Technician in trouble! What solutions are which?

BUT … the teacher has lost the sheet! The technician has to find out quickly the identity of each solution because they are needed for the next class!

This experiment should take 40–70 minutes. 

Equipment

• Eye protection
• 5 dropping bottles labelled A, B, C, D and E (Each bottle (or test tube) contains a colourless liquid).
• A test tube rack containing 6 test tubes
• Glass stirring rod

Solutions A to E:

• A - strong acid (hydrochloric acid), 20 cm³
• B - water, 20 cm³
• C - strong alkali (sodium hydroxide), 20 cm³
• D - * weak acid (carbonic acid or tartaric acid), 20 cm³
• E - ** “phenolphthalein indicator + water” mixture, 20 cm³ *preferably one that doesn’t have a recognisable smell

Health, safety and technical notes 

• Read our standard health and safety guidance here.
• Wear eye protection.
• Phenolphthalein is carcinogenic, mutagenic and a reproductive toxin. Below 1%, though, the solutions are of low hazard. See CLEAPSS Hazcard HC032
• Hydrochloric acid, 0.5 mol dm^–3 HCl (aq), is corrosive. See CLEAPSS Hazcard HC047a
• Sodium hydroxide solution, 0.4 mol dm^–3 NaOH (aq), is corrosive. See CLEAPSS Hazcard HC091a
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Students can identify vinegar (ethanoic acid) by smell.
• Solutions need to be made up on the day it’s needed, as it loses its strength.

Possible approaches

At the start of the exercise, show students the colour changes for phenolphthalein indicator (colourless in acid and water, pink in alkali). Tell students always to replace the glass droppers in the right bottles (or test tubes) – ie point out dangers of contaminating solutions.

A possible sequence of operations:-

Mix each unknown solution with every other unknown solution. The only colour change is pink (E + C = phenolphthalein and alkali, but students won’t know which is which). As long as students know the colour changes of phenolphthalein indicator, they should get this far.

Next, add the other liquids to (E+C):

1. (E+C), add A – pink solution goes colourless quickly, also test tube feels hot.
2. (E+C), add B – pink solution remains pink, just becomes more dilute. Therefore, B = water.
3. (E+C), add D – pink solution eventually goes colourless, but more D is needed than A for neutralisation. Therefore, D = weak acid, and A = strong acid.

Finally, to find out which is the indicator and which is the alkali:

Mix a small amount of A (strong acid) with a small amount of C (strong alkali) and lots of E (indicator) – solution stays colourless.

Then, mix a small amount of A (strong acid) with a small amount of E (indicator) and lots of C (strong alkali) – the solution goes pink.

This proves that C is the alkali.

Alternatively, add a lot of C (strong alkali) to E (indicator) – this gives a weak pink colour.

Then, add a lot of E (indicator) to C (strong alkali) – this gives a strong pink colour.

This confirms that E is the indicator. (The indicator makes the colour.)

Students should be encouraged to do a flow chart or table.