An absorption spectrum shows dark lines against a continuous background.

Acid–base indicators are weak acids that exist in equilibrium between their protonated form (HInd) and deprotonated form (Ind⁻).

The activated complex is a high-energy, unstable arrangement of atoms that exists momentarily as reactants are transformed into products. It represents the "peak"; of the energy barrier in a reaction.

Activation energy ($E_a$) is the minimum energy required for colliding particles to form an activated complex (also known as the transition state) and proceed to products.

Addition polymerization is a reaction in which monomers with double or triple bonds join together without forming any by-products, creating a polymer chain.

An alloy is a mixture of a metal with one or more other elements, which can be either metals or non-metals

An alloy is a mixture of a metal with one or more other elements, which can be either metals or non-metals.

An amphiprotic species is a chemical species that can either donate a proton (acting as an acid) or accept a proton (acting as a base), depending on the reaction conditions.

An anion is formed when an atom gains one or more electrons, resulting in a net negative charge.

Atom economy is a measure of how efficiently the atoms in the reactants are used to form the desired product in a chemical reaction.

Atom economy measures how efficiently the atoms in the reactants are incorporated into the desired product.

The atomic radius is the distance from the nucleus of an atom to the outermost electron.

Average bond enthalpy represents the average energy required to break a given bond across a range of compounds.

Avogadro's law states that equal volumes of all gases, measured under the same conditions of temperature and pressure, contain equal numbers of molecules.

Biofuels are energy sources derived from biological materials such as plants, algae, or organic waste.

The energy required to break one mole of a specific bond in the gaseous state is called its bond enthalpy (or bond dissociation energy)

Bond length is the average distance between the nuclei of two bonded atoms.

Bond polarity refers to the unequal sharing of electrons between two atoms in a covalent bond, resulting in a partial positive charge on one atom and a partial negative charge on the other.

Bond strength refers to the energy required to break a bond.

The bonding triangle is a triangular diagram that represents the relative contributions of ionic, covalent, and metallic bonding to a material.

The Born-Haber cycle is a thermodynamic tool that uses Hess’s Law to calculate the lattice enthalpy of an ionic compound.

Boyle's law states that at constant temperature and for a fixed amount of gas, pressure is inversely proportional to volume.

A Brønsted–Lowry acid is a proton (H⁺) donor, which gives up a hydrogen ion in a reaction.

A Brønsted–Lowry base is a proton (H⁺) acceptor, which gains a hydrogen ion in a reaction.

The Brønsted–Lowry theory, introduced in 1923, defines acids and bases based on their ability to transfer protons (H⁺ ions).

Buffers are solutions that resist changes in pH when small amounts of acid ($H⁺$ ions) or base ($OH⁻$ ions) are added.

A calorimeter is an apparatus designed to measure temperature changes, which can then be used to calculate the heat transferred during a reaction.

A carbocation is a carbon atom that carries a positive charge, formed when a bond is broken heterolytically, leaving the carbon electron-deficient.

A cation is formed when an atom loses one or more electrons, resulting in a net positive charge.

Chain isomers differ in the arrangement of the carbon skeleton.

Charles's law states that at constant pressure and for a fixed amount of gas, volume is directly proportional to absolute temperature (in Kelvin).

A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants (starting substances) on the left side, the products (substances formed) on the right, and an arrow indicating the direction of the reaction.

A chiral carbon (or stereocenter) is a carbon atom bonded to four different atoms or groups.

Cis-trans isomerism occurs in molecules where rotation about a bond is restricted.

Coal is a solid fossil fuel primarily composed of carbon, along with smaller amounts of hydrogen, sulfur, oxygen, and nitrogen.

Combustion is a chemical process where a substance, known as the fuel, reacts with oxygen (O₂), releasing energy in the form of heat and light.

Complete combustion occurs with excess oxygen, producing carbon dioxide (CO₂) and water (H₂O).

A compound is a substance formed when two or more different elements chemically bond together in fixed proportions.

Condensation polymers are created when monomers with two reactive functional groups join together, releasing a small molecule such as water or hydrogen chloride (HCl).

