- Water is essential for life on Earth due to its unique physical and chemical properties.
- These properties allow it to sustain ecosystems, support biological processes, and regulate the climate.
Polarity and Hydrogen Bonding
Polarity
Polarity refers to the uneven distribution of electrical charge in a molecule, leading to regions of slight positive and negative charge.
- Each water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom in a V-shaped structure.
- Oxygen is more electronegative, pulling shared electrons closer to itself, creating a partial negative charge (δ–) at the oxygen end and partial positive charges (δ+) at the hydrogen ends.
- This unequal distribution of charge makes water a polar molecule.
- Due to polarity, adjacent water molecules form hydrogen bonds, weak attractions between the δ+ hydrogen of one molecule and the δ– oxygen of another.
- Individually weak but collectively strong, these bonds give water its unique stability and cohesive strength.
- Without hydrogen bonding, water would exist as a gas at room temperature, and life as we know it would not exist.
- The polarity of water enables hydrogen bonding, which allows droplets to form, supports surface tension, and contributes to the cohesion that insects exploit to walk on water.
Cohesion
Cohesion
The force of attraction between molecules of the same substance (in this case, water).
- Cohesion is the attraction between water molecules caused by hydrogen bonding.
- It allows water molecules to “stick” together, forming droplets and maintaining surface tension.
- This surface tension allows water to resist external forces and form curved surfaces such as the meniscus in a test tube.
Water striders can walk on water because surface tension acts like an elastic film.
- Cohesion is essential in plants.
- It helps maintain an unbroken water column in xylem vessels during transpiration.
Adhesion
Adhesion
Adhesion is the attraction between unlike substances, often involving polar or charged surfaces.
- Adhesion occurs when water molecules are attracted to other materials, especially those with polar or charged surfaces.
- Adhesion allows water to “climb” narrow tubes against gravity through capillary action.
- This process, combined with cohesion, drives transpiration in plants, allowing water to move upward from the roots to the leaves through xylem vessels.
Cohesion is water sticking to itself, while adhesion is water sticking to something else.
ExampleIn plants, adhesion between water and cellulose in xylem walls helps pull water upward against gravity during transpiration.
Solvent Properties
- Water is known as the universal solvent due to its polarity.
- It dissolves many ionic compounds (like NaCl) and polar molecules (like glucose).
- When ionic compounds dissolve, the positive hydrogen poles surround negative ions (Cl⁻), while the negative oxygen poles surround positive ions (Na⁺).
- This process, known as hydration, separates and disperses ions evenly throughout the solution.
Sodium chloride (NaCl) dissolves in water because the δ+ hydrogen atoms attract Cl⁻ ions and the δ– oxygen attracts Na⁺ ions.
Transparency
- Water is transparent, allowing sunlight to penetrate aquatic ecosystems.
- This enables photosynthesis in underwater plants, algae, and phytoplankton.
- The depth to which light can reach determines biological zones:
- Euphotic (sunlit) zone: up to ~200 m, where most photosynthesis occurs.
