HCFCs differ from CFCs because:

HCFCs differ from CFCs because:

• A. The addition of hydrogen allowed it to be oxidized and made HCFCs reactive
• B. The presence of chlorine increased reactivity
• C. They have a larger molecular weight
• D. They are emitted directly from smog sources

Answer: (D)

The main idea is atmospheric stability and how the molecule reacts in the air. HCFCs contain hydrogen, which makes them less stable in the lower atmosphere. In the troposphere, hydroxyl radicals readily attack and oxidize HCFCs, causing them to break apart before they can reach the stratosphere. Because they degrade more quickly, they have shorter atmospheric lifetimes and release less chlorine into the upper atmosphere, leading to a much lower ozone depletion potential than CFCs. So, the presence of hydrogen is the key difference: it drives faster oxidation in the troposphere, preventing HCFCs from persisting long enough to reach and impact the ozone layer as strongly as CFCs do. The other ideas—more chlorine, bigger molecular weight, or emissions from smog sources—aren't the main factors that distinguish HCFCs from CFCs.

The main idea is atmospheric stability and how the molecule reacts in the air. HCFCs contain hydrogen, which makes them less stable in the lower atmosphere. In the troposphere, hydroxyl radicals readily attack and oxidize HCFCs, causing them to break apart before they can reach the stratosphere. Because they degrade more quickly, they have shorter atmospheric lifetimes and release less chlorine into the upper atmosphere, leading to a much lower ozone depletion potential than CFCs.

So, the presence of hydrogen is the key difference: it drives faster oxidation in the troposphere, preventing HCFCs from persisting long enough to reach and impact the ozone layer as strongly as CFCs do. The other ideas—more chlorine, bigger molecular weight, or emissions from smog sources—aren't the main factors that distinguish HCFCs from CFCs.