A sample of any of the following substances disappears after some time when exposed to air like camphor, naphthalene, or dry ice. This phenomenon is called?

Analysis

The question asks for the name of the physical process where specific solid substances (camphor, naphthalene, and dry ice) transition directly into a gaseous state when exposed to air, without passing through the liquid phase.

Concept Explanation

In chemistry, matter typically exists in three states: solid, liquid, and gas. Usually, a solid must melt into a liquid before it can evaporate into a gas. However, some substances have unique properties where the intermolecular forces are weak enough that the solid can transition directly into a gas when heat is absorbed. This process is known as sublimation.

• Sublimation: The direct change of state from solid to gas (e.g., camphor, naphthalene/mothballs, and dry ice/solid $CO_2$).
• Evaporation: The process where a liquid turns into a gas at a temperature below its boiling point.
• Diffusion: The movement of particles from an area of higher concentration to an area of lower concentration.
• Radiation: The transfer of energy through electromagnetic waves.

Step-by-Step Solution

1. Identify the substances: Camphor, naphthalene, and dry ice are all solids at room temperature.
2. Observe the behavior: When left in the open, these substances "disappear" or shrink in size without leaving any liquid residue behind.
3. Apply the definition: Since they bypass the liquid state entirely, the process cannot be evaporation. Because they are changing state due to heat absorption from the environment, the process is defined as sublimation. 4.

Exam Tip

Whenever you see questions involving "mothballs" (naphthalene), "camphor," or "dry ice" disappearing, immediately think of sublimation. These are the classic examples used in JAMB and WASSCE chemistry exams to test your knowledge of changes of state.