Water as the medium for life, and hydrogen bonding

← Explore another subject

Biology · Unity and diversity: molecules · SL · Guide 2025

Your course level

This complete sample covers content shared by SL and HL. Your selected level carries through when you sign up.

Try the highlighter ↓Read the section ↓See this unit’s study tools ↓

Try the real highlighter

Pick up the highlighter, choose a colour, then tap a passage. Trial highlights last until you leave or refresh.

Key idea · Definition · Exam tip · Revisit. Create an account to save highlights in your personal notebook.

Water as the medium for life, and hydrogen bonding

Life began in water and still runs on it.

This section explains what it means to call water the medium for life, and then the one feature of the water molecule that every later property depends on: polarity, and the hydrogen bonds it causes.

Water as the medium for life

A medium is the substance in which something takes place.

The first cells originated in water, and the reason is simple.

Only in a liquid can dissolved molecules move about, meet and react.

In a solid they are locked in place, and most of the molecules of life cannot exist as a gas.

Water was the liquid available on the early Earth, and it dissolves a very wide range of substances.

So the chemistry that led to cells, and the first cells themselves, were surrounded by water and filled with it.

Water is still the medium in which most processes of life occur:

Inside cells. A typical active cell is about 70% water by mass. The cytoplasm is a watery solution, and most reactions of metabolism happen between substances dissolved in it.

Between cells. Transport fluids such as blood plasma and the sap in xylem and phloem are mostly water, carrying dissolved substances around the organism.

Outside the organism. Many organisms live in water all their lives. Those that live on land carry their water with them and must replace what they lose.

The evidence that water is needed, not just present, comes from what happens without it.

Dry seeds, spores and dried mosses can stay alive, but their metabolism falls almost to zero.

It restarts only when water returns.

In an exam answer, "medium" needs its reason.

"Life needs water" earns nothing.

"Reactants are dissolved in water, so they can move, collide and react" earns the mark.

Why a water molecule is polar

A water molecule has two covalent bonds, each an oxygen atom sharing a pair of electrons with a hydrogen atom.

Oxygen attracts shared electrons much more strongly than hydrogen does, so the sharing is unequal.

The electrons spend more time near the oxygen, giving it a small negative charge ( $\delta^-$ ) and leaving each hydrogen with a small positive charge ( $\delta^+$ ).

These are partial charges, not full ones.

No electron has been transferred, so water contains no ions: the molecule as a whole is neutral.

The molecule is bent, not straight, so the partial charges do not cancel out.

One end of the molecule is slightly negative and the other slightly positive.

A covalent bond with unequal sharing like this is a polar covalent bond, and a molecule with oppositely charged ends is a polar molecule.

Hydrogen bonds between molecules

Opposite partial charges attract.

The $\delta^+$ hydrogen of one water molecule is drawn towards the $\delta^-$ oxygen of a neighbouring molecule.

This attraction is a hydrogen bond.

In water it is an intermolecular force: it acts between molecules, not within one.

The order of cause and effect matters.

Unequal sharing of electrons makes the bonds polar.

Polarity gives the atoms partial charges.

The partial charges cause hydrogen bonding.

A hydrogen bond is a consequence of polarity, never its cause.

A single hydrogen bond is weak, far weaker than a covalent bond.

But every water molecule is hydrogen bonded to several neighbours, so liquid water holds vast numbers of hydrogen bonds at any moment, and their combined effect is large.

That combined effect is why water is a liquid at the temperatures found on most of the Earth, when other molecules of similar size are gases.

It also explains cohesion, adhesion, water's solvent properties and its thermal properties, each of which has its own section.

Four polar water molecules with partial charges shown, linked by dashed hydrogen bonds from the hydrogen of one molecule to the oxygen of the next.

① Polar covalent bond within one water molecule

② Hydrogen bond between two molecules

③ Oxygen atom, partial negative charge

④ Hydrogen atom, partial positive charge

Drawing water molecules and hydrogen bonds

You are expected to draw two or more water molecules with the hydrogen bonds between them, using the standard notation.

A marker looks for four things:

each molecule drawn as one O joined to two H atoms by solid lines, in a bent (V) shape

$\delta^-$ written on every oxygen and $\delta^+$ on every hydrogen

each hydrogen bond drawn as a dashed or dotted line

that line running from an H of one molecule to the O of a different molecule, and labelled

✓

EXAM FOCUS

How the IB examines this

Paper 1A, single best answer: you are asked what causes the polarity of water, or where a hydrogen bond forms. The distractors swap the two bonds (a hydrogen bond placed inside a molecule) or turn partial charges into ions.

Paper 1A, labelled diagram: a drawing of several water molecules carries labels, and you choose the label that shows a hydrogen bond or a polar covalent bond. Check whether the line is solid or dashed, and whether it joins atoms of one molecule or two.

Paper 2, draw: two or three marks for water molecules with hydrogen bonds between them. Marks are lost for a straight molecule, missing partial charges, or a dashed line that joins H to H.

Paper 2, explain or outline: two or three marks for the chain from unequal sharing to polarity to hydrogen bonding. Each link is a separate mark, so an answer that jumps straight to "water forms hydrogen bonds" scores one at most.

