The big idea: Water is polar, so it does not only stick to itself. It also sticks to other polar or charged materials. That is adhesion.
The points to remember
- Adhesion is water sticking to a different material. Cohesion is water sticking to water.
- Water adheres to materials that are polar or charged: its δ+ hydrogens and δ− oxygen are attracted to their charges.
- The pull is a hydrogen bond, the same kind of bond that holds water to water.
- Polar surfaces: glass, cellulose cell walls, soil particles. Water does not stick to wax or oil: no charges.
Remember it as: Co-hesion: water with water. Ad-hesion: water added on to something else.
Real example: rain on a window. The drops cling to the glass and slide down slowly instead of falling straight off, because glass is polar and water adheres to it. On a waxed car bonnet the same rain beads up and rolls away: wax has no charges for water to grip.
A trap in the wording: 'Adhesion between water molecules' is wrong: water to water is always cohesion, and surface tension comes from cohesion, not adhesion. Adhesion always needs a second material.
That trap is set with a picture. Look at what this figure can and cannot show:
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Lay the corner of a paper towel in a spill and the water climbs up the towel, against gravity, with no pump. The towel is made of cellulose fibres with tiny gaps between them, and water does the rest.
That climb is capillary action. It happens wherever water meets a narrow space with polar walls.
The points to remember
- In a narrow space, adhesion pulls water up the walls.
- Cohesion makes the rest of the water follow, so a whole column rises: this is capillary action.
- The narrower the space, the higher the water climbs.
- In soil: water climbs the narrow gaps between soil particles, from wet soil deeper down to the roots.
- In plant cell walls: water creeps through the gaps between cellulose fibres, and clings to the polar walls of the xylem.
Remember it as: Narrow gap, big climb.
Draw it yourself, one part at a time:
How much does the width matter? Here are real measurements in clean glass tubes:
Real example: silt and gravel: In silt the gaps between particles are tiny, so water climbs about a metre up from the water table and the soil stays damp for roots in a dry spell. In gravel the gaps are wide and water climbs only a few centimetres.
One limit to keep in mind, because it costs marks when it is forgotten:
Not the tree, not root pressure: Capillary action lifts water a few centimetres, or a metre or so in fine soil. It is not what lifts water to the top of a tall tree, and it is not root pressure.
Water goes up a tree because evaporation from the leaves pulls on the column; adhesion to the xylem walls helps the column hold on. If asked about root pressure or transpiration, do not write 'capillary action'.
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How this is tested: Paper 1A asks for one mark: which everyday effect is a consequence of adhesion (water climbing through soil), as opposed to cohesion (surface tension) or water being a solvent (carrying nutrients in blood).
A written part gives a scene and asks you to explain how water moves through soil or a cell wall. Three marks, three steps: adhesion, cohesion, the narrow space.
A gardener waters seedlings only from a tray under their pots. Within an hour the soil at the top of each pot is damp. Explain how the water reaches the roots near the surface.
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