Question

Name the phase change in each of these events: (a) A diamond film forms on a surface from gaseous carbon atoms in a vacuum. (b) Mothballs in a bureau drawer disappear over time. (c) Molten iron from a blast furnace is cast into ingots ("pigs").

Short answer

(a) Deposition, (b) Sublimation, (c) Solidification

Step-by-step solution

Identify phase change in event (a)

In this event, gaseous carbon atoms in a vacuum are converting into a solid diamond film. This is an example of deposition. Deposition is the phase change where a gas changes directly into a solid.

Identify phase change in event (b)

Mothballs disappearing over time is due to the solid phase of the substance changing directly into a gas. This phase change is known as sublimation.

Identify phase change in event (c)

Molten iron being cast into ingots involves the iron changing from a liquid state to a solid state as it cools. This phase change is called solidification (or freezing).

Key concepts

Deposition

Deposition is a fascinating phase change where a gas transforms directly into a solid, skipping the liquid phase entirely. In the exercise, the formation of a diamond film from gaseous carbon atoms in a vacuum is an example of deposition. This process is the reverse of sublimation. One way to observe deposition is to look at frost forming on surfaces during cold weather. Here, water vapor in the air turns directly into ice without becoming liquid first. Deposition is crucial in natural processes and industrial applications, including creating thin films for electronics and other technologies. Understanding this phase change helps us control and harness various materials more effectively.

Sublimation

Sublimation occurs when a solid changes directly to a gas without passing through the liquid phase. This can be seen when mothballs disappear over time in a drawer. The mothballs, typically made of naphthalene or paradichlorobenzene, slowly transition from solid to gas, releasing a vapor that repels moths. Another common example of sublimation is dry ice, which is solid carbon dioxide. At room temperature, dry ice doesn't melt into a liquid but rather turns directly into carbon dioxide gas. Sublimation is widely used in freeze-drying, which preserves food by removing water content directly from the frozen state.

Solidification

Solidification, also known as freezing, is the process where a liquid turns into a solid. In the exercise, molten iron being cast into ingots is an illustration of solidification. When the hot, liquid iron is poured into molds, it cools and solidifies into a solid metallic structure. Solidification is not limited to metals but applies to many other substances, like water freezing into ice. This phase change is crucial in manufacturing and materials science. By controlling the cooling rate, one can influence the properties of the final solid, making solidification a key process in producing various components and materials.