Question

Use water and mercury as examples to explain adhesion and cohesion.

Short answer

Adhesion refers to the attraction between different types of molecules, while cohesion refers to the attraction between similar types of molecules. In water, both these forces are pronounced, allowing it to exhibit behaviors like capillary action. Mercury, on the other hand, is more cohesive than adhesive, causing it to form spherical droplets and a convex meniscus when in contact with other substances.

Step-by-step solution

Definition of Adhesion and Cohesion

Adhesion is the attractive force between unlike molecules. They are caused by forces acting between two different substances. Cohesion, on the other hand, is the attractive force between like molecules. They are caused by forces acting between similar substances.

Adhesion and Cohesion in Water

Water molecules exhibit both adhesion and cohesion. Cohesion refers to the bonding between water molecules (H2O), due to hydrogen bonding. Water's cohesion allows it to stay together and form droplets or flow in a stream. Adhesion is seen when water molecules stick to other substances, like a glass, forming a concave meniscus or climbing up thin tubes against gravity in a process called capillary action.

Adhesion and Cohesion in Mercury

Mercury molecules also exhibit adhesion and cohesion but in a different way than water. Mercury is more cohesive than adhesive, which means mercury molecules are more attracted to each other than other substances. This is why mercury does not stick to most materials and forms spherical droplets. When poured into a glass, a convex meniscus is formed because the adhesive forces between the mercury and the glass are weaker than the cohesive forces within the mercury.