Hello there, I'm Mr. Forbes and welcome to this lesson from the particle explanations of density and pressure unit.

This lesson is all about measuring density and in it, you're going to be measuring density in different ways.

By the end of this lesson, you're going to be able to measure the density of solids and that's regular and irregular solids.

And you're going to be able to measure the density of liquids through a simple, practical technique.

The key words you need for this lesson are shown here.

The first is density.

That's the mass per cubic metre or cubic centimetre of a material.

Displacement can and that's a device that's got a spout that allows you to measure the volume of a submerged object.

Vernier callipers, which are devices used to measure length precisely.

And a micrometre, which is something that can measure length to very high precision.

You can return to this slide at any point during the lesson.

The lesson's in three parts.

And in the first part you're going to look at a technique for measuring the density of a liquid.

In the second part, you're going to look at a technique to measure the density of an irregular solid using a displacement can.

And in the final part you're going to look at a technique to measure the density of a regular solid, like a sphere or a cube.

So when you're ready, let's start with measuring the density of liquid.

The density of any substance can be found using this equation.

Density is mass divided by the volume.

Written in symbols that looks like this.

And those symbols are rho equals M divided by V, where the mass M is measured in kilogrammes or it can be measured in grammes.

Volume can be measured in cubic metres or cubic centimetres, and the density, rho, is measured in kilogrammes per cubic metre or grammes per cubic centimetre depending on our choice of mass and volume units.

So I'd like to ask you to calculate an example density please, using the equation you've just seen.

So we've got a sample of liquid.

It's got a volume of 400 centimetres cubed and a massive of 300 grammes.

What's the density of that liquid? Pause the video, calculate the density, and restart please.

welcome back.

Hopefully you found the density is 0.

75 grammes per centimetres cubed.

And the calculation you should have performed should look something like this.

Well done if you got that answer.

To find the density of a sample of liquid, we need two things then.

We need to know the volume of the liquid sample and that can be measured simply using a measuring cylinder, which is specifically designed to measure the volume of something.

And the second thing we need to do is to measure the mass of the sample of the liquid as well.

And we can do that by using a top pan balance, something like this.

So to calculate the density, we are going to be measuring the volume and the mass and that'll allow a simple calculation to take place.

Now, obviously we're using a liquid and that needs to be inside the container.

So we need to take into account the mass of that container whenever we're trying to measure the mass of the liquid.

And to do that, what we do is we measure the mass of the empty measuring cylinder we're going to be using and we record that.

Then we measure the mass when the liquid's in the container and that will give us a higher reading.

And obviously the mass of the liquid is going to be the difference in those two values.

The increase in mass is all accounted by the mass of the liquid.

So the mass of the liquid is that increase in mass, and we can calculate that by subtracting the mass of the measurement cylinder and the total mass, something like this.

And that gives me a mass of liquid in that measuring cylinder of 69.

7 grammes.

Let's see if you can do that.

I've got an empty measuring cylinder, a mass of 20.

4 grammes.

It's filled with 50.

9 centimetres cubed of liquid and the mass increases to 75.

4 grammes.

What's the density of the liquid? So you need to do two calculations here.

So pause the video, work out the density and restart please.

Welcome back, hoping you selected the answer, C, 1.

10 grammes per centimetre cubed.

First thing we need to do is to find the mass of the liquid and that was 55.

0 grammes.

Then we put in our density equation and that gives us that final answer of 1.

10.

Well done if you've got that.

Okay, it's time for the first task now.

And you're going to find the density of some samples of liquids.

So the instructions are here, for each liquid, you're going to measure the mass of the empty container, partly fill it with the sample liquid, record the volume of that liquid by looking at the measuring cylinder, measure the new mass, calculate the mass of the liquid in the cylinder, using the technique you've just seen, and then finally calculate the density.

I'd like you to record your results in a table like this.

You might use slightly different liquids than the ones I've got here.

So you're just gonna have the mass of the empty container, mass of the container and liquid, mass of the liquid, volume of liquid, and then finally that calculated density.

I'd like you to do all those calculations.

So pause the video, carry out these instructions, and then restart once you've collected and calculated the densities, please.

Welcome back.

Hopefully, you've got a successful experiment and you calculated values like this.

Well done.

And now, it's time for the second part of the lesson.

And in this one we're going to be measuring the density of an irregular solid.

A solid that's not got a simple shape.

So as I've said, an irregular solid doesn't have a simple shape.

We can calculate its volume mathematically.

Its density still needs to be found using its mass and volume though.

So to measure its mass, we can use a top pan balance as we've done before.

To measure its volume, we can use a displacement can.

So something like this.

We've got a measuring cylinder, sorry, we've got a displacement can, a measuring cylinder and some water.

And we place the object in the displacement can, and we measure the volume of the displaced water, and that will give us the volume of the shape.

Okay, so imagine I've got a pebble of mass 25.

2 grammes, and it displaces a volume of 4.

5 centimetres cubed of water after it's placed in a displacement can.

Can you calculate the density of the pebble? So I'd like you to calculate that density please choosing from those three values.

So pause the video, calculate the density, and restart.

Welcome back.

Hopefully you've got 5.

6 grammes per centimetres cubed.

We've got the mass, we've got the volume from the question there.

