Hello, welcome to our lesson today.

This is part of the unit "Separating substances." We're going to consider how to analyse water samples that contain dissolved solids.

My name is Mrs. Clegg, so let's start the lesson.

So here's the outcomes for today's lesson.

We can apply techniques to analyse water samples containing dissolved substances.

And here are our keywords for today.

Listen out for these and check that you know what they mean.

Dissolve, sample, analysis.

Here are the definitions for each of those.

So a substance has dissolved when its particles separate and spread throughout the particles of a solvent, resulting in it no longer being seen.

So a sample is a smaller portion of a larger quantity of a substance.

An analysis is a process of separating and quantifying the components of a sample to understand its nature and composition.

Find out what it contains, basically.

Today's lesson consists of two parts, obtaining dissolved solids from a sample and analysing data.

Let's get started with the first part of our lesson.

So you might remember that mineral salts and sometimes gases can dissolve in water, and this forms different solutions.

So we've got groundwater, seawater, and rainwater.

The groundwater and seawater contains minerals, salts, and gases, and then rainwater typically just contains dissolved gases.

Now, samples of water may also contain very small solid particulates that don't dissolve in the water, and they're so small and these might be dust, soot, pollen, and they're incredibly difficult to see in the sample of water with one's naked eye.

Pure water, remember, contains just H2O molecules, nothing else.

Now our tap water contains water molecules and dissolved ions, and these would be considered impurities in a water sample.

Anything that's dissolved in a water sample is an impurity.

You might have noticed this happen.

If a sample of a solution is left to dry and all the water evaporates away, any of the dissolved solids that were in that sample are left behind on the surface.

So if you've got a cup that's had a hot drink in it and you've just got the dregs at the bottom there and you leave it for a period of time, the water evaporates and you've got all the dissolved solids in the hot drink, dregs, left as a residue.

A residue just means what's left behind.

Let's have a quick check.

So some spilled water eventually dried but a white mark was left behind.

What does this tell you about the water? Well done if you said C.

It does indeed tell us that the water had dissolved substances in it.

Now, you might have thought A, that the water was salty, because a white mark was left, but there's really not enough information to tell us if that white substance is salt at this point in time.

Other substances will leave a white mark as well.

So we might want to collect those dissolved solids so we can analyse them.

So drying a sample of different types of water allows chemists to obtain and compare the dissolved solids in each.

Let's just check we know what the apparatus is called here.

So we've got the heatproof mat, Bunsen burner, tripod, metal gauze, a beaker containing some water, and over the top of the beaker we put a watch glass, which is like a curved piece of glass, and into it we would put a small volume of our water sample and then we gently heat it over the boiling water, and any dissolved solids in the sample are deposited on the watch glass.

In other words, we're left with a residue as the water evaporates.

By gently heating it over boiling water means we allow the sample's water to evaporate slowly and it's in a controlled manner.

So if we heated the watch glass directly on the gauze with a Bunsen burner, that might cause the watch glass to crack.

And stronger heat also risks the dissolved solids spitting out from the sample and being lost.

And if we want to find out how much dissolved solids is in the sample, that wouldn't be helpful.

It's also dangerous.

So to get accurate data about each water sample, we need to know the mass of the watch glass before we put the sample in.

Sometimes we'll call the watch glass an evaporation dish.

So this one measures 30.

42 grammes.

Then we would gently heat it over the beaker of boiling water.

And once that's done, we would allow it to cool and we would dry the bottom of the watch glass so we're removing any of the condensation.

And then we would measure the mass of the dry water glass again.

And you can see this time the residue from the dissolved solids is present and the mass shown on the display is now 30.

48 grammes, so it's increased.

If we want to ensure a fair comparison of water samples, then we need to use the same volume of each sample each time.

So maybe 5 centimetre cubed, for example.

All the water must be evaporated away and this can be checked by heating to constant mass.

That means that you heat the watch glass again and then you re-measure the dried watch glass' mass and you keep going until the mass no longer changes.

In other words, all the water has evaporated and the mass is constant.

So let's have a quick check.

Scientists want to compare the dissolved solids from different water samples.

What method or methods might they use? Well done if you said B, evaporate all the water away from the sample.

Using litmus, remember, only tells us if the sample's acid or alkaline.

What two other steps might be necessary to ensure the sample is completely dry? So have a look at these and identify two extra steps.

Well done if you said heat the watch glass and sample again.

And finally, measure the mass of the watch glass and sample again.

Okay, I think we're ready for Task A now.

