Water Composition and Polarity

Water Composition

• Water consists of two hydrogen atoms covalently bonded to an oxygen atom, represented by the molecular formula H2O

Polarity of Water

• Electrons are not shared equally in water's covalent bonds, resulting in polarity
• Oxygen's higher electronegativity attracts electrons more strongly, resulting in a slightly negative charge (δ−)
• Hydrogen atoms have a weaker electron attraction, leading to a slightly positive charge (δ+)
• Polarity allows water to form weak associations with other polar molecules or charged ions
• Slightly negative poles (δ−) attract slightly positive poles (δ+) of other molecules, and vice versa.

Hydrogen Bonds

• Hydrogen bonds form as a consequence of polar covalent bonds within water molecules
• Hydrogen bonds are intermolecular associations formed by polar covalent molecules
• Hydrogen bonds form when a δ+ hydrogen atom is attracted to a δ− fluorine or nitrogen atom of another molecule
• They are stronger than other associations due to high electronegativity
• Hydrogen bonding is responsible for water's unique properties like thermal, cohesive, and solvent characteristics

Cohesion

• Water is cohesive, meaning water molecules stick to each other through hydrogen bonds
• Hydrogen bonding allows water to resist low levels of external force, creating surface tension
• Surface tension makes water dense enough for small organisms to move across it
• Water striders use long legs with microscopic hairs to trap air and increase buoyancy

Adhesion

• Water is adhesive, meaning water molecules stick to other polar or charged molecules
• Attraction to charged or polar surfaces, such as cellulose cell walls, allows capillary action, enabling water to flow against gravity
• Capillary action strength depends on the diameter of the pore through which water moves; smaller diameter = more action
• Capillary action is necessary for transporting water up plant stems via a transpiration stream
• Water vapor loss from leaves (evaporation) and water absorption by roots (osmosis) create a pressure gradient
• Water moves along this gradient via capillary action and cohesion, transported up the stem via narrow xylem vessels
• Capillary action also moves water through the soil from deeper water tables
• Soil type influences capillary action strength, affecting agricultural practices

Solvent Properties

• Water's capacity to dissolve many substances makes it important for metabolic reactions
• Solutes dissolved in aqueous solutions are more likely to collide with enzymes and undergo chemical reactions
• Water promotes enzyme activity by absorbing heat from exothermic reactions and maintaining acid-base neutrality
• Substances in metabolic reactions are either hydrophilic or hydrophobic
• Hydrophilic substances freely associate and dissolve in water, including polar molecules and ions
• Hydrophobic substances do not freely associate or dissolve in water, like large non-polar molecules (lipids)

Transport Properties

• Blood plasma in animals transports dissolved solutes (amino acids, simple sugars, wastes, and gases)
• Mineral ions are transported via xylem vessels, while dissolved nutrients are transported via phloem
• Lipids in animals are packaged with proteins to form water-soluble lipoproteins for blood transport

Physical Properties

• Physical properties such as buoyancy, viscosity, thermal conductivity, and specific heat capacity make water the medium of life
• Buoyancy is the upward force on an object in a medium determined by density
• Water is denser than air, allowing objects to float
• An object's capacity to float is determined by its relative weight
• Viscosity is a fluid's tendency to flow; more viscous fluids resist flow more

Viscosity of Water

• Water is more viscous than air due to hydrogen bonds, increasing friction between flowing molecules
• Dissolved solutes increase water's viscosity
• Thermal conductivity is a medium's ability to move heat across a temperature gradient
• Water absorbs and transfers heat more readily than air because water particles are packed tighter

Heat Capacity

• Specific heat capacity describes the energy needed to raise the temperature of 1g of a substance by 1ºC
• Water has a higher specific heat capacity than air (highest of any liquid) since its hydrogen bonds require more energy to break
• High specific heat capacity makes water an effective coolant
• Sweat absorbs a lot of thermal energy, releasing heat by latent heat of vaporization

Biological Adaptations

• Black-throated loons and ringed seals live in the Arctic and spend time on land and in water
• Loons have lighter, less dense bones, allowing them to float, and non-hollow bones, allowing them to dive
• Loons have difficulty walking on land due to rear leg placement, which enable them to propel through water
• Loon feathers form an interlocking structure which creates a barrier to water which prevents heat loss because water has a high thermal conductivity

Marine Animals

• Seals have denser bones than loons, which allows them to stay submerged while diving and makes them less buoyant
• Seals have streamlined bodies to better propel themselves through water
• Seals have an outer coat of air-trapping fur and a thick blubber layer to prevent heat loss
• Ringed seals do not have effective cooling mechanisms due to generally stable water temperatures (due to specific heat capacity/thermal conductivity)

Water Origins

• Earth's water may have originated from asteroids formed further from the Sun, where cooler temperatures allowed water to freeze as ice
• Asteroids colliding with Earth had achieved enough mass (and gravity) to retain water vapor and had cooled enough for it to condense

Extraterrestrial Water

• Liquid water is essential for searching other habitable planets with extraterrestrial lifeforms
• The Goldilocks zone is the range of distance from a star where the right temperatures exist for liquid water to exist on a planet
• Relevant distances depend on the star's size and temperature
• Habitable zone is farther from hotter stars because they're hotter
• Few planets in Goldilocks zones have suitable mass or atmosphere for liquid water to exist on the surface