Paper 2 Section B, short part: an outline of why water is the medium for life. One distinct reason per mark, each with its "so that".

Paper 1B, data: results linking water content to an organism's activity, to read, process and explain. The explanation mark needs "dissolved" or "in solution", not just "water is needed".

◆

Key terms

Medium: the substance in which a process takes place; for life, the water in which substances dissolve, move and react.

Polar covalent bond: a covalent bond in which electrons are shared unequally, so one atom carries a partial negative charge and the other a partial positive charge.

Polar molecule: a molecule with an uneven distribution of charge, so one part is partially positive and another partially negative.

Hydrogen bond: a weak attraction between a partially positive hydrogen atom of one molecule and a partially negative atom, such as oxygen, of another molecule.

Intermolecular force: an attraction between separate molecules, as opposed to a bond within a molecule.

↗

Worked example 1: drawing hydrogen bonds

Draw three water molecules, showing the hydrogen bonding between them. [3]

1. Draw each molecule as O with two H atoms attached by solid lines, in a bent shape.

2. Label every O with $\delta^-$ and every H with $\delta^+$ .

3. Arrange the molecules so an H of one points at the O of the next.

4. Join that H to that O with a dashed line, and label the dashed line "hydrogen bond".

Answer: three bent molecules, each with solid O-H bonds [1]; $\delta^-$ on oxygen and $\delta^+$ on hydrogen shown on the molecules [1]; at least two dashed lines, each from an H of one molecule to the O of a different molecule, labelled as hydrogen bonds [1].

↗

Worked example 2: two kinds of bond

Distinguish between the covalent bonds and the hydrogen bonds in water. [2]

1. Distinguish asks for differences only, and each point must mention both bonds.

2. Say where each bond is: covalent bonds hold O and H together within one molecule, whereas hydrogen bonds link separate molecules.

3. Say what each bond is: a covalent bond is a shared pair of electrons, whereas a hydrogen bond is an attraction between partial charges.

4. Compare strength: covalent bonds are much stronger than hydrogen bonds.

Answer: any two of: covalent bonds are within a molecule whereas hydrogen bonds are between molecules [1]; a covalent bond is a shared pair of electrons whereas a hydrogen bond is an attraction between opposite partial charges [1]; covalent bonds are stronger than hydrogen bonds [1].

Two separate descriptions, one of each bond with no link between them, would not score.

↗

Worked example 3: water and methane (Paper 2 style)

The table compares water with methane, a molecule of similar size.

Water

Methane

Formula

H₂O

CH₄

Relative molecular mass

Sharing of electrons in its bonds

unequal

almost equal

Boiling point / °C

100

−162

(a) State the physical state of each substance at 20 °C. [1]

(b) Explain why hydrogen bonds form between water molecules but not between methane molecules. [3]

1. (a) Water is a liquid and methane is a gas. [1] Both are needed: 20 °C is below water's boiling point and far above methane's.

2. (b) In water, oxygen attracts the shared electrons more strongly than hydrogen, so the bonds are polar [1]. Oxygen carries a partial negative charge and hydrogen a partial positive charge, and the hydrogen of one molecule is attracted to the oxygen of another [1]. In methane the electrons are shared almost equally, so the atoms carry no significant partial charges and there is nothing for a neighbouring molecule to be attracted to [1].

3. (c) Hydrogen bonds hold water molecules together, so water is a liquid at the temperatures found on Earth [1]. Substances can dissolve in a liquid and move, meet and react, which they cannot do in the same way in a gas [1].

Answer: (a) water liquid, methane gas; (b) polar bonds and partial charges in water only; (c) water is liquid, so it can hold reactants in solution.

In (b) the mark scheme wants both molecules explained: an answer about water alone stops at two marks.

The graph below is used in worked example 4.

Rate of carbon dioxide release by moss shoots against water content, for invented data: zero at 5%, 0.4 at 20%, 2.1 at 40%, 5.6 at 60% and 6.0 micromoles per gram per hour at 80%

↗

Worked example 4: water content and metabolism (Paper 1B style)

Some mosses survive being dried out.

Shoots of one such moss were dried to five different water contents, and the rate at which they released carbon dioxide in the dark was measured.

The graph above shows the results, which were written for this question.

(a) State the rate of carbon dioxide release at a water content of 40%. [1]

(b) Calculate the percentage increase in the rate when the water content rises from 40% to 60%. [2]

(d) Explain the results in terms of the role of water in cells. [2]

1. (a) 2.1 μmol g⁻¹ h⁻¹. [1] The unit is part of the answer.

2. (b) $\dfrac{5.6 - 2.1}{2.1} \times 100$ [1] $= 167\%$ [1]. Divide by the starting value, 2.1, not by 5.6.

3. (c) The rate rises as the water content rises, from 0 at 5% to 6.0 μmol g⁻¹ h⁻¹ at 80% [1]. The rise is steepest between 40% and 60% and levels off above 60%, where the rate changes only from 5.6 to 6.0 [1].

