| Date | May 2013 | Marks available | 2 | Reference code | 13M.3.HL.TZ2.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Evaluate | Question number | 7 | Adapted from | N/A |
Question
In 2003, the Integrated Approach to Community Development (IACD) organization introduced the chulli water purifier to homes in Bangladesh that had not previously had access to safe drinking water. It was designed to be made cheaply from local materials. The purifier uses sand filtration to remove organic particles and heat pasteurization to eliminate microbes from water.
Water samples from 15 different locations containing high levels of the bacterium E. coli were passed through the purifier at different flow rates and temperatures to test its effect on contaminated water. The shaded area of the graph below represents the recommended temperature and flow rate for using the purifier.
State the highest temperature at which bacteria were found in water that had passed through the chulli purifier.
Calculate the maximum volume of safe drinking water that could be produced by the chulli purifier in one hour.
Discuss whether 80°C is the best temperature to operate the chulli purifier.
The results suggest that there may be a relationship between the water flow rate and the minimum temperature needed to eliminate microbes. State this relationship.
Evaluate pasteurization as a method of controlling microbial growth.
Markscheme
55°C (units required)
30 litres (accept answers in the range of 28.8 to 30.0 litres)
Working not required.
a. good because it kills/is free of bacteria;
b. no bacteria between 80° and 60° even at higher flow rates;
c. not good as it would use too much energy/be expensive to heat water;
the slower the flow rate, the lower the temperature.
implications:
a. kills bacteria/most pathogens by heating food/liquids to specific/high/ 60° – 72° temperature;
b. (usually pasteurization temperature) does not alter the taste/quality/chemical structure;
limitations:
c. may not kill heat-resistant/all bacteria;
d. requires immediate cooling to prevent (further) microbial growth;
Examiners report
Most had 55° and 30 (or 29.4) litres for (a) and (b).
Most had 55° and 30 (or 29.4) litres for (a) and (b).
Many confused between the independent and dependent variables and/or were distracted by the only data point at 80°; they stated a reversed relationship in (d) (e.g. "the higher the flow rate, the lower the temperature"); their answer to the previous question (c) was influenced by this, but some managed to get the two marks there anyways.
Many confused between the independent and dependent variables and/or were distracted by the only data point at 80°; they stated a reversed relationship in (d) (e.g. "the higher the flow rate, the lower the temperature").
For (e), some read the question as relating to the graph and the chulli purifier, others read it as a theoretical question (F.6.4); among the latter, some confused pasteurization and sterilization; the markscheme allowed for both perspectives and most candidates gained some marks.
