Problem 85 - Chemistry The Molecular Science

Question

Hexane (density \(=0.66 \mathrm{~g} / \mathrm{cm}^{3}\) ), perfluorohexane (density \(=1.669 \mathrm{~g} / \mathrm{cm}^{3}\) ), 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 \(0.97 \mathrm{~g} / \mathrm{mL}\) ), polyvinyl chloride (PVC, density = \(\left.1.36 \mathrm{~g} / \mathrm{cm}^{3}\right),\) and Teflon (density \(\left.=2.3 \mathrm{~g} / \mathrm{cm}^{3}\right) .\) None of these common plastics dissolves in these liquids. Describe what you expect to see.

Step-by-Step Solution

Answer

Hexane will be on top, water in the middle, and perfluorohexane at the bottom. HDPE floats on water, PVC on perfluorohexane, and Teflon sinks to the bottom.

Step 1: Understand the Densities

First, we need to understand the relative densities of the liquids and the solids involved. Hexane has a density of 0.66 g/cm³, perfluorohexane has a density of 1.669 g/cm³, and water has a density of 1.00 g/cm³. The solid objects are HDPE (0.97 g/cm³), PVC (1.36 g/cm³), and Teflon (2.3 g/cm³).

Step 2: Determine Liquid Layers

Based on their densities, these liquids will not mix and will form layers. The least dense liquid, hexane (0.66 g/cm³), will float on top. Water (1.00 g/cm³) will be the middle layer. The densest liquid, perfluorohexane (1.669 g/cm³), will sink to the bottom.

Step 3: Determine the Position of HDPE

HDPE (0.97 g/cm³) will float on water (1.00 g/cm³) but will sink in hexane (0.66 g/cm³). Therefore, HDPE will float on the water layer but below the hexane layer.

Step 4: Determine the Position of PVC

PVC (1.36 g/cm³) is denser than both hexane (0.66 g/cm³) and water (1.00 g/cm³), so it will sink through both those layers. However, it is less dense than perfluorohexane (1.669 g/cm³), so PVC will float on the perfluorohexane layer.

Step 5: Determine the Position of Teflon

Teflon (2.3 g/cm³) is denser than all of the liquids, including perfluorohexane (1.669 g/cm³), so it will sink to the bottom of the cylinder and rest at the base.

Key Concepts

Immiscible Liquids

When we talk about immiscible liquids, we refer to liquids that do not mix together. In this particular exercise, hexane, perfluorohexane, and water are examples of such liquids. They each have their distinct densities which prevent them from blending into a uniform mixture.
Immiscible liquids can be observed in daily life, such as oil and water. When you place these liquids in a container, they create separate layers. This distinct separation is essential for understanding how immiscible liquids behave in experiments.
Understanding immiscibility is crucial for many scientific processes:

• It helps in purifying substances by using separation techniques, such as using a separating funnel.
• Immiscible liquids play a critical role in processes like oil recovery and wastewater treatment.

In summary, immiscible liquids stay distinct due to their chemical properties, which do not support intermixing.

Density Comparison

Density is a measure of how much mass is contained within a specific volume. It is expressed in units like grams per cubic centimeter (g/cm³). In this exercise, understanding density helps us determine where each liquid and solid will position in the graduated cylinder.
Let’s review the densities:

• Hexane: 0.66 g/cm³
• Water: 1.00 g/cm³
• Perfluorohexane: 1.669 g/cm³
• HDPE: 0.97 g/cm³
• PVC: 1.36 g/cm³
• Teflon: 2.3 g/cm³

Density comparison enables prediction of whether a substance will float or sink in another. For instance:

• A less dense substance will always float on a denser one.
• This principle explains why hexane forms the top layer despite it not mixing with other liquids.

In practical applications, understanding density helps in material selection based on buoyancy needs, such as designing ships or predicting how substances interact.

Layer Formation in Liquids

Layer formation in immiscible liquids is governed by their differing densities. When these liquids are poured into a container, each forms a separate horizontal layer based on density. The least dense liquid rises to the top, while the densest settles at the bottom.
In our exercise:

• Hexane, being the least dense (0.66 g/cm³), sits atop the other liquids.
• Water, with a middle density (1.00 g/cm³), takes the center position.
• Perfluorohexane, the densest (1.669 g/cm³), settles at the base.

Solids such as HDPE, PVC, and Teflon also find their position based on their densities relative to the liquid layers. Floating and sinking depend on how their densities compare to the surrounding liquid:

• HDPE floats on the water layer but sinks in hexane.
• PVC floats on perfluorohexane.
• Teflon, being the densest among them, finds its place at the container’s very bottom.

This stratification is exploitable in various industrial and laboratory contexts where separation of components is desired.