1The idea in one paragraph

A water molecule is lopsided. Its oxygen atom pulls the shared electrons towards itself, so the oxygen end is slightly negative and the two hydrogen ends are slightly positive. That makes water polar, and it lets each water molecule attract its neighbours through hydrogen bonds. Almost everything that makes water useful to life comes from those two facts. Hydrogen bonds hold water to water (cohesion) and to other polar surfaces (adhesion); they let water pull charged and polar substances apart and carry them (solvent properties); and they make water dense, sticky to move through, quick to conduct heat and slow to change temperature. Learn polarity and hydrogen bonding properly and the rest of the subtopic follows from them.

2Water as the medium for life

Life began in water and has never left it. The first cells formed in water, and every cell today is mostly water: the cytoplasm is a watery solution, and the chemical reactions of life, which together are called metabolism, happen dissolved in it. A frog, a tree and a bacterium living in a cloud droplet are all, on the inside, bags of solution.

Water is the medium for life for three reasons you will meet in turn. It dissolves the substances cells use, so they can move and react. It holds its temperature steady, so reactions run at a predictable rate. And it is liquid across the range of temperatures found on most of Earth's surface, so it can flow, carry things and fill cells.

Keep that last point in mind when you reach section 9: "is there liquid water?" is the first question asked about any place where life might exist.

3Polar bonds and hydrogen bonds

A water molecule, H₂O, is one oxygen atom joined to two hydrogen atoms by covalent bonds. In each bond a pair of electrons is shared, but not shared equally. Oxygen attracts electrons much more strongly than hydrogen does, so the shared pair spends more of its time near the oxygen. Figure 1 shows the result.

The oxygen end carries a small negative charge, written δ− ("delta negative", meaning a partial charge, much smaller than the full charge on an ion). Each hydrogen carries a small positive charge, δ+. A bond in which the electrons are shared unequally is a polar covalent bond, and a molecule with an uneven spread of charge like this is a polar molecule. Because the molecule is bent rather than straight, the two δ+ hydrogens sit on one side and the δ− oxygen on the other, so the whole molecule has a positive side and a negative side.

Now put two water molecules near each other. The δ+ hydrogen of one is attracted to the δ− oxygen of the other. That attraction is a hydrogen bond. Figure 2 shows the notation the guide expects you to use.

A hydrogen bond is the attraction between a δ+ hydrogen atom of one molecule and a δ− atom (in water, the oxygen) of another. It is between molecules, not within one, and it is caused by the polarity of the covalent bonds.

Two things about hydrogen bonds decide how you should write about them.

One hydrogen bond is weak. It is far weaker than the covalent bond inside the molecule, and in liquid water hydrogen bonds break and re-form constantly as molecules jostle. Never write that water "is held together by strong hydrogen bonds" as if one bond were strong.

There are enormous numbers of them. Each water molecule can form hydrogen bonds with up to four neighbours, two through its hydrogens and two through its oxygen, as in Figure 2. Weak individually, enormous together: that is the whole story of water's properties.

4Cohesion: water holding on to water

Cohesion is the attraction between water molecules, caused by hydrogen bonding. It makes water behave as a connected mass rather than a heap of loose particles. The guide asks for two consequences for organisms, and Figure 3 draws both.

Water is pulled up the xylem under tension. A tree has no pump. Water evaporates from its leaves, and each molecule that leaves pulls on the molecules behind it, because they are hydrogen-bonded together. That pull passes all the way down a continuous column of water in the xylem vessels to the roots. The column is under tension, meaning it is being stretched, like a rope being hauled up a well. It does not snap because cohesion holds each molecule to the next. This is how water reaches the top leaves of a tree 50 metres tall with no energy spent by the tree on lifting it. (You meet the rest of this mechanism, transpiration, in B3.2.)

The water surface is a habitat. At the surface, molecules have water below and beside them but air above, so all their hydrogen bonds pull sideways and downwards. The surface layer behaves like a stretched elastic skin. This effect is called surface tension, and small animals use it. A pond skater stands on the surface film: its long legs spread its weight, the film dips under each foot, and cohesion stops it breaking. Mosquito larvae hang from the underside of the same film to breathe through a tube at the surface. For these animals the surface is not the edge of the habitat. It is the habitat.

5Adhesion: water holding on to other things

Adhesion is the attraction between water molecules and a different substance. Water adheres to any surface that is polar or charged, because the δ+ and δ− ends of water molecules form hydrogen bonds or other attractions with it. Glass, cellulose and the surfaces of soil particles are all polar or charged. Wax and oil are not, which is why water beads up on a waxy leaf.

Adhesion and cohesion working together produce capillary action: water climbing a narrow space against gravity. Adhesion pulls water up the sides of the space; cohesion drags the rest of the water up behind it. The narrower the space, the larger the surface compared with the volume of water, and the higher the water climbs. Figure 4 shows this in glass tubes and then in the two places the guide names.

In soil. Soil is particles with narrow pores between them. Water adheres to the surface of each particle as a thin film and is held in the smallest pores even against gravity, so soil keeps water long after rain has drained through. Capillary action also moves water through the pores from wetter soil towards drier soil, including the soil around roots that are absorbing it.

