IB Biology SL — Water: Notes, Questions & Solutions

IB Biology SL Water Notes, Questions, and Solutions

Short answer: Water is a polar molecule. It forms hydrogen bonds with other water molecules and with other polar or charged substances. These bonds give water cohesion, adhesion, strong solvent power, and a high specific heat capacity. In IB Biology, water is sub-topic A1.1 and the first topic in Theme A.

These notes cover A1.1 at SL, the HL extra points, and exam-style practice questions with model answers.

Key points

• Water's polarity comes from oxygen's stronger pull on electrons. This gives oxygen a partial negative charge and the hydrogens partial positive charges.

• Hydrogen bonds form between water molecules because of this polarity. Each bond is weak, but there are many of them, so the effect is strong.

• Cohesion, or water sticking to water, helps water move up xylem under tension and creates surface tension.

• Adhesion, or water sticking to other polar surfaces, causes capillary action in soil and cell walls.

• Water is a good solvent for polar and ionic substances. That makes it the medium for reactions and transport in cells, blood, and plant sap.

• Water's physical properties — buoyancy, viscosity, heat transfer, and specific heat capacity — shape life in water.

• The hydrogen bonds here also matter in nucleic acids, where they hold DNA base pairs together.

Where Water Fits in the IB Biology Syllabus

Water is sub-topic A1.1 in Theme A, Unity and Diversity, at the molecular level. Teachers teach it first because later topics depend on it. Cells are mostly water, reactions happen in solution, and transport in plants and animals depends on water's properties.

Content	SL	HL
Water as the medium for life	✓	✓
Polarity and hydrogen bonding	✓	✓
Cohesion and its effects	✓	✓
Adhesion and its effects	✓	✓
Solvent properties, reactions, and transport	✓	✓
Physical properties and aquatic animals	✓	✓
Origin of Earth's water and why it stayed	--	✓
Water and the search for life in space	--	✓

1. Water as the Medium for Life

The first cells likely formed in water, and water is still the medium for life's chemistry. Water makes up much of living cells. Inside cells, the cytoplasm is mostly water, and most reactions happen there.

Water matters because it:

• dissolves many substances cells need, so they can move and react

• takes part directly in reactions, such as hydrolysis and sugar making

• carries substances around the body in blood, lymph, xylem, and phloem

• helps maintain a steady temperature in organisms and their surroundings

All these roles come from one feature: water's polarity.

2. Polarity and Hydrogen Bonding

Why water is polar

A water molecule, H2O, contains one oxygen atom bonded to two hydrogen atoms through covalent bonds, sharing electrons. Oxygen pulls on the shared electrons more strongly than hydrogen. So oxygen is slightly negative, and each hydrogen is slightly positive.

The molecule is bent, not straight, so the charges do not cancel. One end is slightly negative and the other slightly positive. Water is a polar molecule, and its covalent bonds are polar too.

What a hydrogen bond is

A hydrogen bond is a weak electric pull between the partly positive hydrogen of one water molecule and the partly negative oxygen of another.

Key facts to remember:

• Hydrogen bonds are between molecules. Covalent bonds are within molecules.

• One hydrogen bond is much weaker than a covalent bond.

• Hydrogen bonds break and form again all the time in liquid water.

• Because there are huge numbers of them, they have a strong effect on water's properties.

Diagrams usually show hydrogen bonds as dashed or dotted lines between molecules, with δ+ and δ− labels. Practice drawing this. A common short-answer task.

3. Cohesion

Cohesion is the attraction between water molecules caused by hydrogen bonding. Water molecules stick together, and this has two main effects.

Transport in xylem

In plants, water evaporates from leaves during transpiration. As water leaves the leaf, it pulls other water molecules with it because cohesion keeps them in a chain. This creates tension, or negative pressure, that pulls a column of water up the xylem from roots to leaves.

Without cohesion, the water column would break, and tall plants could not move water to their leaves. This links to plant transport later in the syllabus.

Surface tension

Hydrogen bonds with nearby molecules pull molecules inward and sideways at the surface of water. This creates surface tension. It lets small organisms such as pond skaters move across the water surface without sinking.

4. Adhesion

Adhesion is the attraction between water molecules and other polar or charged surfaces. Hydrogen bonding causes it.

Capillary action

When water sticks to the walls of a narrow tube, it can move up or along the surface. This is capillary action. It matters in two main ways:

• In soil: water moves through tiny spaces between soil particles by capillary action, so roots can get it.

• In plant cell walls: cellulose is polar, so water sticks to it. This keeps cell walls moist, helps gas exchange in leaves, and helps water move through the walls.

Exam tip: Cohesion is water to water. Adhesion is water to other substances.

5. Solvent Properties of Water

People often call water a universal solvent because it dissolves many substances. Its polarity explains why.

How water dissolves substances

When sodium chloride is added to water:

• the negative oxygen ends are attracted to Na+ ions

• the positive hydrogen ends are attracted to Cl− ions

• water molecules surround each ion and pull the ions apart, so the compound dissolves

Polar molecules such as glucose dissolve in a similar way. They form hydrogen bonds with water.

