Topic Exploration Pack

Practical skills: experiments involving making salts

Learning outcomes...............................................................................................................................2

Introduction..........................................................................................................................................2

Additional teacher preparation.............................................................................................................3

Learner activity 1................................................................................................................................10

Learner activity 2................................................................................................................................11

Learner activity 3................................................................................................................................12

Instructions and answers for teachers

These instructions cover the learner activity section which can be found on page 10. This Topic Exploration Pack supports OCR A Level Chemistry A and A Level Chemistry B (Salters).

When distributing the activity section to the learners, either as a printed copy or as a Word file, you will need to remove the teacher instructions section.

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Learning outcomes1.1.1 Planning

(a) experimental design, including to solve problems set in a practical context [Including selection of suitable apparatus, equipment and techniques for the proposed experiment. Learners should be able to apply scientific knowledge based on the content of the specification to the practical context.]

(b) identification of variables that must be controlled, where appropriate

(c) evaluation that an experimental method is appropriate to meet the expected outcomes.

1.1.2 Implementing

(a) how to use a wide range of practical apparatus and techniques correctly [As outlined in the content of the specification and the skills required for the Practical Endorsement]

(c) presenting observations and data in an appropriate format

1.1.4 Evaluation

(a) how to evaluate results and draw conclusions

(b) the identification of anomalies in experimental measurements

(c) the limitations in experimental procedures

(d) precision and accuracy of measurements and data, including margins of error, percentage errors and uncertainties in apparatus

(e) refining experimental design by suggestion of improvements to the procedures and apparatus.

2.1.4(c) neutralisation as the reaction of: (ii) acids with bases, including carbonates, metal oxides and alkalis (water-soluble bases), to form salts, including full equations

2.1.5(e) redox reactions of metals with acids to form salts, including full equations (Additional guidance: Metals should be from s-, p- and d-blocks e.g. Mg, Al, Fe, Zn. Ionic equations not required. Reactions with acids will be limited to those producing a salt and hydrogen.]

3.1.2(b) the relative reactivities of the Group 2 elements Mg → Ba shown by their redox reactions with: (i) oxygen; (ii) water; (iii) dilute acids [Additional guidance: Reactions with acids will be limited to those producing a salt and hydrogen.]

3.1.3(g) the precipitation reactions, including ionic equations, of the aqueous anions Cl –, Br– and I–

with aqueous silver ions, followed by aqueous ammonia, and their use as a test for different halide ions. [Additional guidance: Complexes with ammonia are not required other than observations.]

3.1.4(a) qualitative analysis of ions on a test-tube scale; processes and techniques needed to identify the following ions in an unknown compound: (i) anions: CO3

2–, by reaction with H+(aq) forming CO2(g); SO4

2–, by precipitation with Ba2+(aq); Cl –, Br–, I–; (ii) cations: NH4

+, by reaction with warm NaOH(aq) forming NH3.Version 1 2 © OCR 2017

5.3.1(j) reactions, including ionic equations, and the accompanying colour changes of aqueous Cu2+, Fe2+, Fe3+, Mn2+ and Cr3+ with aqueous sodium hydroxide and aqueous ammonia, including: (i) precipitation reactions; (ii) complex formation with excess aqueous sodium hydroxide and aqueous ammonia. [Additional guidance: For precipitation, non-complexed formulae or complexed formulae, are acceptable e.g. Cu2+(aq) or [Cu(H2O)6]2+; Cu(OH)2(s) or Cu(OH)2(H2O)4. With excess NaOH, only Cr(OH)3 reacts further forming [Cr(OH)6]3–. With excess NH3, only Cr(OH)3 and Cu(OH)2 react forming [Cr(NH3)6]3+ and [Cu(NH3)4(H2O)2]2+ respectively.

5.3.2(a) qualitative analysis of ions on a test-tube scale: processes and techniques needed to identify the following ions in an unknown compound: (i) anions: CO3

2–, Cl –, Br–, I–, SO42–; (ii)

cations: NH4+; Cu2+, Fe2+, Fe3+, Mn2+, Cr3+

EL(s) the solubility of compounds formed between the following cations and anions: Li+, Na+, K+, Ca2+,Ba2+, Cu2+, Fe2+, Fe3+, Ag+, Pb2+, Zn2+, Al 3+, NH4

+, CO32–, SO4

2–, Cl –, Br–, I–, OH– , NO3–;

colours of any precipitates formed; use of ions as tests e.g. Ba2+ as a test for SO42–; a sequence of

tests leading to the identification of a salt containing the ions above [Additional guidance: Knowledge of the reaction of 3+ cations with CO3

2– is not required.]

