Water — IB Diploma Biology HL
IB Biology A1.1 Water: polarity, hydrogen bonds, cohesion, adhesion, solvent properties, water vs air as a habitat, and (HL) water beyond Earth.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Water is the medium for life: the first cells originated in water and most processes of life still happen in aqueous solution. Oxygen attracts the shared electrons more strongly than hydrogen, so the O–H bonds are polar covalent, and hydrogen bonds form between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.
Cohesion is the attraction of water molecules to each other by hydrogen bonds, while adhesion is the attraction of water to other polar or charged materials. Cohesion lets water be pulled up xylem under tension without the column breaking and creates surface tension used as a habitat. Adhesion causes capillary action in soil and in plant cell walls.
Water is an excellent solvent because its polar molecules are attracted to ions and to other polar molecules, surrounding them so they dissolve. This makes water the medium for metabolism, as most enzymes catalyse reactions in aqueous solution, and for transport in blood plasma, xylem and phloem. Some molecules, such as membrane lipids, work because they are hydrophobic and insoluble.
Water differs from air in buoyancy, viscosity, thermal conductivity and specific heat capacity. Water is far denser and more viscous, so it supports bodies but resists movement. It conducts heat much faster, so endotherms lose heat quickly, and its high specific heat capacity keeps aquatic temperatures stable. The black-throated loon and ringed seal show adaptations to water and air.
Earth's water is thought to have an extraplanetary origin: it was probably delivered by water-containing asteroids early in Earth's history. Earth retained it because its gravity holds the atmosphere and temperatures are low enough for water to condense into liquid. Because life depends on liquid water, the search for extraterrestrial life focuses on planets in the Goldilocks zone.
Exam questions
7 exam-style questions (20 marks), each with its mark scheme.
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18 cards: flip them, mark what you knew, and practise the rest.
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The whole of molecules (unity and diversity) on one page, so you can see where this subtopic fits.
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Quick check questions
Why is a water molecule polar?
Show answer
Electrons in the O–H covalent bonds are shared unequally, so oxygen is δ− and the hydrogens are δ+, and the bent shape means the charges do not cancel.
What is the difference between cohesion and adhesion?
Show answer
Cohesion is attraction between water molecules; adhesion is attraction of water to polar or charged materials.
Name two physical properties of water that differ from air and matter to aquatic animals.
Show answer
Any two of: buoyancy, viscosity, thermal conductivity, specific heat capacity.
Why do membranes depend on phospholipid tails being hydrophobic?
Show answer
The hydrophobic tails do not dissolve in water, so they form a barrier to ions and polar molecules.
HL only What is the Goldilocks zone?
Show answer
The range of distances from a star where temperatures allow liquid water on a planet's surface.
Frequently asked questions
Why is water called the medium for life?
Water is called the medium for life because the first cells originated in water and most processes of life still occur in it. Cytoplasm is mostly water, most enzymes catalyse reactions in aqueous solution, and blood plasma, xylem and phloem carry substances dissolved in water around organisms.
How do hydrogen bonds form between water molecules?
Hydrogen bonds form because water molecules are polar. Oxygen attracts the shared electrons more strongly than hydrogen, so oxygen is slightly negative and hydrogen slightly positive. The δ+ hydrogen of one molecule is attracted to the δ− oxygen of a neighbouring molecule, and this weak intermolecular attraction is a hydrogen bond.
How does cohesion help water move up the xylem?
Cohesion holds water molecules together by hydrogen bonds, so the water in a xylem vessel behaves as a continuous column. When water evaporates from leaves, it pulls on this column, which is under tension, and the whole column moves upwards without breaking. Adhesion to the vessel walls also helps.
Why do seals need blubber in cold water?
Seals need blubber because water has a much higher thermal conductivity than air, so heat is lost from a warm body far faster in water. The thick layer of blubber in the ringed seal insulates the body and reduces this heat loss, and it also adds buoyancy.
Why do scientists look for water when searching for life on other planets?
HL only Scientists look for water because all known life uses liquid water as its medium. Planets in the Goldilocks zone of a star, where temperatures allow liquid water on the surface, are therefore the best places to search, along with places with evidence of water such as Mars.
Written and checked against the IB Biology HL specification · Updated October 2026