Identifying ions

Identifying

ions

Practical video

Supporting resources

Registered charity number: 207890

Contents

Teacher notes ..................................................1

How to use this video .......................................................1

Notes on running the practical experiment ....................................1

Key terms .................................................................2

Prior knowledge ...........................................................2

Common misconceptions ...................................................3

Intended outcomes .........................................................3

Additional resources ............................................ 5

Pause-and-think questions...................................................5

Pause-and-think questions.................................................. 8

Follow-up worksheet .......................................................13

Follow-up worksheet: answers ..............................................15

Teacher notes

These resources support the practical video Identifying ions, available here: rsc.li/3dhnn5B

The value of experiencing live practical work cannot be overstated. Numerous studies provide evidence of its

value in terms of learner engagement, understanding, results and the likelihood of continuing to study chemistry

or work in a related field. This video can be used to complement live practical work, as well as helping learners to

understand the methods, equipment and skills when they cannot access the lab.

How to use this video

The video and additional resources are designed to be used flexibly, but some suggestions follow.

Flipped learning

Learners view the video ahead of the live practical lesson to help it run more smoothly and keep objectives in

focus. This may also help build confidence for some learners and improve their outcomes in the lesson. Use

questions from the set provided as part of the preparation task.

Consolidation and revision

Learners view the video aer the practical – this may be directly aer the lesson or learners can return to it as part

of revision for examinations.

Revisiting a practical with a dierent focus

A practical experiment can support many learning outcomes. Focussing on just one or two of those in a lesson will

help ensure that the aims are achieved. The video could be used to revisit the experiment with a dierent focus.

Home learning

Whether it is remote teaching, homework, or individual learner absence, the video provides an opportunity to

engage with a practical experiment and the associated skills when learners are not in the lab.

Other tips

• Provide your own commentary

Mute the voice over and provide your own commentary. This will allow you to better engage with learners

and adapt to the needs and objectives of your lesson.

• Use questions

A set of pause-and-think questions are provided in two formats, one for teacher-led questions and

discussion and a student worksheet which can be used independently by learners. Select from these or

create your own questions to help engage learners and target specific aims.

Notes on running the practical experiment

Technician notes including the equipment list and safety notes are available as a separate document here:

rsc.li/3dhnn5B. If you are planning to carry out the practical in the classroom, you will need to carry out your own

risk assessment.

The flame tests (wooded splint method) take about 10 minutes to carry out and it is safe for learners to work in

pairs.

The microscale sodium hydroxide test for positive ions reaction takes around 10 minutes and it is safe for learners

to work in pairs. This should be carried out on either the printable sheet (in this booklet) or on the integrated

instruction sheet. In both cases you will need to either put the printed sheet into a plastic wallet or laminate it.

TIP:

Printing the microscale sheet/integrated instructions onto bu coloured paper will make it easier

to see when a white precipitate has been formed.

TIP:

Once completed learners can take a photo of their results using a mobile phone or tablet; so that they

can clear away immediately. This will help avoid learners spilling chemicals on their results table.

Allow 30 minutes for the testing of negative ions, 10 minutes for each test. You may wish to carry out this

experiment in a separate lesson. This will give you time to go over the theory in the same lesson.

Once all the tests have been completed, provide the class with an unknown solution X and ask the students to

identify either the positive ion present, the negative ion present or both.

Further practical activities and context

Details of alternative methods to carry out the flame tests can be accessed from rsc.li/2ZpnROO. This activity

makes a good plenary or starter; asking students to name the metal ion as they see the dierent flame colours.

The flame tests infographics is a great reference sheet and also explains the chemistry behind the flame colours.

The chemistry of fireworks infographic and sparklers infographic provides a nice teaching context. You may

also be interested in the Exhibition Chemistry video Rainbow flame demonstration, which provides a wonderful

display of colours as well as further details of the chemistry of flame tests.

Integrated instructions

Printable integrated instructions are provided for learners. These are available as a separate download here:

rsc.li/3dhnn5B.

Integrated instructions use clear numbering, arrows and simple pictograms, like an eye to show where

observations are required. These have been developed using cognitive load theory. Integrated instructions

remove unnecessary information, and therefore reduce extraneous load on students, which increases their

working memory capacity to think about what they are doing and why. Read more about the use of integrated

instructions here: rsc.li/2SdSqkQ.

