When sodium carbonate (Na2CO3) is heated, it undergoes thermal decomposition to form sodium oxide (Na2O) and carbon dioxide (CO2) gas. The chemical equation for this reaction is:

Na2CO3 (s) → Na2O (s) + CO2 (g)

On the other hand, when hydrogen (H2) is heated, it does not undergo any chemical change. Hydrogen is a diatomic gas, and heating it only causes the molecules to move faster and gain more energy, without changing their chemical composition.

In summary, when sodium carbonate is heated, it decomposes to form sodium oxide and carbon dioxide, while hydrogen remains unchanged when heated.