EL(t) techniques and procedures for making soluble salts by reacting acids and bases and insoluble salts by precipitation reactions

ES(k) the reactions between halide ions (Cl –, Br– and I–) and silver ions (Ag+) and ionic equations to represent these precipitation reactions, the colours of the precipitates and the solubility of silver halides in ammonia.

IntroductionChemistry is a practical subject and the development of practical skills is fundamental to understanding the nature of chemistry. Practical skills are embedded throughout the content of both Chemistry A and Chemistry B (Salters). Learners are required to develop a range of practical skills throughout their study of the course which, alongside the Practical Endorsement, are then assessed indirectly in written examinations.

The introductory section of the topic exploration pack ‘Practical skills: experiments on rates of reaction’ (http://www.ocr.org.uk/Images/371956-practical-skills-experiments-on-rates-of-reaction-topic-exploration-pack.doc) discusses and exemplifies the requirements of the learning outcomes relevant to this indirect assessment of practical skills, Module 1.1 in the specifications. Reference is made to specific questions from the Sample Assessment Materials, available on the qualification websites, which illustrate the general points that are made.

Additional teacher preparationThere are a number of reasons that teachers choose to use class practical activities or practical demonstrations. These include the development of skills which can be assessed directly within the Practical Endorsement framework or indirectly in written examination papers, and to support learner understanding of chemical concepts. Some practical activities may contribute to all three areas, but it is important that teachers are clear about their aims when choosing any particular activity. If the reasons for doing practical work are confused or blurred, or the practical is ‘overloaded’ with too many expected outcomes, then these outcomes may not be achieved. Version 1 3 © OCR 2017

Spending sufficient time, sometimes spread over more than one session, can help learners focus on the different outcomes.

A useful approach to planning a sequence of practical activities is to decide on those that will support the learning of chemical concepts required for the particular topic. These activities might be a combination of class activities and practical demonstrations. Once chosen, the activities can be assessed for their potential to provide opportunities for learners to develop skills that are required to achieve the Practical Endorsement. The activities can further be analysed for their potential to provide learner opportunities to develop practical related skills that may be assessed indirectly in written examination questions. Sometimes this will mean that learners need to be involved in planning the activity. Sometimes these opportunities are best provided by extension questions arising from practical work in which learners process and evaluate data.

In the context of making salts, learners need to develop skills to use the techniques and procedures required to make salts by the reaction of acids with bases and metals, carry out qualitative analysis on a test tube scale of anions and cations and carry out reactions of some d–block ions with aqueous sodium hydroxide and ammonia as described by learning outcomes for Chemistry A 2.1.4(c)(ii), 2.1.5(e), 3.1.2(b), 3.1.3(g), 3.1.4(a), 5.3.1(j) and 5.3.2(a) and Chemistry B (Salters) EL(s), EL(t), and ES(k).

OCR Practical Activity Group (PAG) Suggested Activities

It is important to realise that PAGs are not just useful for helping learners develop their skills to achieve the Practical Endorsement but are also useful in developing practical related skills that are indirectly assessed in written exam papers.

Although PAG 1.2 is mainly about finding the relative atomic mass of magnesium, it involves the formation of a salt from magnesium and sulfuric acid. Learners do not crystallise the salt but are asked in question 5 about the hazards that would be involved in the purification of the magnesium sulfate. The PAG therefore provides an opportunity for learners to develop skills required by learning outcomes 2.1.5(e) and 3.1.2(b).

In PAGs 4.1, 4.2, and 4.3 learners identify cations and/or anions. In PAG 4.1 they are given specific tests to carry out which enable them to identify two anions and a group 2 cation. In PAGs 4.2 and 4.3 learners devise their own sequence of tests to identify cations and anions. These activities provides an opportunity for learners to develop skills required by learning outcomes 3.1.3(g), 3.1.4(a), 5.3.2(a), EL(s) and ES(k).