Results tables

Printable results tables have been provided for the three sets of tests. For the microscale tests for metal ions there

is a table to perform the experiment on and one to record results.

Key terms

Learners will need to have a clear understanding of the following scientific terminology:

Ion – a positively or negatively charged particle.

Metal halide – general term used to describe the group of ionic compounds that form when a metal reacts

with a halogen.

Ionic compound – a compound made up of oppositely charged ions. Ionic compounds are held together by strong

electrostatic forces between oppositely charged ions. These forces are called ionic bonds.

Aqueous solution – a solution where the solvent is water. So an aqueous solution of sodium hydroxide contains

ions, OH

ions and H

O molecules.

Precipitate – a solid that forms from ions in an aqueous solution. The precipitate is insoluble in water.

Ionic equation – a symbol equation which focuses on the ions that react together and ignore the ones that do not

take part in the reaction (spectator ions).

Prior knowledge

Learners should be able to recall:

• Particles can be atoms, molecules or ions.

• An ion is a positively or negatively charged particle.

• An atom or a molecule can lose or gain electron(s) to form an ion.

• When an atom/molecule gains negatively charged electron(s), a negative ion is formed. When an atom/

molecule loses negatively charged electron(s), a positive ion is formed.

• A solution is formed when a solute (salt) is dissolved in a solvent (water).

Learners should be confident writing word and symbol equations

There are some questions included which ask learners to balance symbol equations and write ionic equations.

Depending on where the identification of ions comes in your scheme of work your learners may not have come

across this yet. Adapt the questions to make them relevant to the stage and level that you are at. Some of the

challenge tasks require learners to use and apply their knowledge from other topics.

Common misconceptions

1. When an atom/molecule loses negatively charged electron(s), a positive ion is formed. This is something

learners often struggle with later on in their studies. Introducing the electron now, before learners meet the

other sub-atomic particles, can help to embed the idea that the loss of electrons results in a positively charged

ion, and may help reduce confusion later on.

2. As learners develop their understanding of chemical bonding further, it is common for students to refer

to ionic compounds as molecules or to refer to intermolecular forces when explaining properties of ionic

compounds. To avoid these misconceptions introduce and emphasise the correct use of the terms ‘ion’ and

‘molecule’ from the outset.

3. Learners often find solution chemistry challenging as they fail to appreciate that as well as the ions taking

place in the precipitate reaction, both water molecules and spectator ions are also present.

Diagnostic multiple-choice questionsare a great way to explore learners’ reasoning behind their answers.You can

read more about diagnostic questioning here: rsc.li/3u1kED3.

Intended outcomes

It is important that the purpose of each practical is clear from the outset, defining the intended learning outcomes

helps to consolidate this. Outcomes can be categorised as hands on, what learners are going to do with objects,

and minds on, what learners are going to do with ideas to show their understanding. We have oered some

dierentiated suggestions for this practical. You may wish to focus on just one or two, or make amendments

based your learners’ own needs. (Read more at rsc.li/2JMvKa5.)

Consider how you can share outcomes and evaluation with learners, empowering them to direct their own learning.

Hands on Minds on

Effective at a lower level Students correctly:

• Follow instructions

• Make careful observations

• Carry out a flame test

• Carry out tests for negative ions

• Carry out the sodium hydroxide test

for positive ions

Students can:

• Correctly record test results

in a table

• Use the results to identify an

unknown sample

Effective at a higher level Students correctly:

• Plan and carry out a series of tests to

identify an unknown sample

Students can:

• Explain why dierent metals have

dierent flame colours

• Write ionic equations for the sodium

hydroxide tests and halide tests

How to use the additional resources

Using the pause-and-think questions

Pause-and-think questions are supplied in two formats: a teacher version for ‘live’ questioning and a student

version which can be used during independent study. The time stamps allow you to pause the video when

presenting to a class, or learners to use for active revision.

The questions have been put into four sections: general questions, flame tests, testing for positive ions and

testing for negative ions.

Teacher version

The questions are presented in a table and you can choose to use as many as appropriate for your class and the

learning objectives.

