Problem 105 - Chemistry The Molecular Science

Question

Water does not mix with either benzene or bromobenzene when it is stirred together with either of them, but benzene and bromobenzene do mix. (a) If you pour \(2 \mathrm{~mL}\) bromobenzene into a test tube, then add \(2 \mathrm{~mL}\) water and stir, what would the test tube look like a few minutes later? (b) Suppose you add \(2 \mathrm{~mL}\) benzene to the test tube in part (a), pouring the benzene carefully down the side of the tube so that the liquids do not mix. Describe the appearance of the test tube now. (c) If the test tube containing all three liquids is thoroughly shaken and then allowed to stand for five minutes, what will the tube look like?

Step-by-Step Solution

Answer

(a) Two layers: bromobenzene (bottom) and water (top). (b) Three layers: bromobenzene (bottom), water, and benzene (top). (c) Two layers: water (bottom) and mixed benzene and bromobenzene (top).

Step 1: Bromobenzene and Water Immiscibility

When bromobenzene is mixed with water, they do not mix because they are immiscible. Bromobenzene is denser than water, so it will form the bottom layer in the test tube. After stirring, you would see two clear layers: bromobenzene at the bottom and water on top.

Step 2: Adding Benzene to the Mixture

When benzene is carefully poured into the test tube containing water above bromobenzene, it will form the top layer since benzene is less dense than both water and bromobenzene. Now you will have three distinct layers in the test tube: bromobenzene at the bottom, then water, and benzene at the top.

Step 3: Shaking the Test Tube

Upon shaking the test tube containing benzene, bromobenzene, and water, bromobenzene and benzene will mix because they are miscible. Water remains separate from this mixture. Once settled, two layers will form: a bottom layer of water and a top layer of the mixed benzene and bromobenzene.

Key Concepts

Bromobenzene

Bromobenzene is an organic compound, often used in laboratory settings and chemical engineering. It consists of a benzene ring with one bromine atom attached to it. This gives it unique chemical properties. One of these properties is its density. Bromobenzene is denser than water, with a density of about 1.495 g/mL compared to water's 1.0 g/mL. This means that when bromobenzene is mixed with water, it tends to sink, creating a distinct layer at the bottom in a container like a test tube.

Additionally, bromobenzene is immiscible with water. Immiscibility refers to the inability of two liquids to form a homogeneous mixture. When you try to mix water and bromobenzene, you will find they separate quickly into two layers. Bromobenzene will settle at the bottom due to its higher density, while water will float above it.

Benzene

Benzene is a simple aromatic hydrocarbon composed of a six-carbon ring with alternating single and double bonds. It's a clear, flammable liquid at room temperature and is less dense than both bromobenzene and water. With a density of about 0.879 g/mL, benzene will float above these two when present in the same container.

Benzene has a unique ability to mix well with other organic compounds due to its nonpolar nature. This means benzene can easily dissolve in other hydrocarbons and similar organic molecules like bromobenzene. When mixed with bromobenzene in the presence of water, benzene combines with bromobenzene to form a single layer, excluding water because benzene and water are immiscible.

• Benzene helps to demonstrate the principles of liquid immiscibility when used in experiments.
• Understanding benzene's behavior is crucial for students studying chemistry, especially when dealing with organic reactions and solvent properties.

Density of Liquids

The concept of density is critical for understanding how liquids will arrange themselves when combined in a container. Density itself is a measure of mass per unit volume, often expressed in grams per milliliter (g/mL) for liquids. When dealing with multiple liquids, their relative densities will determine their positioning in a mixture.

In the exercise, bromobenzene, water, and benzene exhibit distinct behaviors due to their differing densities:

• Bromobenzene: This liquid has the highest density, causing it to settle at the bottom.
• Water: With a density in between the two organics, water forms a middle layer above bromobenzene but below benzene.
• Benzene: Being the least dense of the three, benzene floats to the top layer.

When these liquids are thoroughly shaken and allowed to settle, the densities again play a key role in reestablishing their order. Understanding these principles can aid in predicting the behavior of miscible and immiscible liquids.