Try this microscale practical exploring the reactions of various positive ions with sodium hydroxide

In this microscale version of an experiment commonly conducted in test tubes, students test the reactions of different positive ions with sodium hydroxide, with products including a range of different coloured precipitates and a gas.

The main advantages of the microscale version are the tiny quantities of chemicals consumed, and there are no test tubes to wash up. Instead of test tubes, students have a results sheet which looks like a large results table. This is laminated or put inside a plastic document wallet and can be reused many times.

This version is also far quicker than the more traditional test tube version and only takes a few minutes to do.

The main management issue is likely to be students wandering around looking for the various reagents. This can be avoided if you supply enough bottles – ideally one bottle of each chemical for each bench of students. Make sure that there are plenty of bottles of sodium hydroxide, as this is used the most.

Students need to make or be given a second copy of the results table to record their observations.

Equipment

Apparatus

  • Eye protection
  • Results table, either laminated or in a plastic document wallet (see note 11 below)
  • Red litmus paper

Chemicals

Access to bottles of these solutions (see note 12 below):

  • Sodium hydroxide, < 0.5 M (IRRITANT at this concentration)
  • Iron(II) sulfate, 0.2 M, in 0.1 M sulfuric acid
  • Iron(III) nitrate, 0.2 M
  • Copper(II) sulfate, 0.2 M
  • Aluminium nitrate, 0.2 M
  • Calcium chloride, 0.2 M
  • Magnesium chloride, 0.2 M
  • Ammonium chloride, 0.2 M

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout.
  • Sodium hydroxide solution, NaOH(aq), (IRRITANT at concentrations less than 0.5 M) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
  • Iron(II) sulfate solution, FeSO4 (aq) – see CLEAPSS Hazcard HC055B and CLEAPSS Recipe Book RB051. 
  • Iron(III) nitrate solution, Fe(NO3)3 (aq) – see CLEAPSS Hazcard HC055C.
  • Copper(II) sulfate solution, CuSO4 (aq) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031. 
  • Aluminium nitrate solution, Al(NO3)3 (aq) – see CLEAPSS Hazcard HC002B. 
  • Calcium chloride solution, CaCl2 (aq) – see CLEAPSS Hazcard HC019A. 
  • Magnesium chloride solution, MgCl2 (aq) – see CLEAPSS Hazcard HC059b. 
  • Ammonium chloride solution, NH4 Cl(aq) – see CLEAPSS Hazcard HC009a.
  • The results table is available for download below as a PDF or editable Word document. Laminated copies of the results table last longer. Results tables in document wallets are very likely to get wet and dirty. If this practical is done regularly, it is worth laminating the tables.
  • Bottles of chemicals are required for each bench of students. If enough bottles are supplied, students do not need to wander round looking for the reagents. Dropper bottles are best. The concentrations are not crucial, but the sodium hydroxide should be below 0.5 M to minimise the hazard. Exactly which salt is used is also not critical, ie sulfates, chlorides or nitrates could be used, as available.

Procedure

  1. Take a copy of the results sheet. If it is not laminated, put it into a plastic pocket. Put two drops of sodium hydroxide onto each of the empty boxes and then two drops of the positive ion solution, and observe what happens.
  2. Hold a piece of damp red litmus paper over the ammonium chloride and sodium hydroxide box.
  3. Add more sodium hydroxide, dropwise, to the aluminium nitrate and sodium hydroxide box. Observe what happens.

Teaching notes

It is important that students do not add more than a couple of drops of each solution to the boxes. If this happens, the drops spread and mix, obscuring the results.

If the red litmus is not held near the ammonium chloride and sodium hydroxide box soon after the solutions have mixed, it may be difficult to see litmus changing colour.

The solids formed are: iron(II) hydroxide; iron(III) hydroxide; copper(II) hydroxide; aluminium hydroxide; calcium hydroxide; magnesium hydroxide. The gas made is ammonia.

Equations

Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)

Fe3+(aq)+ 3OH–(aq) → Fe(OH)3(s)

Cu2+(aq)+ 2OH–(aq) → Cu(OH)2(s)

Al3+(aq) + 3OH–(aq) → Al(OH)3(s)

followed by the aluminium hydroxide dissolving in the excess hydroxide to give a solution of sodium aluminate:

Al(OH)3(s) + 3OH–(aq) → (Al(OH)6)3–(aq)

Ca2+(aq) + 2OH–(aq) → Ca(OH)2(s)

Mg2+(aq) + 2OH–(aq) → Mg(OH)2(s)

NH4+ + OH– → NH3 + H2O

Expected results

If students use the results table provided, the expect results are given in the table below. The table is shaded for aluminium, calcium and magnesium to improve the visibility of the white precipitates.

Positive ion solutionPositive ion solution and sodium hydroxide solution
Iron(II), Fe2+ Grey/Green solid is formed
Iron(III), Fe3+ Orange solid is formed
Copper(II), Cu2+ Blue solid is formed
Aluminium, Al3+ White solid is formed which dissolves in excess sodium hydroxide
Calcium, Ca2+ White solid is formed
Magnesium, Mg2+ White solid is formed
Ammonium, NH4+ A gas is evolved which turns damp red litmus paper blue

Downloads

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Additional information

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. 

Practical Chemistry activities accompany Practical Physics and Practical Biology.

The experiment is also part of the Royal Society of Chemistry’s Continuing Professional Development course: Chemistry for non-specialists.

© Nuffield Foundation and the Royal Society of Chemistry