Other sources of practical activities

Other practical activities can be used in addition to or in place of the OCR PAG activities. The Royal Society of Chemistry website LearnChemistry, http://www.rsc.org/learn-chemistry, is a rich source of useful activities. There is a useful compilation of the resources on the site mapped to the required skills, apparatus and techniques: http://www.rsc.org/learn-chemistry/resource/res00002010/english-a-level-chemistry-specification-guide.

Details about how to make a soluble salt, ammonium sulfate, from an acid and an alkali can be found at http://www.rsc.org/learn-chemistry/resource/res00001760/preparing-a-soluble-salt-by-

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neutralisation while details about making soluble salts copper(II) sulfate and magnesium sulfate from an acid and an oxide and a carbonate, can be found at http://www.rsc.org/learn-chemistry/resource/res00001762/preparing-salts-by-neutralisation-of-oxides-and-carbonates and http://www.rsc.org/learn-chemistry/resource/res00001917/reacting-copper-ii-oxide-with-sulfuric-acid. CLEAPSS have produced two videos showing how to make copper(II) sulfate crystals. One demonstrates a quick, easy and safe traditional method, https://www.youtube.com/watch?v=PTa8tkJ8rv0. The second demonstrates how the salt can be made on a microscale https://www.youtube.com/watch?v=L1mI4IHQJsc, and this has been produced as a GCSE PAG activity, http://www.ocr.org.uk/Images/340544-pag-activity-chemistry-production-of-salts-suggestion-2.docx, which could easily adapted for A Level.

Details about how to make an insoluble salt, lead(II) chloride, are found at http://www.rsc.org/learn-chemistry/resource/res00001761/preparing-an-insoluble-salt and for making insoluble magnesium carbonate at http://www.rsc.org/learn-chemistry/resource/res00000431/making-magnesium-carbonate-the-formation-of-an-insoluble-salt-in-water.

The LearnChemistry ‘Challenging plants’ resource includes a wide range of learner and teacher sheets which cover the preparation of salts in the context of their use as fertilisers http://www.rsc.org/learn-chemistry/resource/res00000903/challenging-plants-fertilisers-practicals. Choosing one of these examples can provide an interesting alternative for learners who may well have carried out the preparation of more common examples of salts at Key Stage 4. Salts used as fertilisers include copper(I) aminoethanoate, ammonium dihydrogenphosphate, ammonium sulfate, calcium nitrate, copper(II) citrate, copper(II) ethanoate, diammonium hydrogenphosphate, magnesium sulfate, potassium dihydrogenphosphate and zinc sulfate.

All of the resources that focus on the preparation of salts by reacting acids with bases provide opportuntities for learners to practice skills required by learning outcomes 2.1.4(c)(ii) and EL(t). Asking appropriate questions about experiment design in these activities can also help develop the skills required by learning outcomes 1.1.1(a), 1.1.1(c) and 1.1.4(e).

A comprehensive worksheet that covers testing for both cations and anions can be found at http://www.rsc.org/learn-chemistry/resource/res00000464/testing-salts-for-anions-and-cations while the resources http://www.rsc.org/learn-chemistry/resource/res00000758/testing-for-negative-ions and http://www.rsc.org/learn-chemistry/resource/res00001936/analysis look specifically at testing for anions and the resource http://www.rsc.org/learn-chemistry/resource/res00001936/analysis looks specifically at testing for cations. A GCSE PAG activity, http://www.ocr.org.uk/Images/323619-pag-activity-chemistry-identification-of-species-suggestion-2.docx, can be readily adapted for A Level.

Using the wooden splint method to carry out a flame test to detect some cations can be found at http://www.rsc.org/learn-chemistry/resource/res00000759/flame-tests-the-wooden-splint-method while details of a demonstration of flame colours using spray bottles is also available at http://www.rsc.org/learn-chemistry/resource/res00000760/flame-colours-a-demonstration.

Testing for ions provides a good opportunity to develop learner skills in problem solving. Analysis of cations and anions can be set in the context of finding which salts are in sea water http://www.rsc.org/learn-chemistry/resource/res00001785/what-are-the-dissolved-solids-in-seawater. An alternative approach, that is similar to PAGs 4.2 and 4.3, requires learners to Version 1 5 © OCR 2017

distinguish between a group of sodium salts http://www.rsc.org/learn-chemistry/resource/res00000591/which-sodium-salt-is-which and http://www.rsc.org/learn-chemistry/resource/res00000595/which-white-salt. In similar rersources learners devise and carry out a method to distinguish between four solutions http://www.rsc.org/learn-chemistry/resource/res00000664/four-solutions and between three or five white solids http://www.rsc.org/learn-chemistry/resource/res00000676/three-white-solids and http://www.rsc.org/learn-chemistry/resource/res00000663/five-white-solids. Another resource asks them to find which of five solids is a chloride http://www.rsc.org/learn-chemistry/resource/res00000660/find-the-chloride.