Some questions have two timestamps to allow you to adapt the questions for dierent classes or scenarios. Pause

the videos at the earlier timestamp to ask a question before the answer is given, useful for revision or to challenge

learners. Pause at the later timestamp to ask a question reflectively and assess whether learners have understood

what they have just heard or seen. This would be useful when introducing a topic, in a flipped learning scenario or

when additional support and encouragement is needed.

Think about how you will ask for responses. Variation may help to increase engagement – learners could write

and hold up short answers; more complex questions could be discussed in groups.

Not all answers to questions are included in the video. Some of the questions will draw on prior learning or extend

learners’ thinking beyond the video content.

Student version

The same questions are oered as a printable worksheet for learners. Use in situations where there is not a

teacher present to guide discussion during the video, for example homework, revision or remote learning.

Using the structure strips

Writing about chemistry encourages learners to reflect on their understanding, formulate new ideas and make

links between ideas in new ways. Learners also need to practice for longer-answer questions in examinations.

Structure strips provide scaolded prompts and help overcome ‘fear of the blank page’. The learner sticks the

strip into the margin of their exercise book or onto an A4 sheet of paper and writes alongside it. Use this long-

answer question to consolidate learning aer the practical and/or for revision. (Read more at rsc.li/2P0JDlW.)

Long-answer question:

A sample of an unknown ionic solution has been collected for analysis. The sample is colourless.

Describe a series of qualitative tests that could be used to identify the unknown ions in the sample.

In your plan you must include instructions for carrying out the tests and the expected results.

Using Johnstone’s triangle

Johnstone’s triangle helps learners to understand what going on in a chemical reaction and gain a deeper

understanding. It does this by helping learners to make links between three dierent levels of representation: the

macroscopic, symbolic and sub-microscopic levels. The macroscopic is the ‘seen’ level, e.g. practical investigation

or observations, which can be described. The symbolic level is the unseen: how we represent the macroscopic

through word or symbol equation. The sub-microscopic is unseen, at the atomic level, and includes explanatory

models. (Read more at rsc.li/2XhYN9Q.)

As part of the additional resources we have included a completed example of Johnstone’s triangle for the sodium

hydroxide test for iron(); an example for learners to work through for copper(), and a template so you can set

your own questions (editable file at rsc.li/3dhnn5B).

In the example provided we have used a very simple model at the sub-microscopic level. When discussing this

model with your learners, you could ask them how to improve it, for example, by showing the water molecules

present and the spectator ions.

If your learners are not familiar with using Johnstone’s triangle, then it is recommended that you take the

approach ‘I try’ to introduce the triangle; ‘let’s try’ to work through an example together. Finally, ‘you try’ where

learners work through an example on their own. When completing the ‘sub-microscopic’ level it is helpful if

learners have access to modelling resources such as molymods, modelling clay, ionic jigsaw etc.

Using the follow-up worksheet

A practical skills worksheet has been included as part of the additional resources. The first section provides

structured questions at a support level for learners, the second section provides more challenging applied

questions. This worksheet could be used to follow up the practical activity, for example as homework or a revision

exercise.

Pause-and-think questions

Teacher version

Timestamp(s) Question Answer/discussion points

00:22

What is the difference between qualitative

analysis and quantitative analysis? Give

examples of each.

Qualitative analysis is where we can only

identify which ions or molecules are present

in an unknown sample. Chemical tests are

an example of qualitative analysis.

Quantitative analysis is where we can

identify the amount of the substance

present, eg the actual concentration

of an ion. Titrations are an example of

quantitative analysis.

00:33 00:45

What is an ion?

Describe how a positive and negative ion is

formed.

A positively or negatively charged particle.

A positive ion is formed when a particle loses

electrons.

A negative ion is formed when a particle

gains electrons.

01:12

What will we be looking for during the

chemical tests?

How will you know if a test has a positive

result?

Signs that a chemical change has taken

place, eg colour change, formation of a gas,

temperature change, formation of a solid or

a precipitate.

Consider the observed result. If there

is only one possible ion with this result

then the result will be positive, eg a blue

precipitate with sodium hydroxide indicates

the presence of the copper ion. However

if there are several possibilities, eg a white

precipitate is formed with sodium hydroxide

then you will need to do further tests until

there is only one possible answer.

