- 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.
In 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.
- Dysphotic (twilight) zone: 200–1,000 m, reduced light but some visibility.
- Aphotic (midnight) zone: below 1,000 m, no light, photosynthesis impossible.
In coral reefs, sunlight penetration supports photosynthetic algae (zooxanthellae), essential for coral survival.
High Specific Heat Capacity
Specific heat capacity
Specific heat capacity is the amount of energy needed to raise the temperature of 1 gram of a substance by 1°C.
- Water has an exceptionally high specific heat capacity, about 4.2 J g⁻¹ °C⁻¹.
- This is due to strong hydrogen bonding between molecules that must be broken before the temperature can rise.
- As a result, water heats and cools slowly, moderating both climate and body temperature.
- Oceans absorb heat during the day and release it slowly at night, stabilizing global climate patterns.
- Within organisms, water in blood and tissues helps regulate internal temperature.
Water acts like a thermal buffer, preventing rapid temperature fluctuations that could harm life.
Density and Temperature
- Water displays anomalous density behavior.
- It is most dense at 4°C.
- As it cools below this temperature, molecules form a crystalline structure (ice) that is less dense than liquid water, allowing it to float.
- Floating ice provides insulation for aquatic ecosystems, preventing complete freezing.
- This ensures that organisms survive beneath the ice even in extremely cold climates.
Polar bears hunt on sea ice, which remains buoyant because of its lower density compared to liquid water.
When explaining density, always refer to the hydrogen bonding arrangement and molecular spacing in ice versus water.
Gas Solubility in Water
- Water can dissolve atmospheric gases such as oxygen (O₂) and carbon dioxide (CO₂).
- Solubility depends on temperature and pressure:
- Cold water holds more dissolved gases.
- Warm water holds less, leading to oxygen depletion.
- This property is essential for aquatic respiration and photosynthesis.
- However, warming oceans due to climate change reduce oxygen levels, leading to hypoxic zones or “dead zones”.
The cold Arctic Ocean supports abundant fish species due to high oxygen solubility.
High Latent Heat of Vaporization
Latent heat of vaporization
Latent heat of vaporization is the amount of energy required to convert 1 kg of liquid to vapor without a change in temperature.
- Water requires large amounts of energy to change from liquid to gas due to hydrogen bonding.
- This property allows evaporation to remove heat effectively, acting as a cooling mechanism for organisms and ecosystems.
- Humans sweat and dogs pant to cool down.
- Evaporation of water from the body surface carries away excess heat.
- In plants, transpiration has a cooling effect on leaves exposed to sunlight.
- How might the unique properties of water influence the search for life on other planets?
- Could the presence of water alone be enough to suggest the possibility of life?
- Explain how polarity and hydrogen bonding give water its unique properties.
- Compare cohesion and adhesion, giving one biological example of each.
- Discuss the importance of water’s high specific heat capacity to climate stability.
- Analyze how the density anomaly of water supports life in frozen lakes.
- Evaluate the ecological impact of decreasing gas solubility in warming oceans.
