DP Biology SL A1.1 Water

DP Biology SL A1.1 Water - Full notes

RevisionPrep

DP Biology SL | A1.1 Water

DP BIOLOGY · THEME A: UNITY AND DIVERSITY

A1.1

Water

Full notes for SL · IB-style

Contents

1	Water as the medium for life
2	The structure of the water molecule and hydrogen bonding
3	Cohesion and adhesion
4	Water as a solvent
5	Thermal properties of water
6	Physical properties and how organisms use them
7	Polarity in detail
8	Hydrogen bonds: strength in numbers
9	Cohesion in plants: the transpiration stream
10	Surface tension and organisms
11	Adhesion and capillary action
12	Solvent properties and transport in blood
13	Hydrophilic and hydrophobic molecules
14	Specific heat capacity and temperature stability
15	Latent heat of vaporization and cooling
16	Ice: less dense than liquid water
17	Physical properties: buoyancy, viscosity and conductivity
18	Water in metabolism
19	Mixed review: worked problems
20	Additional exam-style practice
21	Water and the search for life
22	Common misconceptions and links
23	Exam-style question with markscheme
24	Check your understanding
25	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain why water is the medium for life
describe the polarity of water and explain hydrogen bonding
explain cohesion and adhesion with examples from plants and animals
explain why water is an excellent solvent and distinguish hydrophilic and hydrophobic substances
explain the high specific heat capacity and latent heat of vaporization of water
explain how buoyancy, viscosity, thermal conductivity and specific heat capacity affect organisms

This booklet covers the SL content of this subtopic. Additional Higher Level material is not included.

Guiding questions

What physical and chemical properties of water make it essential for life? How do these properties come from the structure of a single small molecule?

How this topic is examined

A1.1 is the first subtopic of the course and underpins much of what follows: transport in plants, temperature regulation, metabolism and membrane structure all depend on water. Paper 1 multiple-choice items test the properties of water and their explanations; Paper 2 short-answer questions ask you to link a property to hydrogen bonding and to a biological example.

The marks come from the chain of reasoning: structure (polarity) to bonding (hydrogen bonds) to property (cohesion, solvent, heat capacity) to biological consequence.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Water as the medium for life

Life began in water and every cell is still mostly water.

Think about it

Your body is about 60% water by mass, and most cells are 70% to 80% water. Why might life depend so heavily on one simple compound?

Water is the most abundant molecule in living organisms. The chemical reactions of metabolism take place in water, substances are transported dissolved in water, and many organisms live surrounded by it. The first cells almost certainly arose in water, and water is still described as the medium for life.

Water is a solvent in which a huge range of molecules can dissolve, move and collide, allowing reactions to happen. The cytoplasm of every cell is an aqueous solution of ions, sugars, amino acids and proteins. Blood plasma, plant sap, tissue fluid and digestive juices are all water-based.

To understand why water is so suitable, we need to look at the structure of its molecule. Almost every important property of water can be traced back to one feature: the molecule is polar, and so water molecules form hydrogen bonds with each other and with other polar substances.

Key point

Every property of water in this topic comes from polarity and hydrogen bonding. In exam answers, always start from the structure and work towards the biological consequence.

Quick check

1. Why is water described as the medium for life?

2. Give two fluids in organisms that are water-based.

Answers: 1. metabolic reactions occur in it and substances are transported dissolved in it 2. e.g. cytoplasm, blood plasma, xylem sap, tissue fluid

2 The structure of the water molecule and hydrogen bonding

Unequal sharing of electrons makes water polar.

Think about it

Oxygen and hydrogen share electrons in a water molecule. If they did not share them equally, what would that mean for the charge on each end of the molecule?

2.1 Polarity

A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. In each covalent bond the atoms share a pair of electrons, but they do not share them equally. Oxygen is much more electronegative than hydrogen: it attracts the shared electrons more strongly.

As a result, the oxygen end of the molecule carries a slight negative charge (written δ−) and each hydrogen carries a slight positive charge (δ+). The molecule is also bent, with an angle of about 105° between the two O–H bonds, so the charges do not cancel out. A molecule with an uneven distribution of charge like this is described as polar.

