Hello, my name is Mrs. Collins, and I'm going to be taking you through the learning today.

This lesson forms part of the unit chemical analysis and is called "Chemical tests: aqueous ions." So let's get started on the learning.

During today's lesson, you will learn how to perform and interpret sodium hydroxide precipitate tests for metal ions.

Another test for identifying ammonium, carbonate, sulphate, and halide ions.

Here are the keywords for today's lesson.

Precipitate, ion, cation, and anion.

Pause the video here, check the definitions, and make any notes you feel you need to.

Today's lesson is divided into two parts.

We are going to start talking about cation tests and then we're going to talk about anion tests.

So let's start with cation tests.

And remember, a cation is a positive ion.

Unique tests are crucial for accurately identifying specific ions, preventing confusion from similar reactions.

So here, we've got an example.

We've got sodium hydroxide solution in a beaker.

We've got a pipette that's going to transfer that to the test tube containing the metal cation solution.

Cation tests involve several methods to distinguish between positive ions, including reactions with sodium hydroxide, which can produce distinct precipitates.

Now remember, a precipitate is a solid produced during a chemical reaction that does not dissolve in the solution.

Here we have a true or false question based on the learning so far.

Unique tests are essential for accurately identifying specific ions.

Is that true or false? And justify your answer using the statements below.

So pause the video here and I'll see you when you're finished.

Welcome back.

So the answer to that question is true, and this is because it prevents confusion from similar reactions and falsely identifying a different substance.

Both calcium and zinc ions form white precipitates when sodium hydroxide solution is added.

So we need some means of differentiating between those two ions.

So what we can do is add excess sodium hydroxide solution because zinc hydroxide will dissolve, forming a colourless solution, but calcium hydroxide will not dissolve in that excess sodium hydroxide.

So that's how we can differentiate between the two.

So here's a question based on that learning.

What happens when sodium hydroxide is added to a solution containing zinc ions? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to that question is a white precipitate or form, and the precipitate that forms dissolves in excess sodium hydroxide.

So well done if you've got that correct.

Other metal ions can be identified by adding sodium hydroxide solution.

So copper II ions will form a blue precipitate.

Iron II ions will form a green precipitate.

And iron III ions will form a brown precipitate.

Word and symbol equations can show the reactions that are taking place.

So here's calcium chloride reacting with sodium hydroxide to form sodium chloride and calcium hydroxide.

Now if you look at the symbol equation, you can see the calcium hydroxide.

The state symbol at the end is an s in brackets.

So that tells you it's a solid.

So it's not dissolved in the solution, and therefore that's the precipitate.

But the sodium chloride that's produced has got aq in brackets, which tells you it's aqueous, so it has dissolved in the solution.

Here's iron II chloride reacting with sodium hydroxide to form sodium chloride and iron II hydroxide.

And again, the iron II hydroxide is a solid, so therefore a precipitate.

Lastly, we have iron III chloride reacting with sodium hydroxide to form sodium chloride, and the precipitate iron III hydroxide.

So here's a question based on that learning.

Which of the following ions forms a blue precipitate when sodium hydroxide is added? Pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to that question is copper II plus ions.

So well done if you've got that correct.

Ammonium ions can also be detected by heating with sodium hydroxide, which releases ammonia gas.

So here in the diagram you've got a solution containing ammonium ions and sodium hydroxide that's being gently heated, and the gas is turning damp red litmus paper blue, and that shows you an alkali is present.

So the gas dissolves in the water on the damp litmus paper and turns the paper blue, confirming the presence of ammonium ions.

Here is the equation for that reaction.

So we've got ammonium hydroxide that's breaking down into ammonia and water, and this is a reversible reaction.

Here is a question based on that learning.

So select all the statements below that are correct when testing for ammonium ions.

Pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to the question is the solution is heated and the red litmus paper changes colour.

You could specify the colour it changes to there if you wanted to.

So well done.

So now we're going to complete task A.

A student completes a series of chemical tests to identify some unknown substances.

Fill in the missing spaces in the results table below.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So here are the answers to that question.

So the blue precipitate forms, then that means there are copper II ions present.

If green precipitate forms, then iron II ions are present.

If a white precipitate forms but dissolves in excess sodium hydroxide, that indicates zinc ions are present.

And if a brown precipitate forms, that indicates iron III ions are present.

So well done if you've got that correct.

We are now moving on to part two of the lesson where we're moving on to anion tests.

Remember, anions are negative ions.

Anion tests help to identify negatively charged ions in solutions such as carbonates, sulphates, and halides.

These tests involve adding specific reactants that produce characteristic reactions and precipitates.

So these can be used to identify those ions.

Carbonate ions react with dilute acids to release carbon dioxide gas.

And here in the image, we've got that happening.

So we've got a solution containing carbonate ions and a dilute acid, and you've got carbon dioxide being released, and that's being bubbled through limewater.

And remember, limewater turns cloudy or milky when carbon dioxide is passed through it.

Here we can see the equipment setup.

The presence of carbon dioxide is confirmed if bubbling through limewater turns it cloudy or milky.

Word and symbol equation show the reactions that are taking place.

So here's an example.

Sodium carbonate plus hydrochloric acid forms sodium chloride, water, and carbon dioxide.

Now remember, the general equation for a metal carbonate reacting with an acid is to form a salt, water, and carbon dioxide.

In this case, the salt is sodium chloride.

Carbon dioxide reacts with limewater, which is calcium hydroxide, to form calcium carbonate and water.

And if you look at the symbol equation, you can see the calcium carbonate is a solid.

So that's a precipitate.

So that's why the limewater turns milky or cloudy during the reaction because calcium carbonate is formed as a precipitate.

Here is a question based on that learning so far.

