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- Describe an experiment to determine the density of a liquid and of a regularly shaped solid, and make the necessary calculation using the equation:
density = mass / volume or d = m / V
Determining the density:
Density is a measure of the amount of mass per unit volume. To calculate density, we divide the total mass of a substance by its total volume (the total amount of space it occupies).
If you write that down mathematically, it becomes
Density = mass / volume
which is shortened to
d = m / V,
where d is density in kg/m3, m is mass in kg and V is the volume in m3.
Note that density can also be measured in g/m3 or g/cm3.
To convert kg/m3 to g/m3, all you need to do is multiply by 1000. For example, 1kg/m3 is 1000g/m3, because there are 1000g in 1kg.
To convert g/m3 to g/cm3, you need to divide by 1,000,000 or 106 (there are 106 cm3 in 1 m3, and because the volume is a divisor when calculating density, you have to divide instead of multiply by 106. I know this part can be super confusing, so if you still don’t get it, please drop a comment down below to let me know!)
To convert kg/m3 to g/cm3, all you have to multiply by 1000 and then divide by 106. You may have noticed that multiplying by 1000 and then dividing by 1,000,000 is the same as dividing by 1000, so you can simply divide by 1000 instead, too.
Determining the density of a liquid:
So we’ve already established that to calculate density, we need to know the mass and volume of the substance that we’re calculating the density of.
To calculate the mass of a liquid, first, weigh the container that is going to hold the liquid on an electric balance (I’d recommend using a measuring cylinder or a volumetric flask as a container). Record the mass shown. Then add the liquid to the container (fill it up to the mark if it’s a volumetric flask), and measure and record the mass of the container + liquid.
The mass of container + liquid minus the mass of the container will give you the mass of the liquid.
Now you need the volume of the liquid. Simply read the volume from the graduations on the measuring cylinder, or, if it’s a volumetric flask and you’ve filled it up to the mark, you already know its volume (i.e. if you filled a 250cm3 volumetric flask to the mark, then you’ve got 250cm3 liquid).
Note that 1ml = 1cm3 and 1litre = 1dm3.
Now calculate the density of the liquid using d = m / V and the appropriate units.
Determining the density of a regularly shaped solid:
When I say ‘regularly shaped’ solids, I mean ones that you can calculate the volume of using math – like a cube or cuboid, a cylinder, a prism, a pyramid, a sphere, etc.
I’m going to assume you know how to calculate the volumes of the shapes listed above, but if you don’t, please drop a comment below, and I’ll update the notes to include them!
So the first step is calculating and recording the volume of the solid.
Then find and record its mass by weighing it on an electric balance.
Calculate its density using the formula d = m / V and the appropriate units.
- Describe the determination of the density of an irregularly shaped solid by the method of displacement, and make the necessary calculation.
It’s difficult to calculate the volume of an irregular solid, so we use a slightly different method – we use the displacement of water.
There are two ways to do this – if your solid is small enough to not take up much space in a measuring cylinder, then you can use just the measuring cylinder. If not, then use a displacement beaker.
Calculating volume with a measuring cylinder:
First, fill a measuring cylinder with water to about half its volume and record the volume of the water in the cylinder. Then carefully drop the solid into the water, making sure not to splash the water. Record the new volume. The initial volume reading is the volume of the water, and the final volume reading is the volume of the water + solid, so the difference between the two volume readings is the volume of the solid.
Calculating the volume with a displacement beaker:
Fill the displacement beaker with water, filling it as much as you can without having any water displaced. Place a measuring cylinder at the spout of the beaker (like in the diagram). Carefully place the solid into the beaker, without splashing any water. The measuring cylinder should collect all the displaced water. Read off the value of the volume of displaced water in the measuring cylinder and record it. This is equal to the volume of the solid in the displacement beaker.
Note that for both these methods, you cannot use water absorbent solids (e.g. dry dirt, a sponge) as they will absorb some water. This means that the volume of the displaced water will be less than the volume of the solid.
Okay, so now you have the volume of the irregular solid.
All that’s left is finding its mass using an electric balance, and calculating its density using the formula d = m / V and the appropriate units.
Notes submitted by Sarah.
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