Flame tests

01:22 01:29 What are flame tests used to identify? Metal ions.

01:57

Why can’t we use a dry splint for a flame

test?

A dry splint itself will burn and affect the

colour that we see. A soaked splint will not

burn immediately so we will only see the

flame colour associated with the metal ions

initially.

02:32

Why do we test distilled water first. What

does this show?

This is a control. It shows that the distilled

water that the splints were soaked in does

not affect the flame colour.

02:52 02:58

Record the flame colour for lithium in the

table provided.

Lithium Li

crimson

03:09 03:15

Record the flame colour for potassium in the

table provided.

Potassium K

lilac

03:30 03:42

Record the flame colour for calcium in the

table provided.

Calcium Ca

orange-red

03:52 03:56

Record the flame colour for copper in the

table provided.

Copper Cu

green

04:11 04:14

Record the flame colour for copper in the

table provided.

Sodium Na

yellow-orange

04:25

Check your results or, if you haven’t yet done

so, record the flame colours in your results

table.

Lithium Li

crimson

Potassium K

lilac

Calcium Ca

orange-red

Copper Cu

green

Sodium Na

yellow-orange

04:36

Challenge: Suggest a reason why different

metals have different flame colours.

They have different electronic configurations.

For more information, see the links in the

teacher notes.

Testing for positive ions

04:55

What are the benefits of microscale

chemistry?

Benefits include:

• sustainability

• safety

• more careful observations can be made

Read more about the benefits of microscale:

rsc.li/2ZtlkTM.

05:17 What colour is sodium hydroxide? Colourless

05:30

Name the ions present in sodium hydroxide

solution.

Sodium ion, Na

Hydroxide ion, OH

(also some H

and OH

from the water)

05:47 06:00

Name the green precipitate formed when

sodium hydroxide is added to iron () ions.

Iron() hydroxide

06:04 06:58

Record the results shown with each metal

ion in your results table.

Iron() Fe

green precipitate

Iron() Fe

rust precipitate

Copper() Cu

blue precipitate

Aluminium Al

white precipitate

Calcium Ca

white precipitate

Magnesium Mg

white precipitate

06:58

Name the products formed in the reactions

and write the ionic equations.

Iron() hydroxide

(aq) + 3OH

(aq)  Fe(OH)

(s)

Copper() hydroxide

(aq) + 2OH

(aq)  Cu(OH)

(s)

Aluminium hydroxide

(aq) + 3OH

(aq)  Al(OH)

(s)

Calcium hydroxide

(aq) + 2OH

(aq)  Ca(OH)

(s)

Magnesium hydroxide

(aq) + 2OH

(aq)  Mg(OH)

(s)

07:43

Describe the test for aluminium.

What does a positive result look like?

Add excess sodium hydroxide to the

white precipitate.

If the solid disappears then the aluminium

ion is present.

07:50

Both magnesium ions and calcium ions form

a white precipitate with sodium hydroxide.

Suggest a further test you could do to

distinguish between the two metal ions.

Carry out a flame test. If an orange-red flame

is observed then the calcium ion is present.

(Magnesium has no flame colour in a

flame test.)

Testing for negative ions

08:28

Why do you think that the limewater is put

into a separate test tube at the start of the

carbonate test?

The lime water is to test the gas being

produced. If it was mixed with the original

solution you may not be able to observe any

colour change taking place.

08:34 What does limewater test for? Carbon dioxide.

09:10

Why does the gas not escape the test tube to

the surroundings?

Carbon dioxide is more dense than air.

Therefore most of it remains in the test tube

because diffusion will be slow.

09:29 09:33

What has happened to the limewater?

Name the gas being produced.

The limewater has gone cloudy.

Carbon dioxide gas is being produced.

09:37 09:43

What is the test for carbonate ions? What

does a positive result look like?

Add hydrochloric acid and test any gas

being produced by bubbling it through

limewater. If the limewater goes cloudy it is

a carbonate.

09:37 09:50

Write a word and symbol equation for

the reaction of sodium carbonate with

hydrochloric acid.

What type of reaction is this?