What indicates the presence of carbonate ions in a solution when dilute acid is added? So pause the video here, read through those statements, and I'll see you when you finished.

Welcome back.

So the answer to the question is fizzing.

Well done if you got that correct.

Here is the second question based on that learning.

Which of the following are products when sodium carbonate reacts with hydrochloric acid? So again, pause the video here, and I'll see you when you finished.

Welcome back.

So the products of that reaction are carbon dioxide, sodium chloride, and water.

So remember, when a metal carbonate reacts with an acid, it forms a salt, water, and carbon dioxide.

So well done if you've got that correct.

To test for sulphate ions, add barium chloride solution to an acidified sample.

So here, we've got a beaker containing the barium chloride solution and a test tube containing the acidified sample, and a white precipitate is forming.

The formation of a white precipitate of barium sulphate indicates the presence of sulphate ions.

And we can see this happening in the experiment there.

There's the white precipitate form.

Word and symbol equations show the reactions that are taking place.

So magnesium sulphate and barium chloride forms barium sulphate and magnesium chloride.

And we can see in that equation, the barium sulphate is a solid, so therefore it's a precipitate.

Another example.

Potassium sulphate reacting with barium chloride to form barium sulphate and potassium chloride.

And the barium sulphate, again, is the precipitate.

Here is a question based on that learning.

So this time, true or false.

The formation of a white precipitate when barium chloride is added to a sample shows the presence of chloride ions.

Is that true or false? And then justify your answer using the statements below.

Pause the video here, I'll see you when you're finished.

Welcome back.

So the answer to that question is false, and this is because the white precipitate indicates the presence of sulphate ions.

Here's a second question based on that learning.

This time you need to look very carefully at the state symbols when answering the question.

So what are the products when potassium sulphate reacts with barium chloride? Pause the video here and I'll see you when you're finished.

Welcome back.

So the answer to the question is, the potassium chloride is aqueous and the barium sulphate is a solid.

Remember, the barium sulphate is forming the precipitate, the white precipitate in that reaction.

Well done if you got that correct.

Halide ions are detected by adding silver nitrate solution in the presence of dilute nitric acid.

So silver chloride forms a white precipitate.

So if we add the silver nitrate to a solution containing chloride ions, then a white silver chloride precipitate will form.

If we add it to something containing bromide ions, then a cream silver bromide precipitate will form.

And if we add it to something containing iodide ions, then a yellow silver iodide precipitate will form.

Word and symbol equations can show the reactions that are taking place.

So sodium chloride reacts with silver nitrate to produce sodium nitrate, which is aqueous, and silver chloride, which is a solid precipitate.

And that's white.

Sodium bromide reacting with silver nitrate forms sodium nitrate, which is aqueous, and silver bromide, which is a cream precipitate.

And then sodium iodide reacts with silver nitrate to form sodium nitrate and silver iodide.

And silver iodide is a yellow precipitate.

Here is a question based on that learning.

When silver nitrate is added to a solution of halide ions, a precipitate forms. Which of the following precipitate colours is correct? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to that question is iodide ions are yellow, form a yellow precipitate.

So well done if you got that correct.

Dilute acid is added before the sulphate or halide ion test to react with and remove impurities, such as carbonate ions, which might result in false positives.

So any acid added must not contain the ions, which would also trigger a false positive.

So this means hydrochloric acid is used in sulphate tests because it contains chloride ions and they don't interfere.

And nitric acid is used in the halide tests because its nitrate ions do not interfere.

Here is a question based on that learning.

Why is dilute hydrochloric acid used in sulphate tests and not sulfuric acid? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to the question is, chloride ions don't interfere with the test, but sulphate ions do, and it reacts with the carbonate ions, and so removes them.

Remember, the acid will react with carbonate ions.

Well done if you got that correct.

Dissolved substances are often considered impurities in a water sample.

So if we look at the diagram, we can see we've got tap water there.

We've got water molecules present, but we've also got dissolved ions, and we would call these impurities.

Ion testing ensures potable water quality by detecting harmful ions.

Safe drinking water requires low levels of dissolved salts and contaminants.

And this is important.

So we can test it for cations and anions, confirm the presence and remove these potentially harmful substances.

Here we have a true or false question.

So all dissolved substances in water are considered beneficial and improve the quality of potable water.

So is that true or false? And justify your answer using the statements below.

Pause the video here and I'll see you when you're ready.

Welcome back.

So the answer to that question is false, and that's because many dissolved substances are considered impurities and could be harmful.

So well done if you've got that question correct.

We are now going to go onto task B, and hopefully, you've been provided with some unknown salt samples.

And possible chemicals include sodium chloride, calcium sulphate, and sodium carbonate.

You're gonna complete a series of anion tests to identify the unknown samples, and record your observations in the table, identifying any false positives.

You're then going to write the word and symbol equations for all positive test results.

So pause the video here, carry out the experiment, record your observations, and write the word and symbol equations.

Welcome back.

So here's a potential results table, including a false positive test result.

So pause the video here and compare that with your results, although you may have additional results or alternative results compared to those.

For question two, here are some potential word and symbol equations that you may have.

So pause the video here and check those against yours.

Here is a summary of today's lesson.

Some metal ions form coloured precipitates in a reaction with sodium hydroxide solution.

Ammonium ions can be identified by heating with sodium hydroxide and using damp red litmus paper, which turns blue if positive.

A dilute acid is usually added before testing for carbonates, sulphates, or halides to avoid false positives, Halide ions produce different coloured precipitates when silver nitrate is added to a solution containing them.

A white precipitate is formed when barium chloride is added to an acidified solution containing sulphate ions.

Thank you very much for joining me for today's lesson.