Water: notes and practice questions
- This topic explores the essential physical and chemical properties of water for life and its role as a habitat.
- Water molecules are polar due to unequal electron sharing in covalent bonds.
- Hydrogen bonds form between water molecules due to their polarity.
- Cohesion, a result of hydrogen bonding, enables water transport in xylem and surface tension for habitats.
- Adhesion, due to water's polarity, facilitates capillary action in soil and plant cell walls.
- Water's solvent properties allow hydrophilic molecules to dissolve, crucial for metabolism and transport.
- Key physical properties include high specific heat capacity, buoyancy, viscosity, and thermal conductivity.
How it is examined
Paper 1A multiple choice on properties and their consequences, and Paper 2 short answer built on outline or explain of one property linked to one organism. The diagram of hydrogen-bonded water molecules is a live drawing item. Tariffs are small, 1 to 3 marks. Answers that name the property without naming the consequence for an organism lose the second mark.
- A1.1.1 Water as the medium for life. The first cells originated in water and water is still the medium in which most processes of life happen.
- A1.1.2 Hydrogen bonds follow from the polar covalent bonds inside a water molecule. Polarity comes from unequal sharing of electrons; hydrogen bonding happens between molecules. Students must be able to draw two or more water molecules with the hydrogen bonds between them, using the guide's polarity notation (partial charges marked).
- A1.1.3 Cohesion of water molecules due to hydrogen bonding, and what that means for organisms: transport of water under tension in xylem, and water surfaces as habitats because of surface tension.
- A1.1.4 Adhesion of water to polar or charged materials: capillary action in soil and in plant cell walls.
Origin-of-water hypotheses are limited to asteroids. Comets and other routes are outside the syllabus.
- A1.1.7 Extraplanetary origin of water on Earth and why it was retained. Hypotheses for origin are limited to asteroids; reasons for retention are limited to gravity and temperatures low enough to condense water.
- A1.1.8 The link between the search for extraterrestrial life and the presence of water, including the "Goldilocks zone".
Guiding questions
- What physical and chemical properties of water make it essential for life?
- What are the challenges and opportunities of water as a habitat?
Linking questions
- How do the various intermolecular forces of attraction affect biological systems?
- What biological processes only happen at or near surfaces?
Practice questions
12 questions · 7 easy · 4 medium · 1 hardQuestion 1
EasyPaper 1A · calculator1 markMany organisms live in aquatic environments, such as lakes and oceans, which show less temperature fluctuation than terrestrial environments. Which property of water is responsible for this temperature stability?
A. Water has a high specific heat capacity, requiring large amounts of energy to change its temperature.
B. Strong covalent bonds between water molecules store a large amount of thermal energy.
C. The high cohesion of water molecules allows them to stick together, preventing heat loss to the atmosphere.
D. Water is a universal solvent, which allows dissolved solutes to buffer temperature changes.
Think about how much energy it takes to heat up a large body of water like a swimming pool or the ocean compared to heating up the air or the land on a sunny day. What property relates to the energy required to change a substance's temperature?
Question 2
MediumPaper 2 · calculator15 marksLiving organisms must maintain a stable internal environment, a process known as homeostasis.
(a) Explain the control of blood glucose concentration in humans.
(b) Explain the effect of pH on the activity of enzymes.
(c) Distinguish between the transport of glucose and oxygen in the blood, with reference to the properties of water.
Think about the two key hormones released from the pancreas. What triggers the release of each hormone, what are their target cells, and what is the resulting effect on blood glucose levels?
Consider how pH affects the chemical bonds that maintain a protein's specific three-dimensional shape. What is the term for the irreversible change in an enzyme's structure, and how does this affect its function?
Compare the polarity of glucose and oxygen molecules. How does this property affect how well each substance can dissolve in the main component of blood plasma, which is water?
Question 3
HardPaper 2 · calculator10 marksThe movement of water across the plasma membrane is a fundamental process in both plant and animal cells.
Explain how the net movement of water across a cell membrane is driven by differences in solute concentration.
Describe the role of aquaporins in facilitating the movement of water across the plasma membrane.
Explain how pressure potential affects the movement of water into a plant cell.
Think about the terms hypotonic and hypertonic, and how solutes interact with water molecules to affect their ability to move freely.
Consider the structure of the phospholipid bilayer and why water might need a specialized protein to cross it rapidly.
What happens when a plant cell takes in water and expands against its cell wall? How does this physical pressure change the overall water potential?
Question 4
EasyPaper 1A · calculator1 markWhat explains the polarity of a water molecule?
