Problem 113 - Chemistry

Question

A small drop of oil in water assumes a spherical shape. Explain. (Hint: Oil is made up of nonpolar molecules, which tend to avoid contact with water.)

Step-by-Step Solution

Answer

Oil is made up of nonpolar molecules that avoid contact with water. Owing to surface tension, which demands the least surface area possible, a small drop of oil in water forms a sphere, as the sphere is the shape with minimal surface area for a given volume.

Step 1: Understand the Structure of Oil

Firstly, understanding the structure of oil is important. Oil is made up of nonpolar molecules. A nonpolar molecule is a molecule that shares electrons equally and does not have oppositely charged ends. Therefore, because of this nature, these nonpolar molecules tend to avoid contact with water, which is a polar molecule, with distinctively positive and negative charged ends.

Step 2: Surface Tension

Secondly, let's understand the concept of surface tension. It is an important property that significantly impacts the interaction of water and oil. Surface tension is the elastic tendency of liquids which makes it acquire the least surface area possible. This happens because the liquid’s surface acts like a stretched elastic sheet.

Step 3: Formation of Spherical Shape

Combining both concepts, a drop of oil in water tends to minimize its surface area to achieve the smallest possible contact with water, due to its nonpolar nature. The shape with the least surface area for a given volume is a sphere. Therefore, a small drop of oil in water assumes a spherical shape.

Key Concepts

Nonpolar Molecules

Oil is made up of nonpolar molecules, which is essential to understand why a drop of oil behaves as it does in water. A nonpolar molecule is a type of molecule where the electrons are shared equally among the atoms. This equal sharing results in no distinct charge at either end of the molecule.
This absence of a positive or negative charge means that nonpolar molecules, like those found in oil, do not mix well with polar molecules. Water, in contrast to oil, contains polar molecules. Water molecules have areas with small positive charges and small negative charges, due to its bent shape and unequal sharing of electrons.
Nonpolar molecules avoid interacting with polar molecules due to a difference in charge distribution. This tendency explains why oil tends to cluster together, minimizing its interaction with water.

Surface Tension

To comprehend how oil forms into a spherical shape in water, it's helpful to understand surface tension. Surface tension is a property that causes the surface of a liquid to behave like a thin elastic film. This phenomenon results from the cohesive forces between liquid molecules, particularly at the interface between the liquid and air or between two different liquids.
In the case of water, the molecules at the surface are attracted more strongly to each other than to the air above them. This cohesive force is what creates a tension on the surface, making it shrink to the smallest possible area.
This surface tension plays a crucial role when oil is in water. Due to oil's nonpolar nature, it also forms droplets that strive to have minimal contact with polar water molecules. This is where surface tension becomes significant, as it influences the shape that the oil takes above the water.

Spherical Shape

The spherical shape of an oil drop in water is the result of several interacting factors, primarily driven by the properties of nonpolar molecules and surface tension. A sphere is a shape that has the least possible surface area for a given volume.
For oil in water, forming a sphere minimizes the contact area between the oil (nonpolar) and water (polar). By adopting this shape, the oil drop efficiently reduces interaction with the water molecules, safeguarding its nonpolar molecules from the water's polar nature.
This spherical configuration is thus an optimal state for the oil. It allows it to stay as cohesive as possible, leveraging surface tension and the nature of nonpolar molecules to minimize potential energy and achieve stability.