A conjugate acid–base pair consists of two species that differ by exactly one proton ($H^+$).

A continuous spectrum contains all wavelengths of light, merging seamlessly from one color to the next.

A coordination bond (also known as a dative covalent bond) forms when both electrons in the shared pair come from the same atom.

A covalent bond is a chemical bond formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the bonded atoms.

Crude oil, or petroleum, is a liquid mixture of hydrocarbons formed from marine organisms.

A dipole moment describes the separation of electrical charge in a bond or molecule due to differences in electronegativity.

A dipole moment vector is a vector pointing from the less electronegative atom to the more electronegative atom.

Dipole-dipole forces are intermolecular attractions between the oppositely charged ends of permanent dipoles in polar molecules.

Dipole-induced dipole forces occur when a polar molecule with a permanent dipole induces a temporary dipole in a nearby nonpolar molecule.

Distillation separates two or more liquids with different boiling points.

A double bond forms when two atoms share two pairs of electrons.

Dynamic equilibrium occurs in reversible reactions within a closed system.

Effective nuclear charge is the net positive charge experienced by valence electrons.

An electrochemical cell is a device that uses redox (reduction-oxidation) reactions to either produce or consume electrical energy.

Electrodes are solid surfaces where the redox reactions occur

Electrodes are solid surfaces where the redox reactions occur.

Electrolysis uses electrical energy to drive non-spontaneous redox reactions.

An electrolytic cell is a device that uses electrical energy to force a non-spontaneous redox reaction to occur.

Electron affinity (EA) is the energy change when an atom gains an electron. A more negative value indicates a stronger attraction for the electron.

Electron configuration refers to the arrangement of electrons in an atom’s energy levels, sublevels, and orbitals.

An electron domain is a region in which electrons are most likely to be found (bonding and nonbonding)

Electronegativity is the ability of an atom to attract shared electrons in a covalent bond.

An electrophile is a species that accepts an electron pair to form a covalent bond.

Electroplating is a process that uses an electrolytic cell to coat an object (usually the cathode) with a thin layer of metal.

An element is the simplest form of matter that cannot be broken down into simpler substances by physical or chemical means.

An elementary step is a single molecular event in a reaction mechanism where reactants are converted into products or intermediates.

An emission spectrum displays bright lines corresponding to wavelengths of light emitted by an atom.

The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.

Energy is absorbed by the system from the surroundings, decreasing the temperature of the surroundings.

Enthalpy $ H $ is a state function, meaning its value depends only on the current state of the system (e.g., pressure, temperature, and composition) and not on the path taken to reach that state.

The enthalpy change (ΔH) of a reaction quantifies the heat exchanged at constant pressure during a chemical or physical change.

Entropy, denoted as $S$, is a thermodynamic property that quantifies the dispersal or distribution of energy and matter in a system.

The equilibrium constant $K$ is a ratio that provides a snapshot of the relative concentrations of products and reactants at equilibrium for a reversible chemical reaction.

The equivalence point in a titration is the precise moment when the amount of base added is chemically equivalent to the amount of acid initially present.

Evaporation removes a liquid from a solution by heating it, leaving behind the dissolved solid.

The excess reactant is the substance that remains after the reaction is complete because there’s more of it than needed.

Energy is released from the system into the surroundings, increasing the temperature of the surroundings.

Filtration is used to separate insoluble solids from liquids. For example, sand can be separated from water using filter paper.

Formal charge is a theoretical concept that assigns a charge to each atom in a molecule or ion. It assumes that bonding electrons are shared equally between the bonded atoms.

Fossil fuels are ancient energy sources formed over millions of years from the remains of plants and animals exposed to heat and pressure beneath Earth's surface

A fuel cell is a device that converts chemical energy directly into electrical energy through a redox reaction.

A functional group is a specific arrangement of atoms within a molecule that determines the molecule's characteristic chemical reactions.

Functional group isomers have the same molecular formula but belong to entirely different functional group families.

In gases, particles are far apart and move freely in all directions. This explains why gases expand to fill the shape and volume of their container.

