Hello, my name's Mrs. Clegg, and welcome to today's lesson.

We're going to be looking at how to test water samples using pH.

So, we'll learn a little bit more about the indicators that we can use and why there might be differences in pH between different water samples.

And here's today's learning outcome.

So, by the end of today's lesson, you should be able to describe how to test the pH of different water samples, and you should be able to suggest why they might be different in terms of the dissolved substances that they contain.

So, let's get on.

Here are the key words for today's lesson.

Dissolve, sample, and pH.

And here are the definitions of those words.

You might like to pause the video and take some notes.

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

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

So, you might sample a piece of cheese, for example, to test what it's like before you buy it.

And pH is a number that tells us about the alkalinity or acidity of a solution.

Today's lesson is split into two parts: dissolved substances in water sources, and then determining the pH of water samples.

So, let's get started on the first part of our lesson.

When a substance's particles spread throughout a solvent's particles, we say it has dissolved and formed a solution.

So, an example might be sugar being added to water.

The sugar dissolves in the water, and then we've got a sugary solution.

And if we look at the particle diagram of what's going on here, the pink circles are representing the sugar, the substance, and the blue circles are representing the solvent, in this case, water.

And so, when a solid dissolves its particles separate and spread throughout the particles of the solvent.

This results in it no longer being able to be seen.

And so, here we've got our solution, our sugar solution, and we can't see the sugar particles anymore.

In nature, some substances dissolve in water, and that forms different types of water or different solutions.

So, groundwater, for example, is water that has flowed through soil and porous rocks.

Porous rocks are rocks that have spaces between their grains.

(mouse clicking) And as water moves through the soil and these porous rocks, mineral salts and gases can dissolve into the water.

So, some of the minerals that might dissolve are calcium ions or iron ions.

(mouse clicking) Some of the salts that might dissolve are sodium chloride or potassium nitrate, and some of the gases that we might find dissolved in water are carbon dioxide and oxygen.

(mouse clicking) Seawater is what we find in oceans and seas.

And as the seawater moves across the coastlines or along the ocean floor, a variety of substances can then dissolve into the water.

Mineral salts and gases can also dissolve into seawater.

So, some of the minerals you might find, sodium ions, magnesium ions, some of the salts might be sodium chloride and magnesium sulphate, and some of the gases, carbon dioxide and oxygen.

So, quite a lot of things in seawater that maybe we hadn't guessed before.

Which of the following substances dissolves in both groundwater and seawater? Well done if you said all three.

We can find gases, minerals, and salts dissolved in groundwater and seawater.

Water that's fallen as rain or been collected from the rain is called rainwater.

And you might know somebody that collects rainwater.

They might use the rainwater for watering plants, for example.

As rain falls through the atmosphere, gases dissolve into the water.

Oxygen, carbon dioxide, and then in more built-up areas or industrial areas, polluting gases such as sulphur dioxide and nitrogen oxides may also dissolve as the rain falls.

And did you know very small solid particulates such as dust, soot, pollen, and sometimes even volcanic ash can also enter the rainwater? And these solids don't dissolve in the water, but they're so small that they're nearly impossible to see with the naked eye.

Let's have a quick check.

All sources of water contain the same dissolved substances.

Is that true or is that false? Well done if you said false.

And then, of course, can you justify your answer? Why did you think it was false? (mouse clicking) And the answer is A, rainwater only contains mainly dissolved gases, whereas other types of water will contain dissolved salts, minerals, and gases.

Let's just have a quick look at water again.

So, pure water contains absolutely no dissolved substances, only water molecules, H₂O.

So two molecules of hydrogen and one of oxygen.

So the red spheres there represent oxygen, and the two white spheres represent hydrogen atoms. So pure water literally is H₂O molecules, nothing else.

So let's have a quick check.

Pure water is not a solution.

Is that true or is that false? So, this one's actually true.

And can you justify your answer? So, brilliant if you said B because pure water contains only molecules of H₂O, solutions contain dissolved substances.

So what I'd like you to do now is to match each description to the correct type of water, so we've got descriptions on the left-hand side and the types of water on the right-hand side, so mix and match them.

Pause the video and join us when you are ready.

Okay, let's have a look at the answers.

You might need to pause the video and check your answers, but I'll go through them.

So seawater is found in the oceans, so seawater is found in our oceans, groundwater comes from moving through porous rock, pure water is H₂O molecules only and rainwater only falls from the sky.

Well done.

Let's have a look at question two.

So we've got a table with three columns labelled groundwater, rainwater, and seawater.

And what I'd like you to do is to place each substance into the correct column to say which soluble substances are found in each type of water.

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

Okay, let's have a look at some answers.

So for groundwater, carbon dioxide, sodium chloride, and the dissolved ions calcium and magnesium.

Rainwater, carbon dioxide and sulphur dioxide.

And then seawater, carbon dioxide, sodium chloride, and the dissolved ions calcium and magnesium.