We put those into the simple equation for density and that gives us 5.

6 grammes per centimetres cubed.

Well done if you've got that.

To carry out this experiment, we need to measure the mass accurately, and we need to measure its mass before we place it in the water.

Because once we place something in water, it gets wet.

And if we then place it on a balance, we might end up measuring the mass of some of the water that might be soaked into the object or just clinging into its surface.

So if we place a dry object onto a balance and we get a measurement, let's say of 24.

6 grammes, then if we put in water and take it back out, we've got a wet object and that's obviously gonna have some water as well.

So the mass reading is gonna have increased.

So we need to use the true mass rather than the mass including water in our calculations.

So we need to measure the mass when the object is dry.

Okay, I'd like you to think a bit about that.

Our pupil measures the mass of a metal coin with a top pan balance and it's volume using the displacement can.

They measure the mass of the coin after removing it from the water.

And they use that in the calculation, what's the likely result they're gonna get? So have a look at those three options and decide which of those is gonna be true.

So pause the video, make your selection and restart please.

Welcome back, you should have selected the bottom one.

The calculated value is higher than the true value because the measured mass is gonna be greater than the true mass.

So we're gonna get too large an M in this equation and that's gonna give us a larger rho than you'd normally get.

So we're gonna have a calculated value higher than the true value.

Well done if you selected that option.

Okay, now it's time for you to carry out an experiment to try and find the density of some irregular solids.

The instructions are all here.

We're gonna measure the mass of the dry solid, find a displacement using the displacement can.

And then we're going to measure the volume of displaced water using the measuring cylinder and use that mass and volume reading to get the density of the irregular solid.

Here's a quick video showing you that technique.

For this experiment.

You'll need a displacement can, measuring cylinder, water, which can be coloured to make it easier to see and some sample materials which sink in water.

Fill the displacement can to above the spout and let any excess water escape into another container.

Position the spout above the measuring cylinder.

Here I've used a tripod and mat as a platform.

Very carefully, place the sample object into the displacement can.

Make sure you don't splash.

Allow the water to be displaced from the can into the measuring cylinder.

Once the water stopped leaving the spout, carefully read off its volume using the scale on the measuring cylinder.

This is the volume of the object.

The volume here is 61 centimetres cubed.

Okay, hopefully that helps to explain a bit more.

I need you to then record your results in a table of this format.

So we've got four irregular, solids there for you to find the density of.

Okay, so now it's time to pause the video, follow the instructions and carry out the experiment, then restart when you're done.

Welcome back.

Hopefully, you got some sample results.

Mine obviously are gonna be very different from yours 'cause I used different solids than you've done, but you should have a table, something like this.

And I've rounded my values down to two significant figures.

Well done if you've got something like that.

And now it's time for the final part of the lesson.

We're gonna look at a final experiment where we're going to measure the density of some regular solids.

To find the density of regular solids, we still need the mass and the volume.

Now we can find the mass quite easily and the volume can be calculated from measurements of the dimensions.

So to find the mass, we're gonna be using a top pan balance as usual.

And to find the dimensions, we can use a ruler or vernier callipers or even a micrometre.

And that'll give us the measurements we need to calculate the volumes.

We need to know the equations to calculate the volumes of keyboards and spheres.

And they're these.

The volume of a cuboid, we need its height, its width, and its length.

And the volume is the length, times the width, times the height.

To find the volume of a sphere, we need to find its diameter.

And then we need to halve that to find its radius.

And once we've got its radius, the volume is 4/3 times pi times the radius cubed.

Let's see if you can calculate the density using some measurements.

I've got a cuboid here.

It's got a mass of 14.

80 grammes and it's got the dimension shown.

I'd like you to calculate the density of the cuboid, please.

So pause every video, find the density and restart.

Welcome back.

Hopefully, you selected 2.

08 grammes per centimes cubed.

We've got the volume, we calculate it like this, and then we can use that volume and the mass from the question to get a density of 2.

08 grammes per centimes cubed.

Okay, it's time for the final task of the lesson.

And I'd like you to find the density of some regular solids using these instructions.

You measure the mass with a balance, use a micrometre to measure the correct dimensions, depending if it's cuboid or a sphere, then you calculate the volume using the correct equation.

And finally, calculate the density of the samples using that equation, density is mass divided by volume.

You should have results in a table, something like this.

So I need you to record the shapes and the two corrected dimensions, which ones you measure, and finally fill in all the rest of the data.

So here are the instructions again.

What I'd like you to do is to follow those to complete the task.

So pause the video, do that, and restart when you're done.

Welcome back.

Hopefully, you've got a table completed, something like this where you've recorded the shapes, the masses, the dimensions, then calculated the volume, and finally calculated the density.

Well done if you've got something like this.

And we've reached the end of the lesson now.

So here's a quick summary.

The density of something's found by its mass divided by the volume, and we've got the equation in symbols there.

To measure the density of a sample, you measure its mass with a top pan balance and measure its volume according to the type of substance you've got.

For a fluid, we use a measuring cylinder, for an irregular solid, we use a displacement can and measure the volume of the displaced water.

And for a regular solid, we use a micrometre or vernier callipers to measure the dimensions and calculate the volume according to those equations.

Well done for reaching the end of the lesson.

I'll see you in the next one.