Gay-Lussac's law states that at constant volume and for a fixed amount of gas, pressure is directly proportional to absolute temperature (in Kelvin).

Gibbs free energy ($\Delta G$) is a thermodynamic property that determines whether a chemical reaction is spontaneous under constant pressure and temperature.

The group number of an element indicates the number of valence electrons in its outermost energy level.

A half-equation focuses on one part of a redox reaction: either the oxidation or reduction process.

Heat is the energy transferred between a system and its surroundings due to a temperature difference.

The total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction pathway.

In chemistry, heterolytic fission refers to the breaking of a covalent bond where both bonding electrons are transferred to one of the two bonded atoms.

A homologous series is a group of chemical compounds with the same functional group and similar chemical properties, where each successive member differs by a methylene ($CH_2$) group.

Homolytic fission is the breaking of a covalent bond in such a way that each atom involved in the bond takes one electron, creating two radicals.

Hybridization is the process of combining atomic orbitals (such as s and p orbitals) to create new hybrid orbitals that are better suited for bonding.

Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (N, O, or F), creating a highly polar bond. The partially positive hydrogen atom $δ^+$ is then attracted to a lone pair of electrons on another electronegative atom in a nearby molecule.

The hydrogen fuel cell is one of the most commonly discussed types of fuel cells. It uses hydrogen gas ($H_2$) as a fuel and oxygen gas ($O_2$) from the air to produce water ($H_2O$), electricity, and heat.

Hydrogenation is a chemical reaction where hydrogen ($H_2$) is added to an unsaturated compound. This reaction is classified as a reduction because the molecule gains hydrogen atoms, reducing its degree of unsaturation.

Hydrolysis is the reverse of condensation polymerization, where water reacts with the polymer to break the covalent bonds between monomers.

When a salt dissolves in water, its ions may undergo hydrolysis, reacting with water to form either H$^+$ (acidic) or OH$^−$ (basic) ions.

Incomplete combustion happens with limited oxygen, resulting in byproducts like carbon monoxide (CO) and soot (C).

An intermediate is a species that is formed in one elementary step and consumed in another. It is not present in the overall reaction equation because it does not appear in the final products or reactants.

Intermolecular forces are the electrostatic attractions between molecules.

An ion is an atom or group of atoms that has gained or lost electrons, resulting in a net electrical charge.

An ionic bond is the electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion.

Ionic compounds consist of positively charged ions (cations) and negatively charged ions (anions) held together by strong electrostatic forces of attraction.

Ionic radius is the measure of an ion's size, defined as the distance from its nucleus to the outermost electron, influenced by its charge and electron configuration.

Ionization energy ($IE$) is the minimum energy required to remove an electron from a gaseous atom in its ground state.

Isoelectronic ions are ions that have the same number of electrons but different nuclear charges.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

A lattice structure is a repeating, orderly arrangement of ions in three dimensions.

Le Châtelier's Principle states:

"If a dynamic equilibrium is disturbed by a change in the reaction conditions, the system adjusts to counteract the disturbance and restore a new equilibrium."

A leaving group is an atom or group of atoms that detaches during a substitution reaction.

Lewis acid is a species that accepts an electron pair. It is often electron-deficient, meaning it has an incomplete octet or a positively charged center that makes it eager to gain electrons.

Lewis base is a species that donates an electron pair. It is electron-rich, often possessing a lone pair of electrons that can be shared.

The Lewis formula is a visual representation of the valence electrons in a molecule.

A molecule or ion that donates a lone pair of electrons to a central metal ion to form a coordination bond.

In a chemical reaction, the limiting reactant is the substance that is completely consumed first, stopping the reaction from proceeding further.

A line spectrum consists of discrete lines of specific wavelengths against a dark background.

Liquids have particles that are close together but not in a fixed arrangement. This allows them to flow and take the shape of their container while maintaining a constant volume.

London dispersion forces are weak intermolecular forces caused by temporary dipoles that arise due to momentary fluctuations in electron distribution within atoms or molecules.

When a hydrogen halide (HX) adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms, while the halogen attaches to the carbon with fewer hydrogen atoms.