2.2 Hydrogen bonds

Because of polarity, the slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of a neighbouring molecule. This attraction is a hydrogen bond: an intermolecular force between a hydrogen atom bonded to an electronegative atom and another electronegative atom nearby.

A single hydrogen bond is weak, about 5% of the strength of a covalent bond, and hydrogen bonds between water molecules constantly break and re-form. But each water molecule can form up to four hydrogen bonds, and there are vast numbers of molecules, so collectively hydrogen bonds give water remarkable properties.

Figure 1. Water is polar: the oxygen is δ− and the hydrogens δ+. Water molecules surround and separate charged particles such as Na⁺ and Cl⁻ ions.

Feature	Explanation
Covalent bonds within the molecule	O and H share electrons, but unequally
Polarity	O is δ−, H is δ+; the bent shape stops the charges cancelling
Hydrogen bonds between molecules	attraction between δ+ H of one molecule and δ− O of another
Strength	individually weak, collectively strong because there are so many

Common mistake

Calling hydrogen bonds the bonds inside a water molecule. The bonds within the molecule are covalent; hydrogen bonds form between molecules.

Quick check

1. Why is water polar?

2. Define a hydrogen bond.

3. How many hydrogen bonds can one water molecule form?

Answers: 1. O attracts shared electrons more strongly; bent shape so charges don't cancel 2. attraction between a δ+ H and a δ− electronegative atom of another molecule 3. up to four

3 Cohesion and adhesion

Water sticks to itself and to other polar surfaces.

Think about it

How can water rise more than 100 m from the roots to the top leaves of a tall tree without a pump?

3.1 Cohesion

Cohesion is the attraction between water molecules caused by hydrogen bonding. Because the molecules cling to each other, water behaves as a continuous column and resists being pulled apart.

Transport in xylem. In plants, water is pulled up through xylem vessels from the roots to the leaves. Water evaporates from leaves (transpiration), and cohesion means that as molecules leave the top of the column, they pull the molecules below them upwards. The column is under tension but does not break, because of cohesion. This is the cohesion–tension mechanism of water transport (you will study it in B3.2).

Surface tension. At the surface of water, molecules are pulled inwards and sideways by hydrogen bonds, creating a 'skin'. This surface tension allows small organisms such as pond skaters to walk on water without sinking.

3.2 Adhesion

Adhesion is the attraction between water molecules and other polar or charged substances, again through hydrogen bonding. Water adheres to the cellulose in plant cell walls and to glass.

Adhesion explains capillary action: water rises in narrow tubes because it is attracted to the walls. In plants, adhesion to the cellulose of cell walls helps water move through the walls of leaf cells and helps xylem vessels resist the tension in the water column. Adhesion also explains why water moves through soil, holding a film around soil particles that roots can absorb.

Figure 2. Adhesion to the glass makes water climb the walls of a narrow tube: capillary action.

Property	Caused by	Biological example
Cohesion	hydrogen bonds between water molecules	continuous water column in xylem; surface tension for pond skaters
Adhesion	hydrogen bonds between water and other polar surfaces	capillary action in xylem and cell walls; water held in soil

In the real world

Pond skaters (Gerris) spread their weight on long, water-repellent legs, so the surface tension created by cohesion supports them as they hunt insects on the water surface.

Common mistake

Mixing up cohesion and adhesion. Cohesion is water to water; adhesion is water to a different substance.

Quick check

1. Define cohesion.

2. Explain how cohesion helps water move up xylem.

3. What is capillary action caused by?

Answers: 1. attraction between water molecules due to hydrogen bonds 2. evaporation pulls the column up; cohesion keeps it continuous under tension 3. adhesion of water to the walls of narrow tubes

4 Water as a solvent

Water dissolves polar and charged substances, but not non-polar ones.

Think about it

Salt and sugar dissolve easily in water, but cooking oil floats on top. What is different about these substances?

Water is often called the 'universal solvent' because so many substances dissolve in it. The reason is polarity. Ions such as Na⁺ and Cl⁻ are surrounded by water molecules: the δ− oxygens face positive ions and the δ+ hydrogens face negative ions. This hydration shell separates the ions and keeps them in solution. Polar molecules such as glucose and amino acids dissolve because they form hydrogen bonds with water.

