Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) lodine 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.

Short answer

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

Step-by-step solution

Identify the Phase Transition for Part (a)

When iodine solid turns to iodine gas upon heating, this process is known as sublimation. Sublimation is when a substance changes from a solid directly to a gas without passing through a liquid state.

Determine Exothermic or Endothermic for Part (a)

Sublimation is an endothermic process because it requires the absorption of heat to change the solid into gas.

Identify the Phase Transition for Part (b)

When snowflakes (solid) turn into water (liquid), this is an example of melting. Melting is the transition from solid to liquid.

Determine Exothermic or Endothermic for Part (b)

Melting is an endothermic process as it requires heat absorption to break the bonds in the solid state turning it into a liquid.

Identify the Phase Transition for Part (c)

Droplets of water appearing on grass in the morning typically indicate condensation. Condensation is the transition from gas to liquid.

Determine Exothermic or Endothermic for Part (c)

Condensation is an exothermic process because it releases heat when gas molecules transition into the liquid state.

Identify the Phase Transition for Part (d)

When dry ice, which is solid carbon dioxide, disappears directly into gas, this is another example of sublimation.

Determine Exothermic or Endothermic for Part (d)

Similar to part (a), sublimation of dry ice is an endothermic process as it requires the absorption of heat to change from solid to gas.

Key concepts

Sublimation

Sublimation occurs when a solid changes directly into a gas without passing through the liquid state. This intriguing process can be seen in daily life with substances like dry ice or iodine. When dry ice, which is solid carbon dioxide, is kept at room temperature, it transforms directly into carbon dioxide gas. Similarly, iodine turns into gas when heated.

This process requires an input of energy, making it an endothermic reaction. The molecules in the solid absorb the heat energy, which breaks the bonds holding them together in that solid state, allowing them to become a gas.

• Solid to Gas: No liquid phase involved
• Endothermic: Requires absorption of heat
• Examples: Dry ice, iodine heating

Endothermic and Exothermic Processes

Phase transitions involve either gaining or losing energy, which is classified as endothermic or exothermic processes. To understand these transitions, it's essential to recognize what happens on a molecular level.

• Endothermic Processes: These absorb heat from the environment. Molecules gain energy, either increasing their movement or breaking bonds in a solid. Sublimation and melting are examples, as they require heat to change states.


• Exothermic Processes: These release heat. Molecules lose energy, resulting in a tighter arrangement, such as in condensation where gas turns into liquid. Exothermic processes tend to form stronger attractions between molecules.

Recognizing these processes helps in predicting the heat exchange during phase transitions.

Condensation

Condensation is the process where gas transforms into a liquid. This occurs commonly when humid air meets a cold surface, like grass in the morning or a chilled glass of your favorite drink. When moist air contacts these cold surfaces, it cools, causing water vapor to lose energy and aggregate into liquid droplets.

This makes condensation an exothermic process, as heat is released when the gas phase loses energy. Understanding condensation is crucial in studying atmospheric phenomena and cooling systems.

• Gas to Liquid: Molecules slow down and become closer
• Exothermic: Releases heat to surroundings
• Common Examples: Dew, fogging of a mirror after a shower

Melting

Melting is the transition of a substance from solid to liquid. This is what happens when ice cubes in your drink begin to disappear or when snow transforms into slush in a mild thaw. This transformation requires the input of energy to break the forces holding the molecules in a solid state.

Because of this need for energy, melting is categorized as an endothermic process. The energy absorbed facilitates the molecules to move further apart, hence turning the solid into a liquid.

• Solid to Liquid: Needs heat to break bonds
• Endothermic: Absorbs heat from surroundings
• Everyday Examples: Snow melting, chocolate in your hand

Chemistry Education

Understanding phase transitions is essential in chemistry education because they demonstrate fundamental principles about energy change and molecule interactions. Chemistry, the study of matter and its transformations, requires a strong understanding of the processes involving heat and energy, such as sublimation and condensation.

Learning about these transitions not only fosters a deeper grasp of chemical reactions but also equips students with the skills to analyze everyday phenomena, like why dew forms on grass or how ice packs work.

• Connects theory to real-world applications
• Encourages critical thinking about state changes
• Essential for understanding energy exchanges

Thorough chemistry education lays the groundwork for advanced studies and fosters an appreciation for the intricate workings of the physical world.