Covering Lessons 6 to 10: water quality parameters, dissolved oxygen and BOD, nutrient pollution, heavy metal monitoring, and the chemistry of water treatment. Use this checkpoint to test whether you can connect measurements to environmental decisions and treatment choices.
Module 8 requires you to analyse why environmental monitoring is needed, and to use the named analytical methods. It does not require mastery of Winkler titration, BOD5, drinking-water limits, chloramination, reverse osmosis or the eutrophication pathway. Of the questions here only the AAS item assesses named Module 8 content, so this paper should not be read as a measure of syllabus readiness. It is worth doing as applied chemistry once the core analytical work is secure.
Which pair of parameters is most directly used to judge salt content in water?
What is the correct stoichiometric shortcut in the Winkler method?
Which formula correctly defines BOD5?
Which statement best explains why a low concentration of mercury in water can still become a major ecological issue?
Which statement about AAS is correct?
Which event occurs later in the eutrophication chain, after algal bloom development?
What is the main role of alum in water treatment?
In chlorinated water, which species is the more active disinfectant?
Why might a treatment plant choose chloramines instead of only free chlorine?
What is the major limitation of reverse osmosis desalination named in the course?
Explain how dissolved oxygen can be measured using the Winkler method, and state why sodium thiosulfate volume can be used to calculate the original oxygen concentration. 4 marks
Explain the full eutrophication sequence from nutrient input to fish kill, referring to nitrate or phosphate, algal bloom, decomposition and BOD. 4 marks
Evaluate the most suitable treatment strategy for a large drinking water network supplied by turbid freshwater that contains organic matter and must remain microbiologically safe through long distribution pipes. In your answer, refer to coagulation, chlorination or chloramines, and DBP risk. 5 marks
1. C conductivity and TDS are the key salt-related parameters.
2. D the Winkler shortcut is 1 mol O2 : 4 mol Na2S2O3.
3. A BOD5 is initial DO minus final DO after 5 days.
4. B mercury risk can increase through bioaccumulation and biomagnification.
5. C AAS measures absorption by ground-state atoms after atomisation.
6. D decomposition, rising BOD and oxygen depletion occur later in eutrophication.
7. A alum helps form Al(OH)3 that adsorbs suspended particles.
8. B HOCl is the more active disinfectant species.
9. C chloramines can reduce DBP formation while maintaining residual protection.
10. D reverse osmosis is effective but energy-intensive.
Q11 (4 marks): In the Winkler method, dissolved oxygen in the sample is first trapped through a sequence of redox reactions involving manganese species. This ultimately leads to the formation of iodine in an amount linked stoichiometrically to the original oxygen. The iodine is then titrated with sodium thiosulfate solution. Because the amount of thiosulfate needed is stoichiometrically related to the iodine, and the iodine is linked to the original oxygen, the thiosulfate titre can be used to calculate dissolved oxygen concentration. The key shortcut is 1 mol O2 to 4 mol Na2S2O3.
Q12 (4 marks): Eutrophication begins when excess nutrients such as nitrate and phosphate enter the water. These nutrients promote rapid algal growth and bloom formation. Dense blooms reduce light penetration, so submerged plants may die. Dead algae and plants are then decomposed by microorganisms, which raises biochemical oxygen demand. As oxygen is consumed, dissolved oxygen falls and hypoxia can develop, leading to fish kill.
Q13 (5 marks): A suitable strategy would begin with coagulation, flocculation and sedimentation because the turbid water contains suspended particles that need to be destabilised and removed. Alum is useful because Al3+ hydrolyses to Al(OH)3, which adsorbs suspended particles and helps form flocs. Filtration should then remove remaining particles and some organic matter. Organic removal matters because chlorine reacting with natural organic matter can form DBPs such as trihalomethanes. For a large distribution network, residual disinfection is still needed, so chloramines are often a strong choice because they maintain residual protection while generally producing fewer DBPs than free chlorine, although they act more slowly. Overall, the best strategy is strong pretreatment to remove particles and organics, followed by a residual disinfectant chosen with DBP risk in mind.
Tick when you've finished the review and checked your answers.