Water and Hydrogen Bonding: Study Guide (Biology 1.1)

Why one small molecule matters so much

Most of the mass of a living cell is water. Water moves nutrients, carries heat, takes part in reactions and gives cells their shape. Yet a water molecule is tiny: one oxygen atom and two hydrogen atoms. To see why it behaves so unusually, start with how its atoms share electrons.

Polar covalent bonds: where the partial charges come from

Inside a water molecule, oxygen shares one pair of electrons with each hydrogen atom. These are covalent bonds. But the sharing is not equal. Oxygen is more electronegative than hydrogen: it attracts the shared electrons more strongly.

• The shared electrons spend more time near the oxygen. Oxygen carries a partial negative charge (δ⁻).
• Each hydrogen is left with a partial positive charge (δ⁺).
• The molecule is bent (about 104.5° between the two O–H bonds), not straight. So the charges do not cancel out. One end of the molecule is negative and the other is positive.

A bond with an unequal share of electrons is a polar covalent bond. A molecule with a negative end and a positive end is a polar molecule. Polar does not mean charged overall: a water molecule has no net charge. It is not an ion.

Hydrogen bonds: attractions between molecules

Opposite partial charges attract. The δ⁺ hydrogen of one water molecule is attracted to the δ⁻ oxygen of a neighbouring molecule. This attraction is a hydrogen bond.

Key facts about hydrogen bonds in water:

• Each one is weak compared with a covalent bond, and short-lived. In liquid water they break and re-form constantly.
• Each water molecule can form hydrogen bonds with up to four neighbours. At any moment, liquid water is a shifting network of them.
• Because there are so many, together they have large effects. Every property below comes from this network.
• Hydrogen bonds also form within and between biological molecules, for example between the two strands of DNA and between parts of a folded protein.

From mechanism to biological outcome

Property	Molecular cause	Biological outcome
Cohesion	water molecules hydrogen-bond to each other	a continuous column of water can be pulled up through a plant’s xylem
Adhesion	water hydrogen-bonds to other polar surfaces, such as xylem cell walls	water clings to and creeps up narrow tubes, helping to resist gravity
Surface tension	molecules at the surface are pulled inward and sideways by neighbours	a “skin” strong enough for small insects such as pond skaters to stand on
High specific heat	energy must break hydrogen bonds before molecules move faster	water-rich organisms and lakes change temperature slowly
High heat of vaporization	a molecule must break all its hydrogen bonds to escape as gas	evaporation of sweat or water from leaves removes a lot of energy, cooling the organism
Ice is less dense than liquid	in ice, hydrogen bonds lock molecules into an open lattice	ice floats and insulates the water below, so aquatic life can survive winter
Good solvent	δ⁻ and δ⁺ ends surround ions and polar molecules	cytoplasm and blood can dissolve and transport ions, sugars and amino acids

Cohesion, adhesion and transpiration

In a plant, water evaporates from the leaves through pores called stomata. This loss of water is transpiration. As a molecule leaves, it pulls on the molecules behind it, because cohesion links them. This creates tension that draws the whole column upward from the roots. Adhesion to the xylem walls helps hold the column in place. Note what drives the flow: evaporation provides the pull; cohesion transmits it. Cohesion alone does not lift water.

Specific heat: why water resists temperature change

Specific heat capacity (c) is the energy needed to raise 1 g of a substance by 1 °C. For water, c = 4.18 J g⁻¹ °C⁻¹, which is high compared with most common substances. The relationship is:

q = m × c × ΔT

where q is energy in joules (J), m is mass in grams (g), c is in J g⁻¹ °C⁻¹ and ΔT is the temperature change in °C.

Why is c high? When water absorbs energy, part of it goes into breaking hydrogen bonds rather than into making molecules move faster. Temperature measures the average kinetic energy of the molecules, so the temperature rises slowly.

Reading Figure 2. Both liquids receive the same energy each second. The water line has the smaller gradient: about 0.060 °C s⁻¹, against 0.125 °C s⁻¹ for the oil. After 300 s (15 000 J), water has risen by about 17.9 °C and the oil by 37.5 °C. A smaller gradient for the same energy input means a larger specific heat. Real heaters lose some energy to the air, so measured lines would be slightly lower and may curve; the model shows the idea.

Heat of vaporization: evaporative cooling

To evaporate, a water molecule must break every hydrogen bond holding it in the liquid. Only the fastest-moving molecules have enough energy to escape. When they leave, the average energy of the molecules left behind falls, so the liquid cools. Water needs about 2.4 kJ per gram to evaporate near body temperature, a high value. This is why sweating cools mammals and why transpiration cools leaves.

Ice floats

Liquid water is densest at about 4 °C. As it cools further, hydrogen bonds increasingly hold molecules at fixed angles, and the water starts to expand. In ice, every molecule is hydrogen-bonded to four others in an open lattice, with more space between molecules than in the liquid. Ice is therefore less dense (about 0.92 g cm⁻³) than liquid water (about 1.00 g cm⁻³), and it floats. A floating ice layer insulates the water beneath it, so lakes tend to freeze from the top down, and organisms below can survive.