Sodium carbonate + hydrochloric acid 

Sodium chloride + water + carbon dioxide

(aq) + 2HCl(aq)  2NaCl(aq) + H

O(l)

+ CO

(g)

A neutralisation reaction.

10:00 10:10

Why do you think dilute acid is added at the

start of the sulfate test?

To remove any ions that might interfere with

a positive result.

10:25 10:28

Write a word and symbol equation for the

reaction of sodium sulfate with barium

chloride.

What type of reaction is this?

Sodium sulfate + barium chloride  sodium

chloride + barium sulfate

(aq) + BaCl

(aq)  2NaCl(aq) +

BaSO

(s)

A double displacement reaction.

10:25 10:34

What is the test for sulfate ions?

What does a positive result look like?

Add a few drops of hydrochloric acid and

barium chloride to the sample. If a white

precipitate forms then a sulfate is present.

10:40 10:46

Write the symbols for the halide ions in your

results table:

Chloride

Bromide

Iodide

11:03

What was added to the test tube to test for

the presence of halides?

Nitric acid and sliver nitrate solution.

11:28 11:47 Record the results in your results table.

white precipitate is formed

cream precipitate is formed

yellow precipitate is formed

11:40

Write the ionic equations for the formation

of silver bromide and silver iodide.

(aq) + Ag

(aq)  AgBr(s)

(aq) + Ag

(aq)  AgI(s)

12:08 12:15

Unknown solution B, shown in the video, is

blue.

What test would you use to confirm the

identity of the positive ion?

Since the solution is blue, I would suspect

that a copper ion was present so I would

do either the sodium hydroxide test or the

flame test first.

Pause-and-think questions

Student version

Pause the video at the time stated to test or revise your knowledge of these practical experiments.

Time Question

00:22 What is the difference between qualitative analysis and quantitative analysis?

00:33 What is an ion?

Describe how a positive and negative ion are formed.

01:12 What will we be looking for during the chemical tests?

How will you know if a test has a positive result?

Flame tests

01:22 What are flame tests used to identify?

02:32 Why do we test distilled water first? What does this show?

Record the symbol and flame colours for each metal ion in the table below:

Metal ion Symbol Observation: flame colour

Lithium

Potassium

Calcium

Copper

Sodium

02:58 Lithium.

03:09 Potassium.

03:30 Calcium.

03:52 Copper.

04:11 Sodium.

Testing for positive ions

05:17 What colour is sodium hydroxide?

05:30 Name the ions present in sodium hydroxide solutions.

05:47 Name the green precipitate formed when sodium hydroxide is added to iron () ions

06:04 Record the results shown with each metal ion in the results table below:

Positive ion Symbol Observation when added to

sodium hydroxide solution

Observation with excess sodium

hydroxide solution

Iron()

Iron()

Copper()

Aluminium

Calcium

Magnesium

06:58 Name the products formed in the reactions and complete the ionic equations:

Iron() hydroxide: Fe

(aq) + 3OH

(aq)  ________________

Copper() hydroxide: _________ + 2OH

(a q)  ______________

__________________: Al

(aq) + 3OH

(aq)  ________________

__________________: Ca

(aq) + ________  ______________

Magnesium hydroxide: _________ + __________  ____________

07:43 Describe the test for aluminium. What does a positive test look like?

07:50 Both magnesium ions and calcium ions form a white precipitate with sodium hydroxide. Suggest a

further test you could do to distinguish between the two metal ions.

Testing for negative ions

Negative ion Symbol Test Observation

Carbonate

Sulfate

08:28 Why do you think that the limewater is put into a separate test tube at the start of the carbonate test?

08:34 What does limewater test for?

09:29 What has happened to the limewater?

09:33 Name the gas produced.

09:37 Complete the word and symbol equation:

Sodium carbonate + hydrochloric acid  __________________ + ________________ + ___________________

(aq) +___________  ___________ + H

O(l) + ___________

What type of reaction is this?

09:43 Complete the table above with the test and positive result for carbonate ions.

09:58 Why do you think dilute acid is added at the start of the sulfate test?

10:25 Complete the table above with the test and positive result for sulfate ions.

