Carbon dioxide and global warming: the greenhouse effect
3 marks · NormalA student heats potassium carbonate in a test tube and observes no visible change. Use your knowledge of the thermal decomposition of metal carbonates to explain this result.
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Also accept: Potassium carbonate is more thermally stable than copper(II) or calcium carbonate.; Alkali metal carbonates are very stable to heat.; It requires a very high temperature to decompose, higher than achievable with common lab burners.
Common mistakes: Saying no reaction occurs with no explanation; Mistaking results with those for less stable carbonates
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Marking points
M1 · 1 mark · Recognises that potassium carbonate does not decompose easily.
M2 · 1 mark · States potassium carbonate is more thermally stable than calcium or copper(II) carbonate.
A1 · 1 mark · Explains that the temperature needed is higher than can be achieved in the lab with a Bunsen burner.
Worked solution
Potassium carbonate is very thermally stable, unlike copper(II) carbonate or calcium carbonate. The heat from a laboratory burner is not enough to decompose potassium carbonate, so no reaction or visible change occurs.