In plant cell walls. A cell wall is a meshwork of cellulose fibres, and cellulose is polar. Water adheres to the fibres and is drawn through the narrow spaces between them, so it spreads through the walls of a plant the way ink spreads through paper. In a leaf, water evaporating from the walls of the cells inside is replaced this way from the xylem. Water also adheres to the walls of the xylem vessels themselves, which helps the column in section 4.

Adhesion and cohesion are the two properties students most often swap. Cohesion is water to water. Adhesion is water to something else. When you write either word, add "…due to hydrogen bonding" and name what is sticking to what.

Linking question. What biological processes only happen at or near surfaces? Surface tension and adhesion are two answers already; gas exchange and enzyme action will be others.

6Water as a solvent, and why some molecules must not dissolve

A solvent is a liquid that dissolves other substances, the solutes, to form a solution. Water dissolves an unusually wide range of substances, and it does so because it is polar.

Look at Figure 5. When a crystal of salt goes into water, the δ− oxygens of water molecules cluster around each positive sodium ion, and the δ+ hydrogens cluster around each negative chloride ion. Surrounded by water, the ions are pulled out of the crystal and spread through the liquid. Polar molecules such as glucose and amino acids dissolve the same way, forming hydrogen bonds with water through their own δ+ and δ− groups. Substances that dissolve in water are hydrophilic, "water-loving".

Non-polar substances, such as lipids, carry no charge for water to attract. Water molecules hydrogen-bond with each other instead and push the non-polar molecules together into droplets. These substances are hydrophobic, "water-fearing", and they are insoluble or only slightly soluble.

Why this makes water the medium for metabolism and transport:

A medium for metabolism. Most enzymes catalyse their reactions in aqueous solution, meaning solution in water. Dissolved substrate molecules move freely and keep colliding with the enzyme's active site, which is how reactions happen at a useful rate. Water also takes part in many reactions directly: it is used up when large molecules are broken down by hydrolysis and released when they are built by condensation.

A medium for transport in animals. Blood plasma is mostly water. Glucose, amino acids, mineral ions, hormones and urea travel dissolved in it. The substances that dissolve poorly are the telling cases. Oxygen is non-polar and only slightly soluble, which is why blood carries most of its oxygen bound to haemoglobin in red blood cells rather than dissolved. Fats and cholesterol are hydrophobic, so they travel packaged in lipoproteins, with a coat of hydrophilic molecules facing the water.

A medium for transport in plants. Xylem sap carries mineral ions, such as nitrate and potassium, dissolved in water from the roots. Phloem sap carries sucrose, dissolved, from the leaves to wherever it is needed.

The guide also asks you to understand the other side: some molecules work only because they are hydrophobic and insoluble.

• The plasma membrane is a double layer of phospholipids whose hydrophobic tails face inwards. That hydrophobic core is exactly what stops dissolved substances leaking in and out of the cell. A membrane that dissolved in water would not be a barrier.
• Fats and oils store energy. Because they are insoluble, they sit in droplets and can be packed densely without drawing water into the cell by osmosis.
• The waxy cuticle on a leaf is hydrophobic, so it waterproofs the leaf and cuts water loss.

So "water dissolves everything" is wrong twice over: it does not, and life depends on the things it does not dissolve.

7Living in water, living in air

Water and air are both fluids that animals live in and move through, but their physical properties are wildly different. The guide names four, and Figure 6 shows how far apart the two media are on each.

The specific heat capacity of water is high because of hydrogen bonds. Heating a substance makes its molecules move faster, but in water much of the energy supplied goes first into breaking hydrogen bonds, so the temperature rises only slowly. Put numbers on it: raising 1 kg of water by 5 °C takes 4.18 J g⁻¹ °C⁻¹ × 1,000 g × 5 °C = 20,900 J, while 1 kg of air needs only about 5,000 J. And a litre of water has around 800 times the mass of a litre of air, which is why the air over a lake can swing by 15 °C in a day while the lake barely moves.

The guide asks you to use animals that live in water and in air or on land to illustrate these differences. Two good ones are the black-throated loon (Gavia arctica), a diving bird of northern lakes and coasts, and the ringed seal (Pusa hispida), a seal of the Arctic sea ice. Both spend their lives crossing between the two media, so each property shows up as a problem they have had to solve. Figure 7 sets the solutions side by side.

The loon. Its bones are denser and less air-filled than those of most birds, which reduces buoyancy so it can dive after fish. Its body is streamlined against water's viscosity, and its feet, set far back, drive it forward like a propeller. Those features cost it in air and on land: with so little support from air, a heavy loon needs a long run across the water to get airborne, and with its legs so far back it walks very badly. Its dense, waterproof plumage traps a layer of air, a poor conductor, against the skin, which slows heat loss into the much more conductive water.

The ringed seal. It feeds in near-freezing sea water beneath the Arctic ice, where water draws heat from its body far faster than air would, so it carries a thick layer of blubber under the skin as insulation. The blubber also adds buoyancy, and supported by the water the seal needs no heavy skeleton; out on the ice it moves slowly and awkwardly. Its torpedo-shaped body slips through viscous water, driven by its hind flippers. Its habitat is cold but thermally stable: the sea under the ice stays close to −1.8 °C, the freezing point of sea water, while the air above can be tens of degrees colder.

When the exam says "using named examples", name the organism, name the property, and link the two with a because: "the ringed seal has thick blubber because water's high thermal conductivity would otherwise draw heat rapidly from its body".