So, what equipment should be used to hold the water sample over the boiling water, measure the mass of the water sample, and measure the volume of the water sample? So what equipment would you use there? Pause the video and come back when you've answered those three questions.

Welcome back, let's have a look at the answers.

So, to hold the water sample, we would use a watch glass or an evaporation dish.

We would use a balance or sometimes people call these scales.

And to measure the volume, we could use a measuring cylinder, a pipette or a burette.

Let's have a look at question two.

Match each diagram to the appropriate description.

So we've got five pieces of equipment set up there, and what is their purpose? So pause the video and come back when you're ready.

Let's have a look.

Okay, you might need to pause the video to check your answers, but basically the measuring cylinder is to measure 5 centimetre cubed of the water sample.

The balance is to record the mass of the watch glass.

The watch glass has got condensation on the bottom of it, so that is linked to remove condensation from the watch glass.

The balance here contains the watch glass plus the dissolved solids, so we want to record that mass.

And then finally, the Bunsen burner, gauze, tripod, beaker, and watch glass is to heat the sample until it's dry.

Well done if you've got all those correct.

Part B, order the statements to create a method for obtaining dissolved solids from a sample of water.

So we've got A, B, C, D, E, so put them in order.

Pause the video, come back when you're ready.

Okay, let's have a look.

So we would record the mass of the watch glass, B, then we would measure out the water sample, A, then we would heat the sample until dry, E, then we would remove the condensation from the watch glass, C, and then we would record the mass of the watch glass and the dissolved solids.

Well done if you've got all those in the right order.

Okay, question three now.

So Sophia suspects she has a pure water sample and is testing to see if it contains any dissolved substances.

What should she expect to observe and why? Pause the video and come back when you're ready.

Okay, let's have a look.

So Sophia should expect no residue on the watch glass and no change in mass for the watch glass.

And the why is because pure water contains no impurities, so there's no dissolved substances, it's only got H2O molecules in it.

So all of those, all of the water would evaporate, leaving nothing behind.

Well done.

Question four.

Sam used the following method and equipment setup to see how much dissolved solids are in their garden pond water.

How might Sam's investigation be improved? Here is his method and a diagram of how to set up the equipment.

Would this method tell you exactly what to do to get accurate results? Pause the video, come back when you're ready.

Okay, how did you improve Sam's investigation then? Let's have a look.

So for me, I recorded the mass of the watch glass first.

I let the watch glass dry and removed any condensation from the bottom, and I repeated steps three to five until I got a constant mass.

Hopefully you got those.

Well done if you did.

Let's move on to the second part of our lesson now, analysing data.

Analysis of water samples can include calculating the mass of dissolved solids in each sample.

So let's just run through how to calculate the mass of the dissolved solids in this 5-centimeter-cubed sample.

So we want to find out the mass of the dissolved solids, we know the mass of the watch glass and the dried sample, and we need to take away the mass of the watch glass.

So, 30.

43 minus 30.

38, and that tells us the mass of the dissolved solids in the sample was 0.

05 grammes.

So ratio tables can also help us to calculate the mass of dissolved solids in larger or smaller samples.

So here we've got a table.

We know the volume of the sample is 2 centimetre cubed and we know that contained 0.

005 grammes of dissolved solids.

And we want to know what the mass would be in 10 centimetre cubed.

So we know 2 multiplied by 5 would give us 10, and so we can do the same thing on the other side.

So if we multiply 0.

005 by 5, we'll get 0.

025 grammes.

And that's the amount we would find in 10 centimetre cubed.

Let's have a look at another example.

So here we've got 5 centimetre cubed of sample which contains 0.

05 grammes of dissolved solids, and we want to know how much would be in 1 centimetre cubed.

And so if we divide by 5 on this side, if we divide 5 by 5 we get 1.

And so on the other side, if we divide the mass of dissolved solids by 5, we'll get 0.

01, and that's the amount present in 1 centimetre cubed.

So let's have a quick check.

So we've got 0.

02 grammes of dissolved solid in a 1-centimeter-cubed sample.

What mass of dissolved solids is in a 5-centimeter-cubed sample? So do the ratio table.

So we know 1 centimetre cubed gives us 0.

02.

We want to go to 5 centimetre cubed, so what would we do? We would multiply by 5.

And so we multiply by 5 on this side and get 0.

10.

Let's have another example.

This time we've got a 30-centimeter-cubed water sample in 0.

20 grammes of dissolved solids.

What mass of dissolved solids is present in a 5-centimeter-cubed sample? So I'm gonna give you a moment to have a go and then we'll go through it together.

So pause the video and come back when you're ready.

