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A1.1.3—Cohesion of water molecules

Hydrogen bonds make water molecules cohere, producing continuous columns under tension and surface tension that living organisms exploit for transport and support.

Syllabus
First assessment 2025
Objective
A1.1.3
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paper—
Typical marks1–2

Common command terms

  • Identify
  • Explain
  • Outline

Scoring notes

Common mistake
Defining cohesion as attraction between water and another surface.

Recent exam appearances

May 2025Paper2 HL · TZ18(a)[ 4 ]A1.1.3—Cohesion of water molecules
May 2025Paper1A HL · TZ31[ 1 ]A1.1.3—Cohesion of water molecules
November 2023Paper1 HL · TZ24[ 1 ]A1.1.3—Cohesion of water molecules
November 2023Paper1 HL · TZ14[ 1 ]A1.1.3—Cohesion of water molecules
May 2019Paper2 HL · TZ14(b)[ 2 ]A1.1.3—Cohesion of water molecules
Practice this objective

Coverage 2019–2025 · Updated 14 Jul 2026

Cohesion Keeps Water Molecules Together

Cohesion is attraction between molecules of the same substance; in water it supports continuous columns and surface tension.

Hydrogen bonding resists separation. This allows water to transmit tension through xylem and lets the surface support small organisms.

Distinguish cohesion from adhesion before explaining a transport or surface effect.; separate property, mechanism and biological use

A continuous water column in a plant xylem vessel can be pulled upward as leaves lose water, because water molecules cohere. This gives a concrete prediction from the stated water condition.

Cohesion alone does not lift water without transpiration, adhesion and a continuous pathway. Interpret the result within the stated evidence and scale limits.

Cohesion of water molecules

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, multiple choice, commonly using Identify / Explain / Outline.

Command terms

Identify / Explain / Outline

What earns marks

Strong exam answers begin with polarity or hydrogen bonding, name cohesion, and then link it to a specific biological outcome such as xylem transport or surface tension.

Watch for

Defining cohesion as attraction between water and another surface.

Representative question

Question 1

[Maximum number: 4]

Outline how the cohesive properties of water benefit living organisms.

Water summary

  • Water is life's medium because substances can dissolve, move and react in it.
  • Unequal electron sharing and bent geometry make water polar; hydrogen bonds form between molecules.
  • Many hydrogen bonds produce cohesion, maintaining xylem columns and creating surface tension.
  • Adhesion is water-to-surface attraction and contributes to movement through narrow soil spaces and cellulose walls.
  • Polarity lets water hydrate ions and interact with polar solutes; dissolved particles can be transported and react, while non-polar substances are hydrophobic.
  • Compared with air, water provides more buoyancy and drag, transfers heat faster and changes temperature more slowly. Aquatic adaptations respond to these consequences.

Concept essentials

  • Cohesion is attraction between water molecules.
  • Hydrogen bonding produces cohesive forces.
  • Cohesion maintains continuous water columns in xylem.
  • Surface tension is a consequence of cohesion at an interface.