Aquatic Resources & Ecology – Properties of Water

Vocabulary flashcards summarizing key physical, chemical, and biological properties of water and their significance to aquatic ecology and fisheries science.

Water (H₂O)

A colorless, tasteless, odorless substance composed of two hydrogen atoms and one oxygen atom; exists naturally as liquid, solid (ice) and gas (vapor).

Covalent Bond

Strong chemical bond formed by the sharing of electron pairs between atoms; holds the hydrogen and oxygen atoms together inside a water molecule.

Hydrogen Bond

Weak electrostatic attraction between the partial positive hydrogen of one water molecule and the partial negative oxygen of another; responsible for many of water’s unique properties.

Polar Molecule

A molecule with unequal charge distribution; in water, oxygen is partially negative and hydrogens are partially positive, giving water electrical polarity.

Dipolarity

Condition of having two opposite electrical poles; water’s dipolarity enables strong intermolecular attractions and solvent ability.

Water Cycle

Continuous movement of water through evaporation, condensation, precipitation and runoff, keeping Earth’s total water quantity essentially constant.

Specific Heat Capacity of Water

4.18 J g⁻¹ °C⁻¹; the amount of heat needed to raise 1 g of water by 1 °C, giving water great thermal stability for aquatic life and climate regulation.

Latent Heat of Vaporization

≈ 2,260 J g⁻¹; energy required to convert liquid water to vapor without temperature change, allowing evaporative cooling and moderating pond temperatures.

Latent Heat of Fusion

334 J g⁻¹; energy needed to melt ice to liquid water, causing slow freezing/melting and winter protection for aquatic organisms.

Boiling Point of Pure Water

100 °C (212 °F) at 1 atm; raised when solutes such as salt are present.

Freezing Point of Pure Water

0 °C (32 °F) at 1 atm; lowered when solutes are dissolved (freezing point depression).

Temperature of Maximum Density

4 °C for fresh water; below this, density decreases so ice is less dense and floats.

Density (of Water)

Mass per unit volume; ~1 g cm⁻³ for pure water at 4 °C, increases with salinity and pressure, decreases with temperature above 4 °C.

Thermal Conductivity

0.606 W m⁻¹ K⁻¹ at 25 °C; moderate ability of water to conduct heat, helping distribute warmth evenly in aquatic systems.

Viscosity

0.89 mPa·s at 25 °C; measure of internal friction resisting flow; decreases with higher temperature and increases with salinity.

Surface Tension

72.8 mN m⁻¹ at 25 °C; cohesive force at the water–air interface that lets some insects walk on water and supports small objects.

Cohesion

Attraction between like water molecules via hydrogen bonds, creating surface tension.

Adhesion

Attraction between water molecules and other substances; key to capillary action.

Capillary Action

Ability of water to rise in narrow tubes against gravity due to cohesion and adhesion; important in soil moisture and plant water uptake.

Compressibility

Water’s very low volume change under pressure; keeps density nearly constant even at great depths.

Refractive Index of Water

1.333 at 20 °C; ratio indicating light travels 1.333 × slower than in vacuum, affecting underwater vision and photosynthesis depth.

Electrical Conductivity (EC)

Ability of water to conduct electricity via dissolved ions; measured in µS cm⁻¹, increases from fresh to brackish to seawater.

Transparency

Degree to which light penetrates water; high transparency promotes photosynthesis, while turbidity reduces it.

Turbidity

Cloudiness caused by suspended particles; measured in NTU, high values limit light, hinder feeding, and reduce dissolved oxygen.

Universal Solvent

Water’s ability to dissolve many ionic and polar substances, facilitating nutrient transport and biochemical reactions.

Hydration (Ion Hydration)

Process where water molecules surround and separate ions, enabling salts like NaCl to dissolve.

Salinity

Total concentration of dissolved salts in water, expressed in parts per thousand (ppt) or practical salinity units (PSU).

Practical Salinity Unit (PSU)

Dimensionless unit based on seawater conductivity; 35 PSU approximates typical ocean salinity.

Osmoregulation

Physiological process by which aquatic organisms maintain internal salt and water balance despite external salinity changes.

Amphoteric Nature of Water

Capacity of water to act as both an acid (proton donor) and a base (proton acceptor), helping buffer pH changes.

Autoionization of Water

Spontaneous formation of equal amounts of H₃O⁺ and OH⁻ ions, establishing a neutral pH of 7 at 25 °C.

pH

Logarithmic measure of hydrogen ion concentration; aquatic life generally thrives between pH 6.5 and 9.

Redox Reaction

Chemical process involving electron transfer; in water, affects oxygen levels, corrosion, and nutrient cycling.

Photosynthesis (Water Role)

Process in which water supplies electrons and protons to produce oxygen and glucose in plants and algae.

Dissolved Oxygen (DO)

Concentration of free O₂ in water, essential for respiration of fish and other aquatic organisms.

Buoyancy

Upward force exerted by water proportional to its density; supports aquatic organisms and vessels.

Thermal Stratification

Layering of water masses of different temperature and density, influencing oxygen distribution and nutrient cycling.

High Heat Capacity & Climate Regulation

Water absorbs and releases large amounts of heat, moderating coastal climates and stabilizing pond temperatures.

Sea Smoke (Steam Fog)

Visible vapor formed when cold air moves over warmer water, illustrating latent heat of vaporization.

Hydrostatic Pressure

Pressure exerted by the weight of water above a point; increases by ~1 atm every 10 m depth, challenging deep-sea life.

Hydrogen Bonds in Ice

Fixed lattice hydrogen bonds create an open hexagonal structure, making ice less dense than liquid water.

Ice Expansion

Volume increase of about 9 % when water freezes, exerting force that can crack rocks and pipes.

Density Anomaly: Ice Floats

Because solid water is less dense than liquid, ice remains at the surface, insulating the water below.

Specific Gravity

Ratio of the density of a substance to the density of water; determines whether objects sink or float.

Nephelometric Turbidity Unit (NTU)

Standard unit expressing turbidity based on light scattering by suspended particles.

Dead Sea

Hypersaline lake (~300–350 ppt) where high salinity prevents most aquatic life, hence the name 'Dead.'

Brackish Water

Water with intermediate salinity (0.5–30 ppt), typical of estuaries and mangroves.

Freshwater

Water with very low salinity (<0.5 ppt); habitat for freshwater fish and many inland ecosystems.

Marine Water

High-salinity water (~35 ppt) of the open ocean; supports marine fish and invertebrates.

Don Juan Pond

Hypersaline Antarctic lake (≈ 440 ppt), one of Earth’s saltiest natural water bodies.