Question

Name the phase transition in each of the following situations, and indicate whether it is exothermic or endothermic: (a) Bromine vapor turns to bromine liquid as it is cooled. (b) Crystals of iodine disappear from an evaporating dish as they stand in a fume hood. (c) Rubbing alcohol in an open container slowly disappears. (d) Molten lava from a volcano turns into solid rock.

Short answer

(a) Condensation; Exothermic. (b) Sublimation; Endothermic. (c) Evaporation; Endothermic. (d) Solidification or Freezing; Exothermic.

Step-by-step solution

Situation (a) - Bromine vapor turns to bromine liquid as it is cooled

This phase transition is called condensation, which is the process of a gas turning into a liquid. It occurs when gas particles lose energy, usually due to cooling. Condensation is an exothermic process because energy is released during the transition.

Situation (b) - Crystals of iodine disappear from an evaporating dish as they stand in a fume hood

This phase transition is called sublimation, in which a solid directly turns into a gas. The iodine crystals are heated slightly or the air pressure around them is reduced, causing the particles to gain enough energy to transition directly into the gaseous state. Sublimation is an endothermic process because it requires an input of energy for the transition to occur.

Situation (c) - Rubbing alcohol in an open container slowly disappears

This phase transition is called evaporation, which is the process where a liquid changes into a gas at a temperature below its boiling point. Evaporation happens when the surface molecules of the liquid gain enough energy to overcome the intermolecular forces. Evaporation is an endothermic process because energy is absorbed during the transition.

Situation (d) - Molten lava from a volcano turns into solid rock

This phase transition is called solidification or freezing, in which a liquid changes into a solid. Solidification occurs when a liquid loses enough energy that its particles no longer have the ability to move past each other, forming a solid structure. Solidification is an exothermic process because energy is released during the transition.