02.03 Water
Covalent Bonds and Electron Sharing
- Covalent Bonding: Atoms share electrons to form stable molecules.
- Example: In a water molecule (H₂O), each hydrogen atom shares electrons with the oxygen atom, creating covalent bonds.
Dipoles: Unequal Electron Sharing
- Unequal Sharing of Electrons:
- Electrons are not always shared equally in covalent bonds.
- In water:
- Oxygen attracts electrons more strongly, acquiring a slight negative charge (δ−).
- Hydrogen atoms have a slight positive charge (δ+).
- This creates a dipole, a molecule with opposite charges on different ends.
Hydrogen Bonds
- Definition: A weak bond formed by attraction between a δ+ hydrogen and a δ− region (often oxygen or nitrogen) in another molecule.
- Representation: Shown as dotted or dashed lines in diagrams.
- Strength: Weaker than covalent bonds but significant for molecular properties.
- Example in Water:
- The δ− oxygen of one water molecule is attracted to the δ+ hydrogen of another, forming a hydrogen bond.
Importance of Dipoles and Hydrogen Bonds
- Hydrogen Bonding in Biological Molecules:
- Common in molecules with –OH, –CO, or –NH groups.
- Essential in stabilizing the structures of carbohydrates (e.g., cellulose) and proteins.
Polarity and Solubility
- Polar Molecules:
- Molecules with dipoles (like water and sugars) are hydrophilic (water-attracting).
- Water Solubility: Polar molecules dissolve in water due to attraction between their charges and water’s dipoles.
- Non-Polar Molecules:
- Molecules without dipoles are non-polar and are hydrophobic (water-repelling).
- Role in Cell Membranes: Non-polar molecules’ aversion to water is critical in forming cell membranes (hydrophobic inner layer).
Polar vs. Non-Polar:
- Polar molecules are hydrophilic and soluble in water.
- Non-polar molecules are hydrophobic, a property essential for cellular membrane formation.
Properties of Water and Its Importance for Life
- High Specific Heat Capacity
- Description: Water can absorb or release a large amount of heat with minimal temperature change.
- Importance: Stabilizes temperatures in organisms and environments, supporting life.
- High Latent Heat of Vaporization
- Description: Water requires significant energy to change from liquid to gas.
- Importance: Helps in cooling mechanisms like sweating and transpiration.
- Cohesion and Adhesion
- Cohesion: Water molecules stick together due to hydrogen bonding.
- Adhesion: Water molecules stick to other surfaces.
- Importance: Enables water transport in plants (e.g., through xylem) and surface tension for organisms like insects to walk on water.
- Universal Solvent
- Description: Water dissolves many substances due to its polarity.
- Importance: Facilitates chemical reactions in cells and nutrient/waste transport in organisms.
- Density and Ice Floating
- Description: Water is less dense as a solid than as a liquid, causing ice to float.
- Importance: Insulates aquatic ecosystems, allowing life to survive in cold climates.
- Transparency
- Description: Water is transparent, allowing light to penetrate.
- Importance: Supports photosynthesis in aquatic plants and algae.
- High Surface Tension
- Description: Water molecules at the surface are more tightly bonded.
- Importance: Supports small organisms and plays a role in cellular processes.
Practise Questions
Question 1
Explain how covalent bonds form and provide an example. (4 marks)
Mark Scheme:
- Covalent bonds are formed when atoms share electrons to achieve a stable electron configuration. (1 mark)
- The sharing allows atoms to fill their outermost electron shells. (1 mark)
- Example: In a water molecule (H₂O), each hydrogen atom shares an electron with the oxygen atom, forming covalent bonds. (1 mark)
- These bonds hold the atoms together, creating a stable molecule. (1 mark)
Question 2
What is a dipole, and how does it form in a water molecule? (5 marks)
Mark Scheme:
- A dipole is a molecule with opposite charges on different ends due to unequal electron sharing. (1 mark)
- In water, the oxygen atom attracts shared electrons more strongly than hydrogen atoms. (1 mark)
- This creates a partial negative charge (δ−) on the oxygen atom and a partial positive charge (δ+) on each hydrogen atom. (1 mark)
- The result is a polar molecule with a dipole moment. (1 mark)
- This polarity allows water molecules to interact through hydrogen bonding. (1 mark)
Question 3
Describe how hydrogen bonds form and provide an example. (6 marks)
Mark Scheme:
- Hydrogen bonds form due to the attraction between a partially positive hydrogen atom (δ+) and a partially negative region (δ−) of another molecule. (1 mark)
- These bonds typically occur in molecules containing oxygen (O) or nitrogen (N). (1 mark)
- In water, the δ− oxygen of one molecule attracts the δ+ hydrogen of another, forming a hydrogen bond. (1 mark)
- Hydrogen bonds are weaker than covalent bonds but collectively significant in stabilizing molecular structures. (1 mark)
- Example: Hydrogen bonds stabilize the helical structure of DNA or the secondary structure of proteins. (1 mark)
- Representation: Often shown as dashed or dotted lines in molecular diagrams. (1 mark)
Question 4