Substances that are attracted to water and dissolve in it are hydrophilic ('water-loving'). Substances that are not attracted to water are hydrophobic ('water-fearing'): non-polar molecules such as fats, oils and many hydrocarbons. They do not form hydrogen bonds with water, and water molecules hydrogen-bond to each other in preference, pushing the non-polar molecules together.

This difference has huge biological importance. It explains why cell membranes form: the hydrophobic tails of phospholipids cluster away from water while the hydrophilic heads face it (B2.1). It also determines how substances are transported in blood.

Figure 3. Hydrophobic oil does not mix with water: non-polar molecules are pushed together as water molecules hydrogen-bond to each other.

4.1 Transport in blood

Substance	Solubility	How it is transported in blood
Sodium chloride	soluble (ions)	dissolved in plasma as Na⁺ and Cl⁻
Glucose	soluble (polar)	dissolved in plasma
Amino acids	soluble (charged/polar groups)	dissolved in plasma
Oxygen	only slightly soluble (non-polar)	mostly bound to haemoglobin in red blood cells
Fats and cholesterol	insoluble (hydrophobic)	carried inside lipoprotein complexes with a hydrophilic outer layer

The solvent properties of water also make it the medium for metabolism: enzymes and substrates dissolved in cytoplasm can move and collide. And water itself takes part in reactions: it is a reactant in hydrolysis (breaking polymers into monomers) and a product of condensation (joining monomers), which you will meet in B1.1.

Key point

Hydrophilic = polar or charged, dissolves in water. Hydrophobic = non-polar, does not dissolve. Membranes, blood transport and protein folding all depend on this distinction.

Quick check

1. Why do ions dissolve in water?

2. Define hydrophobic.

3. Why is oxygen carried by haemoglobin rather than dissolved in plasma?

Answers: 1. water forms hydration shells around them, attracted by opposite partial charges 2. non-polar, not attracted to water, insoluble 3. oxygen is non-polar and poorly soluble

5 Thermal properties of water

Hydrogen bonds make water slow to heat and hard to evaporate.

Think about it

A swimming pool is still cool in the afternoon even after a hot morning, while the concrete around it is too hot to stand on. Why?

5.1 High specific heat capacity

Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C. Water has a high specific heat capacity: about 4.18 kJ kg⁻¹ °C⁻¹, several times that of rock or air.

The reason is hydrogen bonding. When water is heated, much of the energy goes into breaking hydrogen bonds rather than making molecules move faster, so the temperature rises slowly. Equally, water releases a lot of energy as it cools.

Biological significance. Bodies of water such as lakes and oceans change temperature slowly, giving aquatic organisms a stable thermal environment. Inside organisms, the high water content of cells and blood buffers changes in temperature, helping enzymes work within a narrow range. Blood can carry heat from active muscles to the skin.

5.2 High latent heat of vaporization

To evaporate, a water molecule must break all its hydrogen bonds and escape. This requires a lot of energy: water has a high latent heat of vaporization (about 2 260 kJ kg⁻¹). When water evaporates from a surface, it takes this energy with it, so the surface cools.

Biological significance. Sweating in mammals and transpiration in plants both use evaporative cooling. A small amount of evaporation removes a large amount of heat, which makes these cooling mechanisms efficient.

WORKED EXAMPLE 1 Using specific heat capacity

Calculate the energy needed to raise the temperature of 2.0 kg of water from 20 °C to 35 °C (specific heat capacity = 4.18 kJ kg⁻¹ °C⁻¹). Explain why the same mass of air would need much less energy.

Solution

Energy = mass × specific heat capacity × temperature changeM1

= 2.0 × 4.18 × 15 = 125 kJA1

Air molecules are not hydrogen bonded, so less energy is needed to increase their kinetic energyR1

Quick check

1. Define specific heat capacity.

2. Why does water have a high latent heat of vaporization?

3. How does sweating cool the body?

Answers: 1. energy needed to raise 1 kg by 1 °C 2. hydrogen bonds must be broken for molecules to escape 3. evaporation removes a large amount of heat from the skin

6 Physical properties and how organisms use them

Compare water and air as environments for life.

