An outdoor activity centre treats fresh water from a stream. The treated water must contain fewer than 10 harmful microbes per 100 cm³.
Two equal samples of the same water are treated using the same filter bed and the same exposure to ultraviolet light.
| Method | Order of treatment | Suspended insoluble particles during ultraviolet treatment in mg/dm³ | Harmful microbes after both stages per 100 cm³ |
|---|---|---|---|
| R | filtration, then ultraviolet light | 3 | 6 |
| S | ultraviolet light, then filtration | 45 | 94 |
Suggest which method should be used. Explain why the order of the stages affects the result. Use data from the table.
87 exam-style questions on AQA GCSE Chemistry 10.1 Using the Earth's resources and obtaining potable water, covering 10.1.1 Using the Earth's resources and sustainable development, 10.1.2 Potable water, 10.1.3 Waste water treatment, and 10.1.4 Alternative methods of extracting metals. Each one has a worked solution and a mark scheme showing where the marks go.