Hello and welcome to today's lesson.

We're going to look at the pH scale today and the different types of indicators that we might use.

This is part of the "Making Salts" unit.

My name's Mrs. Clegg, so let's get the lesson started.

Here's today's lesson outcome.

By the end of it, you should be able to explain the pH scale and appropriately use universal indictor, phenolphthalein, methyl orange, litmus and pH metres to identify different solutions.

And here are today's keywords.

You'd might like to pause the video now and take some notes to refer back to throughout.

Today's lesson's split in into two parts.

We're gonna look at the pH scale first and then we're gonna look at lots of different types of indicators.

So let's get started with the scale.

The pH scale is a numerical scale that ranges from zero to 14, and we use to measure how acidic or alkaline a solution is.

So if we look at this graphic, seven is in the middle and that is neutral and seven and below, we increase the acidity and seven and above, we increase the alkalinity.

And the scale is actually logarithmic, which means each whole number change represents a tenfold change in hydrogen ion concentration.

And the term pH actually stands for potential of hydrogen or sometimes it's referred to as the power of hydrogen.

Now, acids are substances that release hydrogen ions when they're dissolved in water and the concentration of these ions actually determines the acidity of a solution.

So if we look there from seven to zero, we're increasing the concentration of hydrogen ions as the solutions becomes more acidic and the opposite is happening between seven and 14.

So let's just have a quick check.

The higher the concentration of hydrogen ions, complete the sentence.

Well done if you said the lower the pH.

Let's just do another check.

Which of the following statements is true about the pH scale? Well done if you said B and even better if you said D as well.

They're both correct, well done.

Now, bases are substances that can neutralise acid.

When a base is soluble in water, we call it an alkali.

Alkalis release hydroxide ions into the water and that decreases the concentration of hydrogen ions and raises the pH of the solution.

And so we got the opposite happening, from seven down to zero, we got an increasing concentration of hydrogen ions, but we've also got a decreasing concentration of hydroxide ions and the opposite is happening from seven to 14.

Now, a neutral solution has an equal concentration of hydrogen and hydroxide ions.

The hydroxide ions neutralise the hydrogen ions, producing water.

And so pH seven is the point at which this happens, where you've got equal concentrations of hydrogen and hydroxide ion, and here we can see the equation showing what's happening to the hydrogen and hydroxide ions.

So is this true or is this false? Alkalis release hydrogen ions into the water, increasing the pH of a solution.

Well done if you said false.

Can you justify your answer? Well done if you said A.

Alkalis release hydroxide ions into the water, so you could correct the first statement to say, "Alkalis release hydroxide ions into the water, increasing the pH of a solution." Let's have another question.

In a neutral solution, what is the relationship between hydrogen and hydroxide ions? Okay, let's have a look.

So if it was A, if there were more hydrogen ions than hydroxide ions, then the solution would be more acidic.

In B, if there were more hydroxide ions than hydrogen ions, then the solution would be more alkaline.

And if we look at C, the concentrations of hydrogen ions and hydroxide ions are equals, that is correct.

That is the answer.

So let's have a look at task A.

So complete the pH scale below, question one.

Explain what the pH scale is and how it is used to classify substances as acidic, neutral or alkaline.

And then a question about a solution with a pH of four.

If a small amount of alkaline solution is added, how would the pH change and why? So pause the video and come back when you've had a go at those three questions.

Welcome back, let's have a look.

From seven down to zero, we're increasing in acidity, seven is neutral and from seven to 14, we're increasing in alkalinity.

Question two, explain what the pH scale is and how it's used to classify substances as acidic, neutral or alkaline.

Well, the pH scale is a numerical scale ranging from zero to 14 and it measures how acidic or basic a substance is.

Substances with a pH less than seven are acidic, those with a pH of seven are neutral and those with a pH greater than seven are classified as basic or alkaline.

Question three, how did you do on that one? So if a small amount of a strong base is added to a solution with a pH of four, the pH of the solution would increase.

This is because the hydroxide ions from the base would neutralise some of the hydrogen ions in the solution, reducing the acidity and raising the pH closer to neutral.

Let's move on to part two of our lesson today which is all about indicators.

So an indicator is a substance that changes colour depending on the pH of the solution that they're in and they're used to determine whether a solution is acidic, whether it's neutral or whether it's alkaline.

And here are some examples that you might already have come across.

Litmus paper, blue and red, universal indicator paper and phenolphthalein solution.

There are paper indicators and aqueous solution indicators.

So a universal indicator comes as both paper and solution, litmus is paper and solution, phenolphthalein is an aqueous solution and methyl orange is a different indicator which is also an aqueous solution.

Quick check.

Which of these are available in both paper and solution forms? Well done if you said litmus and universal indicator.

So both the paper and solution versions of universal indicator contain actually a mixture of indicators and the paper or solution actually change colour depending on the pH of the sample and then we compare the colour to a scale, a coloured pH scale and we estimate the pH that's made.

So here's an example scale.