Aquatic organisms live surrounded by water; terrestrial organisms are surrounded by air. The physical properties of the two media are very different, and organisms are adapted accordingly. The syllabus compares four properties, using the ringed seal (lives in Arctic water) and the black-throated loon (a diving bird that moves between air and water) as examples.

Property	Water compared with air	Consequence for organisms
Buoyancy	water is much denser, so gives more upthrust	aquatic animals need less skeletal support; seals and loons float easily; loons must work to dive
Viscosity	water is far more viscous (resists movement)	streamlined shapes reduce drag; seals and loons have torpedo-shaped bodies
Thermal conductivity	water conducts heat about 25 times faster than air	animals lose heat quickly in water; seals have thick blubber
Specific heat capacity	water's is much higher	water temperature is stable; air temperature changes quickly; loons face larger temperature swings in air

Figure 4. Water at 4 °C is densest and sinks; ice floats and insulates the water below, so lakes do not freeze solid.

Another unusual property is that ice is less dense than liquid water: as water freezes, hydrogen bonds lock molecules into an open lattice. Ice therefore floats, forming an insulating layer that allows organisms to survive under the ice in winter.

In the real world

Ringed seals keep warm in near-freezing Arctic water with a layer of blubber several centimetres thick, essential because water conducts heat away from the body so much faster than air.

Quick check

1. Why do aquatic animals need less skeletal support?

2. Why do seals need thick blubber?

3. Why does ice float?

Answers: 1. water's buoyancy supports their weight 2. water's high thermal conductivity removes heat quickly 3. hydrogen bonds hold molecules in an open lattice, so ice is less dense

7 Polarity in detail

Unequal sharing of electrons.

Oxygen is more electronegative than hydrogen, so the shared electrons in each O–H covalent bond are pulled towards oxygen. Oxygen carries a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+). Because the molecule is bent (about 104.5°), these charges do not cancel and water is a polar molecule.

WORKED EXAMPLE 2 Explaining polarity (standard)

Explain why water is a polar molecule.

Solution

Oxygen is more electronegative and attracts the shared electrons more stronglyR1

O is δ− and H is δ+R1

Bent shape means the charges are not symmetrically arrangedR1

Common mistake

Saying hydrogen bonds hold the hydrogen and oxygen atoms together within a water molecule. Those are covalent bonds; hydrogen bonds form between molecules.

Quick check

1. Why is carbon dioxide non-polar although its bonds are polar?

2. Draw a water molecule with partial charges.

Answers: 1. it is linear, so the bond polarities cancel 2. O δ− at the apex; each H δ+

8 Hydrogen bonds: strength in numbers

Weak individually, strong collectively.

A single hydrogen bond is about 20 times weaker than a covalent bond, but liquid water contains enormous numbers of them, constantly breaking and reforming. Collectively they explain water's high boiling point, high specific heat capacity, cohesion and surface tension.

WORKED EXAMPLE 3 Comparing (standard)

Explain why water (M = 18) is a liquid at room temperature while methane (M = 16) is a gas.

Solution

Water molecules form hydrogen bonds with each otherR1

Methane is non-polar with only weak London forcesR1

More energy needed to separate water moleculesR1

Quick check

1. How many hydrogen bonds can one water molecule form?

Answers: 1. up to four (two through its H atoms, two through the lone pairs on O)

9 Cohesion in plants: the transpiration stream

Pulling columns of water.

Water evaporates from leaf cells and leaves through stomata. This creates tension that pulls water up the xylem. Cohesion between water molecules (hydrogen bonding) keeps the column continuous, and adhesion to the xylem walls helps prevent it breaking. This is the cohesion–tension theory.

WORKED EXAMPLE 4 Cohesion–tension (exam level)

Explain how water moves up the xylem of a tall tree.

Solution

Transpiration from leaves creates tension (negative pressure)R1

Cohesion due to hydrogen bonding keeps the column intactR1

Adhesion to xylem walls supports the columnR1

In the real world

The tallest trees, such as coast redwoods over 110 m tall, lift water to their tops entirely by the cohesion–tension mechanism, with no pump.

