BITSAT key terms

The reactant in a chemical reaction that is completely consumed first, determining the maximum amount of product that can form; other reactants are said to be in excess.

The quantitative relationship in which one mole equals 6.022 × 10²³ particles of a substance, allowing conversion between mass, number of particles, and volume of gas.

The number of elementary entities (atoms, molecules, or ions) present in one mole of a substance, equal to approximately 6.022 × 10²³ per mole.

The calculation of quantities of reactants and products in a balanced chemical equation, using mole ratios derived from the coefficients of the equation.

A concentration unit expressing the number of moles of solute dissolved per litre of solution; it changes with temperature because solution volume expands or contracts.

A solution that resists a significant change in pH when small amounts of acid or base are added, typically containing a weak acid with its conjugate base or a weak base with its conjugate acid.

A relationship giving the pH of a buffer as pH = pKa + log([conjugate base]/[weak acid]), widely used to design buffers and predict pH shifts.

A logarithmic scale from 0 to 14 measuring hydrogen ion concentration, where pH = −log[H⁺]; values below 7 are acidic and above 7 are basic at room temperature.

A rule stating that when a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the system shifts to counteract that change and restore equilibrium.

A dynamic state in a reversible reaction where the forward and reverse reaction rates are equal, so concentrations of reactants and products remain constant over time.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium; its value indicates whether products or reactants are favoured.

The mixing of atomic orbitals of similar energy to form new equivalent hybrid orbitals (sp, sp², sp³) that explain the observed geometry and bond angles in molecules.

A model predicting molecular shape by assuming electron pairs around a central atom repel each other and arrange themselves as far apart as possible, including lone-pair effects.

The angle formed between two adjacent chemical bonds sharing a common atom; it depends on hybridisation and lone-pair repulsion and helps identify molecular geometry.

A guideline stating that in addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon already bearing more hydrogens, placing the halogen on the more substituted carbon.

A highly reactive species containing an unpaired electron, often formed by homolytic bond cleavage and involved in chain reactions such as halogenation of alkanes.

An electron-deficient species that accepts a pair of electrons during a reaction, typically attacking regions of high electron density; common examples are carbocations and carbonyl carbons.

An electron-rich species that donates a pair of electrons to form a new bond, often attacking electron-poor centres such as carbonyl carbons or carbocations.

The property of a molecule that cannot be superimposed on its mirror image, usually arising from a carbon atom bonded to four different groups, giving rise to enantiomers.

One of a pair of stereoisomers that are non-superimposable mirror images of each other, having identical physical properties except the direction they rotate plane-polarised light.

The law stating that the net force on a body equals its mass times its acceleration (F = ma), linking force, mass, and acceleration in a vector relationship.

The principle that the net work done on an object equals its change in kinetic energy, providing an energy-based method to solve problems involving forces and motion.

The principle that in the absence of non-conservative forces such as friction, the sum of kinetic and potential energy of a system remains constant.

A measure of an object's resistance to rotational acceleration about a given axis, depending on mass distribution; it plays the role in rotation that mass plays in linear motion.

The rotational analogue of linear momentum, equal to the product of moment of inertia and angular velocity; it is conserved when no external torque acts on a system.

The change in observed frequency of a wave when the source and observer move relative to each other, with frequency rising on approach and falling on separation.

The phenomenon in which a system is driven at its natural frequency, producing a large-amplitude response, as in vibrating strings, air columns, and electrical circuits.

The law of refraction stating that the ratio of the sines of the angles of incidence and refraction equals the ratio of refractive indices of the two media.

The complete reflection of light back into a denser medium when it strikes a boundary at an angle greater than the critical angle; the basis of optical fibres.

The emission of electrons from a metal surface when light of sufficient frequency strikes it, providing evidence that light behaves as discrete quanta called photons.

The wavelength associated with a moving particle, given by λ = h/p, where h is Planck's constant and p is momentum; it underlies wave–particle duality.

The value that a function approaches as its input approaches a specified point; it is the foundation of continuity, derivatives, and integrals in calculus.

A property of a function that has no breaks, jumps, or holes at a point, formally requiring that the limit exists, the function is defined there, and both are equal.

The instantaneous rate of change of a function with respect to a variable, geometrically the slope of the tangent line, defined as the limit of a difference quotient.

The limit of a sum of areas of thin rectangles under a curve between two limits, interpreted as signed area and evaluated using the fundamental theorem of calculus.

An arrangement of objects in which order matters; the number of permutations of r objects chosen from n distinct objects is n!/(n−r)!.

A selection of objects in which order does not matter; the number of ways to choose r objects from n distinct objects is n!/[r!(n−r)!].

A numerical measure between 0 and 1 of how likely an event is, computed as favourable outcomes divided by total equally likely outcomes in classical settings.