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 beer.

Short answer

(a) Melting; Endothermic (b) Evaporation; Endothermic (c) Deposition; Exothermic (d) Condensation; Exothermic

Step-by-step solution

(a) Identify the phase transition and exothermic or endothermic process.

When ice is heated, it turns to water. This is a transition from solid to liquid, which is called melting. Melting requires energy to break the bonds between the water molecules in the solid state. Therefore, it is an endothermic process.

(b) Identify the phase transition and exothermic or endothermic process.

When wet clothes dry on a warm summer day, water in the liquid state evaporates into the gaseous state. This phase transition is called evaporation. Evaporation requires energy to overcome the intermolecular forces in the liquid state, making this an endothermic process.

(c) Identify the phase transition and exothermic or endothermic process.

Frost appearing on a window on a cold winter day is a result of water vapor in the air transitioning from the gas phase to the solid phase. This phase transition is called deposition. Deposition releases energy to the surroundings as water vapor molecules join together and form a solid, making this an exothermic process.

(d) Identify the phase transition and exothermic or endothermic process.

When droplets of water appear on a cold glass of beer, the water vapor in the air cools and turns into a liquid form, which is called condensation. Condensation occurs when gas molecules lose energy and come together to form a liquid. This transfer of energy from the gas to its surroundings makes the process exothermic.

Key concepts

Melting

Melting occurs when a substance changes from a solid to a liquid. This happens when heat is introduced to the solid material, such as ice, causing its molecules to vibrate with greater energy. As the temperature reaches the solid's melting point, enough energy is absorbed to break the bonds that hold its molecules in a fixed position.
This phase transition is known as endothermic because it requires energy intake to proceed. You can easily observe melting in everyday scenarios, such as ice cubes turning to water in a drink or chocolate left in a warm place becoming gooey.

• Key Example: Melting of ice into water.
• Energy Type: Endothermic (energy absorbed).
• Bond Type: Bonds in solid break for molecules to move freely in liquid state.

Evaporation

Evaporation involves the transition from a liquid to a gaseous state. This process typically occurs at the surface of a liquid, where molecules with enough energy escape into the air as vapor. Unlike the boiling process, evaporation happens at temperatures below the boiling point of the liquid.
Energy input is necessary for evaporation to take place, making it an endothermic transition. This is because the molecules must overcome intermolecular forces that hold them in the liquid state. A practical example of evaporation is wet clothes drying on a warm day.

• Key Example: Drying of wet clothes on a sunny day.
• Energy Type: Endothermic (energy absorbed).
• Process Occurrence: Occurs at the liquid surface, requiring sufficient energy to overcome intermolecular forces.

Deposition

Deposition is the process where gas transforms directly into a solid without passing through the liquid phase. It is essentially the reverse of sublimation. Deposition releases energy into the surroundings as molecules form a solid, joining swiftly and neatly due to loss of kinetic energy.
This makes deposition an exothermic process. Commonly seen in nature, this phase change is responsible for frost forming on cold surfaces from water vapor, such as on windows during frosty mornings.

• Key Example: Formation of frost on a window.
• Energy Type: Exothermic (energy released).
• Phase Transition: Direct gas to solid formation.

Condensation

Condensation is the transition from a gas to a liquid and occurs when gas molecules lose energy and slow down, allowing them to stick together to form liquid droplets. When water vapor in the air encounters a cooler surface, it transfers heat energy to the cooler environment, changing phase in the process.
This release of energy to the surroundings characterizes condensation as an exothermic process. A typical example is the foggy appearance on the outside of a cold glass, where moisture in the air condenses into liquid water.

• Key Example: Water droplets forming on a cold glass.
• Energy Type: Exothermic (energy released).
• Occurrence: Gas molecules release energy to condense into a liquid form.