Intermolecular Attraction: Hydrogen bonds form between the oxygen atom of one water molecule and the hydrogen atom of a neighboring molecule. These are not chemical bonds but rather strong electrostatic attractions that occur due to water's polarity.
Collective Strength: While a single hydrogen bond is relatively weak and easily broken by thermal energy, the vast number of these bonds in liquid water creates a highly cohesive network. This network is responsible for water's high boiling point and its liquid state at room temperature.
Dynamic Nature: In liquid water, hydrogen bonds are constantly breaking and reforming as molecules move. This allows water to remain fluid while still maintaining the strong internal tension required for biological transport.
High Specific Heat Capacity: Water requires a significant amount of energy () to raise its temperature because much of the heat energy is used to break hydrogen bonds rather than increasing kinetic energy. This property prevents rapid temperature fluctuations in aquatic habitats and within the cells of organisms.
High Latent Heat of Vaporisation: A large amount of thermal energy is needed to evaporate water, as many hydrogen bonds must be broken for molecules to escape as gas. This makes evaporation (such as sweating or transpiration) an extremely effective cooling mechanism for living things.
Thermal Buffering: Because water resists temperature change, it acts as a buffer for the internal environment of organisms. This stability is essential for maintaining the optimal temperature for enzyme-controlled reactions and preventing protein denaturation.
Universal Solvent: Due to its polarity, water can dissolve a wide variety of ionic and polar substances. Water molecules surround the solute particles, with the oxygen attracting cations and the hydrogens attracting anions, effectively isolating them in solution.
Cohesion and Adhesion: Cohesion is the tendency of water molecules to stick together via hydrogen bonds, while adhesion is their ability to stick to other polar surfaces. Together, these forces allow water to move in continuous columns through the xylem of plants against gravity.
Surface Tension: At the air-water interface, water molecules are more strongly attracted to each other than to the air, creating a 'skin' known as surface tension. This property supports small organisms and influences how water droplets form and interact with biological surfaces.
| Feature | Cohesion | Adhesion |
|---|---|---|
| Definition | Attraction between like molecules (water-water) | Attraction between unlike molecules (water-surface) |
| Mechanism | Hydrogen bonding between dipoles | Interaction with polar groups on other surfaces |
| Biological Role | Creates surface tension and continuous water columns | Allows water to 'climb' vessel walls (capillary action) |
| Property | Specific Heat Capacity | Latent Heat of Vaporisation |
| --- | --- | --- |
| Focus | Energy to change temperature | Energy to change state (liquid to gas) |
| Biological Benefit | Temperature stability in habitats/bodies | Efficient cooling through evaporation |
Link Structure to Function: When asked about a property of water, always start by mentioning its polarity and hydrogen bonding. Examiners look for the causal link between the chemical structure and the biological advantage.
Precision in Terminology: Do not confuse 'hydrogen bonds' with 'covalent bonds'. Covalent bonds are within the molecule; hydrogen bonds are between molecules. Using these terms interchangeably is a common way to lose marks.
Explain the 'Why' of Cooling: If discussing sweating, don't just say it cools the body. Explain that because water has a high latent heat of vaporisation, it carries away a large amount of heat energy from the skin as it evaporates.
Density Awareness: Remember that water is less dense as a solid (ice) than as a liquid. This is because hydrogen bonds hold the molecules in a fixed, open lattice structure when frozen, allowing ice to float and insulate the water below for aquatic life.