Many of the tests for cations and anions can be carried out at a microscale. Details can be found on the CLEAPSS website http://science.cleapss.org.uk/resources/resource-search.aspx?search=Microscale and some microscale tests for some cations can also be found at http://www.rsc.org/learn-chemistry/resource/res00000757/microscale-reactions-of-positive-ions-with-sodium-hydroxide.

Activities that involve qualitative analysis of cations and anions can provide opportunities for learners to develop the skills required by learning outcomes 3.1.3(g), 3.1.4(a), 5.3.1(j), 5.3.2(a), EL(s) and ES(k).

One of the problems that can arise in making soluble salts is judging when to stop heating the salt solution so that it will crystallise on cooling. The CLEAPSS video demonstrates how this can be achieved https://www.youtube.com/watch?v=PTa8tkJ8rv0 . . Usually the salt solution should be evaporated slowly until it is about one-fifth of its original volume. Learners should not evaporate the solution too quickly because it may ‘spit’. If the cooled solution does not produce crystals immediately the evaporating basin should be covered with cling film and left overnight.

Asking learners to calculate the amount of reagents that are needed before they start a practical to make a salt is a good way of focussing attention on this the reaction involved and helps to develop skills required by learning outcomes 1.1.1(a) and (c). This is particularly important when making salts such as ammonium dihydrogen phosphate and diammonium hydrogenphosphate when the acid is only partially neutralised.

Emphasising the difference in solubility of compounds helps learners make sense of the precipitation reactions that they use to identify cations and anions and experiments to prepare insoluble salts. This approach provides a general framework for all tests of this type. Some learners struggle with the concepts of ions in solution and of ‘spectator ions’. Asking them what particles are present in a bottle labelled copper(II) sulfate is a way of identifying learners who find the concept of separate and independent ions difficult. Presenting qualitative analysis activities as problem solving tasks can add interest to the practical work and helps develop skills required by learning objectives 1.1.1(a) and (e).

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Learner activity 1

In this activity learners devise a series of tests that can use to identify the composition of six colourless solutions. This activity will help learners develop skills that are required to meet learning outcomes 1.1.1(a) and 1.1.1(c).

Suitable tests are:

1. Add dilute nitric acid to each solution. Sodium carbonate can be identified as it will be the only solution to produce bubbles of a gas.

2. Add barium nitrate solution to the remaining solutions. Ammonium sulfate and sodium sulfate will both form white precipitates.

3. Warm the solutions identified in test 2 with sodium hydroxide solution. Ammonium sulfate will produce an alkaline gas which can be confirmed with damp red litmus paper. The other solution must be sodium sulfate.

4. Add silver nitrate solution to the remaining three solutions. Sodium chloride and potassium chloride will both produce a white precipitate but potassium iodide can be identified because it will produce a pale yellow precipitate. This can be confirmed by adding ammonia solution to the precipitates. The precipitates from the two chlorides will dissolve but the precipitate from the iodide will remain.

5. Carry out a flame test on the solution known to be sodium chloride and potassium chloride. The sodium chloride will produce a yellow flame and the potassium chloride will produce a lilac flame.

Learner activity 2

In this activity, learners use a card sort activity to plan an experiment to make the salt diammonium hydrogen phosphate. This involves choosing steps in the procedure that they will use from the group of cards. Some of the cards are distractors since they include apparatus that are not required and information that is not relevant. Learners then place the steps in the order that will form their experimental procedure.

This activity will help learners develop skills that are required to meet learning outcomes 1.1.1(a), 1.1.1 (c), 2.1.4(c)(ii) and EL(t). A good way of using the activity is to ask groups of learners to discuss together which cards they should select and the sequence of the steps in the experimental procedure. This helps learners to articulate and clarify their ideas and can help identify any misconceptions that they might have.

