An example of a substance that does not change directly from solid to gas when heated is

Analysis

The question asks us to identify which of the listed substances does not undergo sublimation. Sublimation is the process where a substance changes directly from a solid state to a gaseous state without passing through the liquid phase.

Concept Explanation

• Sublimation: Substances that sublime have weak intermolecular forces, allowing them to transition directly to gas upon heating. Common examples include Ammonium Chloride ($\text{NH}_4\text{Cl}$), Iodine ($\text{I}_2$), and Naphthalene.
• Thermal Decomposition: Some substances, when heated, do not sublime but instead undergo a chemical reaction where they break down into simpler substances. This is common with carbonates.

Step-by-Step Solution

1. Analyze Option A ($\text{NH}_4\text{Cl}$): Ammonium chloride is a classic example of a substance that sublimes. When heated, it turns directly into ammonia and hydrogen chloride gases.
2. Analyze Option B ($\text{I}_2$): Iodine crystals are well-known for subliming. When heated, they produce a characteristic purple vapor directly from the solid state.
3. Analyze Option C ($\text{CaCO}_3$): Calcium carbonate (limestone) does not sublime. Instead, when heated strongly, it undergoes thermal decomposition to produce calcium oxide (quicklime) and carbon(IV) oxide gas: $$\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)$$Because it forms a new solid product ($\text{CaO}$) rather than turning entirely into gas, it is not a subliming substance.
4. Analyze Option D ($\text{S}_8$): Sulfur can sublime under certain conditions, though it also melts. However, compared to $\text{CaCO}_3$, it is much more associated with sublimation in chemistry curricula.

Conclusion

Since $\text{CaCO}_3$ undergoes chemical decomposition rather than a physical phase change (sublimation), it is the correct answer.

Exam Tip

When you see questions about "sublimation" in JAMB Chemistry, look out for these common "sublimers": Iodine ($\text{I}_2$), Ammonium Chloride ($\text{NH}_4\text{Cl}$), Naphthalene, and Dry Ice ($\text{CO}_2$). If you see a carbonate like $\text{CaCO}_3$, remember it almost always decomposes into an oxide and $\text{CO}_2$ gas when heated!