The big idea: Because water is polar, its molecules form hydrogen bonds with each other.
These weak attractions make water molecules stick to each other (this is cohesion) and stick to other surfaces (this is adhesion).
These two 'stickiness' properties explain how water is pulled up tall plants and why small animals can rest on a pond surface.
Polarity is the root cause: each δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring molecule.
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Cohesion (blue) = water molecules hydrogen-bonding to EACH OTHER. Adhesion (green) = the same molecules hydrogen-bonding to an adjacent surface, here a xylem-vessel wall.
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Where the stickiness comes from: A hydrogen bond is a weak attraction between the δ+ hydrogen of one water molecule and the δ− oxygen of another.
Each bond is weak, but there are so many of them that, together, they hold water molecules tightly together.
Three closely-related properties all come from water's hydrogen bonds. Be careful to keep them separate — exams often ask you to tell cohesion and adhesion apart.
- Cohesion
- The attraction of water molecules to OTHER water molecules (water sticking to itself).
- Adhesion
- The attraction of water molecules to a DIFFERENT surface (water sticking to something else).
- Surface tension
- A 'skin-like' effect at the water surface, caused by cohesion pulling the surface molecules together.
Cohesion — water to water
- Water molecules pull on each other
- Creates an unbroken column of water in xylem vessels
- Produces surface tension at the water surface
- Lets pond skaters and other small insects rest on water
Adhesion — water to a surface
- Water molecules stick to other materials
- Water climbs the walls of narrow xylem vessels and tubes
- Helps water spread across and wet a leaf or soil particle
- Lets water rise up very thin tubes (capillary action)
Why it matters in plants: In a tall plant, water is pulled up the xylem from roots to leaves.
Cohesion holds the water molecules together in an unbroken column, so a pull at the top drags the whole column up.
Adhesion sticks water to the walls of the narrow xylem vessels, helping to support and lift the column. This is called the cohesion–tension mechanism.
Surface tension supports life: At the surface, water molecules have no water above them, so cohesion pulls them sideways and down, making the surface behave like a stretched 'skin'.
This surface tension is strong enough to support the weight of small organisms such as pond skaters (water striders), which can walk on the water surface.
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How this is tested: On Paper 1A (multiple choice) you are often asked to identify which water property explains an example — for instance, surface tension letting an insect walk on water, or adhesion (not cohesion) explaining water climbing a surface.
On Paper 2 a 4-mark Outline question asks you to give water's cohesive (or general) properties and link each one to a benefit for living organisms.
IB-style question — cohesion and living organisms
Outline how the cohesive properties of water benefit living organisms. [4]
How to score all four marks
- State what cohesion is. Water molecules are attracted to each other by hydrogen bonds, so they stick together.
- Benefit 1 — transport in plants. Cohesion holds water in an unbroken column in the xylem, so water can be pulled up from the roots to the leaves.
- Benefit 2 — surface tension. Cohesion at the surface creates surface tension, a 'skin' on the water.
- Answer the command term (link to a benefit). This surface tension is strong enough to support small organisms (such as pond skaters), letting them live on the water surface.
Final answer
Cohesion (hydrogen bonding between water molecules) keeps water in an unbroken column in the xylem so it can be transported up plants, and it produces surface tension strong enough to support small organisms on the water surface.
✓ What the marker is looking for: Each mark needs a property linked to a benefit, not just a property on its own.
'Cohesion → unbroken column → water pulled up the xylem' is one full mark; just writing 'water is cohesive' is not.