Problem 40 - Chemistry: The Central Science in SI Units, Global Edition

Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) Iodine solid turns to iodine gas when it is heated. (b) Snowflakes turn into water when they fall on an open palm. (c) Droplets of water appear on grass in a cold humid morning. (d) Dry ice gradually disappears when left at room temperature for some period of time.

Step-by-Step Solution

Answer

(a) Sublimation, Endothermic; (b) Melting, Endothermic; (c) Condensation, Exothermic; (d) Sublimation, Endothermic

Step 1: Situation (a) - Iodine Solid to Iodine Gas

In this situation, iodine is turning from a solid directly to a gas when heated. This phase transition is called sublimation. Since heat is being absorbed during this process to turn the solid into a gas, it is an endothermic process. So, the phase transition for (a) is Sublimation, and it is Endothermic.

Step 2: Situation (b) - Snowflakes Turns into Water

In this case, snowflakes are changing from a solid-state (ice) to a liquid state (water) when they fall on an open palm. This phase transition is called melting or fusion. Snowflakes absorb heat from the palm to turn into water, making this an endothermic process. So, the phase transition for (b) is Melting, and it is Endothermic.

Step 3: Situation (c) - Droplets of Water Appear on Grass

In this situation, we see water droplets forming on grass in a cold humid morning. This means the water vapor in the air (gas state) is turning into a liquid state (water droplets) when it cools down. This phase transition is called condensation. As the water vapor releases heat to turn into liquid water, it is an exothermic process. So, the phase transition for (c) is Condensation, and it is Exothermic.

Step 4: Situation (d) - Dry Ice Disappears

In this situation, dry ice (solid-state) gradually disappears when left at room temperature for some period. This means it is turning from a solid-state directly to a gas state (CO2 gas). This phase transition is also called sublimation. During this process, dry ice absorbs heat from the surroundings to turn into CO2 gas, making this an endothermic process. So, the phase transition for (d) is Sublimation, and it is Endothermic.

Key Concepts

Sublimation

Sublimation is a fascinating phase transition where a substance changes directly from a solid to a gas, skipping the liquid phase altogether. This process occurs when the substance absorbs enough energy to break the bonds holding the solid structure, allowing particles to escape directly into the gaseous phase.
Here are some key points about sublimation:

• It requires the absorption of energy, making it an endothermic process.
• Common examples include the sublimation of dry ice (\[ ext{CO}_2(s) \rightarrow ext{CO}_2(g)\]), and the transition of iodine crystals to iodine vapor.
• Sublimation is often used in freeze-drying food, which helps preserve it by removing moisture without going through a liquid phase.

Understanding sublimation not only gives insight into phase transitions but also reveals practical applications in our daily lives.

Melting

Melting, also known as fusion, is the process where a solid turns into a liquid. This phase transition occurs with the application of heat, which provides the energy necessary for particles within the solid to overcome their fixed positions and begin to move past each other freely, thus becoming a liquid.
Some important elements of melting include:

• It is an endothermic process because it requires heat absorption for the transition.
• Every substance has a specific melting point, the temperature at which it changes from solid to liquid.
• The melting of ice to water is a classic example, with ice absorbing heat from its environment.

Mastering the concept of melting provides valuable understanding in areas such as material sciences and meteorology.

Condensation

Condensation is a transition where a gas changes into a liquid. This shift occurs as gas particles lose kinetic energy, often due to a reduction in temperature, causing them to slow down and come closer together until they form a liquid.
Key characteristics of condensation include:

• It is an exothermic process, as energy is released into the environment when the gas transforms into a liquid.
• Commonly seen phenomena include dew on grass and fog, where airborne water vapor cools and condenses.
• Condensation plays a crucial role in the water cycle, aiding cloud formation and precipitation.

By learning about condensation, one gains insight into weather patterns and the behavior of gases.

Endothermic and Exothermic Processes

The terms endothermic and exothermic describe the energy changes associated with chemical and physical processes. When a reaction or process absorbs heat from its surroundings, it's termed endothermic. Conversely, an exothermic process releases heat. Understanding these concepts is central to thermodynamics and the study of energy changes in reactions.
To clarify:

• Endothermic processes involve energy absorption. Examples include melting of ice and sublimation of dry ice.
• Exothermic processes involve energy release. Such as condensation of water vapor, releasing energy.
• Both processes dictate the direction and extent of phase transitions and reactions.

Gaining clarity on these processes allows for a deeper comprehension of how energy influences changes in states of matter and beyond.