Water

AS · 12 min

Water makes up about 70–95% of the mass of a cell, and life almost certainly began in it. Its unusual properties all come from one feature: hydrogen bonding between water molecules. This note explains how hydrogen bonds form and then relates three properties, as the syllabus limits them, to their roles in organisms: solvent action, high specific heat capacity and high latent heat of vaporisation. The same ideas return in transport in plants (cohesion in the xylem) and transport in mammals (blood as a transport medium).

The water molecule is polar

A water molecule ( $\ce{H2O}$ ) consists of one oxygen atom covalently bonded to two hydrogen atoms. Oxygen attracts the shared electrons more strongly than hydrogen does, so the electrons spend more time near the oxygen. This gives:

the oxygen atom a small negative charge, written $\delta^-$ ;
each hydrogen atom a small positive charge, written $\delta^+$ .

The molecule is bent (the two O–H bonds are at an angle of about 104.5°), so these charges do not cancel out: one side of the molecule is slightly negative and the other slightly positive. A molecule with an uneven distribution of charge like this is polar. Having positive and negative ends is called a dipole.

Hydrogen bonds

Because water molecules are polar, the slightly positive hydrogen atom of one water molecule is attracted to the slightly negative oxygen atom of a neighbouring water molecule. This attraction is a hydrogen bond.

Definition

A hydrogen bond is a weak attraction between a slightly positively charged hydrogen atom ( $\delta^+$ ) in one molecule (or part of a molecule) and a slightly negatively charged atom, such as oxygen or nitrogen ( $\delta^-$ ), in another.

Each water molecule can form hydrogen bonds with up to four others. Each hydrogen bond is weak (about one-tenth the strength of a covalent bond), and they are constantly breaking and re-forming in liquid water. But there are so many of them that, together, they have a large effect: they hold water molecules together much more strongly than the molecules of similar-sized compounds such as methane ( $\ce{CH4}$ ) or hydrogen sulfide ( $\ce{H2S}$ ), which cannot form hydrogen bonds. That is why water is a liquid at the temperatures found on most of Earth, while hydrogen sulfide, a heavier molecule, is a gas.

Key result

Property	Caused by	Role in living organisms
Solvent action: dissolves ions and polar molecules	water molecules are polar; they cluster around ions and form hydrogen bonds with polar molecules	medium for metabolic reactions in cells; transport of dissolved substances in blood, xylem and phloem; removal of wastes (urea in urine)
High specific heat capacity: a lot of energy is needed to raise the temperature	energy must break many hydrogen bonds before molecules can move faster	temperature of cells, bodies and aquatic habitats stays stable; enzymes are protected from rapid temperature change
High latent heat of vaporisation: a lot of energy is needed to evaporate water	energy must break hydrogen bonds for molecules to escape as vapour	evaporation of sweat (and transpiration from leaves) has a strong cooling effect for little water lost

Solvent action

A substance dissolves in water if water molecules can surround it and separate its particles.

Ions. When an ionic compound such as sodium chloride is added to water, the slightly negative oxygen ends of water molecules are attracted to the positive sodium ions ( $\ce{Na+}$ ), and the slightly positive hydrogen ends are attracted to the negative chloride ions ( $\ce{Cl-}$ ). Each ion becomes surrounded by a shell of water molecules and is separated from the others: it dissolves.
Polar molecules. Molecules with polar groups, such as glucose and amino acids (with –OH, $\ce{-NH2}$ or $\ce{-COOH}$ groups), form hydrogen bonds with water molecules and so dissolve.
Non-polar molecules, such as triglycerides, cannot interact with water: they are hydrophobic and do not dissolve (see Lipids).

Why solvent action matters

Medium for reactions. Most metabolic reactions take place in solution in the cytoplasm, matrix of mitochondria or stroma of chloroplasts. Dissolved molecules move about freely and collide, so reactions can happen. Water also takes part in many reactions as a reactant (hydrolysis, photosynthesis) or product (condensation, respiration).
Transport. Substances are transported dissolved in water: glucose, amino acids, ions, hormones, urea and carbon dioxide (as hydrogencarbonate ions) in blood plasma; mineral ions in the xylem; sucrose and amino acids in the phloem. Water is the main component of blood and tissue fluid.
Excretion. Soluble waste, such as urea, is removed from the body dissolved in water as urine.
Shape of molecules and membranes. The way water interacts with hydrophilic and hydrophobic groups drives protein folding (hydrophobic R groups inside) and the formation of phospholipid bilayers.

High specific heat capacity

Definition

The specific heat capacity of a substance is the amount of energy needed to raise the temperature of 1 kg of it by 1 °C (1 K). For water it is about $4.2\ \text{kJ kg}^{-1}\ ^\circ\text{C}^{-1}$ , which is high compared with most other liquids.

Temperature is a measure of the kinetic energy of the molecules. To make water molecules move faster, energy must first be used to break some of the hydrogen bonds that hold them together. So a large input of energy causes only a small rise in temperature, and water also loses heat slowly as it cools.