Quick check

1. What would happen if an air bubble entered a xylem vessel?

Answers: 1. the water column breaks (cavitation) and water movement in that vessel stops

10 Surface tension and organisms

Living on the water surface.

Cohesion at the surface creates surface tension: the surface behaves like an elastic film. Small organisms such as pond skaters and water striders can walk on water because their weight is not enough to break the surface, and their legs are water-repellent.

WORKED EXAMPLE 5 Surface tension (standard)

Explain why a pond skater can stand on water.

Solution

Hydrogen bonds between surface water molecules create surface tensionR1

Its low mass and hydrophobic legs do not break the surfaceR1

11 Adhesion and capillary action

Water climbing narrow tubes.

Adhesion is the attraction between water and other polar or charged surfaces. In narrow tubes, adhesion pulls water up the walls and cohesion pulls the rest of the water along: capillary action. This helps water move through soil pores and plant cell walls.

WORKED EXAMPLE 6 Capillarity (standard)

Explain why water rises higher in a narrow glass tube than in a wide one.

Solution

Adhesion to glass is the same per unit of wall, but there is less water to lift in a narrow tubeR1

Greater proportion of water molecules in contact with the wallR1

Quick check

1. Give a biological example of adhesion.

Answers: 1. water adhering to xylem walls; water moving through cellulose cell walls

12 Solvent properties and transport in blood

What dissolves, and what is carried.

Hydrophilic substances (ions, glucose, amino acids) dissolve in blood plasma. Hydrophobic substances need carriers: oxygen is carried by haemoglobin (it is only slightly soluble), fats and cholesterol travel in lipoproteins, and steroid hormones bind to carrier proteins.

Substance	Solubility	How it is transported in blood
sodium chloride	soluble (ionic)	dissolved as Na+ and Cl− ions
glucose	soluble (polar OH groups)	dissolved in plasma
amino acids	soluble (charged groups)	dissolved in plasma
oxygen	poorly soluble	bound to haemoglobin in red blood cells
fats and cholesterol	insoluble (non-polar)	in lipoprotein complexes

WORKED EXAMPLE 7 Transport (exam level)

Explain how cholesterol is transported in the blood.

Solution

Cholesterol is hydrophobic and insoluble in plasmaR1

It is carried in lipoproteins with a hydrophilic outer layer of phospholipids and proteinsR1

Quick check

1. Why is glucose soluble in water?

Answers: 1. it has many polar OH groups that form hydrogen bonds with water

13 Hydrophilic and hydrophobic molecules

Water-loving and water-fearing.

Hydrophilic substances are polar or charged and are attracted to water. Hydrophobic substances are non-polar; they do not form hydrogen bonds with water, so water molecules form hydrogen bonds with each other instead, excluding them. This hydrophobic effect drives the formation of membranes and the folding of proteins.

WORKED EXAMPLE 8 Membranes (standard)

Explain how the properties of phospholipids lead to the formation of a bilayer in water.

Solution

Hydrophilic phosphate heads face waterR1

Hydrophobic tails are excluded from water and face inwardsR1

Quick check

1. Classify: amino acid with a CH3 R group; potassium ion; triglyceride.

Answers: 1. R group hydrophobic (amino acid overall amphipathic); hydrophilic; hydrophobic

14 Specific heat capacity and temperature stability

Resisting temperature change.

Water's specific heat capacity (4.18 J g−1 °C−1) is high because energy is needed to break hydrogen bonds before molecules can move faster. Large bodies of water therefore change temperature slowly, providing stable habitats, and the high water content of organisms buffers their internal temperature.

WORKED EXAMPLE 9 Energy calculation (standard)

Calculate the energy needed to raise the temperature of 2.0 kg of water by 5.0 °C.

Solution

q = 2000 × 4.18 × 5.0M1

= 41 800 J (41.8 kJ)A1

Quick check

1. Explain why coastal climates have smaller temperature ranges than inland climates.

Answers: 1. the sea warms and cools slowly due to its high specific heat capacity

15 Latent heat of vaporization and cooling

Sweating and transpiration.

Evaporating water requires a large amount of energy (about 2257 J g−1) to break hydrogen bonds. When sweat evaporates, this energy is taken from the skin, cooling the body. Transpiration cools leaves in the same way.

