• Emily Rose Seeber

7 problem-solving practicals on identification


Here are seven different problem-solving practicals on identification, across KS3 to KS5, ranging from the easy, to the fiendishly hard.

Get your students thinking...

1. Identifying positive ions

Challenge level: EASY to MEDIUM

Key Stage 3-4

In this activity, students test eight known ionic compounds with either a flame test, sodium hydroxide solution, or ammonia solution and write down their observations. They then attempt to identify eight unknown substances.

Not all of the compounds can be identified using a single test, so students will need to do at least two of the three sets of tests to characterise all of the compounds. Students have to link between different tests to work out the answer.

Practical sheet available HERE.

2. Identify the unknown carbonyl

Challenge level: MEDIUM

Key Stage 5

This practical has become a standard part of A-level organic chemistry, although it's not a core practical. The version I like is from ILPAC Advanced Practical Chemistry.

Students are given an unknown carbonyl (butanone works well) and test to see if it an aldehyde or a ketone using Tollen's reagent. Then students make the 2,4-dinitrophenylhydrazine derivative, filter and recrystallise it in 50:50 ethanol to water, and determine its melting point. They compare this to a data sheet and identify the carbonyl.

There's no need to give students instructions for the practical if you want them to think it through themselves. Just give them the reagents and the data book and let them work it out. It is worth making a 'recrystallisation solvent' for them to use so that they don't need a hint about the ratio of water to ethanol.

3. Assigning labels to unknown solutions (inorganic)

Challenge level: MEDIUM

Key Stage 4

In this activity, students are put into small teams and given six substances, A to F. They are told that the substances are copper(II) chloride, iron(II) sulphate, sulphuric acid, ammonium chloride, iron(III) chloride, and copper(II) sulphate, but that the labels have fallen off.

Students need to use the resources available to them (silver nitrate, nitric acid, barium nitrate, sodium hydroxide and UI paper) to identify all of the substances. There is support available on the worksheet to scaffold the thinking if required.

Practical sheet available HERE.

4. What's in the mixture?

Challenge level: HARD

Key Stage 4

For this activity, just give students a mixture of two ionic compounds and ask them to identify the two compounds. It works best if not every student has the same mixture: have about 5 different ones and distribute them around the room, keeping a key for yourself. Ask them to keep notes on the tests they do and their results: afterwards they can evaluate whether or not they chose the best method.

For added stretch, you could ask them to separate the two ionic compounds as well (they will need solubility data for this).

5. How many steps do you need to identify the unknowns (organic)

Challenge level: MEDIUM to HARD

Key Stage 5

This time, students have some organic unknowns (pentan-2-one, pentan-3-one, pentanal, pentan-1-ol, ethylpropylether, and pentanoic acid) to identify. They know that the unknowns are from this list. The 'winner' is the person who can complete the challenge with the fewest steps.

6. You know nothing: mystery substances

Challenge level: FIENDISH

Key Stage 4

For increased challenge, in this practical, none of the substances are labelled. Students will need to work out which bottles contain the chemicals they need to identify other substances. They are completely in the dark and will need to think carefully and critically to solve the challenge.

The substances they have are:

  • sodium hydroxide solution

  • copper sulphate solution

  • barium chloride solution

  • hydrochloric acid solution

  • sodium sulphate solid

  • nitric acid solution

  • silver nitrate solution

  • sodium carbonate solid

  • ammonium chloride solid

  • potassium bromide solution

  • potassium iodide solution

  • copper chloride solution

  • lead nitrate solution

  • magnesium metal

  • copper metal

  • bromine water

  • chlorine water

Students should have access to any standard lab equipment they would like, including a Bunsen burner and apparatus for a flame test.

Although it looks tricky, it can be solved by students and it makes excellent PBL revision for the GCSE papers. I would suggest that this is a team activity. And that you make them wash up their own test tubes as they go along.

7. Designer medicine

Challenge level: FIENDISH

Key Stage 5

This practical involves students identifying the proportions of aspirin, caffeine and paracetamol in a new painkiller. Firstly, the students carry out a pre-lab in which they determine how each drug reacts with various reagents (including magnesium metal, indicator paper, Tollen's or Fehling's reagent, sodium carbonate solution, iron(III) chloride solution, copper(II) solution, or any others you fancy).

Students then use their findings to work out the amounts of each substance in a sample they are given. They have pure samples of the original three drugs available to them.

There are loads of ways to solve this one, but the most interesting way is to calibrate a colorimeter with different concentrations of paracetamol mixed with iron(III) chloride (which turns a deep purple due to the phenol group on paracetamol acting as a ligand). The proportion of aspirin in the designer drug can then be calculated. The alternative is to titrate with bromine water until the yellow-orange colour remains in the conical flask.

Having calculated proportion of paracetamol, the amount of aspirin can be calculated by titration with sodium hydroxide, or by adding to an excess of magnesium and measuring the volume of hydrogen produced. The mass of caffeine can be worked out by eliminating the other two components.

This one is not easy and takes time, but could be done in teams to make it more accessible, or over a couple of weeks in the summer term for students to complete the analysis and write up in full. It's totally worth the time to give students the opportunity to carry out extensive analytical thinking. And the sense of satisfaction they will gain is epic.

#chemistry #practical #differentiation

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© 2017 by Emily Rose Seeber.