Chapter 6: Water and Its Molecular Structure

Vocabulary practice flashcards covering water's chemical composition, polar covalent geometry, intermolecular hydrogen bonding, unique physical and thermal properties, and solvent behaviors.

Discovery of Water's Composition (Lavoisier, 1783)

The experimental demonstration on June 24, 1783, by French chemist Antoine Lavoisier proving that water is composed of hydrogen and oxygen by burning gas produced from acid on iron or copper ($H_2$) with gas from heated red precipitate of mercury ($O_2$), collecting droplets of distilled water.

Compound

A substance composed of two or more distinct chemical elements combined in a fixed proportion.

Covalent Bond (in $H_2O$)

A strong primary chemical bond formed when two hydrogen atoms share their electrons with one oxygen atom to produce a stable water molecule.

Angular Geometry of Water

The $105^\boxminus$ (or $105^\top$) bent shape of a water molecule that causes an asymmetric distribution of electrical charge, giving the molecule distinct positive and negative ends.

Polar Covalent Bond

A covalent bond exhibiting electrical polarity, where one end carries a partial negative charge and the opposite end carries a partial positive charge, enabling it to attract oppositely charged particles like a magnet.

Hydrogen Bond

A weak secondary intermolecular bond formed by the electrostatic attraction between the positive hydrogen end of one polar water molecule and the negative oxygen end of an adjacent water molecule, having about $5\% \sim 10\%$ the strength of an intramolecular covalent bond.

Boiling and Melting Points of Group 16 Hydrides

The thermal property comparison demonstrating that water ($H_2O$) has an unexpectedly high boiling point ($100\,^\circ\text{C}$) and melting point ($0\,^\circ\text{C}$) relative to heavier group 16 hydrides ($H_2S$, $H_2Se$, $H_2Te$), driven by extensive intermolecular hydrogen bonding.

Cohesion

The intermolecular attraction that causes water molecules to stick to one another, resulting in high surface tension.

Adhesion

The property that causes water molecules to stick to other materials, enabling water to wet surfaces.

Capillary Action

The movement of water through narrow spaces or porous materials (such as spreading through a paper towel) caused by the combined effects of cohesion and adhesion.

Blue Color of Pure Water

The optical phenomenon caused by vibrating water molecules absorbing red light from the visible spectrum, leaving more blue light to scatter back to the viewer's eyes.

Three Physical States of Water

The solid, liquid, and gaseous phases of water characterized by hydrogen bond disruption: all hydrogen bonds remain organized in an open hexagonal matrix in solid ice, some are broken in liquid water, and all are broken in water vapor.

Hexagonal Ice Crystal Structure

An open three-dimensional lattice where each water molecule forms hydrogen bonds with up to four others (two via lone pairs of electrons, two via hydrogen atoms), causing water to expand by $9\%$ and making solid ice less dense than liquid water.

Specific Heat (Water)

A measure of thermal resistance that is exceptionally high in water due to hydrogen bonding, requiring large inputs or losses of thermal energy to change its temperature.

Heat Capacity

The quantity of heat required to raise the temperature of $1\,\text{g}$ of a substance by $1\,^\circ\text{C}$; water possesses the highest heat capacity of all common solids and liquids, moderating temperature extremes on Earth.

Latent Heat of Fusion

The quantity of heat gained or lost per unit mass ($80\,\text{cal/g}$ for water) during a phase transition between solid and liquid without an accompanying change in temperature.

Latent Heat of Vaporization

The quantity of heat gained or lost per unit mass ($540\,\text{cal/g}$ for water, the highest of all common substances) during a phase transition between liquid and gas without an increase in temperature, driving weather and climate.

Density Drivers of Seawater

The primary controls of water density (mass per unit volume), determined in order of importance by: (1) temperature, (2) salinity, and (3) pressure. Pure water reaches its maximum density at $4\,^\circ\text{C}$.

Solution

A homogeneous mixture of two or more components with uniform physical and chemical properties throughout, consisting of a solvent and one or more solutes.

Solvent

The more abundant component in a solution, typically a liquid (such as water) that dissolves other substances.

Solute

The less abundant component in a solution, often a dissolved solid (such as salts) or a dissolved gas.

Mixture

A material composed of two or more components that are closely intermingled but retain their separate chemical identities and do not display uniform properties throughout (e.g., noodle soup or oil in water).

Universal Solvent

A moniker for water describing its capacity to dissolve more substances in greater quantities than almost any other common liquid due to its polar covalent nature.

Dissolution of Ionic Compounds

The process by which polar water molecules reduce electrostatic attraction between ions in an ionic solid (such as $NaCl$), with partially positive hydrogen atoms attracting $Cl^-$ and partially negative oxygen atoms attracting $Na^+$, separating them into an ionic state.

Nonpolar Molecule

A molecule (such as oil) that lacks electrical charges or polarity, preventing attraction by polar water molecules and causing it to form a separate mixture rather than dissolving.

Diffusion

The net random movement of solute particles through a solution from regions of high concentration to regions of low concentration.

Saturation

The dynamic chemical equilibrium in a solution where the rate of solute dissolution equals the rate of precipitation, preventing any additional solute from dissolving.

Precipitation (Chemical)

The formation of solid crystals emerging from a solution when solute molecules aggregate together.

Solubility

The maximum amount of solute that can be dissolved in a given volume of solvent at equilibrium.

Sodium Chloride ($NaCl$) Crystal Structure

An alternating ionic lattice of alternating positively charged sodium ($Na^+$) and negatively charged chloride ($Cl^-$) ions held together by electrostatic attractions prior to hydration by water molecules.