Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) When ice is heated, it turns to water. (b) Wet clothes dry on a warm summer day. (c) Frost appears on a window on a cold winter day. (d) Droplets of water appear on a cold glass of lemonade.

Short answer

(a) Melting; endothermic. (b) Evaporation; endothermic. (c) Deposition; exothermic. (d) Condensation; exothermic.

Step-by-step solution

(a) When ice is heated, it turns to water.

The process of ice turning into water is called melting. Since energy is required to turn the solid ice into liquid water, this phase transition is endothermic.

(b) Wet clothes dry on a warm summer day.

When wet clothes dry, the water in them evaporates. The process of liquid water turning into water vapor is called evaporation. Evaporation is an endothermic process because it requires energy, which in this case comes from the warm surrounding air.

(c) Frost appears on a window on a cold winter day.

Frost forming on a window is the result of water vapor from the surrounding air turning into a solid, which is known as deposition. Deposition is the direct phase change from gas to solid, skipping the liquid phase. Since energy is released during this phase change, it is an exothermic process.

(d) Droplets of water appear on a cold glass of lemonade.

When droplets appear on a cold lemonade glass, it is due to the moisture from the warm air surrounding the glass condensing onto its cool surface. This process of water vapor turning into liquid drops on the glass is called condensation. Condensation is an exothermic process because energy is being released as the water vapor turns into liquid droplets.

Key concepts

Melting

When we observe a scoop of ice cream left out on a hot day, we witness the process of melting. This phenomenon occurs when solid matter, like ice, absorbs heat from its surroundings and transitions into a liquid. In scientific terms, melting is an endothermic process. This means it requires an input of energy to break the structured bonds that hold the solid's molecules rigidly in place.

During the melting of ice, the energy absorbed is used to weaken the hydrogen bonds between water molecules in the ice. As these bonds loosen, the molecules move more freely, and the ice transitions into water. Since this phase change involves the absorption of heat, we classify it as an endothermic reaction according to the principles of thermodynamics.

Evaporation

Playing outside on a balmy day, you may notice how a puddle of water slowly disappears. That's evaporation in action, a process where liquid water is transformed into water vapor, a gas. Like melting, evaporation is also an endothermic process because it requires heat energy to occur.

However, evaporation doesn't need to reach a boiling point to take place. It occurs at any temperature when molecules at the surface of the liquid gain enough energy to break away and enter the atmosphere as gas. This can be facilitated by factors like warm temperatures, airflow, and low humidity, explaining why a breeze or the warmth of the sun can help wet clothes dry quicker. Hence, when those clothes hang on the line on a warm summer day, evaporation is actively whisking moisture away.

Deposition

Imagine a frosty windowpane on a chilly morning. Here you're seeing deposition, which is essentially the inverse of sublimation. In deposition, gas transitions directly into a solid without becoming a liquid first. A familiar example is frost, where water vapor in the air turns directly into ice crystals on a cold surface.

This process is exothermic, meaning it gives off energy. As the water vapor, which has higher energy due to its gaseous state, comes into contact with a surface cooled by freezing temperatures, it loses energy rapidly and adheres as ice. The energy released might not always be noticeable in the way we feel warmth from a fire, but it's significant in the context of molecular energy exchange and the never-ending dance of matter changing states.

Condensation

Picture a cold soda can on a humid day, beaded with water droplets. This scenario illustrates condensation, where water vapor in the air changes into liquid water. Condensation is the opposite of evaporation and is also an exothermic process. As gas cools, it can't hold as much energy, and molecules slow down and cluster together.

This clustering leads to liquid, which is denser and has less energy than gas. That's why you often see condensation on cooler surfaces, like that soda can or a bathroom mirror during a hot shower. The droplets form because the cooler surface prompts the warm, moist air to lose thermal energy and transition to a liquid. The ubiquity of this process in daily life—from dewy grass in the morning to foggy car windows—demonstrates how temperature and energy transfers are cornerstones of atmospheric interactions.