WORKED EXAMPLE 10 Cooling by sweat (exam level)

A runner evaporates 500 g of sweat. Estimate the heat removed.

Solution

500 × 2257 JM1

= 1.13 × 106 J (1.13 MJ)A1

In the real world

Dogs cannot sweat much through their skin; they cool down by panting, which evaporates water from the tongue and airways.

Quick check

1. Why does sweating cool the body less effectively in humid air?

Answers: 1. less evaporation occurs when the air is already saturated with water vapour

16 Ice: less dense than liquid water

Insulating lakes.

When water freezes, hydrogen bonds hold molecules in an open hexagonal lattice, so ice is less dense than liquid water and floats. A layer of ice insulates the water below, which stays liquid (around 4 °C at the bottom), allowing aquatic organisms to survive the winter.

WORKED EXAMPLE 11 Floating ice (standard)

Explain why ice floats and why this is important for aquatic life.

Solution

Hydrogen bonds hold molecules further apart in ice: lower densityR1

Ice insulates the water below, which does not freezeR1

17 Physical properties: buoyancy, viscosity and conductivity

Comparing water and air.

Property	Water compared with air	Effect on organisms
buoyancy	much greater	supports large aquatic animals (whales); less skeletal support needed
viscosity	much greater	streamlined bodies needed to swim efficiently
thermal conductivity	about 25 times greater	aquatic animals lose heat quickly; need insulation (blubber)
specific heat capacity	about 4 times greater (per kg)	stable aquatic temperatures

WORKED EXAMPLE 12 Comparing (exam level)

Compare the challenges faced by a seal in water and a bird in air, using the physical properties of water and air.

Solution

Seal: high thermal conductivity of water causes heat loss; blubber insulates; high viscosity: streamlined bodyR1 R1

Bird: low buoyancy of air: needs lift from wings; low viscosity reduces dragR1

18 Water in metabolism

Reactant and product.

Water is a reactant in hydrolysis reactions (digestion of polymers) and photosynthesis, and a product of condensation reactions (building polymers) and aerobic respiration. Many enzymes require an aqueous environment to function.

WORKED EXAMPLE 13 Hydrolysis (standard)

Write a word equation for the hydrolysis of maltose.

Solution

maltose + water → glucose + glucoseA1

Quick check

1. Name a reaction in which water is produced.

Answers: 1. condensation (for example forming a peptide bond) or aerobic respiration

19 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 14 Mixed problem (exam level)

Explain how three properties of water make it a suitable habitat for fish.

Solution

High specific heat capacity: stable temperaturesR1

Ice floats and insulates: water below stays liquidR1

Solvent: dissolved oxygen and nutrients available; buoyancy supports the bodyR1

Quick check

1. Distinguish between cohesion and adhesion.

Answers: 1. cohesion: attraction between water molecules; adhesion: between water and other substances

20 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 15 Paper 1 question (standard)

Which property of water allows sweat to cool the body? A. high specific heat capacity B. high latent heat of vaporization C. cohesion D. low viscosity

Solution

BA1

WORKED EXAMPLE 16 Paper 2 question (exam level)

Outline how hydrogen bonding explains (a) cohesion, (b) the high boiling point of water.

Solution

(a) hydrogen bonds attract water molecules to each otherR1

(b) energy needed to break many hydrogen bonds before molecules escapeR1

21 Water and the search for life

Why 'follow the water'?

Because all known life depends on liquid water as a solvent and reaction medium, the search for life beyond Earth focuses on places where liquid water exists or existed: Mars (ancient lakes and subsurface ice), Europa and Enceladus (subsurface oceans). The 'Goldilocks zone' is the range of distances from a star where liquid water can exist on a planet's surface.

