Class practical

This is a microscale version of the common test-tube practical, the reaction of various positive ions with sodium hydroxide.

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 re-used many times.

Lesson organisation

This version is 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.

Apparatus Chemicals

Eye protection

Each working group requires:

Results table either laminated or in a plastic document wallet (Note 1)

Red litmus paper

Access to bottles of these solutions (Note 2):

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

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

Sodium hydroxide solution, NaOH(aq), (IRRITANT at concentrations less than 0.5 M) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Iron(II) sulfate solution, FeSO4(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Iron(III) nitrate solution, Fe(NO3)3(aq) - see CLEAPSS Hazcard.

Copper(II) sulfate solution, CuSO4(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Aluminium nitrate solution, Al(NO3)3(aq) - see CLEAPSS Hazcard. 

Calcium chloride solution, CaCl2(aq) - see CLEAPSS Hazcard. 

Magnesium chloride solution, MgCl2(aq) - see CLEAPSS Hazcard. 

Ammonium chloride solution, NH4Cl(aq) - see CLEAPSS Hazcard.

1 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.

2 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

a 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.

b Hold a piece of damp red litmus paper over the ammonium chloride and sodium hydroxide box.

c 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
Here are the expected results, if students use the worksheet you can download from this page. Some of the boxes on the worksheet are shaded to increase the visibility of the white precipitates.

Positive ion solution

Positive 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

Health & Safety checked, July 2016 

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated July 2016