Properties of Water: Cohesion and Adhesion | A-level Biology | OCR, AQA, Edexcel

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They key points covered in this video include:

1. Cohesion of Water
2. Surface Tension
3. Adhesion of Water

Cohesion of Water

The polar nature of water impacts the movement of water, due to many hydrogen bonds forming between the water molecules. The hydrogen bonds mean that the molecules of water are attracted to each other and tend to stick together - this is known as cohesion. Cohesion is the attraction between molecules of the same type. Cohesion can easily be seen by the behaviour of water droplets just before they fall. Other liquids such as oil do not contain hydrogen bonds between the molecules, so they are less cohesive and separate quicker. Due to cohesion, if water is taken from the end of a tube, water molecules follow behind. This is important as it allows plants to be able to draw water up xylem vessels in a continuous stream against the pull of gravity.

Surface Tension

The cohesive property of water results in unique behaviour of water at the interface between water and air. Water molecules at the surface of water are more attracted to other water molecules than they are to molecules in the air. This uneven attraction pulls the water molecules inwards towards the water below so the surface of the water is placed under tension. This tension results in a thin 'skin' on top of the surface of water that is difficult to break through. This allows the surface of water to act as a habitat for insects like pond-skaters.

Adhesion of Water

Water molecules don't just stick to themselves - they also stick to other molecules in a process known as adhesion. Adhesion is the attraction between non-alike molecules. Water displays adhesive properties when it is attracted to other polar or charged surfaces. Adhesion can be seen by the behaviour of water when narrow glass straws are placed in water. The water molecules are adhere to the polar glass and so they rise up the straws against gravity - this is capillary action.

Summary

Cohesion is the attraction between molecules of the same type
Water displays cohesive properties as water molecules are attracted to each other by hydrogen bonds
Cohesion allows plants to transport columns of water molecules up from the roots to the leaves against the force of gravity
Cohesion between water molecules at the surface of the water results in surface tension
Surface tension makes the surface of water difficult to break through, allowing it to provide a habitat for small insects
Adhesion is the attraction between molecules that are not the same type
Water displays adhesive properties as water molecules are attracted to other polar substances