Fig. 1 shows a sealed respirometer used by students to investigate decay of leaf litter. The flask contains damp leaves and soil bacteria. A small tube of p...
Question 1 Report
Fig. 1 shows a sealed respirometer used by students to investigate decay of leaf litter. The flask contains damp leaves and soil bacteria. A small tube of potassium hydroxide solution absorbs carbon dioxide made by the bacteria. As the bacteria respire, oxygen is used and the coloured drop moves along the capillary tube. The students place a second, identical apparatus in a refrigerator. They plan to compare the distance moved in 20 minutes. The apparatus is checked carefully because potassium hydroxide is corrosive and must not touch the leaf litter.
(a) Name the chemical in Fig. 1 that absorbs carbon dioxide. [2]
(b) Use Fig. 1 to describe why the coloured drop moves when the bacteria respire. [3]
(c) Explain why the students should use the same mass of leaves in each apparatus. [3]
(d) Complete the prediction: the drop in the refrigerator apparatus will move a __________ distance in 20 minutes. Explain your answer. [3]
Answer Details
(a) Potassium hydroxide solution [2] absorbs the carbon dioxide.
(b) [3] Bacteria use oxygen in respiration. The carbon dioxide they produce is absorbed by potassium hydroxide. This decreases the gas volume or pressure in the flask, pulling the coloured drop along the capillary tube.
(c) [3] The mass of leaves affects the amount of material available to bacteria. Different leaf masses could result in different respiration rates. Using the same mass makes the comparison fair, so temperature is the variable being changed.
(d) [3] The drop in the refrigerator apparatus will move a shorter distance in 20 minutes. Low temperature slows enzyme-controlled reactions in bacteria, so less oxygen is used in the same time. The pressure falls by less, so the drop moves less far.
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