Chemical Change
29 concepts in Chemistry
Chemical change is at the heart of chemistry — it is the process by which substances transform into entirely new substances with different properties. Students learn to distinguish chemical changes from physical changes by looking for evidence such as color change, gas production, temperature change, and precipitate formation. They study different types of chemical reactions including synthesis, decomposition, single replacement, double replacement, and combustion. Balancing chemical equations teaches students that atoms are conserved during reactions, reinforcing the law of conservation of mass. Students also explore factors that affect reaction rates, such as temperature, concentration, surface area, and catalysts. Understanding chemical change is essential for fields ranging from medicine and agriculture to environmental science and materials engineering.
Suggested learning path: Start by distinguishing chemical from physical changes, then learn to write and balance chemical equations, classify reaction types, and finally study factors that influence reaction rates.
Chemical Reaction
A process in which one or more substances (reactants) are transformed into entirely different substances (products) through the breaking and forming of chemical bonds, accompanied.
Reactant
A starting substance that is consumed and chemically transformed during a chemical reaction, appearing on the left side of a chemical equation before the reaction.
Product
A product is a substance that is formed as the result of a chemical reaction.
Chemical Equation
A written representation of a chemical reaction using chemical formulas for reactants and products, coefficients to indicate mole ratios, state symbols to show physical states.
Balancing Equations
The process of adjusting the coefficients (the numbers placed before chemical formulas) in a chemical equation so that the number of atoms of each element.
Conservation of Mass
A fundamental law stating that in any chemical reaction, the total mass of all reactants exactly equals the total mass of all products, because atoms.
Exothermic Reaction
A chemical reaction that releases energy (usually as heat or light) to the surroundings, resulting in an increase in surrounding temperature and a negative enthalpy.
Endothermic Reaction
A chemical reaction that absorbs energy (usually as heat) from the surroundings, resulting in a decrease in surrounding temperature and a positive enthalpy change ($\Delta.
Activation Energy
The minimum kinetic energy that reactant particles must possess upon collision in order to break existing bonds and initiate a chemical reaction, represented as the.
Catalyst
A substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy, without being permanently consumed or chemically.
Reaction Rate
The speed at which reactants are converted into products in a chemical reaction, quantified as the change in concentration of a reactant or product per.
Collision Theory
A model explaining that chemical reactions occur only when reactant particles collide with sufficient kinetic energy (at least equal to the activation energy) and in.
Chemical Equilibrium
A dynamic state in a reversible reaction where the forward and reverse reactions proceed at equal rates, so the macroscopic concentrations of reactants and products.
Le Chatelier's Principle
When a system at chemical equilibrium is subjected to an external stress — such as a change in concentration, pressure, or temperature — the equilibrium.
Acid
A substance that donates $\text{H}^+$ ions (protons) when dissolved in water, increasing the hydrogen ion concentration and lowering the pH below 7.
Base
A substance that accepts $\text{H}^+$ ions (protons) or donates $\text{OH}^-$ ions when dissolved in solution, raising the pH above 7.
pH
A logarithmic scale ranging from 0 to 14 that quantifies the hydrogen ion concentration $[\text{H}^+]$ in an aqueous solution, where values below 7 indicate acidic.
Neutralization
A chemical reaction in which an acid and a base combine to produce water and an ionic compound called a salt, effectively canceling out the.
Oxidation
The loss of electrons by an atom, ion, or molecule during a chemical reaction, resulting in an increase in its oxidation state.
Reduction
The gain of electrons by an atom, ion, or molecule during a chemical reaction, resulting in a decrease in its oxidation state.
Redox Reaction
A chemical reaction in which electrons are transferred from one substance (the reducing agent, which is oxidized) to another (the oxidizing agent, which is reduced).
Electrolyte
A substance that dissociates into free ions when dissolved in a solvent (typically water), producing a solution that can conduct electric current.
Buffer
A buffer is a solution that resists large changes in pH when small amounts of acid or base are added.
Salt
In chemistry, a salt is an ionic compound made of cations and anions, often formed when an acid reacts with a base in a neutralization.
Electrochemistry
Electrochemistry is the study of redox reactions that involve electric current, either producing electricity from a spontaneous reaction or using electricity to force a nonspontaneous.
Electrochemical Cell
An electrochemical cell is a system with two half-reactions separated into electrodes and connected through an external circuit so electrons can flow.
Electrolytic Cell
An electrolytic cell uses an external power source to force a nonspontaneous redox reaction to occur. Electrical energy is converted into chemical change.
Equilibrium Constant
The ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient.
Enthalpy
A thermodynamic quantity representing the total heat content of a system at constant pressure, where the change in enthalpy ($\Delta H$) equals the heat absorbed.