Quick check

1. Why is liquid water, rather than ice or vapour, important for life?

Answers: 1. it is a solvent and medium for reactions; molecules can move and react

22 Common misconceptions and links

Students often think…	Actually…
hydrogen bonds are the bonds inside water molecules	they form between molecules; the bonds inside are covalent
cohesion and adhesion are the same	cohesion: water to water; adhesion: water to other substances
all substances dissolve in water	non-polar (hydrophobic) substances do not
water heats slowly because it is dense	energy is used breaking hydrogen bonds

How A1.1 connects to the rest of the course

Topic	Connection
B1.1 Carbohydrates and lipids	condensation and hydrolysis; hydrophobic lipids
B2.1 Membranes (HL)	phospholipid bilayers form because of hydrophobic interactions
B3.2 Transport	cohesion–tension in xylem; transport in blood
D3.3 Homeostasis	sweating and thermoregulation
C1.1 Enzymes	reactions occur in aqueous solution

23 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 5. Temperature of lake water and nearby land over 24 hours.

(a) State the type of bond that forms between water molecules. [1]

(b) Explain why water has a high specific heat capacity. [2]

(d) Explain the significance of this difference for aquatic organisms. [2]

(e) Explain how cohesion allows water to be transported in xylem. [3]

Markscheme

Part	Answer	Marks
(a)	hydrogen bonds	A1
(b)	much energy used to break hydrogen bonds	R1
	so temperature rises slowly	R1
(c)	land range much larger (about 25 °C vs about 4 °C)	A1
	lake changes slowly; land rapidly	A1
(d)	stable environment	R1
	enzymes / metabolism not disrupted; less need for thermoregulation	R1
(e)	hydrogen bonds hold water molecules together	R1
	evaporation (transpiration) pulls water up	R1
	column remains continuous under tension	R1

Examiner's comments. In (b), many answers said water 'holds heat well' without mentioning hydrogen bonds. In (e), the best answers linked transpiration, cohesion and tension in sequence.

24 Check your understanding

Section A: multiple choice

1. Water is polar because

A. oxygen attracts the shared electrons more strongly than hydrogen B. it has ionic bonds C. hydrogen is more electronegative D. it contains no covalent bonds

2. Hydrogen bonds form between

A. two oxygen atoms B. two hydrogen atoms in one molecule C. a δ+ hydrogen and a δ− oxygen of another molecule D. water and oil

3. Cohesion is the attraction between

A. water and glass B. water molecules C. water and ions D. oil and water

4. Capillary action is mainly due to

A. cohesion only B. high specific heat C. evaporation D. adhesion

5. Which substance is hydrophobic?

A. glucose B. a triglyceride (fat) C. sodium chloride D. an amino acid

6. Sweating cools the body because water has a high

A. latent heat of vaporization B. density C. viscosity D. buoyancy

7. Ice floats because

A. ice has fewer molecules B. ice is polar C. ice has covalent bonds between molecules D. hydrogen bonds hold molecules in an open lattice

8. Animals lose heat faster in water than air because water has a higher

A. viscosity B. buoyancy C. thermal conductivity D. solvent capacity

Section B: structured

9. Explain how the structure of water leads to hydrogen bonding. [3]

10. Distinguish between cohesion and adhesion, giving a biological example of each. [4]

11. Explain how the solvent properties of water affect the transport of substances in blood. [4]

12. Compare the thermal properties of water and air and explain their importance to a named animal. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. Unequal sharing makes O δ− and H δ+; bent shape; attraction between H of one molecule and O of another.

10. Cohesion: water-water (xylem column); adhesion: water-other (capillary action in cell walls).

11. Ions and polar molecules dissolve (glucose, amino acids, NaCl); non-polar O₂ bound to haemoglobin; lipids in lipoproteins.

12. Water: higher specific heat and conductivity; stable but heat loss fast; seal: blubber; loon: larger temperature swings in air.

25 Summary and topic map

Idea	Remember
Polarity	O δ−, H δ+; bent molecule
Hydrogen bonds	between molecules; weak but numerous
Cohesion	water-water; xylem, surface tension
Adhesion	water-other; capillary action
Solvent	hydrophilic dissolve; hydrophobic do not
Thermal	high specific heat; high latent heat of vaporization
Water vs air	buoyancy, viscosity, conductivity, heat capacity

Before the exam, make sure you…

•start every property from polarity and hydrogen bonding
•give a named biological example for each property
•distinguish cohesion from adhesion
•explain blood transport by solubility

Water on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 6. Topic map for Water.

revisionprep.comIB-style notes · question bank · mock papers