The duck pond problem

The local Forensic Science Laboratory is too stretched at present to carry out a quick analysis, and therefore they have approached our school for help. How can the pond be returned to a habitable condition?

This session should take 70 minutes.

Equipment

• Eye protection
• Titration apparatus (Surveying equipment for the pond.)
• A sample of muddy water with some acid added
• Sodium hydroxide solution (0.4 mol dm^–3)
• A suitable indicator

Health, safety and technical notes 

• Read our standard health and safety guidance here.
• Always wear eye protection when working with acids.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete. 
• Sodium hydroxide solution, 0.4 mol dm^–3 NaOH (aq), is an irritant. At 0.4 mol dm^–3 or below then eye protection will still be needed but can be safety spectacles. See CLEAPSS Hazcard HC091a.
• Any remaining acid/alkali (if not being kept for further use) should be neutralised with weak acid or alkali (as appropriate) before being washed to waste. 

Possible approaches

If you’ve got a local duck pond, the teacher needs to set the scene by asking who walked in past the duck pond this morning?

Did anyone notice all the dead fish floating on the surface?

If not, you need to talk about the local river (relate to waste dumped in river).

The groups need to determine the equivalent amount of 0.4 mol dm^–3 sodium hydroxide needed to neutralise the acid.

The problem could then be developed to include estimation of the amount of water in the pond and the original concentration of acid dumped in the pond (assuming initial volume dumped is negligible in comparison to large volume of pond!).

Finally, students need to address the problem of how the pond can be returned to a habitable condition.

Possible extension

Consider biological aspects of pond/river/lake pollution What are the visible signs of pollution?

What has happened to lakes in Scandanavia that have become polluted?

Consider sources of pollution - ie might not be just liquid waste, eg - sulphuric acid plant - aluminium processing plant - electroplating company

Students could then carry out other tests to identify the source of the pollution, eg sulphate tests.

Would it be conclusive - sulphate positive test anyway?

Could give background to processes.