Oil and water are immiscible. Which is the most likely reason? (a) Oil molecules are denser than water. (b) Oil molecules are composed mostly of carbon and hydrogen. (c) Oil molecules have higher molar masses than water. (d) Oil molecules have higher vapor pressures than water. (e) Oil molecules have higher boiling points than water.

Let's analyze each answer choice and compare their relevance to the immiscibility of oil and water. (a) Oil molecules are denser than water. While it is true that oil typically has a lower density than water, this alone does not explain why the two substances are immiscible. Instead, it is the molecular interactions that determine whether two substances mix well or not. (b) Oil molecules are composed mostly of carbon and hydrogen. Being composed of carbon and hydrogen atoms is a characteristic of hydrocarbon-based substances like oil. However, it's not the composition itself that matters, but rather the nature of the molecular interactions. (c) Oil molecules have higher molar masses than water. While oil molecules do often have higher molar masses than water molecules, this does not inherently cause immiscibility. Molecular size and weight are factors that can contribute to the observed macroscopic properties but are not the primary causes of immiscibility between oil and water. (d) Oil molecules have higher vapor pressures than water. Vapor pressure refers to the pressure exerted by a vapor in equilibrium with its liquid phase. Oil and water do have different vapor pressures, but this difference is not the primary cause of immiscibility. (e) Oil molecules have higher boiling points than water. Boiling point is a physical property that depends on the strength of molecular interactions in a substance. While it is true that oil molecules have higher boiling points than water molecules, this alone does not explain the immiscibility between the two substances.

Based on the analysis above, we find that choice (b) is the most relevant answer to the question, "Which is the most likely reason oil and water are immiscible?" The reason is that oil molecules are primarily nonpolar, while water molecules are polar. Nonpolar molecules, like oil (which is composed mainly of carbon and hydrogen), do not have significant charge separation and interact through weak van der Waals forces. In contrast, polar molecules like water experience stronger interactions due to their uneven charge distribution. This difference in molecular interactions is the primary reason for immiscibility, as a substance's preference to interact with molecules of similar polarity results in a lack of mixing between polar and nonpolar substances. Therefore, the best answer is: (b) Oil molecules are composed mostly of carbon and hydrogen.