Problem 77 - Introductory Chemistry Atoms First

Question

Oil is insoluble in water. Which step in the threestep process for dissolving is responsible for this? Explain why this step is the problem.

Step-by-Step Solution

Answer

The step responsible for oil being insoluble in water is the third step in the dissolving process: combining the solute (oil) and solvent (water) particles to form a solution. This is because of the different polarities of oil and water molecules. Oil is non-polar and water is polar, and due to the "like-dissolves-like" principle, they do not mix well, preventing the formation of a homogeneous solution.

Step 1: Understanding the dissolving process

The process of dissolving involves three steps: 1. Separating the solute particles from each other. 2. Separating the solvent particles from each other. 3. Combining the solute and solvent particles to form a solution. In the case of oil and water, oil is the solute and water is the solvent.

Step 2: Analyzing interactions between oil and water molecules

To determine which step is responsible for oil being insoluble in water, we need to understand the interactions between oil and water molecules. Oil is a non-polar molecule and is composed of hydrophobic particles, while water is a polar molecule and is composed of hydrophilic particles. This means that oil particles and water particles have different chemical properties.

Step 3: Identify the step responsible for oil being insoluble in water

Now that we know the interactions between oil and water molecules, we can identify the step responsible for the insolubility. Since oil is non-polar and water is polar, the problem lies in the third step of the dissolving process: combining the solute and solvent particles to form a solution. The "like-dissolves-like" principle states that polar substances tend to dissolve other polar substances and non-polar substances tend to dissolve other non-polar substances. Because oil and water have different polarities, they do not mix well, and oil is unable to dissolve in the water. Thus, the step responsible for oil being insoluble in water is:

Step 3: Combining the solute and solvent particles to form a solution

This step is the problem because oil and water have different polarities. Due to the "like-dissolves-like" principle, oil particles do not mix well with water particles, preventing the formation of a homogeneous solution.

Key Concepts

Solute and Solvent Interactions

The process of dissolving a solute in a solvent involves several interactions between their particles. In essence, the success of the dissolving process is determined by the compatibility of these interactions.
At a molecular level, to dissolve a solute, the solvent must effectively separate and surround the solute particles. These interactions between solute and solvent are crucial because they determine if a solution will form.

• Firstly, the solute particles must separate from each other.
• Secondly, the solvent particles must also separate to make room for the solute.
• Finally, the solute and solvent must mix, allowing solvent particles to surround and interact with solute particles.

If the interactions are favorable – meaning the solvent particles are attracted to the solute particles – a solution forms. This principle is guided by the "like-dissolves-like" rule, where polar solutes dissolve in polar solvents and non-polar ones in non-polar solvents.

Polar and Non-Polar Substances

Polar and non-polar descriptors are essentially the resulting characteristics of molecule types based on their electron distribution. Polarity in molecules depends on the differences in electronegativity between atoms in a molecule and their geometric arrangement.

• Polar molecules have an uneven charge distribution. Water, for instance, has a bent shape and a difference in electronegativity between hydrogen and oxygen, giving it a positive and negative pole.
• Non-polar molecules, like oil, have a more even charge distribution and are often symmetrical, meaning electronegativities between atoms are balanced.

This difference in character is why polar and non-polar substances are generally not compatible. Polar molecules will be attracted to each other due to their charged poles, while non-polar molecules lack such attractions. This is why oil, a non-polar substance, and water, a polar substance, do not mix well.

Insolubility of Oil in Water

One of the most common examples of insoluble substances is oil in water. This phenomenon is mainly due to the differing polarities.
Within the context of the dissolving process, the incompatibility arises in the final step: when combining the solute (oil) with the solvent (water). The key problem here is their different polarities.

• Water molecules are polar and form hydrogen bonds with each other. As such, they preferentially interact with other polar molecules or ions.
• Oil molecules are non-polar and can only dispense other non-polar substances.

When oil droplets are introduced into water, water molecules tend to cling to one another rather than interact with the oil droplets. This leads to oil not dissolving but rather forming separate layers or droplets on the water's surface. This lack of interaction is a classic demonstration of the principle "like dissolves like." Thus, the third step in the dissolving process can't occur effectively due to these incompatible solute and solvent interactions, resulting in the well-observed phenomenon of oil being insoluble in water.