Water Movement in Soils: Ch. 1-Adhesion and Cohesion

Summary

The video explains the principles of capillarity, demonstrating how water moves upward in porous materials due to adhesive and cohesive forces. In the first demonstration, water rises into a ceramic rod from a dish because of attraction between the rod's surface and water molecules. This creates tension, as pressure in the water above the free surface is lower than atmospheric pressure. The second demonstration shows water rising between two closely spaced glass plates, with similar forces at play, emphasizing how tighter spacing increases the height of the water's rise due to internal tension.

Takeaways

• 🧪 The glass plates are one foot high and two feet wide with a half-inch space between them, representing a vertical cross-section through the soil.
• 🌊 Capillarity is demonstrated through the movement of water into dry, coarse materials due to adhesive and cohesive forces.
• 📉 Liquid is pulled upward from free water into a porous ceramic rod because of the attraction between the water molecules and the ceramic's surface.
• 🌍 Cohesion (between water molecules) and adhesion (between water and solid surfaces) allow water to move against gravity.
• 💧 The pressure in water inside the ceramic rod, above the free water level, is lower than atmospheric pressure, creating tension.
• 📊 In another demonstration, water rises between two closely spaced glass plates because of the adhesive force between water and glass.
• 📏 The height of water rise is greater when the plates are closer together, enhancing the adhesive forces.
• 🔽 The pressure in water that rises above the free water surface is less than atmospheric pressure, again referred to as tension.
• ⬆️ The higher the water rises, the greater the internal tension created within the water column.
• 🔬 Both demonstrations illustrate how capillarity works, a critical principle for water movement in soils and porous materials.

Q & A

What is the main concept discussed in the script?

The script discusses the principles of capillarity and how adhesive and cohesive forces allow water to move against gravity, particularly in soil and between glass plates.

What does the model with glass plates represent?

The model with glass plates represents a vertical cross-section through soil, used to demonstrate the principles of capillarity.

How does water move into dry, coarse materials in the first demonstration?

In the first demonstration, water moves into dry, coarse materials through a porous ceramic rod due to the adhesive forces between water and the rod’s surface and the cohesive forces between water molecules.

What is the role of adhesive and cohesive forces in capillarity?

Adhesive forces cause water to stick to surfaces like glass or the ceramic rod, while cohesive forces make water molecules attract each other. Together, these forces allow water to move upward against gravity.

What is meant by 'tension' in the context of the first demonstration?

In the first demonstration, 'tension' refers to the condition where the pressure in the water contained in the ceramic rod is less than atmospheric pressure, which helps move water upward.

How does the second demonstration show the effect of adhesive forces?

In the second demonstration, water rises between two closely spaced glass plates because of the adhesive forces between the glass and water, along with cohesive forces between water molecules.

What happens to the pressure of water above the free water surface in both demonstrations?

In both demonstrations, the pressure of water above the free water surface is less than atmospheric pressure, creating tension that allows water to rise.

Why is the height of water rise greater when the glass plates are closer together?

The height of water rise is greater when the glass plates are closer together because the adhesive forces between the glass and water are stronger when the spacing is tighter, enhancing capillary action.

What is the significance of free water in the dish in the first demonstration?

The free water in the dish serves as the source from which water is pulled upward into the porous ceramic rod due to capillary action and the adhesive and cohesive forces at play.

How do adhesive and cohesive forces allow water to move against gravity?

Adhesive forces cause water to stick to solid surfaces like glass or ceramic, while cohesive forces keep water molecules together. Together, these forces create upward movement of water, overcoming the downward pull of gravity.