Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (c) Rubbing alcohol in an open container slowly disappears.

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1. The phase transition in this situation is from liquid to gas. This process is known as evaporation.

2. In evaporation, the liquid molecules gain enough energy to overcome the intermolecular forces and become gas molecules. This energy is usually provided by the surroundings, such as heat from the environment.

3. Since energy is absorbed from the surroundings during evaporation, this process is endothermic. An endothermic process is one in which heat is absorbed from the surroundings.

4. Therefore, the phase transition of rubbing alcohol in an open container slowly disappearing is evaporation, and it is an endothermic process.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Evaporation

Evaporation is the phase transition from liquid to gas that occurs at the surface of a liquid. It happens when molecules at the surface gain enough energy to overcome intermolecular forces and escape into the air. This process is typically endothermic, as it requires heat energy from the surroundings to provide the necessary energy for the molecules to transition into the gas phase.

Endothermic Processes

Endothermic processes are chemical or physical changes that absorb heat from their surroundings. In the case of evaporation, the liquid absorbs heat energy, which leads to a decrease in temperature of the remaining liquid. This is why evaporation can cool the surface of the liquid, making it feel cooler to the touch.

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence their physical properties, including boiling and melting points. In liquids, these forces must be overcome for molecules to escape into the gas phase during evaporation. The strength of these forces determines how easily a liquid can evaporate; weaker forces lead to faster evaporation rates.

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Related Practice

Textbook Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (d) Molten lava from a volcano turns into solid rock.

Textbook Question

The boiling points, surface tensions, and viscosities of water and several alcohols are listed in the following table:

b. How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 vs. 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol?

Textbook Question

Liquids can interact with flat surfaces just as they can with capillary tubes; the cohesive forces within the liquid can be stronger or weaker than the adhesive forces between liquid and surface:

(b) Which of these diagrams, i or ii, rep- resents what happens when water is on a nonpolar surface?

Textbook Question

Ethyl chloride (C₂H₅Cl) boils at 12 °C. When liquid C₂H₅Cl under pressure is sprayed on a room-temperature (25 °C) surface in air, the surface is cooled considerably. (a) What does this observation tell us about the specific heat of C₂H₅Cl(g) as compared with that of C₂H₅Cl(l)?