Water as a solvent

When an ionic compound such as sodium chloride dissolves, the δ⁻ oxygen ends of water molecules surround each Na⁺ ion, and the δ⁺ hydrogen ends surround each Cl⁻ ion. These hydration shells keep the ions apart in solution. Polar molecules such as glucose dissolve because they form hydrogen bonds with water. Substances that dissolve readily are hydrophilic. Nonpolar substances, such as oils and fats, cannot form these attractions and do not dissolve well: they are hydrophobic. This difference matters later, when you study cell membranes.

Worked example 1: temperature buffering in a rock pool

Question. On a sunny morning, 500 g of water in a shallow rock pool and 500 g of dry sand beside it each absorb 4.18 kJ of energy from sunlight. Calculate the temperature rise of each. Take c(water) = 4.18 J g⁻¹ °C⁻¹ and c(dry sand) = 0.80 J g⁻¹ °C⁻¹. Assume no energy is lost.

1. Convert the energy to joules: q = 4.18 kJ × 1000 = 4180 J.
2. Rearrange q = mcΔT to make ΔT the subject: ΔT = q ÷ (m × c).
3. Water: ΔT = 4180 J ÷ (500 g × 4.18 J g⁻¹ °C⁻¹) = 4180 ÷ 2090 = 2.0 °C.
4. Sand: ΔT = 4180 J ÷ (500 g × 0.80 J g⁻¹ °C⁻¹) = 4180 ÷ 400 = 10.45 °C, which is 10 °C to 2 significant figures (matching c = 0.80).

Check the units. J ÷ (g × J g⁻¹ °C⁻¹) leaves °C, which is right for a temperature change.

Interpretation. For the same energy, the sand warms about five times as much as the water (10.45 ÷ 2.0 ≈ 5.2). Animals living in the pool face much smaller temperature swings than animals on the sand. The same reasoning explains why an organism made mostly of water can keep a fairly steady body temperature. The model ignores evaporation and heat loss, so real changes would differ, but the comparison holds.

Worked example 2: designing and reading a transpiration experiment

Question. A student uses a potometer (a device that measures how fast a cut shoot takes up water) to test the hypothesis: “Moving air increases the rate of transpiration.” Three similar leafy shoots of the same species are tested in still air and then with a fan on its lowest setting. Results (water uptake, mL h⁻¹):

Condition	Trial 1	Trial 2	Trial 3	Mean
Still air (control)	0.42	0.45	0.39	0.42
Fan on	0.81	0.86	0.79	0.82

(a) Identify the independent variable, dependent variable, control group and three controlled variables. (b) Calculate the percentage change in mean uptake. (c) Explain the result in terms of hydrogen bonding, and state how confident the conclusion can be.

(a) Variables.

1. Independent variable: air movement (fan off or fan on).
2. Dependent variable: rate of water uptake, in mL h⁻¹, used as a measure of transpiration.
3. Control group: the still-air condition, which gives the baseline to compare against.
4. Controlled variables (any three): light intensity, air temperature, humidity, species, leaf area of each shoot, time allowed to settle before readings.

(b) Calculation.

1. Mean still air: (0.42 + 0.45 + 0.39) ÷ 3 = 1.26 ÷ 3 = 0.42 mL h⁻¹.
2. Mean fan on: (0.81 + 0.86 + 0.79) ÷ 3 = 2.46 ÷ 3 = 0.82 mL h⁻¹.
3. Percentage change = (0.82 − 0.42) ÷ 0.42 × 100 = 0.40 ÷ 0.42 × 100 = +95% (about double).

(c) Explanation. The fan carries away humid air next to the leaf, so water vapour diffuses out of the stomata faster. Each molecule that evaporates pulls on the next through cohesion (hydrogen bonds between water molecules), and adhesion holds the column against the xylem walls. Faster evaporation therefore means faster uptake at the cut stem.

How confident? The ranges do not overlap (0.39–0.45 against 0.79–0.86), so the difference is unlikely to be chance alone. The data support the hypothesis for this species under these conditions. They do not prove it in general: there were only three shoots, and uptake is not exactly equal to transpiration (a little water is used by the plant).

Correlation versus causation. Suppose a separate field survey found that plants at windier sites lose more water. That is only a correlation. The windier sites might also be sunnier or drier, and either could cause the extra water loss (a confounding variable). The fan experiment is stronger evidence for cause, because only the air movement was changed while the other variables were held constant.

Common misconceptions

• “Hydrogen bonds hold the H and O together inside a water molecule.” No. Those are covalent bonds. Hydrogen bonds are the weaker attractions between molecules (or between parts of a large molecule).
• “Polar means charged.” A water molecule has partial charges but no overall charge. It is not an ion.
• “High specific heat means water heats up quickly.” The opposite: a high specific heat means more energy is needed per degree, so water warms and cools slowly.
• “Ice floats because of trapped air.” Pure, clear ice also floats. The open hydrogen-bonded lattice makes ice less dense.
• “Cohesion pulls water up the plant.” Evaporation from the leaves provides the pull. Cohesion links the molecules so the pull reaches the roots.
• “Evaporation cools because the escaping water is cold.” The escaping molecules are the most energetic ones. The liquid left behind has a lower average energy.
• “Water dissolves everything.” Nonpolar substances such as lipids are hydrophobic and dissolve poorly.
• “A correlation shows that one variable causes the other.” Without controlling other variables, a third factor could explain both.

Where this leads

Hydrogen bonding returns throughout the course: in the structure of proteins and nucleic acids (Topics 1.3–1.6), in membranes and the movement of water across them (Unit 2), and in plant transport. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.