Zadanie 1a
Wzorowane na: Chemistry HL Paper 3, November 2023, TZ1, pytanie 1(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Informacja do zadań 1a-1d.
A food chemistry group investigated how the processing and storage of fruit juice affects its vitamin C (ascorbic acid) content.
Polecenie
Heat treatment at 90 °C is used to destroy spoilage micro-organisms in pineapple juice. The concentration of vitamin C was measured over 56 days in untreated juice and in heat-treated juice from the same batch.
(i) Identify the dependent variable in this investigation.
(ii) Calculate the decrease in the initial concentration of vitamin C, in μg cm−3, that was caused by the heat treatment.
(iii) Calculate the average rate of decrease of the vitamin C concentration in the heat-treated juice over the 56 days, in μg cm−3 day−1.
(iv) Deduce, referring to the graph, whether heat treatment changes the rate at which the vitamin C concentration falls during storage.
(v) The absolute uncertainty in each concentration measurement was ±3 μg cm−3. Deduce, giving a reason, whether the untreated or the heat-treated juice has the larger percentage uncertainty in its vitamin C concentration.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) concentration «of vitamin C» ✔
(ii) «312 − 196 =» 116 «μg cm−3» ✔
(iii) «(196 − 168) μg cm−3 / 56 days =» 0.50 «μg cm−3 day−1» ✔
(iv) no AND the two lines fall by a similar amount «about 28–31 μg cm−3» over the 56 days / the gradients of the two lines are about the same ✔
(v) heat-treated AND the same absolute uncertainty is divided by a smaller concentration ✔ «e.g. at day 0: 3/196 × 100 = 1.5 % compared with 3/312 × 100 = 0.96 %»
Schemat punktowania
One mark per sub-part. (i) 'Vitamin C' alone or 'heat treatment' earns nothing. (ii) Accept values in the range 110-122. (iii) Accept values in the range 0.45-0.55; ignore a negative sign. (iv) Accept 'yes AND only a slightly different rate'; accept 'no AND same trend'. (v) A worked numerical example is acceptable, as is the reverse statement about the untreated juice; 'error' may stand for 'uncertainty'.
Komentarz
The heat treatment lowers the starting concentration but does not change how fast vitamin C is lost afterwards.