\begin{tabular}{l} Which is the substance that makes rainwater slightly \\ acidic? \\ Nitric acid \\ Fossil fuels \\ Carbon dioxide \\ Sulfuric acid \\ \hline\end{tabular}

Rainwater becomes slightly acidic primarily due to carbon dioxide. When CO2 from the atmosphere dissolves in rainwater, it reacts to form carbonic acid, giving rain its mildly acidic quality with a pH typically around 5.6. This process is a natural part of the water cycle and helps regulate the pH of lakes and rivers, influencing the ecosystem within. Interestingly, while carbon dioxide is a natural contributor, human activities—like burning fossil fuels—can increase the concentration of other acids such as sulfuric and nitric acids in rainfall. This leads to the infamous problem of acid rain, which can seriously harm forests, freshwater sources, and even historical buildings!