Have you ever picked up something that looks really heavy only to find out that it is really light? This obviously makes you looked like a fool in front of your friends but try to style it out into a fancy dance move (it totally worked BTW). Well, this is the concept of density in action – and (amazingly) the topic of this activity. You will be defining density and explaining how to measure it experimentally by the end of this activity – like a boss!
So – density a measurement of the mass of each particle AND how many of those particles there are in that object. If you have a lot of heavy particles in 1 cm3 then you will have a high density, but if you have only a few light particles in 1 cm3 then you will have a low density.
The easiest way of picturing this is by looking at the solid, liquid and gas particle models.
In a solid, the particles are close together in a regular lattice. They are vibrating all over the place, but they ten d to stay in their one specific spot. This makes solids denser because there are a lot of particles in a small area. What do you think about liquids and gases then?
In liquids, the particles are all touching still but they are free to flow over each other. This can lead to small gaps forming in their shapes and that means that you can’t fit as many particles in the same space. This means that the density is slightly less than a solid – this is why water is so heavy, because it has a high density.
In a gas, the particles are free to go wherever they want (they are freeeeee). They tend to move quickly and have large gaps in between the particles. This means that they have a low density because the particles are far apart.
So that is an overview of density – now how about we look at how to measure density in a lab. This is a common question, so you need to remember this method of measuring density. Let’s start off by thinking about how you would measure density. Well we know it is the mass of the particles and how many there are in an area. This means that we need to use a divide – what do you think it should be?
This is the equation:
The sign is the Greek letter Rho – in this case it stands for density (because d was already used…), m is obviously mass and V is volume (as in how much space it takes up, not how loud it is…).
Mass is measured in kg, volume is measured in m3 and density is always measured in kg/m3. Did you spot it? The unit of measurement for density is just the unit of measurement of mass / the unit of measurement of volume. That is because the slash mean divided by, so all we are saying is that the unit is kg divided by meters cubed.
EDPLACE TOP TIP TIME!
If you know the unit of measurement of the thing you are trying to find, you can normally work out what needs to go into the equation to find the answer. For example, if you are asked to work out a speed in m/s, then you know you know the equation you need is m divided by s. Cheat method 1 done!
Now let’s look at how to find the density of an irregular object.
Let’s say you have a rock – how will you work out the density of that object using this equipment?
IMAGE OF ALL EQUIPMENT HERE
What do we need to know in order to calculate the density? It’s the mass and volume, right? So mass is nice and easy, you just put it on the balance and hay presto – there is you mass. For volume, it would be easy if it was a regularly shaped object, but it’s not. This is a problem that was solved by Archimedes back in the ancient Greek times. He sat in the bath and saw the water spill out – what he had realised was that he had an amount of water out of the bath that was the same as his volume – so all we need to do is make a fancy bath that will collect the water as it falls out.
This is an Archimedes can – and it measures the water displaced by an irregular object. In this case, we would put the rock in and measure the amount in millilitres (ml) that come out. 1 ml = 1 cm3.
All you need to do then is take your measurements and divide the mass by the volume and you’ve got your density! Be aware though - this only works for objects that sink.