That's the neutral colour.

Then when a solution is increasingly acidic, it sort of goes yellow, orange, red and when a solution becomes increasingly alkaline, it will go through the stages of sort of blue and purple.

A qualitative conclusion would be using the colour and using the terms increasingly acidic or neutral or increasingly alkaline.

A quantitative conclusion of value is more useful than a qualitative conclusion and that's because the values indicate the amount of hydrogen ions and hydroxide ions present in a sample.

However, using universal indicator paper or solution is very subjective, because it relies upon a visual comparison of colours to a reference chart.

And you might interpret a colour different the way that your partner does or I would and so on.

So how can make this more accurate? Well, we could use a pH metre, a digital metre and that would give us a more accurate and precise measurement of a sample's pH.

And if you look at the display, that actually said 5.

70.

So we've got a much more precise figure there and this is better than a colour that's then compared to a reference chart.

So let's have a look.

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

So it's the green bit at the bottom we're looking for.

What do you think that pH might be? So I think it's pH nine.

You might think differently.

Let's think about using indicator solution.

So what's the pH of this sample? I think it's probably pH seven.

You might've used litmus paper before and the litmus paper comes in two versions, red litmus and blue litmus.

And they really only tell us if a sample is acidic or alkaline.

Now, if you use solution, litmus solution, it can tell us if a sample is neutral as it turns purple.

If you look at these various measurements.

So in an acidic solution, the blue litmus paper turns red at the end, but the red paper stays red.

In a neutral solution, the blue paper would stay blue and the red paper would stay red and in an alkaline solution, the blue paper stays blue and the red paper turns blue.

So we need to remember all those different variations.

So let's have a quick check to see if you can interpret those.

So what would happen if you placed blue litmus paper and red litmus paper in an acidic solution? What would happen? Amazing if you said the blue litmus would turn red and the red would stay red.

Let's have a look at another one.

A piece of red litmus paper is placed in a cleaning solution.

The litmus paper turns blue.

This tells us the cleaning solution is.

Well done if you thought it was alkaline.

Let's look at a different indicator now.

Phenolphthalein has two colour states, pink in alkaline solutions and colourless is acidic or neutral solutions.

And methyl orange has three colour states.

So it will be red or orange in acidic conditions and yellow in alkaline or neutral solutions.

So let's have a quick check.

Methyl orange turns yellow in acidic solutions and red in alkaline solutions.

Is this correct or not? Well done if you said false and can you justify your answer? Well done if you said B, methyl orange turns red in acidic solutions and yellow in alkaline solutions.

Remember, methyl orange also turns orange in acidic solutions.

So another check.

Which indicator would you use if you wanted to observe a colour change across quite a wide pH range? Would one would be the best? Well done if you said universal indicator, and that's really why it's called universal, 'cause it covers such a wide pH range.

It covers them all.

Let's have a look at task B.

So for task B, you've been provided with three samples, A, B and C.

They could be acidic, neutral or alkaline and you need to find out which.

So place a few drops of each sample in the columns as shown and then the indicators across in the rows.

So have a look at the diagram and follow that.

And then I would like you to record your results in a suitable table and then when you've done that, I'd like you to talk about how you think using a pH metre would change the way you did this investigation.

So pause the video and come back when you've carried out the test, you've got a table of results and you've thought about using a pH metre.

Okay, how did you do? Let's have a look.

So this is my table, so your results might look like this.

My sample A was green with the universal indicator solution.

My sample B was blue with universal indicator and my sample C was red with universal indicator.

So it looks like I've got a neutral solution, an alkaline solution and an acidic solution there.

Have a look at the colours for each of the other indicators and see if your conclusion matches mine.

And then if we think about using a pH metre, how would that change things? It would actually give us a very precise numerical value for the pH of each sample instead of relying on the colour changes and our own interpretation of those colours versus the reference charts.

So a digital metre would give us that much more precise numerical value.

Well done, we've come to the end of the lesson.

I hope you feel a lot more confident now about pH scales and the colours that different indicators go.

Let's have a look at the summary.

So the pH scale ranges from zero to 14 and that tells us how acidic or basic a substance is.

Universal indicator provides us with a full range of colours that correspond to different pH levels.

Now, that's a positive in one way, but it can also be a limitation in terms of how we interpret each of those colours.

Phenolphthalein changes from colourless to pink in alkaline conditions.

Methyl orange has three colours, so it's red or orange in acidic conditions and yellow in basic or alkaline conditions.

The litmus paper, remember we had the red version and the blue version.

So litmus paper turns red in acidic conditions and blue in basic condition and the solution can also tell us if a solution is neutral, 'cause it will turn purple.

And finally, pH metres are a more accurate way of measuring the pH of a solution.

So if you're ever asked to improve a method that's using one of the indicators, like universal indicator, phenolphthalein, litmus paper and methyl orange, it's easy to improve that method to make it more precise by using a digital pH metre.

Well done.

Thanks for studying with us today and I'll see you next time.