Question

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.

Short answer

The most likely reason oil and water are immiscible is (b) Oil molecules are composed mostly of carbon and hydrogen. This is because the nonpolar nature of oil molecules (primarily composed of carbon and hydrogen) and the polar nature of water molecules result in different types of molecular interactions that prevent them from mixing.

Step-by-step solution

Analyze each answer choice

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.

Determine the best answer and explain the reasoning

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.

Key concepts

Molecular Interactions

Molecular interactions play a crucial role in determining whether different substances will mix or remain separate. These interactions are primarily governed by the polarity of the molecules, which refers to the distribution of electrical charges across a molecule.

Polar molecules, like water, exhibit an unequal distribution of charge, with one part of the molecule being slightly positive and the other slightly negative. This charge separation allows polar molecules to engage in strong dipole-dipole interactions or hydrogen bonding with each other.

In contrast, nonpolar molecules, such as the hydrocarbons in oil, have an even charge distribution and interact through weaker forces called van der Waals forces. This disparity in interaction strengths leads to a preference for polar molecules to cluster together and nonpolar molecules to do the same, preventing them from mixing uniformly. Thus, understanding molecular interactions is key to explaining why substances are immiscible.

Immiscibility

Immiscibility is the term used to describe the inability of two substances to mix and form a homogeneous solution. This commonly observed phenomenon between oil and water is often caused by differences in polarity.

The phrase "like dissolves like" helps to understand this concept better: polar substances tend to mix well with other polar substances, while nonpolar substances mix well with other nonpolar substances. In the case of oil and water, the oil's nonpolar nature, stemming from its hydrocarbon composition, doesn't favor mixing with water's polar molecules.

Besides polarity, other factors such as molecular size and temperature might affect miscibility, but they are secondary to the fundamental intermolecular forces at play. For students tackling problems related to this concept, focusing on the differences in molecular polarity is key to understanding and predicting immiscibility.

Hydrocarbons

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. They are largely nonpolar, making them a primary example of substances that do not readily mix with water.

The vast majority of hydrocarbons are hydrophobic, meaning they repel water. This quality is due to their saturated nature, where carbon and hydrogen atoms form stable, nonpolar bonds.

Hydrocarbons can be found in a variety of forms, from simple structures like methane (CH₄) to complex ones like benzene (C₆H₆). Despite their structural diversity, hydrocarbons share the common trait of being immiscible with water, which is crucial for applications ranging from biological processes to industrial applications.

• For example, the waterproof nature of hydrocarbons is employed in packaging materials.
• In biological systems, they contribute to cell membrane structures, influencing how cells interact with their environment.

Understanding the characteristics of hydrocarbons assists in our knowledge of their behavior and applications.