4. (d) Carbon dioxide is released by cell respiration, whose reactions take place between substances dissolved in water in the cells [1]. With little water the reactants and enzymes cannot move and collide, so the reactions slow or stop; with more water they are in solution and react faster [1].

Answer: (a) 2.1 μmol g⁻¹ h⁻¹; (b) 167%; (c) a rise that levels off above 60%; (d) metabolism needs its reactants in solution.

A describe answer about a graph needs figures quoted from it, as in (c).

✦

Exam tip

In a drawing, examiners look for the dashed line going from H of one molecule to O of another, with $\delta^+$ and $\delta^-$ on the atoms.

A dashed line from H to H, or one drawn inside a single molecule, earns nothing.

Common mistakes

Saying water is polar because it forms hydrogen bonds. It is the other way round: polarity comes first, and hydrogen bonds are its consequence.

Writing that boiling breaks the O-H bonds. Boiling only separates molecules by breaking hydrogen bonds; the molecules stay intact.

Calling a hydrogen bond a covalent bond. No electrons are shared; it is an attraction between opposite partial charges.

Writing full charges ( $\ce{O^2-}$ , $\ce{H+}$ ) on a water molecule. The charges are partial, shown as $\delta^-$ and $\delta^+$ , and the molecule has no overall charge.

Answering "water is the medium for life" with "organisms need to drink". The point is that the reactions of life take place in solution in water.

Check your understanding

1. What causes the polarity of a water molecule? [1]
A. The transfer of electrons from hydrogen to oxygen, forming ions
B. Hydrogen bonds between neighbouring water molecules
C. The unequal sharing of electrons between oxygen and hydrogen
D. The equal sharing of electrons in a bent molecule

2. Which row describes the bonding in liquid water? [1]

Holds the atoms together within a molecule

Acts between neighbouring molecules

polar covalent bond

hydrogen bond

polar covalent bond

hydrogen bond

polar covalent bond

hydrogen bond

3. Between which two atoms does a hydrogen bond form in liquid water? [1]
A. An oxygen atom and a hydrogen atom of the same molecule
B. The hydrogen atoms of two neighbouring molecules
C. The oxygen atoms of two neighbouring molecules
D. A hydrogen atom of one molecule and the oxygen atom of another

4. Which statements about hydrogen bonds in liquid water are correct? [1]

I. Each one is weaker than a covalent bond.
II. Each one forms between atoms carrying opposite partial charges.
III. Each one is a pair of electrons shared between two molecules.

A. I only
B. I and II only
C. II and III only
D. I, II and III

5. Which statement explains why water is described as the medium for life? [1]
A. Water supplies the energy that drives the reactions of metabolism.
B. Water molecules are the catalysts of most reactions in cells.
C. Most reactions of metabolism take place between substances dissolved in water.
D. The first cells arose on dry land and later moved into water.

6. State what is meant by a polar covalent bond. [1]

7. Outline two reasons why water was a suitable medium for the first cells. [2]

8. A student writes: "When water boils, the hydrogen and oxygen atoms are pulled apart." Explain why this statement is wrong. [2]

9. Draw two water molecules with a hydrogen bond between them. [3]

10. A student's drawing of three water molecules has these features: each molecule is drawn as a straight line H-O-H; every oxygen is labelled $\delta^+$ and every hydrogen $\delta^-$ ; dashed lines join hydrogen atoms of neighbouring molecules.
(a) Identify three errors in the drawing, and state the correction for each. [3]
(b) In a correct drawing, state the type of bond shown by each solid line. [1]
(c) A single hydrogen bond is weak. Explain why hydrogen bonding still has a large effect on the properties of water. [2]

11. Students measured the water content of plant material. Each sample was weighed, dried in an oven at 80 °C, and reweighed at intervals until its mass stopped changing. The results were written for this question.

Sample

Fresh mass / g

Dry mass / g

Water content / % of fresh mass

Lettuce leaf

12.50

0.60

95.2

Potato tuber

20.00

4.20

79.0

Dry barley grains

8.00

7.04

Barley grains soaked for 24 hours

15.00

6.90

54.0

(a) Calculate the water content of the dry barley grains. [2]
(b) Explain why each sample was reweighed until its mass stopped changing. [1]
(c) Suggest why the oven was set at 80 °C and not at 250 °C. [1]
(d) Dry barley grains release almost no carbon dioxide, but soaked grains release it rapidly. Explain this difference. [2]

12. Explain how hydrogen bonds form between water molecules. [5]

Try the questions before checking your work.

Continue your study journey

Your free account opens the rest of this free unit, its confidence checklist, study planning and saved highlights. Other units require a paid plan. Practice availability depends on the subject and unit.

Create free account and continue reading Already have an account? Continue here

Put this topic into practice.

These tools belong to this free unit. Create an account to open them and save your progress.

Practice questions

Try the authored questions, check the answer guidance and record your own marks.

Create account to practise

Learning checklist

Check your confidence and choose what to revisit. Your ratings stay separate from practice results.

Create account for checklist

Study booklet

Read and print this unit’s existing notes, available practice and learning checklist together.

Create account for booklet