Chemistry of the Environment Structured Questions | Cambridge IGCSE Chemistry

Chemistry 0620 · Question 1[9 marks]

Aquatic ecosystems are vulnerable to pollution from sewage discharges, synthetic fertilisers, and industrial effluents.

(a)

A sample of river water is tested for Biochemical Oxygen Demand (BOD₅).

[3]

(i)

Define what is meant by Biochemical Oxygen Demand.

[1]

(ii)

Explain why untreated domestic sewage creates an extremely high BOD.

[2]

(b)

Synthetic fertilisers contain nitrate ( $\ce{NO3^-}$ ) and phosphate ( $\ce{PO4^{3-}}$ ) ions.

[3]

(i)

State the pathway by which these ions travel from agricultural fields into natural river systems.

[1]

(ii)

Explain why elevated concentrations of nitrates and phosphates in lakes cause deoxygenation of the water.

[2]

(c)

Industrial factories near an estuary release trace quantities of methylmercury and cadmium compounds into the water. The diagram illustrates the biomagnification of these heavy metals up the aquatic food chain.

[3]

(i)

Explain why the concentration of toxic heavy metals increases at higher trophic levels (e.g. predatory fish and seabirds).

[2]

(ii)

State one adverse health effect on humans who consume fish contaminated with high levels of toxic heavy metals.

[1]