Welcome to today's lesson.

My name is Mrs. Clegg.

We're learning about precipitation reactions today, and these are part of the Understanding chemical reactions unit.

We're gonna find out what a precipitate is.

We're gonna look at word equations and symbol equations of precipitation reactions, and we're also gonna find out how precipitation reactions can be useful.

And here are today's keywords: precipitation, precipitate, aqueous solution, insoluble, and chemical qualitative analysis.

So listen out for those words during our lesson today.

And here are those words written into a sentence.

You might like to pause the video and take some notes about what these words mean so that you can refer back to them.

Precipitation is the process where a precipitate is actually formed, and a precipitate is an insoluble solid that's formed when two solutions react.

An aqueous solution, and we put AQ in brackets, usually in reactions, to show it's an aqueous solution.

That's formed when a substance is dissolved in water.

Insoluble is a way of describing a substance that will not dissolve in a particular solvent.

And chemical qualitative analysis is a chemical test that confirms the presence of a substance.

It doesn't tell you how much of the substance is present.

So that word qualitative is just literally a yes or a no.

And here's today's outcome.

So by the end of the lesson, you should be feeling much more confident about being able to describe what happens during precipitation reactions and be able to write chemical equations to show these.

Today's lesson is split into two parts: precipitation reactions and uses of precipitation reactions.

So let's get started with the first part.

Precipitation reactions occur when two solutions combine to form an insoluble solid product.

So in the video I'm about to show you, watch out for two colourless solutions being combined together to form an insoluble solid product.

And the insoluble solid product is lead iodide, and it's yellow and it's a solid.

So we are seeing a precipitate forming.

Here we go.

And the appearance of a precipitate is evidence that a chemical reaction has actually occurred.

So when we mix solutions of lead nitrate and potassium iodide, we get this yellow precipitate of lead iodide and a solution of potassium nitrate.

We use state symbols to identify if a reaction has produced a precipitate.

So let's have a look at the word equation for this reaction occurring in the video.

Lead nitrate, which is aqueous, and potassium iodide, which is aqueous, meaning that those are dissolved in solvent.

Combine together to form lead iodide, which is a solid, and potassium nitrate, which is aqueous.

It's dissolved in the solvent.

And here's the chemical equation and there's the state symbols showing us that a solid has been formed.

Let's have a quick check.

So what state of matter is a precipitate? Which one of those? Well done if you said solid.

As a precipitate is a solid, it will settle to the bottom of the container over time, as you can see in the picture.

Now, when a precipitate forms, it makes the solution that's formed cloudy or opaque.

And that could be white or it could be coloured.

We can no longer see through it.

Let's have a quick check.

State symbols can be used to identify a precipitation reaction.

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

Now can you justify your answer? So I'm going to give you two statements and I want you to choose one of those that would help explain why the sentence above is true.

Well done.

So a substance with an S in brackets is a precipitator.

So we know what a precipitator is now.

And if we wanted to collect the precipitate, we use the technique called filtration.

So let's have a look at the equipment we use for filtration.

We've got our beaker.

We've got our reaction mixture in the beaker, which we would pour out into the funnel and conical flask.

We've got the filter paper inside the the funnel.

The precipitate would collect inside the filter paper, and the filter paper would let through the filtrate.

So check that you could label that diagram and that you know where you would find the precipitate and you would know where to find the filtrate and what the filtrate is.

So let's have a quick check.

So which process do we use to separate the precipitate from the reaction mixture? Well done.

Filtration.

So on this slide, I'm going to show you lots of chemical reactions and I want you to identify which ones are precipitation reactions.

So which one of those are precipitation reactions? And when you've done that, could you explain how you've identified them as precipitation reactions? You might like to pause the video now and come back when you are ready.

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

Let's see how you've done.

So A would definitely be a precipitation reaction because silver chloride is a solid.

B would not be because we don't have a solid being formed.

C would be because barium sulphate is a solid.

And D would not be because there's no solid being formed.

Question two: explain how you identified these as precipitation reactions.

So here's an answer.

So in a precipitation reaction, a solid is always formed.

And in the equation, the product with the state symbol will be the precipitate.

So hopefully you have got that.

Let's move on to question three.

So in question three you will see some boxes, and I want you to complete the missing parts of the word and symbol equations for the following reactions.

