A small quantity of solid ammonium chloride (NH4Cl) heated gently in a test tube, the solid gradually…

Analysis of the Question

The question describes a process where a solid substance (ammonium chloride) is heated and changes directly into gases without passing through a liquid state. Upon cooling, these gases recombine to form a solid deposit on the cooler parts of the test tube. We need to identify the specific physical process occurring here.

Concept Explanation

The phenomenon described is sublimation.

• Sublimation is the process where a solid changes directly into a gas upon heating, without becoming a liquid first.
• When the gas cools down, it undergoes deposition (the reverse of sublimation), turning back into a solid.
• Ammonium chloride ($NH_4Cl$) is a classic example of a substance that sublimes. When heated, it dissociates into ammonia ($NH_3$) and hydrogen chloride ($HCl$) gases, which then recombine as they cool to form the white solid deposit.

Step-by-Step Solution

1. Identify the change of state: The solid disappears (turns to gas) upon heating. This eliminates precipitation (which involves forming a solid from a solution) and evaporation (which involves a liquid turning into a gas).
2. Evaluate the options: • Distillation: This involves boiling a liquid and condensing the vapor. Since we started with a solid, this is incorrect. • Sublimation: This matches the definition of a solid turning directly into a gas and back to a solid. • Precipitation: This occurs in liquid solutions, not by heating a dry solid. • Evaporation: This is the conversion of a liquid to a gas.
3. Conclusion: The observation of the solid disappearing and reappearing as a white cloud on the cooler part of the test tube is the textbook definition of sublimation.

Exam Tip

Whenever you see questions involving substances like Ammonium Chloride ($NH_4Cl$), Iodine ($I_2$), Naphthalene (mothballs), or Camphor, immediately think of sublimation. These are the most common substances used in JAMB/WASSCE questions to test this concept.