Statement I: Limestone decomposes when it is heated in air. Statement II: Increase in the content of C02 in the atmosphere in recent years is mainly due to the using of limestone in the manufacture of cement.

Statement I is true as limestone (calcium carbonate) undergoes thermal decomposition when heated, breaking down into calcium oxide (lime) and carbon dioxide [1]. This endothermic reaction is a fundamental process in various industries [1]. Statement II is false because, while cement production is a significant contributor to global CO2 emissions (accounting for approximately 7-8% of anthropogenic emissions), it is not the 'main' cause of the recent atmospheric CO2 increase. The primary drivers of the recent rise in atmospheric CO2 are the combustion of fossil fuels for energy and transportation, followed by deforestation and land-use changes. Although the calcination of limestone in cement kilns releases substantial 'process CO2', it remains a secondary source compared to the global scale of fossil fuel consumption.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > Figure 1.4 > p. 8