(1) I’m with you on the toy hamster point. (A pet hamster in a sealed tank will be a cruel and bad idea for a question. A cockroach instead perhaps?). 

(2) CO2 dissolves in water slowly (stale water is more acidic). If that’s a good datalogger then it will show [CO2]↓. O2 won’t dissolve as much in one day and it won’t show on datalogger.

(3) Depending on the food, bacteria and fungi might break it down to a certain degree in 24 hours, via aerobic respiration in this case most likely. Looks like the answer chose not to consider this factor. 

To a P6 kid, I might stress on point (1) and point (2), and put very little weight on point (3).

Thanks Cogito for your detailed explanation. 

So based on your words, if we consider all 3 points, both oxygen and carbon-dioxide will decrease. If we only consider your points (1) and (2), then it fits the given answer (oxygen unchanged and carbon-dioxide decreased).

The question here is how do we know which points to be considered? I am not sure if point (2), ie, “carbon-dioxide is dissolves in water” is in the P5 Science syllabus? 

Short answer: I don’t know what the correct answer should be.

Long answer: There are science questions, where there is a different correct answer at every level/depth of consideration. Some of these will simply start endless debates. I usually would advise my son to take the learning points from the question and move on. This question seems to me to qualify for that category.

Thanks Cogito for the wise suggestion! Really appreciate it!