10:25 Complete the word and symbol equation:

Sodium sulfate + barium chloride  ________________ + ________________

(aq) + BaCl

(aq)  _____________ + _____________

10:28 What type of reaction is this?

10:38 Add the symbols for the halide ions to the results table below.

11:28 Record the results for the halide tests in the results table.

Negative ion Symbol Test Observation

Halide

Chloride

Bromide

Iodide

11:40 Write the ionic equations for the formation of silver bromide and silver iodide.

_________ + _________  __________

12:08 Unknown solution B is blue. Which metal ion would you expect it to contain? Which test would you do

use to confirm you prediction?

Identifying ions

Structure strip

Identifying ions

Structure strip

Identifying ions

Structure strip

Identifying ions

Structure strip

Identifying ions

Structure strip

What is a qualitative test

and what is it used for?

What is a qualitative test

and what is it used for?

What is a qualitative test

and what is it used for?

What is a qualitative test

and what is it used for?

What is a qualitative test

and what is it used for?

Describe the hydroxide

test for positive ions.

What would a positive

test result look like?

Give an example.

Describe the hydroxide

test for positive ions.

What would a positive

test result look like?

Give an example.

Describe the hydroxide

test for positive ions.

What would a positive

test result look like?

Give an example.

Describe the hydroxide

test for positive ions.

What would a positive

test result look like?

Give an example.

Describe the hydroxide

test for positive ions.

What would a positive

test result look like?

Give an example.

What is the limitation of

the sodium hydroxide

test?

What is the limitation of

the sodium hydroxide

test?

What is the limitation of

the sodium hydroxide

test?

What is the limitation of

the sodium hydroxide

test?

What is the limitation of

the sodium hydroxide

test?

Describe the flame test to

confirm the positive ion.

What would a positive

test result look like?

Give an example.

Describe the flame test to

confirm the positive ion.

What would a positive

test result look like?

Give an example.

Describe the flame test to

confirm the positive ion.

What would a positive

test result look like?

Give an example.

Describe the flame test to

confirm the positive ion.

What would a positive

test result look like?

Give an example.

Describe the flame test to

confirm the positive ion.

What would a positive

test result look like?

Give an example.

Describe the series of

tests used for negative

ions.

Describe the carbonate

test.

What would a positive

test result look like?

Describe the series of

tests used for negative

ions.

Describe the carbonate

test.

What would a positive

test result look like?

Describe the series of

tests used for negative

ions.

Describe the carbonate

test.

What would a positive

test result look like?

Describe the series of

tests used for negative

ions.

Describe the carbonate

test.

What would a positive

test result look like?

Describe the series of

tests used for negative

ions.

Describe the carbonate

test.

What would a positive

test result look like?

Describe the sulfate test.

What would a positive

test result look like?

Describe the sulfate test.

What would a positive

test result look like?

Describe the sulfate test.

What would a positive

test result look like?

Describe the sulfate test.

What would a positive

test result look like?

Describe the sulfate test.

What would a positive

test result look like?

Describe the halide test.

What would a positive

test result look like for

chloride, bromide and

iodide?

Describe the halide test.

What would a positive

test result look like for

chloride, bromide and

iodide?

Describe the halide test.

What would a positive

test result look like for

chloride, bromide and

iodide?

Describe the halide test.

What would a positive

test result look like for

chloride, bromide and

iodide?

Describe the halide test.

What would a positive

test result look like for

chloride, bromide and

iodide?

Structure strip: suggested answer content

Identifying ions

Structure strip

What is a qualitative test

and what is it used for?

A qualitative test is used to identify the chemical composition of an unknown sample. A positive test result, such

as a colour change, will confirm that a particular substance is present.

The unknown colourless sample is an ionic solution, so we are going to use a series of qualitative tests to

identify the positive and negative ions.

Describe the hydroxide

test for positive ions.

What would a positive

test result look like?

Give an example.

There are two tests that can be used to determine the positive ion. One of these tests is the hydroxide test:

• Add 2 drops of sodium hydroxide to 2 drops of the unknown solution.

• A coloured precipitate will form.

• Observe the colour of the precipitate. This will identify the possible positive metal ion in the unknown

solution. For example, a rust coloured precipitate would be a positive result for iron() ions.

