Why is Li2CO3 decomposed at a lower temperature whereas
Question:

Why is $\mathrm{Li}_{2} \mathrm{CO}_{3}$ decomposed at a lower temperature whereas $\mathrm{Na}_{2} \mathrm{CO}_{3}$ at higher temperature?

Solution:

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.

$\mathrm{Li}_{2} \mathrm{CO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{Li}_{2} \mathrm{O}+\mathrm{CO}_{2}$

Therefore, lithium carbonate decomposes at a low temperature while a stable sodium carbonate decomposes at a high temperature.