Water: structure, properties and importance

25 min

The pot that cools itself

In many homes drinking water is kept in a large unglazed clay pot, and on a hot afternoon the water inside is noticeably cooler than the air. Water seeps slowly through the porous clay and evaporates from the outside of the pot, and each gram that evaporates carries away a large amount of heat. You cool yourself the same way when you sweat.

That one property, the large amount of energy needed to turn liquid water into vapour, comes from the way water molecules cling together. So do almost all the other properties that make water the medium of life: water makes up roughly 60–70 % of the human body. This lesson explains those properties at the level of the molecule.

Definition

Polar molecule: a molecule with an uneven distribution of charge. In water, oxygen attracts the shared electrons more strongly than hydrogen does, so the oxygen is slightly negative ( $\delta^-$ ) and the hydrogens slightly positive ( $\delta^+$ ).

Hydrogen bond: a weak attraction between a slightly positive hydrogen atom of one molecule and a slightly negative atom (such as oxygen or nitrogen) of another. Each is weak, but there are enormous numbers of them.

Specific heat capacity: the energy needed to raise the temperature of 1 g of a substance by 1 °C. For water it is $4.18\,\mathrm{J\,g^{-1}\,{}^\circ C^{-1}}$ .

Latent heat of vaporisation: the energy needed to turn 1 g of liquid into vapour without a change in temperature; for water at body temperature about $2.4\,\mathrm{kJ\,g^{-1}}$ .

Cohesion: attraction between molecules of the same substance; adhesion: attraction between molecules of different substances.

Properties of water and why they matter

Property	Explained by	Importance to living things
Excellent solvent for ions and polar molecules	Water molecules cluster around charged and polar particles (their $\delta^-$ oxygen towards cations, $\delta^+$ hydrogens towards anions) and separate them	Reactions in cells happen in solution; blood plasma, xylem and phloem sap carry dissolved substances; non-polar substances (lipids) do not dissolve, which allows membranes to form
High specific heat capacity	Energy is used to break hydrogen bonds before the molecules can move faster	Body and cell temperatures, and the temperatures of lakes, change slowly; a stable environment for enzymes
High latent heat of vaporisation	Many hydrogen bonds must be broken for a molecule to escape as vapour	Evaporation of sweat and transpiration from leaves cool the organism greatly for little water lost
Cohesion and surface tension	Hydrogen bonds between water molecules	Columns of water are pulled up the xylem without breaking; small animals such as pond skaters stand on the surface
Adhesion	Hydrogen bonds between water and other polar surfaces, such as cellulose	Helps water rise in narrow xylem vessels (capillarity) and move through cell walls
Maximum density at 4 °C; ice floats	In ice, hydrogen bonds hold molecules in an open lattice, further apart than in liquid water	In cold climates, ice forms an insulating layer on top of lakes and organisms survive in the liquid water below
Transparent	Visible light passes through	Light reaches photosynthesising algae and plants under water
Reactant (metabolite)	Takes part in reactions	Used in photosynthesis and in every hydrolysis reaction (for example in digestion); made in condensation reactions and respiration

Worked example 1: how much does sweating cool you?

During an hour of hard work in the sun, a 70 kg person evaporates 0.5 kg of sweat. Take the latent heat of vaporisation as $2400\,\mathrm{kJ\,kg^{-1}}$ and the specific heat capacity of the body as $3.5\,\mathrm{kJ\,kg^{-1}\,{}^\circ C^{-1}}$ . (a) How much heat does the evaporation remove? (b) By how much would the body temperature have risen if this heat had not been removed?

(a) Heat removed $= m \times L = 0.5 \times 2400 = 1200\,\mathrm{kJ}$ .
(b) Use $Q = m c \Delta T$ , so $\Delta T = \frac{Q}{m c} = \frac{1200}{70 \times 3.5} = \frac{1200}{245} = 4.9\,{}^\circ\mathrm{C}$ .
A rise of about 5 °C, from 37 °C to 42 °C, would be life-threatening. Losing only half a litre of water prevents it, because water's latent heat of vaporisation is so high. The water must, of course, be replaced by drinking.

Three large unglazed clay pots, used for storing drinking and cooking water, stand side by side in a yard. Water seeping through the porous walls evaporates from the outside and cools the water inside. Photo: Salifu Wumpini Hussein, CC BY-SA 4.0, via Wikimedia Commons

Worked example 2: why lake temperatures are stable

Calculate the energy needed to warm $1\,\mathrm{m^3}$ of lake water (mass 1000 kg) by 1 °C, and compare it with the energy needed to warm $1\,\mathrm{m^3}$ of air (mass 1.2 kg, specific heat capacity $1.0\,\mathrm{kJ\,kg^{-1}\,{}^\circ C^{-1}}$ ) by 1 °C.

