Predict the sign of ΔSsys for each of the following processes. Which are positive? (a) Gasoline vapors mix with air in a car engine. (b) Hot air expands. (c) Humidity condenses in cold air. Which are negative? (a) Gasoline vapors mix with air in a car engine. (b) Hot air expands. (c) Humidity condenses in cold air.

Positive: a and b

Negative: c

Explanation:

The entropy (S) is the measure of the randomness of the system, and it intends to increase. The randomness can be determined by the energy of the molecules, their velocity and how distance they are between the other molecules.

When the entropy increases, ΔS is positive, when the entropy decreases, ΔS is negative. So, when gasoline mix with air in a car engine, the process intends to continue, the randomness increases and ΔS is positive. When hot air expands, the distance between the molecules increases, so ΔS is positive.

But, when humidity condenses, the molecules stay closer, so there's a decrease in the randomness, then ΔS is negative.