Cohesion and adhesion | Middle school chemistry | Khan Academy

Understanding Water Droplets and Their Behavior

The Formation of Water Droplets

• Water forms beautiful droplets due to the molecular behavior of water, which can be understood by examining it at a molecular level.

• Water molecules exhibit high-speed movement and attract each other like magnets, creating a force of attraction that keeps them together without sticking permanently.

• This attractive force is known as cohesion, defined as the attraction between particles of the same substance, leading to spherical droplet formation.

• Cohesion in water is stronger than in other liquids (e.g., alcohol), resulting in more rounded droplets compared to flatter shapes seen with less cohesive liquids.

Surface Tension and Droplet Shape

• While surface tension contributes to droplet shape, external forces (like those from a leaf) can flatten the bottom of a droplet.

• In microgravity environments (e.g., NASA footage), water droplets form perfect spheres due to lack of external pressure.

Why Does Water Spill When Pouring?

Adhesion and Spillage

• When pouring water from one glass to another, spillage occurs because water molecules stick to the glass due to adhesion.

• Adhesion refers to the attraction between particles of different substances; here, it describes how water molecules are attracted to glass particles.

Meniscus Formation

• In narrow containers like test tubes, adhesion causes the water level at the edges (where it touches glass) to rise higher than in the middle, forming a meniscus.

Applications of Cohesion and Adhesion

Fog Nets for Water Collection

• Fog nets utilize both cohesion and adhesion principles; when airborne water molecules contact vertical wires, they adhere and coalesce into larger drops that fall due to gravity.

Summary of Key Concepts

• Two main properties discussed are cohesion (attraction among similar particles leading to spherical droplets) and adhesion (attraction between different substances).

• Remembering these concepts: "cohesion" relates to cooperation among similar particles while "adhesion" connects with adhesives that bond different materials.