The lesson covered thermal stability in chemistry, focusing on how compounds react when heated. It explained that some compounds, like potassium and sodium carbonates, do not decompose upon heating, whereas carbonates from calcium to copper decompose into metal oxides and carbon dioxide. The session also detailed the decomposition of metal nitrates, noting that while potassium and sodium nitrates form nitrites and oxygen, nitrates from calcium to copper yield metal oxides, nitrogen dioxide, and oxygen.