4.1 Water
Water is an inorganic compound essential for all living cells. Its unique properties enable biochemical reactions, solute transport, capillary action, and thermal regulation in living organisms.
Learning Objectives
- Describe the polarity of water molecules and how it leads to the formation of hydrogen bonds.
- Explain the role of water as a universal solvent in transporting solutes into cells for biochemical reactions.
- Differentiate between cohesive force and adhesive force, and describe how they produce capillary action in narrow tubes such as xylem vessels.
- State the specific heat capacity of water and explain its importance in maintaining the body temperature of organisms.
Molecular Properties and Forces of Water
Hydrogen bonding arises from water polarity, while cohesive and adhesive forces together produce capillary action in a xylem tube.
Short Explanation
Composition of water
Water is an inorganic compound consisting of hydrogen (H) and oxygen (O) elements. Unlike organic compounds such as carbohydrates, proteins, lipids, and nucleic acids, water consists solely of hydrogen and oxygen elements.
Polarity of water
Water molecules are polar molecules. Shared electrons between oxygen and hydrogen are attracted towards the oxygen atom because oxygen is more electronegative. This gives oxygen a partial negative charge \(\delta^-\) and hydrogen a partial positive charge \(\delta^+\).
Hydrogen bonds
This polarity produces hydrogen bonds between neighbouring water molecules.
Universal solvent
Because of its polarity, water acts as a universal solvent. This property enables solutes such as glucose and electrolytes to dissolve and be transported across plasma membranes into cells for biochemical reactions.
Cohesive and adhesive forces
Cohesive force is the attractive force between water molecules that causes them to attach to each other. Adhesive force is the attractive force between water molecules and other surfaces.
Capillary action
Capillary action is produced by the combined effect of cohesive and adhesive forces, allowing water to enter and move along narrow spaces such as xylem vessels.
Specific heat capacity
Water has a high specific heat capacity of \(4.2\text{ kJ kg}^{-1}\text{ }^\circ\text{C}^{-1}\). This means that \(4.2\text{ kJ}\) of heat energy is required to raise the temperature of one kilogram of water by \(1\text{ }^\circ\text{C}\).
Try to Answer First
Answer in your mind, then press “Check Answer”.
What chemical bonds form between polar water molecules and are broken when water changes from liquid to water vapour?
What is the difference between cohesive force and adhesive force in water?
What is the value of the specific heat capacity of water, and what does it indicate?
Common Mistakes
- Confusing cohesive force with adhesive force — for example, stating that cohesive force attaches water to the walls of xylem vessels.
- Confusing covalent bonds within a water molecule with hydrogen bonds between water molecules when describing water boiling or evaporation.
- Forgetting the unit when stating specific heat capacity, or writing incorrect unit prefixes instead of \(4.2\text{ kJ kg}^{-1}\text{ }^\circ\text{C}^{-1}\).
- Thinking water is an organic compound because it is found in all living organisms, whereas it is strictly an inorganic compound.
Concept Misunderstandings
Water molecules are non-polar because the overall net charge of a water molecule is zero.
Although neutrally charged overall, water is polar because oxygen is more electronegative, attracting shared electrons more strongly and creating partial negative \(\delta^-\) and partial positive \(\delta^+\) regions.
Capillary action is driven solely by adhesive force.
Capillary action is the result of both cohesive forces, which hold water molecules together, and adhesive forces, which pull water molecules along the surface.
Summary
- Water is an inorganic molecule composed of hydrogen and oxygen.
- Due to the high electronegativity of oxygen, water is polar and forms hydrogen bonds, serving as a universal solvent that transports glucose and electrolytes into cells.
- Water molecules demonstrate cohesive forces with each other and adhesive forces with other surfaces, generating capillary action inside narrow spaces like xylem vessels.
- With a high specific heat capacity of \(4.2\text{ kJ kg}^{-1}\text{ }^\circ\text{C}^{-1}\), water absorbs extensive heat with minimal temperature increase, preserving internal temperature stability in organisms.
Short Activity
Objective: test your understanding of water polarity, forces and specific heat capacity.
A. Objective Quiz
Why is water classified as a polar molecule?
Which phenomenon is produced by both cohesive and adhesive forces acting together in narrow spaces like xylem vessels?
B. Fill in the Blanks
The force of attraction that attaches water molecules to other surfaces is known as the force.
Water has a high specific heat capacity of kJ kg⁻¹ °C⁻¹.
C. Matching / Drag and Drop
Match each property or force of water to its correct biological role or definition. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Property or Force to Its Role
Write the correct property or force next to each role.
| Role / Description | Property / Force |
|---|---|
| 1. Allows glucose and electrolytes to dissolve and be transported into cells | |
| 2. Attraction between water molecules that binds them to each other | |
| 3. Attraction between water molecules and the surfaces of other structures | |
| 4. Absorbs substantial heat energy with a small temperature change to maintain body temperature |
Answer choices: Universal solvent • Cohesive force • Adhesive force • High specific heat capacity