Mass concentration specifies the mass of solute (in grams) dissolved in one cubic decimeter (dm³) of solution.

A mass spectrum is a graph that plots the relative abundance of ions (y-axis) against their mass-to-charge ratio (m/z) (x-axis).

The Maxwell–Boltzmann energy distribution curve provides a visual representation of the distribution of kinetic energies among particles in a system.

Metallic bonding is a type of chemical bonding that arises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions.

Metallic character describes an element's tendency to lose electrons and form positive ions.

The methanol fuel cell is another promising type of fuel cell that uses methanol ($CH_3OH$) as a fuel source. It operates similarly to the hydrogen fuel cell but offers the advantage of using a liquid fuel, simplifying storage and transport.

A mixture contains two or more elements, compounds, or both that are physically combined but not chemically bonded.

Molar concentration, commonly called molarity, measures how many moles of solute are dissolved in one cubic decimeter (dm³) of solution.

Molar mass, denoted as $M$, is the mass of one mole of a substance.

The molar volume of a gas is the volume occupied by one mole of an ideal gas under specific conditions of temperature and pressure.

The mole (mol) is the SI unit for the amount of substance, defined as $6.02 \times 10^{23}$ particles (atoms, molecules, or ions).

A mole ratio is the ratio of the amounts (in moles) of reactants and products in a balanced chemical equation. These ratios are determined by the stoichiometric coefficients in the equation.

The molecular formula specifies the exact number of atoms of each element in a molecule.

Molecular geometry is the three-dimensional arrangement of atoms in a molecule, determined by the repulsion between electron pairs around the central atom.

The molecularity of an elementary step refers to the number of reacting particles (atoms, ions, or molecules) that must collide simultaneously to drive a chemical change.

Natural gas is a gaseous fossil fuel composed mainly of methane (CH₄), with smaller amounts of ethane, propane, and other hydrocarbons.

Neutralization is a chemical reaction where an acid reacts with a base to produce a salt and water.

A nucleophile is an electron-rich species that donates a pair of electrons to form a covalent bond. The term “nucleophile” originates from the Greek words nucleus (meaning core) and philos (meaning loving), indicating its affinity for positively charged or electron-deficient centers.

Nucleophilic substitution reactions occur when a nucleophile (an electron-rich species) attacks an electrophile (an electron-deficient species) and replaces a leaving group.

A nucleus is a dense, positively charged core that contains protons and neutrons, collectively called nucleons.

Atoms are driven by their quest for stability, often achieved by attaining a noble gas electron configuration, commonly referred to as the octet rule.

An orbital is a region of space around the nucleus where there is a high probability of finding an electron.

The order of a reaction with respect to a reactant is the power to which the concentration of that reactant is raised in the rate equation.

The order of a reaction describes how the concentration of a reactant influences the rate.

The oxidation state (or oxidation number) is a numerical value assigned to an atom in a compound. It reflects the hypothetical charge the atom would have if all bonds in the molecule were purely ionic.

Paper chromatography is a technique used to separate the components of a mixture based on differences in their solubility and polarity.

The percentage composition of an element in a compound describes the proportion of the compound's mass contributed by that element.

In a chemical reaction, the percentage yield compares the actual amount of product obtained (experimental yield) to the maximum amount predicted by stoichiometry (theoretical yield).

The period number of an element indicates the highest principal energy level ($n$) that contains electrons.

The period number of an element indicates the highest energy level ($n$) that contains electrons in its ground state.

The pH scale is a logarithmic measure of the concentration of hydrogen ions ($ [H^+] $) in an aqueous solution.

Pi bonds form when p orbitals on adjacent atoms overlap sideways.

Polymers are large molecules composed of repeating structural units called monomers, linked together by covalent bonds to form long chains.

Positional isomers occur when the functional group or substituent is attached to different positions on the same carbon chain.

The principal quantum number (n) is a fundamental concept in quantum mechanics that describes the main energy levels, or shells, of an atom.

A pure substance consists of only one type of particle with a fixed composition and uniform properties throughout.

