GCSE · Chemistry · Edexcel · Spec 1CH0

Flame tests for cations

Hold a pinch of salt in a flame and the colour tells you which metal ion is inside.

Chemistry · Flame tests

Which metal ion is hiding in the salt?

Five unknown salts have each been held in a blue Bunsen flame, and the flame colours are listed below. Pick a sample, then pick the metal ion you think it contains. Haven't met these colours yet? Have a go anyway. The reason appears the moment you place each one.

Still to sort

Lithium (0)

Where the line is: Red (crimson). The one people muddle with calcium's orange red.

Sodium (0)

Potassium (0)

Calcium (0)

Where the line is: Orange red. A close neighbour of lithium's red (crimson), so read the exact words.

Copper(II) (0)

5 of 5 still to sort.

Read the flame, name the ion.

Exam line: Give the colour and the ion together. Red (crimson) goes with lithium, orange red goes with calcium.

Watch out: Copper(II) gives a blue green flame, not just green.

Predict, then check

Picture the flame as a blank screen. What happens if the screen isn't blank?

Your Bunsen burner is roaring with an orange flame. You put the loaded loop in it anyway. What goes wrong?

Chemistry · Flame test method

Put the flame test in order

the order the steps are done in

1 · First step5 · Last step

Dip the loop in the sample
Record the colour you see
Acid on the loop
Loop to the edge of the blue flame
Check in the flame

Exam line: Acid, check, sample, flame, record. Most people forget the check.

Chemistry · Limits of the test

A yellow flame from a mixture

A student flame-tests a mixture of salts. The method is spot on, but the flame comes out bright yellow.

Which is closest to what you think right now?

Yellow means sodium, and sodium is the only metal ion in there.A mixture ruins the test, so the colour tells you nothing at all.Sodium may be in there, but its strong yellow could be covering other colours, so I can't tell whether other ions are present.

How sure are you?

I'm sureI think soI'm guessing

Yellow means sodium, and sodium is the only metal ion in there.

This is a common misconception

Half right: yellow does point to sodium. The 'only' is the trap. In a mixture the colours of different metal ions can mask each other, and sodium's strong yellow can overpower other colours. Other ions could be there but hidden, so one colour doesn't prove one ion.

A mixture ruins the test, so the colour tells you nothing at all.

This is a common misconception

That's too gloomy. The yellow still points to sodium. What you can't do is tell whether other ions are present, because their colours can be masked. That's a limitation of the test, not a total failure.

Sodium may be in there, but its strong yellow could be covering other colours, so I can't tell whether other ions are present.

This matches the evidence

Exactly. Colours of different metal ions can mask each other, and sodium's strong yellow is the classic one that overpowers the rest.

One colour means one ion only when there is just one ion. In a mixture, sodium shouts in yellow and can drown out everyone else.

Chemistry · Write it up

Describe the test, then give a result

Describe how to carry out a flame test on a solid sample, and state the flame colour for one named ion. [6 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Cleans the wire loop in hydrochloric acid, removing any ions on it (1 mark)
Checks the loop is clean by holding it in the flame; no colour shows it is clean (1 mark)
Puts the clean loop into the solid sample (1 mark)
Holds the loop at the edge of the blue Bunsen flame (1 mark)
Observes and records the colour of the flame (1 mark)
Gives one ion with its correct flame colour, for example sodium and yellow (1 mark)

Model answer

I would clean the wire loop in hydrochloric acid, then hold it in the flame and make sure no colour shows. Once it's clean I put it into the solid, hold it at the edge of the blue Bunsen flame and record the colour I see. Sodium compounds, for instance, give a yellow flame.

Write the steps in order and name the flame. Finish with an ion and its colour, paired up correctly.

Exam line: Method in order, blue flame, then a colour paired with a named ion.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Hold a pinch of salt in a flame and let the colour tell you which metal ion is inside.