Water: $Q = m c \Delta T = 1000 \times 4.18 \times 1 = 4180\,\mathrm{kJ}$ .
Air: $Q = 1.2 \times 1.0 \times 1 = 1.2\,\mathrm{kJ}$ .
The same volume of water needs about $\frac{4180}{1.2} \approx 3500$ times more energy. So while air temperature over Lake Kivu or Lake Muhazi swings between day and night, the water temperature barely changes, giving fish and other organisms a stable environment.

CrashCourse: "Water - Liquid Awesome" (Crash Course Biology #2). Listen for how hydrogen bonds explain cohesion, adhesion, high specific heat and the floating of ice.

Warning

Common mistakes

Confusing hydrogen bonds with the covalent O–H bonds inside a molecule. Hydrogen bonds are weak attractions between molecules.
Saying water has a "high boiling point so it cools us". The cooling comes from the latent heat absorbed when it evaporates.
Confusing specific heat capacity (energy to warm) with latent heat (energy to change state).
Saying oil does not dissolve in water "because it is heavier". It is lighter; it does not dissolve because it is non-polar.

Tip

Exam technique. Every water question has the same first step: "Water is polar, so hydrogen bonds form between molecules." Then link: hydrogen bonds → property (for example cohesion) → biological importance (water pulled up xylem). Three links, three marks.

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Practice: water

Explain why a water molecule is polar.
Explain how sodium chloride dissolves in water.
How much energy is needed to raise the temperature of 250 g of water from 20 °C to 37 °C?
A plant loses 2 kg of water by transpiration in a day. How much heat does this remove from the leaves? (Use $2.4\,\mathrm{kJ\,g^{-1}}$ .)
Explain how cohesion helps a tall tree supply water to its top leaves.
Explain why ice floating on a lake helps fish survive a cold winter.
Suggest why organisms that live in water experience a more stable temperature than those on land.

Show answersHide answers

Oxygen attracts the shared electrons more strongly than hydrogen, so the oxygen end is slightly negative and the hydrogen ends slightly positive, and the molecule is bent (104.5°), so the charges do not cancel.
The $\delta^-$ oxygen atoms of water molecules surround the $\mathrm{Na^+}$ ions and the $\delta^+$ hydrogens surround the $\mathrm{Cl^-}$ ions; these attractions pull the ions apart and keep them separate in solution.
$Q = m c \Delta T = 250 \times 4.18 \times 17 = 17\,765\,\mathrm{J} \approx 17.8\,\mathrm{kJ}$ .
$2\,\mathrm{kg} = 2000\,\mathrm{g}$ ; $2000 \times 2.4 = 4800\,\mathrm{kJ}$ (4.8 MJ).
Hydrogen bonds hold water molecules together, so when water evaporates from the leaves, the column of water in the xylem is pulled up as a continuous chain; adhesion to the vessel walls helps stop it breaking.
Ice is less dense than liquid water, so it floats and forms a layer on top that insulates the water below, which stays liquid (at about 4 °C at the bottom), so fish can survive.
Water has a high specific heat capacity, so a large amount of energy is needed to change its temperature; the water warms and cools far more slowly than the air.

Quick check

Why does sweating cool the body so effectively?

Key points

Water is polar, so hydrogen bonds form between its molecules.
Hydrogen bonding explains its solvent power, high specific heat capacity ( $4.18\,\mathrm{J\,g^{-1}\,{}^\circ C^{-1}}$ ), high latent heat of vaporisation, cohesion, adhesion, surface tension and the low density of ice.
These properties make water a transport medium, a temperature buffer, a coolant, a habitat and a reactant.

Sources and credits

1.2.3: Water, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for polarity, hydrogen bonding and the properties of water. Adapted by Ganzaa.
Figure 1, the worked examples (the body and sweat figures are typical values for estimates), practice and quick check: written by Ganzaa.
Photos: Salifu Wumpini Hussein, CC BY-SA 4.0; Richard Bartz, CC BY-SA 2.5; Baby Devi 26, CC BY-SA 4.0; all via Wikimedia Commons.
Video: "Water - Liquid Awesome: Crash Course Biology #2", CrashCourse, embedded from YouTube.

1The pot that cools itself

2Properties of water and why they matter

The pot that cools itself

Properties of water and why they matter