Question

Explain the following observations: (a) The surface tension of \(\mathrm{CHBr}_{3}\) is greater than that of \(\mathrm{CHCl}_{3} .\) (b) As temperature increases, oil flows faster through a narrow tube. (c) Raindrops that collect on a waxed automobile hood take on a nearly spherical shape. (d) Oil droplets that collect on a waxed automobile hood take on a flat shape.

Short answer

(a) CHBr3 has a greater surface tension than CHCl3 due to its larger, heavier bromine atoms, which result in stronger dispersion forces in the molecule. (b) Oil flows faster through a narrow tube as temperature increases because the increase in kinetic energy weakens its intermolecular forces, resulting in decreased viscosity. (c) Raindrops on a waxed automobile hood form a nearly spherical shape due to the surface tension of water, which minimizes the droplet's surface area to achieve a lower energy state. (d) Oil droplets on a waxed automobile hood form a flat shape due to the difference in intermolecular forces between oil and water, causing the oil to spread out and maximize contact with the hydrophilic waxed surface.

Step-by-step solution

(a) Comparing surface tension of CHBr3 and CHCl3

Surface tension is a property of liquids that arises due to the unbalanced forces at the surface molecules. In order to compare the surface tension of CHBr3 and CHCl3, we need to consider the strength of the intermolecular forces between the molecules of the two compounds. CHBr3 has larger, heavier bromine atoms as compared to the smaller, lighter chlorine atoms in CHCl3. Larger halogen atoms have more electrons and thus can have stronger dispersion forces. Due to the stronger intermolecular forces in CHBr3, it has a greater surface tension than CHCl3.

(b) Viscosity of oil and temperature

Viscosity is a measure of a fluid's resistance to flow, and it is influenced by factors such as temperature and intermolecular forces. As the temperature of the oil increases, the kinetic energy of its particles increases, leading to the weakening of the intermolecular forces between them. This results in a decrease in the oil's viscosity, allowing it to flow faster through the narrow tube as the temperature increases.

(c) Spherical shape of raindrops on a waxed automobile hood

When raindrops fall on a waxed automobile hood, they form a nearly spherical shape. This occurs due to the surface tension of water, which causes the water molecules to attract each other and minimize the surface area of the droplet. A sphere has the least surface area for a given volume, so the surface tension forces the water droplet to assume a spherical shape to minimize its surface area and achieve a lower energy state.

(d) Flat shape of oil droplets on a waxed automobile hood

Oil droplets on a waxed automobile hood take on a flat shape due to the difference in the intermolecular forces between oil and water. Oil is hydrophobic and does not mix with water; therefore, it has weaker intermolecular forces with water or other hydrophilic substances like the wax on the automobile hood. These weaker intermolecular forces make it more difficult for oil to form a spherical shape like water droplets and instead cause it to spread out into a flatter shape on the waxed surface, maximizing the contact between oil and wax.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles, such as molecules, atoms, or ions. These forces govern many physical properties of substances, such as their boiling point, melting point, and surface tension.
When comparing two substances like \( \text{CHBr}_3 \) and \( \text{CHCl}_3 \), the difference in their intermolecular forces is due to the size and mass of the atoms involved.

• Larger atoms, like bromine in \( \text{CHBr}_3 \), provide more electrons that can contribute to dispersion forces, which are a type of weak intermolecular force.
• Dispersion forces are stronger in heavier and larger atoms, making the surface tension of \( \text{CHBr}_3 \) greater than that of \( \text{CHCl}_3 \).

Understanding the role of intermolecular forces helps explain why certain substances have higher or lower surface tensions.

Viscosity

Viscosity is a measure of a fluid's resistance to flow. It essentially describes how 'thick' or 'thin' a fluid is. Thick honey has a high viscosity, while thin water has a low viscosity.
Several factors can influence viscosity, including:

• **Temperature**: As the temperature of a fluid increases, the particles inside it gain kinetic energy and move faster.
• This movement weakens the intermolecular forces, reducing the viscosity of the fluid.

This is why oil, for example, flows faster when it's heated—the increase in temperature allows the oil molecules to overcome the forces holding them together more easily.

Shape of Raindrops

The shape of raindrops is influenced notably by surface tension and external conditions, like the medium they land on. When raindrops collect on a waxed automobile hood, they tend to form nearly perfect spheres.
Here's why:

• **Surface Tension of Water**: Water molecules are attracted to each other, and this cohesive force creates surface tension.
• Minimized Surface Area**: Due to surface tension, water droplets try to minimize their surface area for a given volume. The geometric shape that provides the smallest surface area is a sphere.
• **Non-reactive Surface**: On a waxed surface, water doesn't spread out due to the lack of adhesion; instead, it retains its spherical shape.

Such formations also lower energy states, making a spherical formation energetically favorable.

Hydrophobic Interactions

Hydrophobic interactions occur when nonpolar substances group together in water, forming clusters to minimize exposure to the water. The term 'hydrophobic' literally means "water-fearing."
In the context of oil on a waxed automobile hood:

• **Nonpolar Oil**: Oil is nonpolar and will not mix with polar substances like water.
• On a waxed surface, oil spreads out due to its weak attachment to the wax, a phenomenon unlike what is observed with water, due to its hydrophobic nature.
• **Flattened Shape**: The oil forms a flatter shape rather than a droplet, due to the lack of cohesive forces that water molecules have. This allows it to maximize its contact with the waxed surface, minimizing its interaction with polar water.

Understanding these properties helps us see why certain substances behave differently on similar surfaces.