Fantastic, if you sorted those out.

Let's move on to the second part of our lesson today, which is determining the pH of water samples.

Most water samples are clear or colourless, and that might suggest that they don't contain any dissolved substances.

Substances that dissolve in water are considered impurities, and they can affect the solution's acidity or alkalinity.

Universal indicator is very simple to use, and it can help us determine a sample's acidity or alkalinity, in other words, its pH.

And we have universal indicator paper where we literally add a drop of the sample to the paper.

We've got universal indicator solution, so we would add a few drops of the indicator to the sample.

The universal indicator paper or the solution change colour, depending on what substances are dissolved in the water and how much is present.

And then we compare the colour of the sample to what we call a reference chart, and that indicates the pH of the sample that's been tested.

So, let's have a look.

What's the pH of a sample that shows this colour using the universal indicator paper? So, universal indicator paper is a sort of yellow colour.

When it's been dipped into a sample, the bottom of it has changed colour.

So, what is the pH of that sample? Use the reference chart to help you.

I think it's pH nine.

Let's use universal indicator solution this time.

So what's the pH of a sample that shows this colour with universal indicator solution? What do you think? I think it's that one, so it's pH seven.

Now, you might have said different pHs, and using universal indicator paper or universal indicator solution is what we call subjective.

It relies upon a visual comparison of colours to the reference chart, so how I interpret the colour might be slightly different from how you do.

So can you think of a more accurate way to do this? We can use a pH metre, and that will give a much more accurate sample measurement.

It gives us a specific value for pH rather than a colour.

And if you look at the display, it says 5.

70.

Now, if we look at the universal indicator paper comparison chart, it would be very difficult for us to have that level of accuracy.

Let's have a quick check.

So, which of the following can be used to determine the pH of a water sample? Okay, so all three there, pH metre, universal indicator paper, universal indicator solution, can be used to determine the pH of a water sample.

Now, litmus paper's an interesting one.

You can get red litmus paper and blue litmus paper, and actually all they do is tell us if a solution is acidic or whether it's alkaline, they don't tell us the pH value.

So, why are we interested in the pH of a water sample? Well, actually, it helps indicate if it's got any dissolved substances present.

Seawater, for example, tends to be alkaline because of the carbonate ions and other salts that are dissolved in seawater.

Whereas rainwater tends to be acidic, because of the carbon dioxide and other gases that are dissolved in it.

And remember, pure water has no dissolved substances so it is neutral, its pH is seven.

All that is present in pure water is H₂O.

Let's have a look at Task B now.

So Jun needs to determine the pH of a sample.

Write a method for how he should use universal indicator paper to complete his task.

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

Okay, let's have a look.

So, you might have written different things but, hopefully, you've got the same components as me.

So use a dropping pipette or a stirring rod to place a drop of the sample onto a strip of universal indicator paper.

Compare the colour that develops to the universal indicator paper's reference chart, and the reference chart colour that matches the colour of the sample's universal indicator paper indicates the sample pH.

So, in Task Two I want you to use your method to find out the pH of four water samples, and record these in the table below.

Pause the video and come back when you've completed your table.

Now, if you don't have access to different water samples, use the information below to complete the results table yourself.

Okay, let's have a look at the answers.

So, the results will depend on the samples that have been used.

If we use the samples from the previous slide, this is what you would get.

So seawater, the universal indicator paper would go a light blue colour.

Groundwater, a light yellowy green.

Pure water, a greeny blue.

And rainwater, a light orange.

So, like this.

Now, sometimes it can be hard to describe the actual words that you want, so it can be useful if you've got colouring pencils and shading to actually record the colour.

So, let's have a look at the pH then.

What do we think the pHs were? So, using the reference chart, this is what I think the pHs were.

Yours might be slightly different because you may have used different samples, so your pHs and colours might be different because you used different samples.

So, we've come to the end of the lesson now.

Let's have a look at the summary of what we've learned today.

So pure water is necessary for lab tests, and it must be neutral, pH seven.

It contains nothing else, just water molecules.

Different substances dissolve in different types of water.

So we talked about groundwater, having those minerals from the porous rocks and the salts dissolved into it.

We talked about the oceans, having the sodium ions and the magnesium ions, and then the salts, sodium chloride and magnesium sulphate, and gases, oxygen and carbon dioxide, dissolved in them.

Substances that dissolve in different waters can affect the pH of the water.

They can make it more acid or more alkaline.

And the pH of water samples can be determined using appropriate methods and instruments, such as a pH metre.

So we talked about using universal indicator paper and universal indicator solution, and pH metres.

Then the pH data can be used to analyse if the water contains impurities.

So if it's not pH seven, then there are other substances dissolved in the water.

So I hope you feel a lot more confident now about being able to describe how to test the pH of different water samples, and to be able to suggest why they might differ in terms of the dissolved substances that we find.

I look forward to working with you next time, well done (mouse clicking).