Teaching using experimental setup

This question tests teachers’ ability to identify the scope and limitations of a given experimental set-up. In this case, the primary objective of the experimental set up is to demonstrate the conditions under which seeds can respire. For respiration to occur seeds need water and oxygen. 

Dried seeds in the first retort will not respire. Hence the composition of the air trapped inside this retort does not change. The level of the liquid (caustic potash solution) remains what it is throughout the experiment. In the case of the third retort, the seeds are soaked and they have oxygen available. Hence, they will respire and as they do so they will consume oxygen and give out carbon dioxide. The CO2 given out will be absorbed by the caustic potash solution. This will create a partial vacuum inside the retort and make the liquid rise in the tube. The seeds in the second retort will also respire and in the process consume oxygen and give out carbon dioxide. But since the salt water will not absorb the carbon dioxide the level will remain unchanged. Since the level of salt water solution does not change, it shows that that amount of carbon dioxide given out by the respiring seeds is the same as the amount of oxygen taken in thereby keeping the pressure inside the retort the same. Hence (i) and (iii) can be demonstrated using this experiment.

• In order to demonstrate that heat is produced during respiration there needs to be a thermometer to measure the change in temperature. Hence (iv) cannot be demonstrated.  Choosing an option containing this may indicate that the teacher thinks that the heat produced during respiration will in some way alter the chemicals used in the beakers.
• To show that respiration can only occur with the help of oxygen the teacher would need to set up another retort with no oxygen in it. And since this is not done (ii) cannot be demonstrated. Choosing this option may indicate that the teacher has a misconception that the reagents in the beaker will in some way dissolve the oxygen in one of the retorts with soaked seeds.
• Option B and C are thus incorrect as each contains one incorrect statement. Option D is the most incorrect options since neither (ii) nor (iv) can be demonstrated for reasons listed above.