A. The hydrogen atoms completely transfer their electrons to the oxygen atom.
B. The oxygen atom and hydrogen atoms have equal electronegativity.
C. The oxygen atom has a higher electronegativity, causing an unequal sharing of electrons with the hydrogen atoms.
D. The molecule has a linear shape, which distributes charge evenly.
Consider the properties of atoms that determine how electrons are shared in a covalent bond. The term for an atom's attraction for electrons in a bond is key here.
Question 5
MediumPaper 1A · calculator1 markWhat is/are the role(s) of hydrogen bonding in the transport of water in the xylem of a plant?
I. It causes cohesion between water molecules, allowing them to be pulled up under tension.
II. It causes adhesion of water molecules to the xylem walls, helping to support the water column against gravity.
III. It generates a high pressure in the roots to push water up the xylem.
A. I only
B. I and II only
C. II and III only
D. I, II and III
Consider the properties of water that arise from its polarity and the formation of hydrogen bonds. How do these properties contribute to the movement of water through the narrow tubes of the xylem?
Question 6
EasyPaper 1A · calculator1 markWhich of the following processes depends directly on the adhesive properties of water?
A. Capillary action in plant cell walls
B. The formation of surface tension on a pond
C. The cooling of skin during the evaporation of sweat
D. The transport of glucose in blood plasma
Consider which property involves water molecules being attracted to different, polar substances, rather than to each other.
Question 7
MediumPaper 2 · calculator10 marks(a) Triglycerides are the main component of adipose tissue in mammals. They are non-polar molecules and are therefore hydrophobic.
Explain the chemical reasons why non-polar molecules are hydrophobic.
(b.i) Outline how the hydrophobic nature of triglycerides makes them well-suited for long-term energy storage in animals.
(b.ii) Steroid hormones, such as oestradiol, are lipids.
State how the hydrophobic nature of oestradiol allows it to enter a target cell.
(c) Explain how the amphipathic properties of phospholipids lead to the formation of a stable cell membrane.
Think about the types of bonds water forms with itself and why it cannot form these bonds with non-polar molecules.
Consider how water affects the mass and volume of stored molecules, and how it affects osmosis.
Think about the structure of the cell membrane and how non-polar molecules cross it.
Define amphipathic and describe how the different parts of the phospholipid interact with water.
Question 8
EasyPaper 1A · calculator1 markWhich characteristic of the black-throated loon (Gavia arctica) is a consequence of the viscosity of water?
A. Streamlined body
B. Solid bones
C. Oiled plumage
D. Lungs
Consider which physical property of water creates resistance or drag when an animal tries to move through it.
Question 9
MediumPaper 1A · calculator1 markWhich process generates the pulling force (tension) that drives the transpiration stream in plants?
A. Evaporation of water from the surfaces of mesophyll cells
B. Hydrogen bonding between water molecules in the xylem
C. Active transport of mineral ions into the root xylem
D. Adhesion of water molecules to the lignified xylem walls
Consider which process creates the initial negative pressure at the top of the plant, rather than the properties of water that simply maintain the continuous column.
Question 10
EasyPaper 1A · calculator1 markWhich of the following correctly describes the partial charges on a water molecule and the type of bond that forms between adjacent water molecules?
A. Oxygen is , hydrogen is , and hydrogen bonds form between molecules.
B. Oxygen is , hydrogen is , and covalent bonds form between molecules.
C. Oxygen is , hydrogen is , and hydrogen bonds form between molecules.
D. Oxygen is , hydrogen is , and covalent bonds form between molecules.
Remember that oxygen has a stronger pull on the shared electrons than hydrogen, and consider what type of bond connects separate water molecules together.
Question 11
EasyPaper 1A · calculator1 markDuring a dry period, water is drawn upwards through the microscopic spaces between soil particles towards the roots of a plant.
Which property of water is primarily responsible for this capillary action?
A. Adhesion
B. Cohesion
C. High specific heat capacity
D. Solvent properties
Think about the interaction between water molecules and the surfaces of the soil particles.
Question 12
EasyPaper 1A · calculator1 markWhich statement correctly describes the bonding between adjacent water molecules?
A. Non-polar covalent bonds form between the oxygen and hydrogen atoms of adjacent molecules.
B. Hydrogen bonds form between the slightly negative oxygen of one molecule and the slightly positive hydrogen of another.
C. Polar covalent bonds form between the slightly positive oxygen of one molecule and the slightly negative hydrogen of another.
D. Hydrogen bonds form due to the equal sharing of electrons between adjacent water molecules.
Remember the difference between the bonds inside a single water molecule and the bonds that connect multiple water molecules together. Also, consider which atom attracts electrons more strongly.
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Where marks are lost
- Command terms are consistently under-read. Students answer describe when the question said explain, so they give an account with no reasons and cap at half marks. The reverse also happens on outline, where a student writes an essay for a 2-mark summary and runs out of time.
