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I can identify metal ions based on the colour of their flame tests and understand how instrumental analysis methods are used for detecting and identifying elements.

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Licence

This content is © Oak National Academy Limited (2023), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Key learning points

Positive ions burn with distinct colours that are visible when heated above a blue Bunsen flame.
Instrumental methods are accurate, sensitive, and rapid for detecting and identifying elements and compounds.
Unique flame emission spectra are produced for each positive metal ion.
A spectrometer can split light from coloured flames into flame emission spectra.
A spectrometer can measure more than one metal ion in a single solution and the concentration of each.

Common misconception

Flame colours are unique, even in mixtures.

Explain that strong colours, like sodium's yellow, can mask others. Use spectrometry to identify masked elements.

Keywords

Flame test - A test where a chemical put into a flame produces a characteristic colour (tests for metal ions).
Spectroscope - An instrument that separates light into its component wavelengths.
Ion - An ion is an atom or group of atoms with a positive or negative charge due to the loss or gain of electrons.
Concentration - The amount of a solute dissolved in a certain volume of solvent.
Calibration curve - A graph used to determine the concentration of a substance in a sample.

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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 instrumental analysis and flame tests.

In today's lesson, we're going to learn how to identify metal ion based on the colour of their flame tests, and also understand how instrumental analysis methods are used for detecting and identifying elements.

Here are the key words for today's lesson.

Flame test, spectroscope, ion, concentration and calibration curve.

Now, some of those words will be familiar to you, so concentration and ion for example, but others will be new.

So pause the video here, read through those definitions, and write down any notes you feel you need to.

Today's lesson is divided into two separate parts.

So we're going to look at flame tests, first of all, and then instrumental analysis.

So let's start with part one of today's lesson, flame tests.

Now flame tests are a very quick and simple method used in chemistry to identify the presence of certain metal ions, so we're looking for those metal cation, the positive metal ions that might be present in a solution.

So when heated, these metal ions emit characteristic colours that can be observed using a blue Bunsen and burner flame.

So characteristic colours means for example, that certain metals like copper will produce a green flame, whereas other metal ions will produce a different coloured flame, and we can use this to help identify the presence of certain metal ions.

Flame tests have been used in analytical chemistry for over a century, so they're quite an old method for finding out which metal ions might be present in a solution.

And early scientists discovered that different elements produce distinct colours when heated, and this led to the development of these tests as a diagnostic tool.

So if you've got a solution and we want to work out what metal ion is present, flame tests might be a process that we can use, an analytical process that we might use.

So in those images there you can see that we're heating copper to a very high temperature and you can see that the flame is green, the presence of metal copper ions produces that flame colour.

So now we're going to carry out a question based on that learning, and this is a true or false question.

So the statement is, \"Flame tests can be carried out in an orange or a blue flame.\" So is that statement true or false? And then justify your answer.

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

Welcome back.

So hopefully you've recognised that answer is false, and it's because flame tests must be carried out in a blue flame, and that's because we are looking for distinctive colours of the metals.

And if you do it in an orange flame, then the temperature is not going to be hot enough, but also that orange colour will interfere with our analysis.

When metal ions are heated, their electrons absorb energy and move to a higher energy level.

As the electrons return to their original levels, they release energy in the form of visible light producing the characteristic colour, and you can see that in the diagram there.

So as the electron returns back to its original energy level, you can see that pink arrow is showing light being released, and that's of a specific colour depending on the different metal ion.

Each flame colour is unique to the metal ion presence.

So lithium compounds result in a red flame, sodium compounds result in a yellow flame, potassium compounds result in a lilac flame, calcium compounds result in an orange red flame, and copper two compounds result in a blue green flame.

So you can see each of those flames is quite distinct, and can help us identify the metal ion present.

So here we have a question based on that learning.

\"Which of the following metals and flame colours are correct?\" So pause the video here, read through each of those, and decide which ones are correct.

Welcome back.

So hopefully you recognise that potassium produces a lilac flame, and copper produces a blue green flame.

Now proper preparation and handling of samples are essential for accurate flame tests.

So there's a very specific method that you need to use when you're carrying out flame tests.

There's the equipment there shown in the diagram.

So we've got a wire loop, we've got a sample that contains the metal ion.

We've got our blue Bunsen burner flame, remember, and then we're taking the sample on the wire loop and placing it in the flame.

So let's have a look at the method for that.

So dip the wire loop in hydrochloric acid, that removes any ions that might be present on the wire loop.

Check that the wire loop is clean by holding in a flame and if no colour appears, that shows it's clean so there are no metal ion already on there.

Dip the clean wire loop into the solid sample of the compound being tested.

Put the loop into the edge of the blue flame from the Bunsen burner, and observe and record the flame colour produced.

So flame tests can be limited by the presence of a mixture.

So if you've got a mixture of different metal ions present, that makes it really difficult to identify the colour of the flame.

So here we've got strong yellow colour from the sodium overpowering other colours, making it difficult to identify if further ion are present.

So you can just about see there we've got the yellow from the sodium, but there's also a little bit of red, and we're not sure if that red is from the crimson from lithium, or if it's actually the orange red from calcium.

It's very difficult to tell.

So here we have a question based on that learning.

So \"What is a limitation of flame tests when analysing mixtures of metals?\" So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So hopefully you've realised that a limitation of flame tests is the presence of a mixture can result in the colours of different metal ions masking each other, and that's a key word, masking.