Hydrophilic and hydrophobic substances

• Hydrophilic substances are polar or charged. They are attracted to water and usually dissolve in it. Examples include glucose, amino acids, and ions.

• Hydrophobic substances are non-polar. They do not form hydrogen bonds with water and do not dissolve well. Examples include lipids such as fats and oils.

Why this matters for reactions and transport

Because so many substances dissolve in water, it is the medium for most reactions in the cytoplasm. It also allows organisms to transport substances around.

Substance	Dissolves in water	Carried in blood as
Glucose	Polar, so soluble	Dissolved in plasma
Amino acids	Polar or charged, so soluble	Dissolved in plasma
Sodium chloride	Ionic, so soluble	Dissolved as Na+ and Cl− ions in plasma
Oxygen	Non-polar, so only a little soluble	Mostly bound to haemoglobin in red blood cells
Fats and cholesterol	Non-polar, so insoluble	Carried in lipoprotein complexes

In plants, water carries dissolved mineral ions in the xylem and dissolved sugars, mainly sucrose, in the phloem.

6. Physical Properties of Water and Aquatic Animals

The physical properties of water are vastly different from those of air, and these disparities significantly impact life in water. You need to know four properties.

Property	Compared with air	Effect on aquatic animals
Buoyancy	Water is much denser, so it supports bodies more	Animals need less skeletal support; diving animals may need ways to avoid floating
Viscosity	Water is more viscous, so it resists movement more	Streamlined body shapes reduce drag when swimming
Heat transfer	Water carries heat away from the body faster	Warm-blooded animals lose heat quickly and need insulation, such as blubber
Specific heat capacity	Water needs more energy to change temperature	Aquatic habitats stay more stable than land

Why water's specific heat capacity is high

Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. Water's temperature rises when energy breaks hydrogen bonds, allowing molecules to move faster. This means:

• lakes and oceans change temperature slowly, so aquatic organisms get a stable setting

• organisms, which are mostly water, resist rapid body temperature changes

Example organisms

The IB guide often uses named animals to show these properties. Check the current guide for the exact examples.

• Black-throated loon (Gavia arctica): a diving bird. Unlike most birds, it has solid, heavy bones, which cut buoyancy and help it dive. Its streamlined body reduces drag.

• Ringed seal (Pusa hispida): a mammal in cold Arctic waters. A thick layer of blubber insulates it against heat loss, because water carries heat away so quickly. Its streamlined shape helps it swim well.

Comparing these animals is a good way to show how water's properties affect organisms.

HL Extension: Water Beyond Earth

HL students also study:

• Where Earth's water came from. Much of it may have come from space, for example from asteroids or comets, early in Earth's history. Earth kept its water because of gravity and its distance from the Sun, which lets liquid water exist.

• Water and the search for life elsewhere. Scientists search for planets in the Goldilocks zone where temperatures support liquid water because life depends on it.

SL students do not need these points for the exam, but they help with context.

How Water Links to Other Topics

• Nucleic acids (A1.2): hydrogen bonds hold complementary base pairs together in DNA.

• Cell membranes: the hydrophilic and hydrophobic parts of phospholipids explain bilayer formation.

• Transport in plants: cohesion and adhesion explain how water moves up the xylem.

• Transport in animals: how substances dissolve helps explain how they move in blood.

• Proteins and enzymes: hydrogen bonds help give proteins their shape.

The syllabus links these ideas often, and they can appear in longer exam answers.

Common Mistakes

• Saying hydrogen bonds are within a water molecule. They form between molecules. The bonds within the molecule are covalent.

• Mixing up cohesion and adhesion. Cohesion is water to water. Adhesion is water to other surfaces.

• Saying water is a universal solvent without explaining why. Link it to polarity and attraction to ions or polar molecules.

• Mislabelling partial charges. Oxygen is δ− and hydrogen is δ+.

• Confusing specific heat capacity with heat transfer. High specific heat capacity means water resists temperature change. High heat transfer means it moves heat quickly.

• Stating a property without its effect. Exam questions usually ask why it matters. Always connect the property to an effect on organisms.

Exam Technique for Water Questions

Pay attention to the command term:

• State or outline: give the fact clearly and briefly.

• Describe: give a clear account, such as how a hydrogen bond forms.

• Explain: give the cause and the effect. For instance, cohesion, caused by hydrogen bonding, enables water to be pulled up the xylem.

• Compare and contrast: give similarities and differences, ideally in a table, such as water vs air.

A good structure for most water questions is property → molecular cause → biological effect.

Practice Questions with Solutions

1. Explain why water is described as a polar molecule. [2]

Solution: Oxygen pulls on the shared electrons more strongly than hydrogen. So oxygen is slightly negative and the hydrogens are slightly positive. Because the molecule is bent, the charges are uneven.

2. Define hydrogen bond in the context of water molecules. [1]

Explanation: A slight attraction between the positive hydrogen of one water molecule and the negative oxygen of another.

