Passage VI

Ozone (O3) is concentrated in a part of Earth's atmo­sphere called the ozone layer. O3 absorbs harmful high­-energy solar radiation. Unusually low amounts of O3 have been detected over Antarctica each spring since 1978 in an area of the ozone layer called the ozone hole. Two scien­tists hypothesize about the cause of the ozone hole.

Scientist 1

Chemicals known as chlorofluorocarbons (CFCs), such as CF2Cl2 and CFCl3, are responsible for the forma­tion of the ozone hole. CFCs have been widely used as refrigerants and aerosol propellants since their introduction in the 1930s. CFCs accumulate in the atmosphere, where solar radiation (Iight) breaks them down to produce chlo­rine atoms (Reaction 1):

(1) CF2Cl2 + light → CF2Cl + Cl

The unstable Cl atoms react with O3 to produce chlorine monoxide and O2 (Reaction 2):

(2) Cl + O3 → ClO + O2

The O2 is then broken down into O atoms (Reaction 3):

(3) O2 + light → 2 O

The unstable O atoms then react with CIO to produce more Cl atoms (Reaction 4):

(4) O + ClO → Cl + O2

Reactions 2-4 make up a chain reaction, because the Cl atoms produced in Reaction 4 can then react with more O3 in Reaction 2:

(2) Cl + O3 → ClO + O2

(3) O2 + light → 2 O

(4) O + ClO → Cl + O2

Thus, the Cl atom produced by 1 CFC molecule can break down large amounts of O3. Due to competing reactions, this chain reaction does gradually stop if the number of Cl atoms produced decreases, but this slowdown takes many years.

Scientist 2

The total amount of O3 in Earth's atmosphere is constant. O3 concentrations vary over Antarctica periodically because wind currents cause local changes in the amounts of all the chemicals in the atmosphere. Though CFCs can break down O3 by Reactions 1-4, the ClO produced in Reaction 2 mostly reacts with the abundant nitrogen oxides in the atmosphere. Reactions 5-7 show how ClO reacts with NO to regenerate O3:

(5) ClO + NO → Cl + NO2

(6) NO2 + light → N + O

(7) O + O2 → O3

ClO can also react with nitrogen dioxide to form chlorine nitrate (Reaction 8):

(8) ClO + NO2 → ClNO3

CINO3 does not react with O3, so it stops the chain reaction shown in Reactions 2-4 from breaking down large amounts of O3.

29. According to both Scientists 1 and 2, which of the fol­lowing substances must initially be present for CIO to be produced by CFCs in Earth's atmosphere?

Answer and Explanation

Your Answer is

Correct Answer is D

Explanation

Look at the fourth line of Scientist 2 Though CFCs can break down O3 by Reaction 1-4...that is to say, Scientist 2 agrees with Reaction 1-4. Look at Reaction 2 Cl + O 3 → ClO + O2, so to generate ClO, O3 must participate