Water is essential for life due to its unique properties arising from its molecular structure and polarity. Water molecules are polar due to an uneven distribution of charge, with oxygen exerting a pull on shared electrons making it slightly negative and hydrogen slightly positive. This allows water to form hydrogen bonds between molecules, giving water high melting and boiling points, heat capacity, and the ability to dissolve many other polar substances. Water's polarity allows it to act as a solvent and universal medium for critical biological and chemical processes on Earth.

1. Topic 2: Molecular Biology
2.2 Water
Romy Friedman
DP Biology

2. Syllabus

3. Essential idea:
Water is the medium of life

4. Water (H2O)
• There is a ratio of 2:1 for Hydrogen: Oxygen
• Water is a compound formed from two elements:
oxygen and hydrogen

5. Forming a water molecule
covalent bonding
• Hydrogen has 1 valence electron
• An oxygen shares an electron with two hydrogen
atoms, forming two covalent bonds
• Oxygen has 6 valence electrons

6. Polarity of water
• Water is a unique substance earth, and
therefore essential to life, because of
its polar nature

8. Partial Charge
• Oxygen exerts a great pull over the shared
electrons. They move a little closer to the oxygen
• That means that the oxygen part of the molecule is
slightly negative
• The hydrogen end of the molecule is slightly
positive

9. Hydrogen bonding in water

10. Polarity relates to
solubility in water
Hydrophillic
• A substance that is attracted to water and forms
intermolecular bonds with water molecules
• Ionic compounds and substances with polar molecules
Hydrophobic
• Means that waters molecules are more strongly attracted to
each other than to the substances
• Hydrophobic substances are insoluble in water

11. Water as the Universal Solvent

12. The Nature of Science
• Theories are used to explain natural
phenomena
• Hydrogen bonding explains the unique
behaviour of water that allows life on
earth

13. 4 Property types of Water
• Cohesive
• Adhesive
• Thermal
• Solvent

14. Water is Cohesive
Explanation
• Water molecules cohere because hydrogen
bonds form between them
Example of benefits to organisms
• Transpiration stream

15. Water is Adhesive
Explanation
• The polarity of water molecules makes them
adhere to surfaces that are polar (and therefore
hydrophillic)
Example of benefits to organisms
• Transpiration stream & gas exchange in plants

17. Water has Thermal
Properties
Explanation
• High melting point & boiling point
• High latent heat of vaporization
• High specific heat capacity
Example of benefits to organisms
• Liquid (aquatic) habitats that are stable
• Effective coolant in leaves or sweat

18. Water as a habitat

19. Water as a Coolant

20. Water is a Solvent
Explanation
• Many molecules dissolve in water due to its
polarity including ions or polar molecules
Example of benefits to organisms
• Medium for metabolic reactions

23. Water as the medium of life