What makes a substance acidic?

In this series of short experiments, students explore what makes substances acidic, and establish that the familiar acidic (and alkaline) properties of some substances require the presence of water.

The practical begins with a short teacher demonstration, in which students observe what happens when magnesium ribbon is added first to pure ethanoic acid, and then to an aqueous solution of the acid. Students may then repeat the test for themselves, using tartaric acid crystals instead of ethanoic acid.

This activity is most suitable for students who already have a good knowledge of the familiar properties of acids and alkalis. The sequence of experiments is relatively brief and several of the tests performed show no reaction.

The activity involves using pure ethanoic acid, also known as glacial acetic acid (CORROSIVE), and steps 1–4 (see the procedure below) should be demonstrated, perhaps with assistance from one or more responsible students. Steps 5–8 can be done by the students themselves or you may prefer to continue demonstrating.

The students can be asked a series of questions to challenge their knowledge and understanding at appropriate points during the experiments.

Equipment

Apparatus

• Eye protection (goggles)

For the demonstration (steps 1–4)

• Boiling tubes, 2 (must be dry) (see note 10 below)
• Rack to hold boiling tubes
• Teat pipette (must be dry) (see note 10) – not needed if the ethanoic acid is provided in a dropper bottle
• Glass rod
• pH colour chart
• Universal indicator paper, small strips, x2 (must be dry) (see note 11)

For the remaining experiments (steps 5–8)

• Test tubes, x4 (all test tubes must be dry) – use boiling tubes if continuing as a demonstration
• Test tube or boiling tube rack
• Spatula
• Forceps
• Universal indicator paper, small strips, x4
• Access to universal indicator pH colour chart

Chemicals

For the demonstration (steps 1–4)

• Magnesium ribbon, 2 cm lengths
• Pure ethanoic acid (glacial acetic acid) (CORROSIVE), 10 cm³ in a small dropper bottle
• Tap water (ensure pH = 7)

For the remaining experiments (steps 5–8)

• Universal indicator solution (HIGHLY FLAMMABLE) in a small dropper bottle
• Small containers of:
  • Tartaric acid crystals (IRRITANT)
  • Bismuth nitrate crystals (OXIDISING, IRRITANT)
  • Hydrated aluminium chloride (IRRITANT)
• Magnesium ribbon, 1 cm lengths (use 2 cm lengths if continuing to demonstrate), x4
• Tap water (ensure pH = 7)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection (goggles) throughout. These are required for this reaction due to the corrosive glacial ethanoic acid and ethanoic anhydride used.
• Tartaric acid, HOOCCH(OH)CH(OH)COOH(s), (IRRITANT) – see CLEAPSS Hazcard HC036C.
• Bismuth nitrate, Bi(NO₃)₃ (s), (OXIDISING, IRRITANT) – see CLEAPSS Hazcard HC073D. 
• Pure ethanoic acid (glacial acetic acid), CH₃ COOH(l), (CORROSIVE) – see CLEAPSS Hazcard HC038a.
• Ethanoic anhydride (acetic anhydride), (CH₃ CO)₂ O(l), (CORROSIVE) – see CLEAPSS Hazcard HC039. Pure ethanoic acid (also known as glacial acetic acid) may need the addition of about 5–10% (by volume) of ethanoic anhydride (acetic anhydride) to ensure it is completely dry. This should be done in a fume cupboard using dry apparatus.
• Hydrated aluminium chloride, AlCl₃.6H₂ O(s), (IRRITANT) – see CLEAPSS Hazcard HC002a. Anhydrous aluminium chloride should not be used by students.
• Magnesium ribbon, Mg(s) – see CLEAPSS Hazcard HC059b. Magnesium ribbon should be kept under teacher control.
• Universal indicator solution (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000.
• It is essential for the success of this activity that all test tubes and teat pipettes are completely dry.
• The universal indicator paper needs to be carefully dried in a dessicator over a good drying agent such as fresh anhydrous calcium chloride. The indicator paper is sufficiently dry when, if tested with a drop of dry ethanoic acid, the colour observed is definitely not the usual colour of the paper in acid. Indicator papers are expensive, and technicians may wish to pre-cut these into smaller test pieces for the class.

Procedure

Demonstration

1. Add a 2 cm depth of pure ethanoic acid to a clean, dry boiling tube. Drop in a small piece of dry universal indicator paper, and note the colour change. Make a careful comparison with the pH colour chart. Does the colour match?
2. Add a 2 cm length of magnesium ribbon to the same tube. Is there any sign of the expected reaction?
3. Add about a 5 cm depth of tap water (at pH=7) to a second clean, dry boiling tube. Carefully add a 2 cm depth of pure ethanoic acid. Stir to mix. Drop in a small piece of dry universal indicator paper. Note the colour change and compare with the pH colour chart. Is there a clear match this time?
4. Add a 2 cm length of magnesium ribbon to this second boiling tube. What happens to the magnesium ribbon this time?

Class experiment or teacher demonstration

5. Repeat the tests in 1–4 above using tartaric acid crystals instead of ethanoic acid.
6. One-third fill each of two clean, dry test tubes with tap water (at pH = 7). Add two drops of Universal Indicator solution to each test tube.
7. Add a small spatula measure of aluminium chloride into one test tube, and shake to dissolve. Use forceps to add a crystal or two of bismuth nitrate to the second test tube. Note the colour changes to the indicator in each test tube.
8. Finally, ask the students to predict what they would expect to see if a piece of magnesium ribbon is now dropped into each of the two test tubes. Add the pieces of magnesium – are their predictions correct?

Teaching notes

This introduction is a lead-in to an explanation of acidity and alkalinity in terms of the properties of hydrogen ions and hydroxide ions, and eventually to the Brǿnsted–Lowry theory of acidity. For most pre-16 students, the first of these is as far as it needs to be taken.

Pure ethanoic acid is an unpleasant corrosive substance, made even more so when ethanoic anhydride is added to ensure it is dry. For this reason it should only be used at this level in a demonstration.

If the ethanoic acid is completely dry, it will turn the indicator paper an unusual colour, often purplish, a colour not shown on the colour chart. Students may wish to see this as the (expected) red colour, and it may be necessary to show them a comparison to a standard colour change in acid such as hydrochloric acid. Similarly, when magnesium ribbon is added, trapped air bubbles sticking to the ribbon may be seen as the expected hydrogen evolution, and the teacher needs to be alert to this. However, the contrast when water is added is very marked and should leave no doubt of the difference.

Students may also see the testing of tartaric acid crystals as a ‘so what’ situation! They ignore the name aspect, and simply see testing solid crystals as pointless, thus missing the intention. This may be a good reason to also make this a demonstration.

The final tests on substances not labelled as acids, but which do show acidic properties when in water, need careful explanation. Bismuth nitrate in particular produces only a weakly acidic solution, and the reaction with magnesium may be very slow.

Student questions

1. Is this what you expect when a compound labelled ‘acid’ is tested?
2. What usually happens when magnesium is added to a compound labelled ‘acid’?
3. What happens to the properties of pure ethanoic acid when water is added?
4. Does tartaric acid react as an acid before water is added? Does it react as an acid after water is added?
5. Neither of these compounds (aluminium chloride and bismuth nitrate) has the word ‘acid’ in its name, but do they show acidic properties? If they do, what happens when magnesium ribbon is added to the tubes?

Additional resources

Try this related practical investigating the role of water in acidity. The experiment is more complex, but provides a useful opportunity to reinforce and develop students’ understanding of acid behaviour.