What is the limitation of

the sodium hydroxide

test?

There is more than one metal ion that will produce a white precipitate. If a white precipitate is formed and it

does not dissolve in excess sodium hydroxide then further tests will need to be carried out to determine whether

the unknown solution contains calcium or magnesium.

Describe the flame test to

confirm the positive ion.

What would a positive

test result look like?

Give an example.

A further test to confirm the positive ion present is a flame test. The flame test could also be used to distinguish

between calcium and magnesium.

• Dip a pre-soaked splint in the unknown solution.

• Hold it in a roaring blue Bunsen flame.

• Observe the colour of the flame.

The flame colour will identify the positive metal ion present in the unknown solution. For example, a lilac flame

would be a positive result for potassium.

Describe the series of

tests used for negative

ions.

Describe the carbonate

test.

What would a positive

test result look like?

Now, we will use a series of tests to identify the negative ion.

The carbonate test

• Put 1 cm

of the unknown solution into a test tube.

• Put 1 cm

of limewater into a separate test tube.

• Add a few drops of hydrochloric acid (HCl). If it fizzes then the carbonate ion is present.

• Confirm the presence of carbonate by bubbling the gas through the limewater. If a carbonate ion is present

the limewater will turn cloudy.

Describe the sulfate test.

What would a positive

test result look like?

The sulfate test

• Put 1 cm

of the unknown solution into a test tube.

• Add a few drops of hydrochloric acid HCl and then add barium chloride (BaCl

• If a white precipitate appears, then the sulfate ion is present.

Describe the halide test.

What would a positive

test result look like for

chloride, bromide and

iodide?

The halide test

• Put 1 cm

of the unknown solution into a test tube.

• Add a few drops of nitric acid (HNO

) and silver nitrate (AgNO

• If a white precipitate forms then the chloride ion is present. If the precipitate is cream, then the bromide ion

is present, if it is yellow then the iodide ion is present.

Follow-up worksheet

1. A student carried out some flame tests.

Complete the flame test results table. The first row has been done for you.

Metal ion Symbol Observation: flame colour

Potassium K

Lilac

Sodium

Crimson

Calcium

Green

Unknown solution X

_______________________

Yellow

2. Describe how you would test for positive ions using the sodium hydroxide chemical test.

3. Complete the sodium hydroxide test results table.

Positive ion Symbol

Observation when added to sodium

hydroxide solution

Iron() Fe

Brown precipitate

Copper() Blue precipitate

4. Complete the equations, using the example to help.

Iron() + hydroxide  iron() hydroxide

(aq) + 2OH

(aq)  Fe(OH)

(s)

(a) Iron() + hydroxide  __________________________

(aq) + __OH

(aq)  Fe(OH)

(__)

(b) Copper() + hydroxide  ________________ () hydroxide

(aq) + __OH

(aq)  Cu(OH)

(__)

5. A student carried out some tests to identify the ions present in an unknown solution. After adding a few

drops of dilute nitric acid and silver nitrate to the sample, a cream coloured precipitate appeared.

The student concluded that a ___________________________ ion was present in the solution.

Challenge

6. A student was asked to identify an unknown sample. She carried out some tests and here are the results.

(a) Complete the table.

Test Result Conclusion

Add 2 drops of sodium

hydroxide to 2 drops of solution

A white precipitate is formed

Add excess sodium hydroxide

to the drop

No change to the white

precipitate

Flame test An orange-red flame is

observed

Add a few drops of hydrochloric

acid to sample

No changes observed

Add a few drops of hydrochloric

acid then a few drops of barium

chloride to sample

No change observed

Add a few drops of nitric acid

then a few drops of silver

nitrate to sample

A white precipitate is formed

(b) The unknown sample is

________( ) + ______ OH

(aq)  _______( )

(aq) + __________ ( )  ____________( )

7. A sample of an unknown ionic solution has been collected for analysis. The sample is colourless.

Describe a series of qualitative tests that could be used to identify the unknown ions in the sample.

In your plan you must include instructions for carrying out the tests and the expected results.

This question has a structure strip to support your written answer. Find more resources to support you here

rsc.li/3a7LS37.