Don't forget to include the state symbols, and you might find the information in the compounds table from the additional material useful.

Pause the video and come back when you are ready.

Okay, are you ready to have a look at the answers? So we've got ammonium sulphate and calcium nitrate reacting together to form ammonium nitrate.

So we know that calcium sulphate is going to be formed.

And if we look at the symbol equation, we can see that as well.

Okay, so let's look at what should be in the boxes.

So calcium sulphate and it's a solid.

And then the chemical symbol for ammonium sulphate is this.

Let's look at B.

So we've got calcium nitrate plus something is changed to form potassium nitrate and calcium fluoride.

So we know that we must be reacting calcium nitrate with potassium fluoride.

The formula for calcium fluoride is CaF2, and that's a solid.

Okay, do the same thing for part C and D now.

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

Okay, let's have a look at how you've done.

So if we look at the chemical equation, the balanced symbol equation, we can see zinc chloride and sodium sulphide react together to form zinc sulphide and sodium chloride.

And we can also see from the symbol equation that zinc sulphide is a solid, so it's a precipitation reaction.

Okay.

And the symbol for sodium sulphide is this.

Let's have a look at D.

So we've got copper sulphate plus sodium hydroxide react together to form sodium sulphate and copper hydroxide.

And you see copper hydroxide there is the solid that's forming the precipitate.

So copper sulphate, there's the symbol.

Sodium sulphate and copper hydroxide.

Don't forget the state symbols.

Well done if you've got those.

If you need to pause the video at any point, do so to check yours.

Okay, do the same thing for E and F now.

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

Okay, let's have a look.

So iron chloride and something combine to form iron hydroxide and sodium chloride.

So in the symbol equation, we've got sodium hydroxide.

So we know iron chloride and sodium hydroxide are reacting to form iron hydroxide and sodium chloride.

So let's have a look at this symbol equation.

Iron chloride there and iron hydroxide.

Where have you got the brackets there? The subscript three is multiplying whatever's inside the brackets.

So there's one iron atom there and three hydroxides.

Let's look at F.

Sodium carbonate plus something makes copper carbonate plus something.

So if we look at the symbol equation, we know that we've got sodium carbonate and copper chloride, and that's aqueous.

And they form copper carbonate and sodium chloride, which is aqueous.

And there's the symbol equation.

Well done if you got those.

That's quite tricky that.

Well done.

Let's move on to the second part of the lesson, uses of precipitation reactions.

So let's see how these can be useful to us.

They're used in the treatment of water.

Here we've got some untreated river water, which contains unwanted dissolved substances that can make water unsafe for us to drink.

So lead and copper compounds, for example.

Now, precipitation reactions would actually help to remove these unwanted substances from the water.

They form insoluble compounds that can then be removed by filtration.

Water would still need further processing before it's safe to drink.

So we often sterilise the water by adding chlorine.

High levels of lead, copper, and cadmium compounds in drinking water can be poisonous or toxic, and they need to be removed, and precipitation can help with this as well.

So the lead compound reacts with sodium hydroxide, which forms a white insoluble precipitate, lead hydroxide.

And then we can remove that precipitate by filtration, and the water is then left safe to drink.

And here's the reaction.

So we've got lead nitrate and sodium hydroxide combining together to form lead hydroxide, which is a solid, and sodium nitrate.

Hard water is a problem in some areas, and that causes limescale in kettles, water pipes, and taps.

If you look at the tap, you can see the white deposit there, the limescale.

And it can also mean that water tastes different.

So hard water tastes different to soft water and it also can cause soap scum.

We need more detergent in hard water areas to create a good lather, or to get lots of bubbles for washing things.

And the hard water is caused by calcium and magnesium compounds in the water.

So precipitates formed, calcium carbonate and magnesium hydroxide, can actually be removed by filtration, and that reduces the problems with its use and makes the water much more pleasant to drink.

And here's a chemical reaction.

Calcium chloride plus sodium carbonate react together to form the solid calcium carbonate and sodium chloride.

And here we've got magnesium sulphate plus sodium hydroxide produce magnesium hydroxide, which is a solid, and sodium sulphate.

So those two compounds, calcium carbonate and magnesium hydroxide, are precipitates, and we can remove them by filtration.

So precipitation reactions can identify if a substance is present.

We can use them as what we call a chemical qualitative test.