Compare the properties of polar and non-polar molecules and their role in biological systems. (6 marks)
Mark Scheme:
- Polar molecules have dipoles, with opposite charges on different ends due to unequal electron sharing. (1 mark)
- Polar molecules are hydrophilic (water-attracting) and dissolve in water. (1 mark)
- Example: Water and sugars are polar molecules that interact with water through hydrogen bonding. (1 mark)
- Non-polar molecules lack dipoles and are hydrophobic (water-repelling). (1 mark)
- Non-polar molecules play a key role in forming cell membranes, with their hydrophobic regions creating a barrier. (1 mark)
- This difference between polar and non-polar properties is crucial for biological processes like transport and compartmentalization. (1 mark)
Question 5
Explain the significance of hydrogen bonds in biological molecules. (5 marks)
Mark Scheme:
- Hydrogen bonds stabilize the structures of many biological molecules, including proteins and DNA. (1 mark)
- In proteins, hydrogen bonds contribute to secondary structures like alpha-helices and beta-pleated sheets. (1 mark)
- In DNA, they hold the complementary base pairs (A-T and G-C) together, maintaining the double-helix structure. (1 mark)
- In carbohydrates, hydrogen bonds strengthen structures like cellulose, providing rigidity to plant cell walls. (1 mark)
- These bonds are also essential in water’s cohesion and solvent properties, supporting life processes. (1 mark)
Question 6
Why do polar molecules dissolve in water, and why are non-polar molecules hydrophobic? (6 marks)
Mark Scheme:
- Polar molecules dissolve in water because their charges interact with water’s dipoles. (1 mark)
- The partially positive (δ+) regions of polar molecules attract water’s oxygen, and the partially negative (δ−) regions attract water’s hydrogens. (1 mark)
- This interaction forms hydrogen bonds, allowing polar molecules to disperse in water. (1 mark)
- Non-polar molecules are hydrophobic because they lack dipoles and cannot form hydrogen bonds with water. (1 mark)
- Water molecules preferentially bond with each other, excluding non-polar molecules. (1 mark)
- This property is critical in forming cell membranes, where non-polar lipid tails create a hydrophobic barrier. (1 mark)
Question 7
Describe how dipoles contribute to the properties of water. (6 marks)
Mark Scheme:
- Water is a polar molecule with dipoles due to the unequal sharing of electrons between oxygen (δ−) and hydrogen (δ+). (1 mark)
- The polarity allows water molecules to form hydrogen bonds with each other. (1 mark)
- These bonds give water its cohesive properties, supporting surface tension. (1 mark)
- Water’s dipoles enable it to dissolve many polar substances, making it a universal solvent. (1 mark)
- Dipoles are also responsible for water’s high specific heat capacity, stabilizing temperatures in organisms and environments. (1 mark)
- This unique property supports biochemical reactions and life processes. (1 mark)
Question 8
How does the polarity of water influence its role in biological systems? (5 marks)
Mark Scheme:
- The polarity of water allows it to dissolve polar substances and ions, enabling it to act as a universal solvent. (1 mark)
- Water facilitates biochemical reactions by dissolving reactants and transporting nutrients. (1 mark)
- The formation of hydrogen bonds contributes to water’s cohesion, supporting processes like water transport in plants. (1 mark)
- Polarity enables water to moderate temperature through its high specific heat, stabilizing environments for organisms. (1 mark)
- These properties make water essential for processes like photosynthesis, cellular respiration, and homeostasis. (1 mark)
Question 9
Explain the role of non-polar molecules in the formation of cellular membranes. (5 marks)
Mark Scheme:
- Non-polar molecules, such as the hydrophobic tails of phospholipids, repel water. (1 mark)
- In water, phospholipids arrange into a bilayer, with hydrophobic tails facing inward and hydrophilic heads outward. (1 mark)
- This arrangement creates a selectively permeable barrier that separates the cell’s internal and external environments. (1 mark)
- The hydrophobic core prevents the free passage of polar molecules and ions, ensuring controlled molecular exchange. (1 mark)
- This structure is critical for processes like signal transduction, transport, and maintaining cellular integrity. (1 mark)
Question 10
Summarize the biological significance of hydrogen bonds and polarity in water. (6 marks)
Mark Scheme:
- Hydrogen bonds give water its cohesive and adhesive properties, supporting processes like capillary action in plants. (1 mark)
- Polarity enables water to dissolve polar and ionic substances, making it a universal solvent. (1 mark)
- Hydrogen bonding contributes to water’s high specific heat capacity, stabilizing temperatures in organisms and ecosystems. (1 mark)
- Polarity and hydrogen bonds support water’s role in transporting nutrients and waste in biological systems. (1 mark)
- Hydrogen bonds are essential for the stability of biological molecules like DNA and proteins. (1 mark)
- Together, these properties make water indispensable for life. (1 mark)