Problem 43 - Chemistry and Chemical Reactivity

Question

Hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}, \text { density }=0.766 \mathrm{g} / \mathrm{cm}^{3}\right),\) perfluorohexane \(\left(\mathrm{C}_{6} \mathrm{F}_{14}, \text { density }=1.669 \mathrm{g} / \mathrm{cm}^{3}\right),\) and water are immiscible liquids; that is, they do not dissolve in one another. You place \(10 \mathrm{mL}\) of each in a graduated cylinder, along with pieces of high-density polyethylene (HDPE, density = 0.97 g/cm^{3 ), poly- } vinyl chloride (PVC, density = \(1.36 \mathrm{g} / \mathrm{cm}^{3}\) ), and Teflon (density \(=2.3 \mathrm{g} / \mathrm{cm}^{3}\) ). None of these common plastics dissolves in these liquids. Describe what you expect to see.

Step-by-Step Solution

Answer

Hexane on top, followed by water, and perfluorohexane at the bottom; HDPE between hexane and water, PVC between water and perfluorohexane, and Teflon at the bottom.

Step 1: Understand Density and Immiscibility

The densities of hexane (0.766 g/cm³), perfluorohexane (1.669 g/cm³), and water (approximately 1 g/cm³) will determine their positions in the graduated cylinder because the liquids are immiscible, meaning they do not mix. Hexane, being the least dense, will float on top, followed by water, and perfluorohexane will settle at the bottom.

Step 2: Predict the Plastic Pieces' Positions

The positions of the plastic pieces—HDPE, PVC, and Teflon—are determined by comparing their densities with those of the liquids. HDPE (0.97 g/cm³) will float on water but sink below hexane. PVC (1.36 g/cm³), being denser than water but less dense than perfluorohexane, will float on perfluorohexane but sink in water. Teflon (2.3 g/cm³), being denser than all liquids, will sink to the bottom of the graduated cylinder.

Key Concepts

Immiscibility

In chemistry, immiscibility refers to the inability of two or more liquids to mix and form a homogeneous mixture. This is a fundamental characteristic that affects how liquids interact with each other.

• Immiscible liquids, such as hexane, perfluorohexane, and water, form distinct layers when placed together.
• This behavior arises due to differences in the molecular structure and polarity of the substances involved. Nonpolar substances, like hexane, tend not to mix with polar substances, such as water, due to the lack of attractive forces between them.
• Each immiscible liquid displays its own unique physical properties, including density, which influences its position when placed with other immiscible liquids in a container, like a graduated cylinder.

Understanding immiscibility helps predict the order and layering of substances based on their inherent properties.

Relative Density

Relative density is the comparison of the density of one substance with another. In the case of immiscible liquids, understanding relative density allows us to predict which liquid will float or sink when placed together.

• Density is defined as mass per unit volume, and it determines whether a substance will float or sink in another substance.
• Hexane, with a density of 0.766 g/cm³, is less dense than water (around 1 g/cm³) and therefore floats on its surface.
• Perfluorohexane has a density of 1.669 g/cm³, making it denser than both water and hexane, causing it to settle at the bottom.

Utilizing these principles of relative density, we can arrange substances logically and accurately predict the layering within a graduated cylinder.

Liquid-Liquid Interactions

Liquid-liquid interactions describe how different liquids behave and arrange themselves when they come into contact without mixing. This phenomenon aids in understanding the behavior of mixtures like the one described in the exercise.

• The interplay of forces, including van der Waals forces, hydrogen bonding, and dipole interactions, influences the miscibility of liquids.
• If the intermolecular forces between molecules of different liquids are weaker than those within each liquid, they remain distinct, leading to a layered configuration.
• The structure of molecules, whether polar or nonpolar, significantly affects these interactions, as seen in the behavior of hexane, water, and perfluorohexane.

Comprehending liquid-liquid interactions enriches our understanding and ability to predict how substances will combine and orient themselves within a mixed environment.