That doesn't tell us how much of the substance is present, it just tells us if it's there.

So if we add sodium hydroxide to certain metal solutions, precipitates will form.

So a blue colour actually shows that a copper compound is present.

If we look at this table, it gives us a summary of some of the colours that are formed.

So if we get a white precipitate, then we've got aluminium or calcium or magnesium or lead compounds being formed.

If it's blue, then copper, a copper compound is formed, and if it's green or brown, then iron, an iron compound is produced.

So there's quite a few metal compounds there that you can see are white.

So we would need to use further tests to tell the difference between those white precipitates.

So let's have a quick check.

Which test tube do you think contains a copper compound? Well done if you said C.

A blue colour would tell us copper is there.

What metal compounds could be in Alex's water samples? This is what Alex found out.

Alex added sodium hydroxide to each sample and observed the following.

The first sample, sample A, formed a green precipitate.

Sample B formed a white precipitate, and sample C formed a blue precipitate.

Sample D formed a brown precipitate.

And sample E did not form a precipitate at all.

So if Alex converts this into a table of results like this, which metal compounds could be present in Alex's water samples? So pause the video and write down what metal compounds you think could be present.

Come back when you're ready.

Let's have a look at the answers.

So if the precipitate is green, that would tell us an iron compound is present.

If it's white, it could be aluminium, calcium, magnesium, or lead.

We're not sure which, we would need further tests.

If it's blue, then it's copper.

If it's brown, then it's iron as well.

And if we get no precipitate, then it tells us there's no aluminium, calcium, magnesium, lead, copper, or iron present at all.

Question two: explain why Alex should not drink his water after removing the metal compounds as precipitates.

So Alex has done the reaction and he's worked out what precipitates are present.

Why should he still not drink it? And then question three: Alex goes to visit his friend in London where they have hard water.

Explain why the water tastes different and list some of the problems that this water may cause.

So pause the video and come back when you've done question two and three.

Let's have a look at the answer.

So why shouldn't Alex drink his water? He would not know if he'd removed all the metal compounds, as the tests are qualitative.

Remember, qualitative means just we know something is present or not present.

We don't know how much of something is present.

The water requires further processing to make sure it's safe to drink.

So we would need to add chlorine, for example, to sterilise it.

And then Alex visits his friend in London.

Why does the water taste different? And what are the problems? So this water is hard, and it's caused by calcium and magnesium compounds in the water, which gives it a different taste to soft water.

And some of the problems are limescale in things like kettles, taps, pipes, et cetera, and soap scum when you're washing, and you need to use more soap to get a lather or bubbles.

So for the next question, your teacher will give you some unknown solutions to test and you need to carry out your own identification of these substances.

So we want know the colour of the precipitate and the metal compound present.

So here's the method to use.

We've got our test tube with the test solution.

We're going to add some drops of sodium hydroxide using a pipette.

So add the sodium hydroxide solution to the test solution.

Observe to see if precipitate forms and record your results in the table.

So carry out the practical and come back when your table is complete.

So your results might look like Alex's.

So the blue precipitate would tell us copper was present.

No precipitate tells us there's none of these present.

If it's brown, can you remember? It's possibly iron metal compound present.

If it's brown, then it's iron, an iron metal compound that's present.

If it's white, it could be aluminium, calcium, magnesium, or a lead compound present.

And if it's green, it could also be an iron metal compound present.

Well done.

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

Let's have a look at what we've learned in our summary.

So precipitation reactions occur when two solutions combine to form an insoluble solid.

So remember, right at the beginning of today's lesson I showed you a small video which had lead nitrate and potassium iodide, both colourless solutions mixing together and forming that bright yellow lead iodide precipitate and potassium nitrate.

The appearance of a precipitate is evidence that a chemical reaction has occurred.

That's one of the ways in which we can tell a chemical reaction has occurred.

Precipitation reactions are important in treating our water to remove unwanted substances from our water.

And we looked at filtration there to remove the precipitate.

And then precipitation reactions are used in qualitative analysis to identify the presence of certain substances in a solution.

Well done.

I hope you feel much more confident about what precipitate is and that you could talk about some of the reactions that occur, which are precipitation reactions and how you know they're precipitation reactions.

And also some of the uses, especially in water treatment for precipitation reactions.

Fantastic.

I look forward to seeing you next time.