Identifying group 2 carbonates and judging thermal safety
A technician has three unlabelled white solids: magnesium carbonate, calcium carbonate and barium carbonate. They are in a damp cupboard, and the labels have fallen off. Only small samples (about 0.5 g) are available. The technician has a Bunsen burner, dilute hydrochloric acid, limewater, a conductivity meter and platinum wire for flame tests. A senior teacher warns that flame tests alone are unreliable here: magnesium gives a very faint flame, calcium gives brick-red, and barium gives apple-green but barium compounds are toxic. The technician needs a safe identification scheme using the minimum number of tests, and must decide which solid is most likely to decompose on gentle heating and which is safest to heat in a test tube. The solids are all insoluble in water but dissolve in acid with effervescence.
1. Design a safe minimum-test identification scheme for the three solids and justify why the conductivity meter is unlikely to help.
Read the worked answer
- First add a few drops of dilute hydrochloric acid to a small sample of each solid in separate test tubes. All three carbonates will effervesce. Pass the gas through limewater; it turns milky, confirming CO2 and hence a carbonate. This test identifies the class but not the metal.
- Convert small portions to chloride solutions by adding excess dilute hydrochloric acid and warming gently to expel CO2. Then carry out a flame test with a clean platinum wire: calcium gives a brick-red flame, barium gives an apple-green flame, and magnesium gives no strong persistent colour or only a very faint one. Clean the wire with hydrochloric acid between tests. Because barium compounds are toxic, use tiny samples and work in a fume cupboard or well-ventilated area; do not inhale dust.
- The conductivity meter is not useful for distinguishing the three metals here. After acid treatment, all three form soluble chloride electrolytes, and conductivity depends mainly on ion concentration and mobility rather than on which group 2 metal is present. Calcium and barium ions have similar mobilities, and the result would not be a unique identification. The flame test plus carbonate test is more discriminating and uses fewer resources.
2. Use periodic trends to predict which carbonate decomposes most readily on gentle heating and discuss which solid is safest to heat in a test tube.
Read the worked answer
Group 2 carbonates decompose on heating: MCO3(s) → MO(s) + CO2(g). Thermal stability increases down the group. The magnesium ion is small and has a high charge density, so it polarises the large carbonate ion strongly and weakens a C–O bond, making decomposition easier. Down the group, the cation becomes larger and its charge density falls, so it polarises the carbonate less; barium carbonate is therefore the most stable. The prediction is that magnesium carbonate decomposes most readily on gentle heating, calcium carbonate needs a stronger heat, and barium carbonate is the most resistant.
The safety judgement is not simply the opposite of thermal stability. Barium compounds are toxic, so heating barium carbonate creates a dust hazard even though it is thermally stable. Magnesium carbonate decomposes easily but gives magnesium oxide and carbon dioxide, which are much less hazardous. Calcium carbonate is a reasonable compromise: it is less toxic than barium carbonate and more stable than magnesium carbonate. If only a small test-tube heating is required, use a very small sample of calcium carbonate, heat gently, and point the tube away from people; avoid heating barium compounds unless necessary and use a fume cupboard.
3. Explain why the technician should not rely on flame tests on the original solid carbonates, and state one precaution needed when interpreting the magnesium result.
Read the worked answer
Flame tests work best when the metal is present as a volatile compound, usually a chloride. The original carbonates are not volatile and do not give reliable flame colours; the solid may just glow or decompose. Converting each carbonate to a chloride with dilute hydrochloric acid gives a more volatile species in the flame, so the colour is more characteristic.
The magnesium result needs care because the flame colour is very faint and may be masked by the platinum wire or by sodium contamination from fingers or glassware. A 'no strong colour' result should be treated as consistent with magnesium only after the wire has been cleaned and a known magnesium sample has been tested as a control. The technician should not claim a positive identification from a faint colour alone; the acid test confirming carbonate plus the absence of a clear calcium or barium flame is the safer judgement.