notesonly.in

Conceptual Problem: A chemistry student tests a rainwater sample near a heavy industrial city in India and finds that its pH is 4.2. Explain what this number means in terms of hydrogen ion concentration compared to pure rainwater (pH 7.0), and write the chemical equation showing how sulfur dioxide gas transforms into the acid found in this rain. Step 1: Understand pH scale. Pure water is neutral with a pH of 7.0. A pH below 7.0 means the solution is acidic, and every drop of 1 unit on the pH scale represents a 10-fold increase in hydrogen ion concentration. Step 2: Compare pH 7.0 and pH 4.2. The difference is 2.8 units, meaning the rainwater is hundreds of times more acidic than normal, clean rain, confirming it is indeed acid rain. Step 3: Recall the source of sulfur dioxide gas from factories burning coal. Step 4: Write the balanced chemical equation for the formation of sulfurous acid when sulfur dioxide dissolves in moisture: SO2 + H2O -> H2SO3. Step 5: Write the further oxidation equation in the atmosphere that leads to sulfuric acid: 2SO2 + O2 + 2H2O -> 2H2SO4.