Okay, let's have a look.

So the volume of our sample is 30.

We want to know what's present in 5, so we divide by 6, and we divide by 6 on the other side, and we get 0.

033 grammes of dissolved solids.

Well done if you've got that.

Now by doing these sorts of analysis, we can actually compare different water samples.

So here's a data table.

So pure water has no dissolved solids.

What do you notice between seawater and groundwater? Seawater contains more dissolved solids than groundwater samples.

Did you think that? And we can also look at the mass of dissolved solids in a water sample to give us an idea about purity.

So here's the same table again.

Which one is pure? And what do you think about seawater and groundwater? So pure water is the most pure because it contains no dissolved solids.

And what about seawater and groundwater, which one is the least pure? Well done if you said seawater because it contains the highest mass of dissolved solids.

Well done.

So let's have a quick check.

A comparison of water purity can be deduced by analysing data about the dissolved solids in a sample.

Deduced just means found out.

Well done if you said true.

And how do you justify your answer? Well done if you said B.

A water is more pure if it contains fewer dissolved solids.

It wouldn't be A because that is literally just telling us how acid or alkaline a substance is.

Okay, so it's time for Task B.

So have a go using the method we talked about before, using the beaker of water with the watch glass over the top and heating very, very gently, to try and obtain a sample of dissolved solids.

Have a go.

And then look at these results that a student has collected and find out what mass of dissolved solids would be in 250 centimetre cubed of the same type of water.

So you've got to do two calculations there.

You've got to find out how much is in the 5-centimeter-cubed sample, first of all, and then work out how many would be in 250 centimetre cubed of the same water sample.

Pause the video and join us when you're ready.

Okay, so this was the method that we used before.

Hopefully you managed to have a go and collect some dissolved solids.

What did they look like? What colour were they? Let's have a look at the student's results.

So we're trying to find out the mass of dissolved solids in 250 centimetre cubed of the same type of water.

So first of all, let's work out how much was in a 5-centimeter-cubed sample.

So if we do 30.

47 minus 30.

41, we'd get 0.

06 grammes.

So let's do our ratio table.

We've got a 5-centimeter-cubed sample gives us 0.

06.

So if we multiply 5 by 50, we get 250.

So we do the same on the other side, so 0.

06 times 50 will give us 3.

0 grammes of dissolved solids.

Amazing if you got that right.

Question two, so Sophia suspected her water sample was pure and tested to see if it contains any dissolved substances, and here's her results table.

Now she did get 0.

01 gramme of dissolved solids.

Now she expected to see no dissolved solids in her sample.

What may have caused her results? Pause the video and come back when you've got some ideas.

Okay, welcome back, let's have a look.

So there are two different ways of looking at this.

Basically, her sample wasn't pure and it did contain some dissolved solids.

Or she didn't actually ensure all of the water evaporated.

In other words, she didn't heat the sample to constant mass and maybe didn't remove the condensation from the bottom of the watch glass.

Let's have a look at question three.

So we've got 5-centimeter-cubed samples of four different types of water.

We know the pH and we know the mass of the dissolved solids.

What I want you to do is put them into order from most pure to the least pure.

Pause the video and join us when you're ready.

Okay, who's ready for the answers? So tap water first of all, river water, groundwater, and seawater.

So seawater contained the largest mass of dissolved solids, so that was the least pure, amazing.

Let's have a look at question four.

So some students investigated different water samples by heating 10 centimetre cubed of each sample until it was dry.

The evaporation dish had a mass of 94.

56 grammes and their results are in the table below.

Which sample is pure water and give a reason, explain your answer.

And which sample do you think might be seawater? And explain your answer.

Pause the video and join us when you're ready.

Okay, let's have a look.

So sample A is the pure water because the mass is the same as the mass of the evaporation dish on its own, so there's no dissolved solids there.

And the seawater is likely to be sample C because the mass of dissolved solids and the evaporation dish after heating is the highest.

Well done, we've come to the end of the lesson, so let's have a quick look at our main learning points from today.

We can use evaporation setups to collect dissolved solids from a water sample.

Remember, we're putting a watch glass on top of a beaker of water and heating that.

We don't heat the watch glass directly because it might shatter and it might cause spitting of the dissolved solids.

We can collect data from evaporated water samples and we can find out the mass of dissolved solids in each sample.

The lower the mass of dissolved solids in a sample, the more pure the water sample.

The higher the mass, the more impure the water sample.

So remember we looked at pure water and then we also looked at seawater, and that was usually the most impure.

Well done, I look forward to working with you again.