Sequence of steps which could make up the experimental procedure:

Pipette 10 cm3 of ammonia solution into a conical flask Add 3 drops of methyl orange indicator Add phosphoric acid from a burette to find the end point Note the titre volume Pipette 20cm3 of ammonia solution into an evaporating basin Add the titre volume of phosphoric acid Evaporate the solution to 1/5th of its original volume Cool the solution Filter off the crystals

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Dry the crystals using another filter paper Weigh a watch glass Scrape the dry crystals onto the watch glass Weigh the crystals and watch glass

Cards that are not required:

Add 2 x titre volume of phosphoric acid Add ½ titre volume of phosphoric acid Heat the solution to remove all of the water Weigh a filter paper

Learner activity 3

In this activity learners answer questions about an experiment to prepare the salt magnesium sulfate. This activity will help learners develop skills that are required to meet learning outcomes 1.1.1(a), 1.1.4(a) and 1.1.4(d).

Sample answers to questions:

1. Percentage uncertainty for burette reading = 2 × (0.05/40) x 100 = 0.25%

2. Amount of sulfuric acid used = 40/1000 x 1 = 0.04 moles

Mass of magnesium carbonate needed to react with this = 0.04 x 84.3 = 3.37 g

So a mass in excess of 3.37 g of magnesium carbonate should be added – e.g. 4 g

3. The learner could warm the mixture and stir it to help the reaction occur.

4. The learner should filter off the excess magnesium carbonate.

5. The learner could cool a few drops of solution down quickly on a watch glass to see if

crystals form

6. Maximum mass of magnesium sulfate = 0.04 x 120.4 = 4.82 g

7. The learner could explain his lower than expected result by suggesting that some of the

magnesium sulfate remained dissolved when the solution was cooled down or that some

solid remained on the filter paper.

8. The learner could explain her higher than expected result by suggesting that the crystals

were still damp when weighed.

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Topic Exploration Pack

Practical skills: experiments involving making saltsLearner activity 1You are provided with six colourless solutions. The solutions are:

ammonium sulfate potassium chloride potassium iodide sodium carbonate sodium chloride sodium sulfate

Devise a series of tests that you could carry out to decide which solution is which. Explain

how your tests will distinguish between the solutions. You should aim to produce a series of

tests with the minimum number of steps.

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Learner activity 2 You are going to plan how to carry out an experiment to make the salt diammonium hydrogenphosphate from phosphoric acid and ammonia solution.

H3PO4 + 2NH3 → (NH4)2HPO4

The method involves first carrying out a titration between phosphoric acid and ammonia solution using methyl orange indicator when another salt, ammonium dihydrogen phosphate, is formed.

H3PO4 + NH3 → NH4H2PO4

Steps in the procedure are given on cards. Place the cards in sequence to show how you will carry out the experiment. You do not need to use all of the cards.

Cool the solution Weigh the crystals and watch glass Filter off the crystals

Pipette 10 cm3 of ammonia solution to a conical flask

Pipette 20 cm3 of ammonia solution into an

evaporating basinHeat the solution to

remove all of the water

Add 3 drops of methyl orange indicator

Scrape dry crystals onto the watch glass

Evaporate the solution to about 1/5th of its original

volume

Add the titre volume of phosphoric acid

Add 2 x titre volume of phosphoric acid

Add ½ titre volume of phosphoric acid

Dry the crystals using another filter paper Note the titre volume Add phosphoric acid from

a burette to find end point

Add phosphoric acid from a burette to find end point Weigh a watch glass

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Learner activity 3 A learner sets out to make some magnesium sulfate.

He uses a burette to add 40 cm3 of 1 mol dm-3 sulfuric acid to a 100 cm3 beaker.

1. What is the percentage uncertainty associated with this measurement?

The learner adds an excess of magnesium carbonate to the sulfuric acid.

2. What would be a suitable mass of magnesium carbonate for the learner to add?

3. What might the learner do to increase the rates of reaction between sulfuric acid and

magnesium carbonate occur?

4. What should the learner do next?

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The learner heats the solution of magnesium sulfate.

5. How could the learner find out when to stop heating the solution?

The learner allows the solution to cool, filters off the crystals that form, dries the crystals

using filter paper, and then weighs them.

6. What is the maximum mass of magnesium sulfate that could be produced?

7. The learner found that in his experiment the crystals weighed 4.00 g. How might the learner explain his result?

8. Another learner carried out the same experiment. She found that her crystals weighed 5.00 g. How might the learner explain her result?

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