Consequences for living organisms:

The body temperature of organisms (which are mostly water) does not change rapidly when the temperature of the environment changes or when metabolic reactions release heat. This keeps the internal temperature stable, close to the optimum for enzymes, and helps homeostasis.
Aquatic habitats (oceans, lakes, ponds) change temperature only slowly through the day and the year, providing stable environments for the organisms living in them.
Water in blood carries heat from active organs (such as liver and muscles) around the body without large temperature changes, spreading heat evenly.

High latent heat of vaporisation

Definition

The latent heat of vaporisation is the amount of energy needed to change a liquid into a vapour (gas) without changing its temperature. For water at body temperature it is about $2.4\ \text{kJ g}^{-1}$ ( $2400\ \text{kJ kg}^{-1}$ ), which is very high.

For a water molecule to evaporate, it must break all of its hydrogen bonds with neighbouring molecules and escape from the surface. This takes a lot of energy, which the molecule takes from its surroundings, so evaporation cools the surface from which it happens.

Sweating in mammals: water in sweat evaporates from the skin, taking a large amount of heat energy from the body. Because the latent heat is so high, a large amount of heat is lost for a relatively small loss of water.
Panting in dogs: evaporation of water from the moist surfaces of the mouth and airways.
Transpiration in plants: evaporation of water from the leaves cools them in hot conditions.

It also means that large bodies of water do not evaporate away easily, so aquatic habitats are relatively stable.

Explain the solvent properties of water (4 marks)

Explain how the structure of water molecules makes water a good solvent, and why this is important in the transport of glucose in the blood.

Solution

Water molecules are polar: the oxygen is slightly negative ( $\delta^-$ ) and the hydrogens slightly positive ( $\delta^+$ ).
Water molecules are attracted to ions (they surround and separate them) and form hydrogen bonds with polar molecules.
Glucose is polar (it has many –OH groups), so it forms hydrogen bonds with water and dissolves.
Dissolved glucose is carried in the plasma, which is mainly water, so it can be transported around the body to respiring cells by mass flow.

Sweat and cooling (calculation)

The latent heat of vaporisation of water at skin temperature is $2.4\ \text{kJ g}^{-1}$ .

(a) A runner evaporates 0.5 kg of sweat during a race. Calculate the heat energy removed. (b) The runner's body contains about 42 kg of water, with a specific heat capacity of $4.2\ \text{kJ kg}^{-1}\ ^\circ\text{C}^{-1}$ . If this energy had not been lost, by how much would it have raised the temperature of the body water?

Solution

(a) $0.5\ \text{kg} = 500\ \text{g}$ , so energy removed $= 500 \times 2.4 = 1200\ \text{kJ}$ .

(b) Energy $= m \times c \times \Delta T$ , so

\Delta T = \frac{1200}{42 \times 4.2} = \frac{1200}{176.4} = 6.8\ ^\circ\text{C}

A rise of nearly 7 °C would be fatal. The two properties work together: the high specific heat capacity means the body warms only slowly, and the high latent heat means a modest amount of sweat removes a great deal of heat.

Comparing liquids (interpreting data)

The table compares water with two other liquids.

Liquid	$M_r$	Boiling point / °C	Specific heat capacity / kJ kg⁻¹ °C⁻¹
water	18	100	4.2
hydrogen sulfide	34	−60	not liquid at room temperature
ethanol	46	78	2.4

Explain why water has a higher boiling point and specific heat capacity than these liquids, despite having the smallest $M_r$ .

Solution

Water molecules form extensive hydrogen bonds with one another (up to four per molecule). Energy is needed to break these bonds before molecules can move faster (raising temperature) or escape as vapour (boiling). Hydrogen sulfide cannot form hydrogen bonds (sulfur is much less electronegative than oxygen), so its molecules are held together only weakly and it boils at −60 °C. Ethanol forms some hydrogen bonds (through its single –OH group), but fewer per molecule than water, so it needs less energy to heat and boil.

Exam-hard: linking properties to a whole organism

Suggest why a large mammal living in a desert can survive temperatures that would rapidly kill a small mammal, using your knowledge of the properties of water.

Solution

A large mammal contains a large mass of water, and water has a high specific heat capacity, so a lot of energy is needed to raise its body temperature; it heats up slowly during the day.
A small mammal has less water (smaller mass) and a larger surface area to volume ratio, so it gains heat more quickly per unit mass and its temperature rises faster.
The large mammal can lose heat by evaporating water (sweating, panting), and the high latent heat of vaporisation means a lot of heat is lost per gram of water.
A small mammal would have to evaporate a larger proportion of its body water to lose the same proportion of its heat, risking dehydration, so small desert mammals usually avoid the heat (for example by burrowing) instead.