So if you're answering an exam question on this, that's the word you need to use.

Well done if you got that correct.

So now we're going to do task A.

And for the first part of task A, you've been provided with some unknown salt samples, and you're going to complete a series of flame tests to identify the unknown samples, and you need to record your observations in a table.

So your results for this investigation may look something like this, You can see we've got the sample in the first column.

We've recorded the flame colour as we saw it during our observations, and then from that we've identified the metal ion that's present.

We're now going to move on to part two of the lesson, instrumental analysis and flame emission spectroscopy.

So instrumental methods are advanced techniques used in chemistry to detect and identify elements and compounds with high accuracy and high sensitivity.

So high accuracy means it's likely to identify the substance in the sample correctly, and sensitivity means it can detect very small amounts of that substance in the sample, and that's really useful.

The diagram underneath shows how this system works.

So you've got the stimulus passing through the sample.

On the other side of the sample you've got a detector, so that will detect how much light, heat, current, or voltage is passing through.

And for light, for example, it might detect the wavelength of light that's passing through the sample.

There's then a sample processor, so that will amplify and digitise the results of that experiment, and then that will provide a readout and it's the readout that the scientists will then interpret to see what was present.

So Flame Emissions Spectroscopy or FES is a method used for analysing metal ions and it uses a flame photometer.

So instrumental methods are more accurate than traditional methods.

They're more sensitive than traditional methods, and they're faster than those traditional chemical tests, so if you compare them to flame tests, for example.

So here is a question based on that learning.

\"Which are the advantages of instrumental methods in chemistry compared to traditional chemical tests? So pause the video here and answer the question.

Welcome back.

So hopefully you identified that they are more accurate and faster.

They're actually more sensitive and more reliable than traditional chemical tests, which is why the other two answers are incorrect.

So well done if you got that correct.

So in flame emission spectroscopy, a sample is introduced into a flame and this causes the metal ions to emit light, and this light is then analysed using a spectroscope.

And what the spectroscope does, it splits it into a spectrum of lines that correspond to different elements, and you can see that happening there in the diagram.

Now each metal ion emits a unique set of lines known as a flame emission spectrum, and these spectra act like fingerprints enabling the identification of multiple metal ions present in a sample.

Now remember the masking from the flame test.

We don't have that problem here.

We can have multiple ions present, and we can detect those in the unknown.

So that appears as a series of lines there on the screen, and that can help us identify what's present and what's not present in the sample.

We're now going to answer a true or false question based on that learning.

So read through the question, pause the video here, answer the question, and I'll see you when you return.

Welcome back.

So the statement is, \"Each metal ion emits a unique flame emission spectrum.\" Is that true or false? That's true, and that's because a specific wavelength of light are emitted by each element.

So well done if you got that correct.

So flame emission spectroscopy can also be used to measure the concentration of metal ions in the solution.

A reading is taken from the flame photometer for different concentrations of a metal ion in solutions.

So we know the concentrations and we take those measurements, and then we plot those readings on a calibration curve.

And that means when we get an unknown reading from the flame photometer, we can use this curve to work out the concentration of sodium mines, for example, in an unknown sample.

So here we have a question on that learning.

It's a true or false question again.

\"So flame emission spectroscopy can only identify metal ions and has no other uses.\" Is that true or false? And then justify your answer using the statements underneath.

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

Welcome back.

So hopefully you've identified that that answer is false, and that is because FES can measure the concentration of metal ion, which allows us to plot a calibration curve.

So well done if you got that correct.

So now we're going to do task B.

So for question one, \"State why it's important that a test for a chemical substance must be unique.\" And two, \"Identify which metals were in the unknown sample for the flame emission spectroscopy.\" So look at those four different spectra there, and identify which of those three metal ions are present in that unknown solution.

So pause the video here and answer the questions.

Welcome back.

So let's go through those answers one at a time.

So question one, \"A unique test ensures accurate identification of a substance without confusion, preventing misidentification.\" Now the important word there is accuracy.

So it allows an accurate identification of a substance without confusing it with a different substance.

So well done if you've written something similar.

And question two, \"The sample contains strontium and titanium as these contain the wavelengths present.

Not all of the wavelengths manganese producers are present in the unknown sample.

So manganese is not present in that unknown.\" So well done if you've got that correct.

Question three.

\"What concentration of sodium ions is present when the flame photometer has a reading of 10 units?\" So look at the flame photometer reading of 10 units, go across to the line that best fit, and identify the concentration of sodium ion present.

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

Welcome back.

So let's go through that answer.

So the answer is 0.

05 grammes per decimeter cube.

Make sure your answer includes the units.

So the graph shows that a concentration of 0.

05 grammes per decimeter cube of sodium ions would produce a flame photometry reading of 10 units.

So well done if you got that correct.

Here is a summary of today's lesson.

Positive ions burn with distinct colours that are visible when heated above a blue Bunsen flame.

Instrumental methods are accurate, sensitive, and rapid for detecting and identifying elements and compounds.

A spectrometer can split light from coloured flames into flame emission spectra.

Unique flame emission spectra are produced for each positive metal ion, and a spectrometer can measure more than one metal ion in a single solution and the concentration of each.

So thank you very much for joining me for today's lesson.

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