Use these synoptic questions to explore structure and bonding, equilibrium, spectrometry and spectroscopy in an interesting context

This resource accompanies the article What am I swimming in? The worksheet originally appeared in the book Chemistry for the gifted and talented.

Learning objectives

  1. Describe how the water in swimming pools is disinfected.
  2. Calculate the concentration of various ions present in the water.
  3. Interpret data obtained from a mass spectrometer.
  4. Suggest/determine the structure of organic molecules.  
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    Synoptic questions exploring the topics of structure and bonding, equilibrium, spectrometry and spectroscopy in the context of swimming pools.


Chlorine (Cl2) is now rarely added directly to pools in the UK because of the hazards associated with its storage and use. Other chlorinating agents are used instead.

In a typical swimming pool, sodium chlorate(I) is added to disinfect the water. The chlorate(I) ion is the conjugate base of the weak acid HOCl. The equilibrium constant Ka for HOCl = 2.9 x 10-9 at 298 K. Both the chlorate(I) ion and the acid HOCl are involved in the disinfecting.

Some data were obtained from a swimming pool in Taunton, on a particular day in September. The combined concentration of HOCl(aq) was measured as 0.50 parts per million (ppm). The pH was 7.3.

Notes to teachers

This activity is synoptic in nature, with questions on many different aspects of chemistry. It is context driven, looking at a collection of interesting chemistry that can be derived from swimming pools.

The main subject content areas are:

  • Structure and bonding
  • Equilibrium
  • Spectrometry
  • Spectroscopy

There are two versions of the student sheet. One is aimed at stretching the most able and the other comes with hints on how to answer the questions, making it accessible to a wider range of students.

The activity comes in two parts and these can be given separately to the students.

  • Part 1 can be used at the end of teaching equilibrium or at the end of a course.
  • Part 2 should be attempted at the end of a course.

The accompanying presentation should be used to set the scene and stimulate discussion. 

  • Equilibria (Kc, Ksp, Ka buffers, Le Chatelier’s principle) and entropy (solvent effects).
  • Mass spectra, oxidation numbers, curly arrow mechanisms, nuclear magnetic resonance (NMR) spectroscopy, delocalisation, tautomerism and relative molecular mass calculations.

Please note that most A level courses and the Scottish Advanced Highers do not require students to carry out Ksp calculations. However, information included within the accompanying presentation should allow students to access the questions.

Tautomerism is beyond the scope of most post-16 specifications, however enough information is included in the question to allow students to answer.

Prior knowledge

Kc, pH, Le Chatelier’s principle, buffers, spectra, NMR and curly arrows.

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