Tip

Hydrogen bonding also gives water cohesion (water molecules stick to each other) and adhesion (water sticks to other polar surfaces such as cellulose). These explain how water moves up the xylem as a continuous column, and they are covered in Water uptake and transpiration. Ice being less dense than liquid water is also a consequence of hydrogen bonding, but it is not on this part of the syllabus.

Watch out

A hydrogen bond is not the covalent bond between hydrogen and oxygen within a water molecule. It is the attraction between molecules.
"Water is a good solvent because it is a liquid" earns nothing. The reason is that it is polar.
Do not confuse specific heat capacity (energy to raise temperature) with latent heat of vaporisation (energy to evaporate). Each has a different biological significance: stability of temperature versus cooling by evaporation.
Sweat cools the body by evaporating, not by being on the skin or by being cool itself.

Exam tip

The syllabus asks you to "explain how hydrogen bonding occurs between water molecules": polar molecule, $\delta^-$ oxygen, $\delta^+$ hydrogen, attraction between $\delta^+$ H of one molecule and $\delta^-$ O of another.
Then relate properties to roles, limited to solvent action, high specific heat capacity and latent heat of vaporisation. For each, give the property, the reason (hydrogen bonds or polarity) and a role in an organism.
A diagram showing two or three water molecules with $\delta^+$ and $\delta^-$ labelled and the hydrogen bond drawn as a dashed line between H and O of different molecules is often credited.

Summary

Water is polar: oxygen is $\delta^-$ , hydrogens are $\delta^+$ .
Hydrogen bonds form between the $\delta^+$ H of one molecule and the $\delta^-$ O of another; weak individually but very numerous.
Solvent action: polar water surrounds ions and hydrogen-bonds with polar molecules; medium for reactions and transport.
High specific heat capacity (about $4.2\ \text{kJ kg}^{-1}\ ^\circ\text{C}^{-1}$ ): hydrogen bonds absorb energy, so temperature is stable in organisms and habitats.
High latent heat of vaporisation: evaporation breaks all hydrogen bonds, so sweating and transpiration cool effectively with little water lost.

Practice questions

Question

Explain why a water molecule is polar.
Describe how a hydrogen bond forms between two water molecules.
Explain why sodium chloride dissolves in water but a triglyceride does not. (3 marks)
State two roles of water as a solvent in mammals.
Define specific heat capacity and explain why water's is high.
Explain how the high specific heat capacity of water benefits fish living in a lake. (2 marks)
Calculate how much energy is needed to raise the temperature of 1000 kg of pond water by 5 °C (specific heat capacity $4.2\ \text{kJ kg}^{-1}\ ^\circ\text{C}^{-1}$ ).
Explain why sweating is an effective way of losing heat. (3 marks)
How many grams of sweat must evaporate to remove 1000 kJ of heat (latent heat $2.4\ \text{kJ g}^{-1}$ )?
Blood is mainly water. Explain how two properties of water make blood an effective transport medium in mammals. (4 marks)

Answers

Oxygen attracts the shared electrons in the O–H bonds more strongly than hydrogen; oxygen is slightly negative and hydrogens slightly positive; the molecule is bent, so the charges are unevenly distributed.
The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of another water molecule; this weak attraction is a hydrogen bond.
Water is polar; the $\delta^-$ oxygen of water is attracted to $\ce{Na+}$ and the $\delta^+$ hydrogens to $\ce{Cl-}$ , so water molecules surround and separate the ions. A triglyceride is non-polar/hydrophobic; it cannot form hydrogen bonds or interact with water, so it does not dissolve.
Any two: medium for metabolic reactions in cells; transport of dissolved substances in plasma (glucose, ions, hormones, urea); excretion of urea in urine; transport of carbon dioxide as hydrogencarbonate ions.
The energy needed to raise the temperature of 1 kg of the substance by 1 °C. Water's is high because energy must be used to break hydrogen bonds between molecules before their kinetic energy (temperature) can increase.
Lake water changes temperature only slowly (needs a lot of energy to warm or must lose a lot to cool), so the habitat temperature is stable; fish (ectotherms) and their enzymes are not exposed to rapid temperature changes.
$E = 1000 \times 4.2 \times 5 = 21\,000\ \text{kJ}$ ( $2.1 \times 10^{4}\ \text{kJ}$ ).
Water has a high latent heat of vaporisation; to evaporate, water molecules must break hydrogen bonds, which requires a lot of energy; this energy (heat) is taken from the skin/body, so a lot of heat is lost for a small loss of water.
$1000 \div 2.4 = 417\ \text{g}$ (3 s.f.).
Solvent action: water is polar, so it dissolves ions and polar molecules (glucose, amino acids, urea, hydrogencarbonate ions), which can be transported in solution in plasma to and from cells. High specific heat capacity: blood can absorb heat from active organs (e.g. liver, muscle) without a large temperature rise and distribute it around the body, helping keep body temperature stable.

The water molecule is polar

Hydrogen bonds

Solvent action

Why solvent action matters

High specific heat capacity

High latent heat of vaporisation

Practice questions

Console