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) Condensation, exothermic. (b) Sublimation, endothermic. (c) Evaporation, endothermic. (d) Solidification, exothermic.

Step 1: (a) Identify the phase transition

The given situation is bromine vapor turning into bromine liquid when cooled. This transition represents the process of condensation, where a gas is turned into a liquid.

Step 2: (a) Determine if the process is endothermic or exothermic

In the condensation process, energy is released as the molecules come closer together to form a liquid phase. Therefore, this process is exothermic.

Step 3: (b) Identify the phase transition

The situation describes crystals of iodine disappearing from an evaporating dish. This represents the process of sublimation, where a solid directly changes into a gas without first becoming a liquid.

Step 4: (b) Determine if the process is endothermic or exothermic

Sublimation requires energy input to transition from a solid to a gas. As a result, this process is endothermic.

Step 5: (c) Identify the phase transition

Rubbing alcohol in an open container slowly disappearing represents the process of evaporation, where liquid alcohol becomes a gas.

Step 6: (c) Determine if the process is endothermic or exothermic

Evaporation is an endothermic process because energy is absorbed by the alcohol molecules to transition from a liquid to a gas phase.

Step 7: (d) Identify the phase transition

Molten lava from a volcano turning into solid rock represents the process of solidification, where the liquid lava is cooled and becomes a solid.

Step 8: (d) Determine if the process is endothermic or exothermic

Solidification is an exothermic process because energy is released when the liquid becomes a solid. In summary: (a) Bromine vapor turning into bromine liquid is condensation, an exothermic process. (b) Iodine crystals disappearing is sublimation, an endothermic process. (c) Rubbing alcohol evaporating is evaporation, an endothermic process. (d) Molten lava solidifying is solidification, an exothermic process.

Key Concepts

Condensation

Condensation is the transformation of a substance from its gaseous form into its liquid state. This phase transition typically occurs when a gas is cooled down or when it comes into contact with a cold surface, causing it to lose energy. In the context of the textbook exercise, bromine vapor cooling to form liquid illustrates condensation. The process is exothermic, meaning it releases heat. This release happens because as gas molecules condense, they lose kinetic energy and release it as thermal energy, contributing to the surrounding environment.

For example, when you see water droplets on the outside of a cold glass, that's condensation in action. The cooler temperature of the glass chills the humid air around it, resulting in water vapor condensing into liquid. Understanding condensation is critical not only in chemistry but also in understanding everyday phenomena like dew formation and the functioning of refrigerators.

Sublimation

Sublimation is a phase transition in which a solid changes directly into a gas without passing through the liquid phase. The exercise mentions the disappearance of iodine crystals, which is a classic example of sublimation. This process is endothermic as it requires an input of energy to overcome the forces holding the solid together. The energy is absorbed by the solid's molecules, which then disperse as a gas.

Examples of Sublimation

• Dry ice turning into carbon dioxide gas.
• The gradual disappearance of snow without melting on a cold day.

The concept of sublimation is not only fascinating but also practical, playing a crucial role in freeze-drying, preserving biological specimens, and even in air fresheners.

Evaporation

Evaporation is the process in which a liquid changes into a gas at temperatures below the boiling point. This phase transition, as depicted by rubbing alcohol in the textbook exercise, involves the conversion of the liquid alcohol to a gaseous state over time. Evaporation is an endothermic process because the molecules of the liquid absorb energy to escape into the atmosphere as a gas.

Everyday Examples of Evaporation

• The drying of wet clothes in the sun.
• Water disappearing from a puddle on a hot day.

Understanding evaporation is important for many areas, including meteorology, where it explains part of the water cycle, and in cooling mechanisms, such as sweating in humans or perspiration in plants.

Solidification

Solidification, also known as freezing, refers to the transition of matter from a liquid into a solid state. This process often occurs when the temperature of the liquid drops below its freezing point. In the given exercise example, molten lava from a volcano turning into solid rock is an instance of solidification. The process is exothermic, which means it releases energy, typically in the form of heat, as the liquid's molecules slow down and arrange into a more stable crystalline structure.

Examples of solidification can be found in:

• Water freezing into ice cubes in a freezer.
• The formation of snowflakes from water vapor in clouds.

Solidification is a significant concept in fields such as metallurgy and materials science, where controlling the solidification process determines the properties of metals and other materials.