3. Explain one way water's cohesive properties are important in plants. [2]

Solution: Cohesion between water molecules, caused by hydrogen bonding, keeps a continuous column of water in the xylem. When plants transpire, they pull the column of water up under tension.

4. Why is water's high specific heat capacity useful for living organisms? [2]

The water's temperature change requires a lot of energy because it needs to break hydrogen bonds. This helps organisms and their habitats resist fast temperature change, so internal conditions stay stable.

5. Give one reason water is an effective solvent for ionic compounds such as sodium chloride. [2]

Solution: Water is polar. Negative oxygen attracts Na+ ions and positive hydrogens attract Cl− ions. Water molecules surround and separate the ions, so the compound dissolves.

6. Distinguish between cohesion and adhesion. [2]

Solution: Cohesion is the attraction between water molecules. Adhesion is the attraction between water molecules and other polar or charged surfaces. Hydrogen bonding causes both.

7. Explain why oxygen is transported mainly bound to haemoglobin, while glucose is transported dissolved in plasma. [3]

Solution: Glucose is polar, so it forms hydrogen bonds with water and dissolves easily in plasma. Oxygen is non-polar, so it is only a little soluble in water, and not enough dissolves to meet the body's needs. Binding to haemoglobin lets blood carry much more oxygen.

8. Outline how the physical properties of water affect a ringed seal living in Arctic waters. [3]

Solution: Water has high heat transfer, so it carries heat away from the seal quickly, and the seal has thick blubber for insulation. Water is viscous, so a streamlined body shape reduces drag when swimming. Water is buoyant, which helps support the seal's body.

9. Describe how capillary action helps plants obtain water. [2]

Solution: Water sticks to polar surfaces, such as soil particles and cellulose in plant cell walls. This lets water move through narrow spaces in the soil towards roots, and through cell walls in the plant.

10. Suggest why non-polar substances such as lipids do not dissolve in water. [2]

Solution: Non-polar molecules have no partial charges, so they cannot form hydrogen bonds with water molecules. Water molecules are more attracted to each other than to the lipid, so the lipid stays separate.

FAQ

Is water on the IB Biology SL syllabus?

Yes. Water is sub-topic A1.1 and is studied at both SL and HL. HL students also study where Earth's water came from and its role in the search for life elsewhere.

What's the difference between a covalent bond and a hydrogen bond in water?

Covalent bonds join oxygen to hydrogen within a water molecule by sharing electrons. Hydrogen bonds are weaker attractions between water molecules.

Why does water have a high specific heat capacity?

Water molecules need energy to break hydrogen bonds so that they can move faster and increase the temperature.

Why is water important for transport in organisms?

Water dissolves many substances, including glucose, amino acids, and ions, allowing them to be carried in blood, xylem, and phloem. Cohesion and adhesion also help water move through plants.

What are hydrophilic and hydrophobic substances?

Hydrophilic substances are polar or charged and dissolve in water. Hydrophobic substances are non-polar and do not dissolve well.

How is water tested in the IB Biology exam?

Water can appear in Paper 1 multiple-choice and data questions, and in Paper 2 short-answer and longer questions. Also often linked to membranes and transport.

Sorty is a revision website that can quiz you on water's properties and mark your answers. It also works as an IGCSE website for quick biology revision.

Q&A

Question: Why do so many different water properties come from the same molecular feature?

Short answer: Most of water's key biological properties come from polarity. Oxygen is negative and hydrogen is positive, so water molecules make bonds with each other and other substances. These hydrogen bonds explain cohesion, adhesion, surface tension, capillary action, high specific heat capacity, and water's ability to dissolve many ionic and polar substances.

Question: How should an exam answer link cohesion to water movement in xylem?

Short answer: A strong answer should include the property, the molecular cause, and the biological effect. For example, hydrogen bonds cause cohesion between water molecules. This keeps water molecules together in a continuous column in the xylem. When water evaporates from leaves during transpiration, tension pulls this column upward from roots to leaves.

Question: Why is adhesion important if cohesion already keeps water molecules together?

Short answer: Cohesion keeps water molecules attracted to each other, but adhesion lets water stick to other polar or charged surfaces. This is important in soil and plant cell walls, where water can move through narrow spaces by capillary action. In plants, adhesion to cellulose also maintains cell walls moist and supports water movement through tissues.

Question: Why do aquatic animals need adaptations related to heat loss and movement?

Short answer: Water transfers heat away from the body faster than air, so warm-blooded aquatic animals such as ringed seals need insulation, such as blubber. Water is also more viscous than air, so it resists movement more. Streamlined body shapes reduce drag and help aquatic animals swim efficiently.

Question: Why are hydrogen bonds important beyond the water topic?

Short answer: Hydrogen bonds are important in several later biology topics. In nucleic acids, they hold complementary DNA base pairs together. In proteins, hydrogen bonds help maintain shape. In membranes, the contrast between hydrophilic and hydrophobic regions helps explain why phospholipids form bilayers.