Follow-up worksheet: answers

1. A student carried out some flame tests.

Complete the flame test results table. The first row has been done for you.

Metal ion Symbol Observation: flame colour

Potassium K

Lilac

Sodium Na

Yellow

Lithium Li

Crimson

Calcium Ca

Orange-red

Copper Cu

Green

Unknown solution X

Sodium

Yellow

2. Describe how you would test for positive ions using the sodium hydroxide chemical test.

Add 2 drops of sodium hydroxide to 2 drops of an unknown solution and observe.

3. Complete the sodium hydroxide test results table.

Positive ion Symbol

Observation when added to sodium

hydroxide solution

Iron() Fe

Green precipitate

Iron() Fe

Brown precipitate

Copper() Cu

Blue precipitate

4. Complete the equations, using the example to help.

Iron() + hydroxide  iron() hydroxide

(aq) + 2OH

(aq)  Fe(OH)

(s)

(a) Iron() + hydroxide  iron() hydroxide

(aq) + 3OH

(aq)  Fe(OH)

(s)

(b) Copper() + hydroxide  copper() hydroxide

(aq) + 2OH

(aq)  Cu(OH)

(s)

5. A student carried out some tests to identify the ions present in an unknown solution. After adding a few

drops of dilute nitric acid and silver nitrate to the sample, a cream coloured precipitate appeared.

The student concluded that a Chloride (or Cl

) ion was present in the solution.

Challenge

6. A student was asked to identify an unknown sample. She carried out some tests and here are the results.

(a) Complete the table.

Test Result Conclusion

Add 2 drops of sodium

hydroxide to 2 drops of solution

A white precipitate is formed Al

, Ca

or Mg

ions could be

present

Add excess sodium hydroxide

to the drop

No change to the white

precipitate

or Mg

ions could be

present

Flame test An orange-red flame is

observed

ion is present

Add a few drops of hydrochloric

acid to sample

No changes observed The carbonate or CO

ion is

not present

Add a few drops of hydrochloric

acid then a few drops of barium

chloride to sample

No change observed The sulfate or SO

ion is not

present

Add a few drops of nitric acid

then a few drops of silver

nitrate to sample

A white precipitate is formed The chloride or Cl

ion is

present

(b) The unknown sample is

Calcium chloride (or CaCl

)

(aq) + 2OH

(aq)  Ca(OH)

(s)

(aq) + Cl

(aq)  AgCl(s)

7. A sample of an unknown ionic solution has been collected for analysis. The sample is colourless.

Describe a series of qualitative tests that could be used to identify the unknown ions in the sample.

In your plan you must include instructions for carrying out the tests and the expected results.

This question has a structure strip see ‘suggested answer content’ (rsc.li/3a7LS37).

Results tables for ﬂame tests

Metal ion Symbol Observation: flame colour

Lithium

Potassium

Calcium

Copper

Sodium

Metal ion Symbol Observation: flame colour

Lithium

Potassium

Calcium

Copper

Sodium

Metal ion Symbol Observation: flame colour

Lithium

Potassium

Calcium

Copper

Sodium

Metal ion Symbol Observation: flame colour

Lithium

Potassium

Calcium

Copper

Sodium

Metal ion Symbol Observation: flame colour

Lithium

Potassium

Calcium

Copper

Sodium

Microscale reactions of positive ions with sodium hydroxide

Positive ion Symbol Positive ion solution and sodium hydroxide solution

Iron() Fe

Iron() Fe

Copper() Cu

Aluminium Al

Calcium Ca

Magnesium Mg

Results table for positve ion tests

Positive ion Symbol

Observation when added to

sodium hydroxide solution

Observation with excess sodium

hydroxide solution

Iron()

Iron()

Copper()

Aluminium

Calcium

Magnesium

Positive ion Symbol

Observation when added to

sodium hydroxide solution

Observation with excess sodium

hydroxide solution

Iron()

Iron()

Copper()

Aluminium

Calcium

Magnesium

Results tables for negative ion tests

Negative ion Symbol Test Observation

Carbonate

Sulfate

Halide

Chloride

Bromide

Iodide

Negative ion Symbol Test Observation

Carbonate

Sulfate

Halide

Chloride

Bromide

Iodide