A truck was carrying a substance in a tank. The molecules of that substance were moving away from each other. The truck parked overnight in a place where energy transferred out of the substance . In the morning . the substance was a gas. How were the molecules moving in the morning ?

In the morning the molecules were moving away from each other with a smaller speed than when the truck was carrying the substance.

2. The reason is that the molecules contain less average kinetic energy, thus their average speed is lower.

Explanation:

Our model of the state (phases) of the substances describes the molecules of solids as not moving away from each other, they are relatively fixed with respect to each other, just vibrating; the molecules of liquids as free to slide pass each other, which permits they to flow, but not moving away from each other; and the the molecules of the gases as moving away from each other, which is the cause of them occupying the whole space of the container that holds them.

According to that, the substance that the truck was carryng was a gas, because the molecules were moving away from each other.

After parking overnight, where energy transferred out of the substance, in the morning, the substance was a gas. Hence, the molecules were moving away from each other too. This answer the first question.

Since energy was transferred out of the substance, in the morning the temperature of the substance decreased (but no enough to make it a liquid). The average kinetic energy of the gases depends only on the temperature. So, the average kinetic energy of the molecules in the morning was smaller than when the truck was carrying the substance. Since the kinetic energy is proportional to the square of the speed of the molecules, the smaller average kinetic energy means that the average speed of the molecules was smaller too. Hence, in the morning the molecules were moving away from each other with a lower speed than when the truck was carrying the substance.

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