Problem 26 - Chemistry

Question

Draw diagrams showing the capillary action of (a) water and (b) mercury in three tubes of different radii.

Step-by-Step Solution

Answer

The capillary action of (a) water forms a concave meniscus because its adhesive forces are stronger than its cohesive forces. This causes the water to curve upwards at the tube edges, and the curvature increases as the tube diameter decreases. The capillary action of (b) mercury forms a convex meniscus because its cohesive forces are stronger than the adhesive forces. This causes the mercury level to be higher in tubes of larger diameters.

Step 1: Draw Capillary Tubes of Different Diameters

Start by drawing three vertical lines to represent tubes of different diameters. These tubes can vary in diameter by drawing them of different widths.

Step 2: Draw the Capillary Action of Water

Next we depict the capillary action of water. Water has a concave meniscus due to its adhesive forces being stronger than its cohesive forces. This means that water forms an upward curve at the tube edges. Draw these curves in each tube, bearing in mind that as the tube diameter decreases, the curvature of the water level increases, accordingly.

Step 3: Draw the Capillary Action of Mercury

Mercury behaves differently in capillary tubes due to its strong cohesive forces overpowering the adhesive forces. Consequently, mercury forms a convex meniscus, or a downward curve at the tube edges. Draw these curves in each of the tubes, showing a higher mercury level in tubes of larger diameters.

Key Concepts

Cohesive Forces

Cohesive forces are the attractive forces that act between molecules of the same substance. In liquids, these forces hold the molecules together, creating surface tension. Cohesion is responsible for the tendency of a liquid to resist separation. For example, water droplets form beads because of the cohesion between water molecules. These forces are at work in capillary tubes as well. In the case of mercury, the cohesive forces are strong enough to overpower any adhesive forces acting between mercury and the glass tube. This results in a convex meniscus or a downward curve at the edges when mercury is placed in a capillary tube.
Understanding the balance between cohesive forces and their opposites, the adhesive forces, is crucial in explaining the behavior of different liquids in narrow spaces.

Adhesive Forces

Adhesive forces are the attractive forces between unlike substances. In the case of capillary action, these forces occur between the liquid and the walls of the tube. These forces determine how a liquid interacts with the surface of a material.

• When adhesive forces between the liquid and the tube material are stronger than the cohesive forces within the liquid, the liquid will spread out and climb up the walls of the container.
• Water is a prime example of this behavior. In glass capillary tubes, water molecules are more attracted to the glass (adhesive force) than they are to each other (cohesive force), resulting in a concave meniscus.

This causes the water to rise in the tube, an effect known as capillary action. This explains why water climbs up a plant's roots and stems, defying gravity.

Meniscus

The meniscus is the curved surface of a liquid in a container, such as a tube or a glass. It reflects the balance between cohesive and adhesive forces at play. When observing a liquid in a thin tube, this curvature can be either concave or convex, depending on the relative strengths of these forces.

• A concave meniscus occurs when adhesive forces are stronger. The liquid, such as water, adheres to the walls and climbs them slightly, creating an upward curve.
• A convex meniscus forms when cohesive forces predominate, as seen with mercury in glass, where the liquid molecules prefer to stick together rather than to the tube walls, resulting in a downward curve.

Understanding the nature of the meniscus helps to explain the principles of capillary action and how different liquids interact with surfaces. The meniscus is key to accurately measuring liquid volumes in laboratory settings, and its shape can affect the outcome of experiments.