Problem 34 - 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: Part (a): Bromine vapor turning to bromine liquid

This involves the process of condensation, where a substance changes from a gaseous phase to a liquid phase as it cools down. Since energy is being released in this phase transition, it is an exothermic process.

Step 2: Part (b): Crystals of iodine disappearing

The given situation describes the process of sublimation, where a substance changes directly from a solid phase to a gaseous phase without passing through a liquid phase. Since energy is being absorbed in this phase transition, it is an endothermic process.

Step 3: Part (c): Rubbing alcohol disappearing

This involves the process of evaporation, where a liquid substance changes to a gaseous phase. The rubbing alcohol absorbs heat from its surroundings and gradually changes to vapor. As it absorbs energy, it's an endothermic phase transition.

Step 4: Part (d): Molten lava turning into solid rock

This situation describes the process of solidification (or freezing), where a liquid substance becomes a solid as it cools down, such as molten lava turning into solid rock. Since energy is being released during this phase transition, it is an exothermic process.

Key Concepts

Condensation

Imagine a hot shower steaming up a bathroom mirror. The steam, which is water vapor, cools down and turns into tiny droplets of water on the mirror's surface. This is called condensation, a phase transition where gas transforms into a liquid. This process can be seen in nature when dew forms on grass or when fog develops. In condensation, molecules lose energy and slow down, clumping together to form a liquid.

Condensation is important in various industrial processes, like distillation to purify liquids, and it plays a critical role in the water cycle, creating clouds and precipitation. This phase transition is exothermic, meaning it releases heat into the surroundings, and that's why the outside of a cold glass of lemonade feels chilly on a hot day.

Sublimation

Ever notice how snow can sometimes disappear from a sidewalk without melting? This is an example of sublimation, a phase transition where a solid turns directly into gas without first becoming a liquid. Dry ice, which is solid carbon dioxide, demonstrates sublimation when it creates fog-like effects at parties.

Sublimation requires energy because the solid particles must gain enough energy to overcome their rigid structure and disperse as a gas. That's why sublimation is an endothermic process, meaning it absorbs energy from its surroundings. This knowledge can be particularly useful, for example, in freeze-drying foods and pharmaceuticals, preserving both their structure and nutrients.

Evaporation

On a sunny day, you may see puddles from a recent rain start to shrink and vanish. What's at work here is evaporation, which is when a liquid turns into gas. When this happens naturally, it's because molecules on the surface of the liquid gain enough energy from heat (like sunlight) to escape into the air.

Evaporation is also critical for cooling; when your body sweats, the evaporation of that sweat from your skin helps you maintain a safe body temperature. As a liquid becomes gas, it absorbs heat, which categorizes evaporation as an endothermic process. This concept is harnessed in technologies like cooling towers for power plants and in the simple act of hanging wet clothes out to dry.

Solidification

When you put water in your freezer, you later find ice. This transformation from liquid to solid is known as solidification. It can also be seen when molten metals cool down to form solid ingots or when lava cools and hardens into rock. During solidification, the motion of the molecules slows down and they arrange themselves into fixed positions, forming a solid.

Solidification is a key process in the manufacturing of many goods, from chocolate to cast iron skillets. This phase transition is an exothermic process because it releases heat to the surroundings. So when water freezes, the surrounding area gets a slight warming effect as the liquid turns to ice.

Exothermic Process

An exothermic process is like a warm hug—it gives off heat. In chemistry, this encompasses any reaction or phase transition that releases energy into the environment. Examples include combustion, like a campfire; chemical reactions in hand warmers; and, as previously mentioned, phase transitions such as condensation and solidification

Understanding exothermic reactions is essential for controlling processes that require or release heat and is a principle foundation of thermodynamics. This knowledge is applied in developing energy-efficient materials and systems, safely managing chemical reactions, and harnessing energy sources.

Endothermic Process

On the flip side, an endothermic process is like an energy sponge—it soaks up heat. This can be any chemical reaction or phase transition where energy is absorbed from the surroundings. Notable examples include photosynthesis in plants, melting ice, and evaporative cooling.

Endothermic processes are important in creating products that require specific energy inputs, such as certain polymers and drugs. They also play a significant role in the natural world, influencing weather patterns and ecosystems. Recognizing endothermic reactions helps us manage energy utilization and storage in chemical engineering and other industries.