Problem 27 - Chemistry and Chemical Reactivity

Question

If a piece of filter paper (an absorbent paper used in laboratories) is suspended above a beaker of water and just touching the surface, water will slowly move up the paper. What is the name given to this phenomenon, and how is this behavior explained?

Step-by-Step Solution

Answer

It's called capillary action, explained by adhesive and cohesive forces.

Step 1: Identify the Phenomenon

The observed phenomenon where water moves up against gravity in narrow spaces is called 'capillary action'. This is a key feature in the behavior of liquids in porous materials.

Step 2: Understand the Mechanism

Capillary action occurs due to the combination of adhesive forces and cohesive forces. Adhesive forces cause the liquid molecules to stick to the material of the filter paper, while cohesive forces make the liquid molecules attract to each other.

Step 3: Discuss Adhesive and Cohesive Forces

Adhesive forces between the water molecules and the paper's fibers pull the water upwards. Simultaneously, cohesive forces among the water molecules themselves enable the water to cohere, maintaining a continuous stream as it rises.

Step 4: Equation for Capillary Action

The height to which a liquid can rise in a capillary tube is often described by the Jurin's Law formula: \[ h = \frac{2\gamma \cos{\theta}}{\rho g r} \]where \(h\) is the height the liquid rises, \(\gamma\) is the surface tension, \(\theta\) is the contact angle, \(\rho\) is the density of the liquid, \(g\) is the acceleration due to gravity, and \(r\) is the radius of the capillary.

Key Concepts

Adhesive Forces

Adhesive forces are a key component in the process of capillary action, particularly because they involve the attractive interactions between different types of molecules. In this context, adhesive forces occur between the molecules of the liquid (like water) and the molecules of the surface they come into contact with, such as the fibers of a piece of filter paper. These forces are responsible for the water molecules clinging to the surface of the filter paper, enabling the liquid to "climb" upwards, defying gravity.

Think of adhesive forces like a magnet that pulls different materials together. A strong adhesive force compared to the weight of the liquid can cause it to move in the opposite direction of gravitational pull.

• Role in capillary action: It helps lift the liquid molecules along the surface.
• Strength of Force: If adhesive forces are stronger than the cohesive forces within the liquid, the liquid will wet the surface and rise.
• Example: Water spreads across the surface of glass due to strong adhesive forces between water molecules and glass.

Cohesive Forces

Cohesive forces are another central player in capillary action. Unlike adhesive forces, cohesive forces involve the mutual attraction between molecules of the same substance - in this case, water molecules. These forces are what makes water droplets form and stick together, maintaining a continuous flow as they climb through narrow spaces like the pores of filter paper.

When considering why water moves cohesively, imagine it as a string of beads; cohesive forces keep the beads (molecules) together as a chain. Without these forces, the liquid wouldn't form a stream and wouldn't rise as efficiently.

• Unity of Molecules: Cohesive forces help the liquid maintain its integrity as a whole as it is drawn upwards.
• Surface Tension: These forces contribute to the liquid surface tension, affecting the height it can achieve in capillary action.
• Comparison to adhesive forces: When cohesive forces are strong, they can counteract adhesive forces and inhibit rise if the balance tips favorably to cohesion.

Jurin's Law

Jurin's Law offers a practical explanation and prediction for the height a liquid will rise in capillary action. It considers how capillary action is affected by various factors, including liquid density, surface tension, contact angle, and the radius of the capillary. According to Jurin's Law, the equation to determine the height (\[ h = \frac{2\gamma \cos{\theta}}{\rho g r} \]
) expresses this behavior quantitatively.

Let's break down this formula:

• Surface Tension (\(\gamma\)): Greater surface tension means a higher rise.
• Contact Angle (\(\theta\)): The angle where the liquid interface meets the solid surface. A smaller contact angle leads to a higher rise.
• Density (\(\rho\)): As density increases, the height decreases.
• Radius of the Tube (\(r\)): Narrower tubes enhance the height to which the liquid rises.

By combining these elements, Jurin's Law reinforces our understanding that capillary action results from a delicate balance between adhesive and cohesive forces, tuned by the liquid's physical properties and the geometry of its container.