Alkali Carbonates are stable to heat except that Li2CO3 is unstable. Thermal stability increases moving down the group.
As we move down the alkali metal group, the
electropositive character increases. This causes an increase in the stability
of alkali carbonates. However, lithium carbonate is not so stable to heat. This
is because lithium carbonate is covalent. Lithium ion, being very small in
size, polarizes a large carbonate ion, leading to the formation of more stable
lithium oxide.
Therefore, lithium carbonate decomposes at a low temperature while a stable
sodium carbonate decomposes at a high temperature.
Li2CO3 on heating gives Li2O + CO2.