A radical is an atom, molecule, cation, or anion that contains an unpaired electron. This unpaired electron makes radicals highly reactive as they seek to pair up their lone electron to achieve a more stable configuration.

The rate constant, $ k $, serves as a measure of how fast a reaction proceeds, and it varies with temperature.

The rate equation describes the relationship between the rate of a chemical reaction and the concentrations of the reactants involved.

The rate of reaction measures how quickly reactants are consumed or products are formed during a chemical reaction. It is defined as:

The change in concentration of a reactant or product per unit time.

The slowest step in the mechanism is called the rate-determining step (RDS).

The reaction quotient, $Q$, measures the relative concentrations of products and reactants in a chemical reaction at a given moment in time, whether or not the system is at equilibrium

The reaction quotient ($Q$) represents the ratio of product concentrations to reactant concentrations at any point during the reaction:

$$Q = \frac{[\text{products}]}{[\text{reactants}]}$$

Recrystallization exploits differences in solubility.

Reduction in organic chemistry refers to the gain of hydrogen atoms or the loss of oxygen atoms in a molecule.

Relative abundance refers to the percentage of a specific isotope of an element present in a naturally occurring sample.

The relative atomic mass of an element is a weighted average of the masses of its isotopes, based on their natural abundances, compared to 1/12th the mass of a carbon-12 atom. Since it is a ratio, $A_r$ has no units.

The relative formula mass ($M_r$) is the sum of the relative atomic masses ($A_r$) of all the atoms in a chemical formula.

Resonance arises from the delocalization of electrons, where electrons are not confined to a single bond or atom but are spread across multiple atoms in a molecule.

Resonance energy refers to the additional stability a molecule gains due to electron delocalization.

Resonance structures are alternative Lewis structures showing different possible arrangements of bonds and lone pairs.

The retardation factor, $R_f$, is a dimensionless ratio that describes how far a substance travels relative to the solvent front in chromatography.

A reversible reaction is one in which reactants can form products (the forward reaction), while products can simultaneously revert back into reactants (the reverse reaction).

The salt bridge is a tube or porous material filled with an ionic solution (e.g., KNO₃ or Na₂SO₄). It allows ions to flow between the two half-cells, maintaining electrical neutrality and completing the circuit.

Sigma bonds form when atomic orbitals overlap head-on along the bond axis—the imaginary line connecting the nuclei of two bonded atoms.

A single bond occurs when two atoms share one pair of electrons.

In solids, particles are tightly packed in a fixed arrangement. They vibrate in place but do not move freely, giving solids a definite shape and volume.

A chemical reaction or physical process is spontaneous if it occurs without external intervention under a given set of conditions.

The standard cell potential, denoted as $E^\circ_{\text{cell}}$, is the voltage produced by an electrochemical cell under standard conditions.

The standard enthalpy of combustion ($ΔH_c^\circ$) is the energy change when one mole of a substance is completely burned in oxygen under standard conditions (298 K and 1 atm).

The standard enthalpy of formation ($ΔH_f^\circ$) is the energy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Strong acids and bases ionize completely in water. All their molecules dissociate into ions, leaving no undissociated molecules in the solution.

The structural formula shows how atoms are connected in a molecule.

Successive ionization energy refers to the energy required to remove electrons one at a time from an atom.

Surroundings are everything outside the system that interacts with it (e.g., the reaction flask, water bath, or air).

The system is the specific part of the universe under study (e.g., the reactants and products involved in the reaction).

Temperature is a direct measure of the average kinetic energy of the particles in a substance.

Temperature measures the average kinetic energy of the particles in a system.

The theoretical yield is the maximum amount of product that can be formed based on the limiting reactant. It assumes the reaction proceeds to completion with no side reactions or losses.

A transition state represents the highest-energy arrangement of atoms during an elementary step. It is the point where bonds are partially broken and formed, and the system is at the peak of its energy profile.

A triple bond occurs when two atoms share three pairs of electrons.

Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs repel each other and arrange themselves as far apart as possible.

Volatility describes how easily a substance evaporates.

Weak acids and bases only partially ionize in water. At equilibrium, the solution contains both ions and undissociated molecules.