube then you can use mathematics. If your object is irregularly shaped then displacement is probably the best option. You can either lower it into a measuring cylinder and watch the increase in volume or you can place it into a eureka can and collect the displaced water in a measuring cylinder.
Gases have densities too. The density of gases decreases as temperature rises (because the molecules flee away from each other) and increases with pressure (because the particles are being squashed together). Air has a density of about 0.0013 g/cm3 at STP (standard temperature and pressure). Helium has a density of 0.00018 g/cm3 which is why a balloon full of helium floats in air.
Questions…
- What shape is a Toblerone tube?
- What is the force of gravity on Earth (in N/kg)?
- If an object has a mass of 27 g and a volume of 9 cm3 what is its density?
- Copper has a density of 8.4 g/cm3 so what is the mass of 50 cm3 of copper?
- [Harder] A cube of gold has a volume of 1cm3 and is hammered out into a circular disc 1 mm thick. What is the radius of the resulting disc?
This was a great detailed refresher on density. Thank you.
You are very kind – thank you.