A forensic scientist compares the decomposition of a dye in two sealed water samples from a river. One sample is exposed to ultraviolet light and the other ...
Question 1 Report
A forensic scientist compares the decomposition of a dye in two sealed water samples from a river. One sample is exposed to ultraviolet light and the other is kept in darkness. The dye concentration is measured every hour with a colorimeter. Both samples begin with the same dye concentration, volume and temperature. The scientist wants to decide whether light changes the rate, without allowing water to evaporate from either bottle.
(a) Identify the independent variable. [1]
(b) Use the results below to calculate the mean decrease in dye concentration for the illuminated sample: from 80 mg dm-3 to 44 mg dm-3 in 3 h. [2]
(c) Explain why a lower dye concentration after the same time indicates a faster decomposition rate. [2]
(d) Give two control variables for this comparison. [2]
Answer Details
(a) Light condition / exposure to ultraviolet light [1] (b) 80 - 44 = 36 mg dm-3 [1]; 36 / 3 = 12 mg dm-3 h-1 [1] (c) More dye has reacted/decomposed [1]; during the same time interval, so the amount changed per hour is greater [1] (d) Any two: initial dye concentration; volume of water; temperature; type of bottle; time measured; same colorimeter method [2]
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