Problem 58 - Chemistry and Chemical Reactivity

Question

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

Step-by-Step Solution

Answer

Liquids form layers: hexane, water, perfluorohexane. HDPE floats on water; PVC floats on perfluorohexane. Teflon sinks to the bottom.

Step 1: Identify the Density of Each Liquid

For each substance, identify its density: Hexane with a density of 0.766 g/cm³, perfluorohexane with a density of 1.669 g/cm³, and water with a density of approximately 1.0 g/cm³.

Step 2: Layering of Liquids Based on Density

Arrange the liquids in order of increasing density. The least dense liquid, hexane (0.766 g/cm³), will be at the top, water (1.0 g/cm³) will be in the middle, and perfluorohexane (1.669 g/cm³) will be at the bottom.

Step 3: Determine the Buoyant Behavior of HDPE

Compare the density of HDPE (0.97 g/cm³) with the densities of the liquids. HDPE will float on the water layer since water's density is greater than HDPE’s density, but it will sink in hexane.

Step 4: Determine the Buoyant Behavior of PVC

Check PVC's density (1.36 g/cm³) compared to the densities of the liquids. PVC will sink in both hexane and water because its density is greater than both, but it will float on perfluorohexane.

Step 5: Determine the Buoyant Behavior of Teflon

Compare Teflon's density (2.3 g/cm³) with those of the liquids. Since Teflon's density is greater than all of them, it will sink through all the layers and rest at the bottom of the graduated cylinder.

Key Concepts

Buoyancy

Buoyancy is a simple but important concept. It refers to the ability of an object to float in a fluid, which can be a liquid or a gas. Buoyancy is determined by the relationship between the object's density and the fluid's density. If an object is less dense than the fluid it is in, it will float. However, if it is more dense, it will sink. This principle is why boats float on water and why some objects sink.
In our exercise, high-density polyethylene (HDPE) shows buoyant properties when placed in water, because its density (0.97 g/cm³) is less than that of water (approximately 1.0 g/cm³). However, it sinks in hexane due to hexane being even less dense than HDPE. Understanding buoyancy helps in various practical applications, from designing ships to engineering submarines.

Immiscible Liquids

Immiscible liquids are those that do not mix or dissolve into one another. Instead, they form distinct layers based on their densities when placed together. This happens due to conflicting polarities and different chemical properties, leading to segregation into separate layers.
In our example, hexane, water, and perfluorohexane are immiscible. When poured into a graduated cylinder, they naturally form layers according to their densities, with hexane on top (density of 0.766 g/cm³), water in the middle (density of around 1.0 g/cm³), and perfluorohexane at the bottom (density of 1.669 g/cm³). Mixing them does not combine them into a single phase, but rather causes temporary turbulence that eventually settles back into the layered order.

Chemical Properties

Chemical properties help determine how substances interact with each other. They include properties like reactivity, flammability, and acidity. For liquids in our exercise, their chemical properties affect how they interact, resulting in being immiscible.
For example, hexane and perfluorohexane are both hydrocarbons, but have vastly different polarities compared to water, making them immiscible. Their resistance to mixing stems from their molecular structure, where non-polar molecules like hexane don't mix with polar molecules like water. Understanding these properties sheds light on why some substances repel each other, maintaining their unique characteristics even when in close contact.

Plastic Materials

Plastic materials, such as HDPE, PVC, and Teflon, have unique densities and properties that define their behavior in different situations. In our exercise, these plastics do not dissolve in the liquids due to their distinct chemical stability.

• HDPE: With a density of 0.97 g/cm³, HDPE floats in water but sinks in hexane.
• PVC: Having a density of 1.36 g/cm³, PVC sinks in both hexane and water layers but can float on perfluorohexane.
• Teflon: This material is denser than all three liquids, at 2.3 g/cm³, causing it to sink through and settle at the bottom of the graduated cylinder.

These plastics' non-reactivity with the liquids makes them durable, commonly used materials in various industries, from packaging to insulation.