Problem 40 - Chemistry The Central Science

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.

Step-by-Step Solution

Answer

(a) The phase transition is condensation (gas-to-liquid), which is exothermic. (b) The phase transition is sublimation (solid-to-gas), which is endothermic. (c) The phase transition is evaporation (liquid-to-gas), which is endothermic. (d) The phase transition is solidification/freezing (liquid-to-solid), which is exothermic.

Step 1: a) Bromine vapor turns to bromine liquid as it is cooled.

In this situation, the phase transition occurring is gas-to-liquid, which is called condensation. As bromine vapor turns into liquid, energy is released, making this an exothermic process.

Step 2: b) Crystals of iodine disappear from an evaporating dish as they stand in a fume hood.

In this case, the iodine crystals are going directly from a solid to a gaseous state, skipping the liquid phase. This phase transition is called sublimation. Energy is required for this process, making it an endothermic process.

Step 3: c) Rubbing alcohol in an open container slowly disappears.

Rubbing alcohol, which is usually a liquid, is evaporating from the container. This means it is transitioning from the liquid phase to the gas phase, a process called evaporation. Similar to sublimation, evaporation requires energy input, making it an endothermic process.

Step 4: d) Molten lava from a volcano turns into solid rock.

The molten lava, which is initially in a liquid state, solidifies and turns into a solid rock. This phase transition is called solidification or freezing. In this case, energy is released as the transition occurs, making it an exothermic process.

Key Concepts

Condensation

Condensation is the process where gas particles cool down and transition into a liquid state. This physical change usually takes place when the gas is in contact with a cooler surface or when it's compressed to a higher pressure. During condensation, energy is released into the surroundings as heat, because the particles are slowing down and the attractive forces between them cause them to stick together, forming a liquid.

For example, when you see water droplets on the outside of a cold drink, that's condensation in action. The water vapor in the air cools down when it hits the cold surface of the drink container and turns back into liquid water. Condensation is an exothermic process, meaning it releases heat to its environment, as demonstrated in the exercise with the conversion of bromine vapor to liquid.

Sublimation

Sublimation might seem like magic, but it's a genuine scientific phenomenon where a solid directly changes into a gas without passing through the liquid phase. Substances that undergo sublimation require energy to break the bonds between the particles in the solid state so they can move away from each other and become a gas. This process is endothermic, meaning it absorbs heat energy from its surroundings.

The disappearance of iodine crystals in an evaporating dish is a classic example of sublimation. As the solid iodine particles gain enough energy, they leap straight into the gas phase and disperse into the air, never becoming liquid. Dry ice, which is solid carbon dioxide, is another common substance that sublimates at room temperature, turning directly into carbon dioxide gas.

Evaporation

Evaporation is an everyday process in which liquid turns into vapor, which is a gas state. Unlike boiling, evaporation occurs on a liquid's surface and typically happens at temperatures below the liquid's boiling point. This phase change is how puddles dry up after rain and how clothes dry on a line. When molecules at the surface gain enough energy, usually from heat, they can overcome the attraction to other liquid molecules and escape into the air as gas.

Evaporation is also an endothermic process since the liquid absorbs heat energy from its surroundings to allow the surface molecules to vaporize. An example from the exercise is the slow disappearance of rubbing alcohol from an open container, where it transitions from a liquid to a gas, gradually lowering the amount of liquid as the gas dissipates into the air.

Solidification

Solidification, also known as freezing, is when a liquid becomes solid. During this change, liquid particles lose energy, move closer together, and arrange into a fixed structure, which is the solid state. As the particles settle into a more stable structure, they release energy to the environment, which is why the process is classified as exothermic.

An intuitive illustration of solidification is when water freezes to become ice. As the temperature drops, the water loses heat, its molecules slow down, and eventually, they lock into place to form a solid—ice. A more dramatic example provided in the exercise is the cooling and hardening of molten lava from a volcano into solid rock. As the lava cools, it transitions from a liquid to solid state, expelling heat and turning into igneous rock.