What you need to know

Heated metal ions emit characteristic colours, so a flame test can identify certain metal ions (cations).
The test must be done in a blue Bunsen flame. An orange flame is not hot enough, and its orange colour interferes.
Start by dipping the wire loop in hydrochloric acid to remove any ions on it.
Next, hold the loop in the flame. No colour shows it is clean.
Have a goYour lab partner holds the acid-washed loop in the flame, sees a flash of colour and shouts 'Brilliant, it's working, add the sample!' Are they right?
No. A clean loop shows no colour, so a flash of colour means it isn't clean yet.
A colour at the check stage is bad news, not success: no colour is what shows the loop is clean.
Then dip the clean loop into the solid sample, put it in the edge of the blue flame, and record the colour.
Lithium compounds give a red (crimson) flame, sodium compounds give yellow, and potassium compounds give lilac.
Calcium compounds give an orange red flame. Copper(II) compounds give a blue green flame, which is quite distinctive.
Have a goA salt gives an orange red flame. Calcium or lithium, and what makes you sure?
Calcium. Lithium gives red (crimson), not orange red.
Both are reddish, so the exact colour words separate them: calcium is orange red and lithium is red (crimson).
In a mixture of metal ions, the colours can mask each other.
Sodium's strong yellow can overpower other colours, making it difficult to tell whether other ions are present.
Have a goA mixture of a sodium compound and a potassium compound goes in the blue flame. What might you see, and why could that fool you?
Possibly just yellow, because sodium's strong yellow can overpower potassium's lilac.
Colours in a mixture can mask each other, so seeing only yellow doesn't show potassium is absent.

The big picture

A flame test identifies certain metal ions (cations) from the colour they give a blue Bunsen flame: lithium red (crimson), sodium yellow, potassium lilac, calcium orange red and copper(II) blue green. The loop must be clean and the flame blue. The test struggles with mixtures, because sodium's strong yellow can hide other colours.

Key points

1A flame test uses the characteristic colour that heated metal ions give a blue Bunsen flame to identify the ion.

2Method in order: acid on the loop, check it is clean in the flame, dip in the solid, edge of the blue flame, record the colour.

3Lithium red (crimson), sodium yellow, potassium lilac, calcium orange red, copper(II) blue green.

4Limitation: in a mixture, colours can mask each other, and sodium's strong yellow can overpower the rest.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

Sam flame-tests an unknown solid like this: dips the wire loop in hydrochloric acid, then goes straight into the solid, then holds the loop in the edge of a blue Bunsen flame. The flame is yellow. Sam writes: 'Sodium compound. Done!' Is the method complete, and what should Sam have done?

⚠ Watch out

Mixing up lithium and calcium. Lithium is red (crimson) and calcium is orange red, so read both words. Also: copper(II) is blue green, not just green.

🧠

Memory hook

Sodium shouts in yellow, so loudly that it can drown out every other ion in a mixture.

✓

Check yourself

Cover the page and run the test in your head. What happens to the loop before the sample goes on, which flame do you need, and which colour goes with each of the five ions?

Flashcards

(14)

What is a flame test used for?

To identify certain metal ions (cations): heated, they emit characteristic colours that you can see in a blue Bunsen flame.

Which Bunsen flame must a flame test use?

A blue one.

Why is an orange Bunsen flame no good for a flame test?

It is not hot enough, and its orange colour would interfere with seeing the flame colour.

What do you do to the wire loop first?

Dip it in hydrochloric acid to remove any ions on the loop.

How do you check the loop is clean?

Hold it in the flame. If no colour shows, it is clean.

After the clean check, what happens to the loop?

Dip it into the solid sample, put it into the edge of the blue flame, then observe and record the colour.

Lithium compounds give which flame colour?

Red (crimson).

Sodium compounds give which flame colour?

Yellow.

Potassium compounds give which flame colour?

Lilac.

Calcium compounds give which flame colour?

Orange red.

Copper(II) compounds give which flame colour?

Blue green, which is quite distinctive.

Which two ions give the reddish flames that are easiest to mix up?

Lithium (red, crimson) and calcium (orange red).

What is the limitation of flame tests on mixtures?

The colours of different metal ions can mask each other, so it can be difficult to tell whether other ions are present.

Which ion's colour can overpower others in a mixture?

Sodium: its strong yellow can hide other colours.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Which metal ion is hiding in the salt?

Put the flame test in order

A yellow flame from a mixture

Describe the test, then give a result

More Edexcel GCSE Chemistry topics