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Arrange the following carbonates in the correct increasing order of their thermal stability: BeCO3, MgCO3, CaCO3, K2CO3.

1. BeCO3 < MgCO3 < CaCO3 < K2CO3
2. K2CO3 < CaCO3 < MgCO3 < BeCO3
3. MgCO3 < BeCO3 < CaCO3 < K2CO3
4. CaCO3 < BeCO3 < MgCO3 < K2CO3

Correct answer: BeCO3 < MgCO3 < CaCO3 < K2CO3

Solution

The smaller and more charge-dense the cation, the more it polarises CO3²-, making decomposition easier. Be²+ > Mg²+ > Ca²+ in polarising power, and K+ (charge +1, large size) has the least polarising power, making K2CO3 the most thermally stable.

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