Chemical Change
24 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 where substances (reactants) are transformed into different substances (products).
Reactant
A starting substance that is consumed and transformed during a chemical reaction.
Product
A new substance formed as a result of a chemical reaction, with different properties from the original reactants.
Chemical Equation
A written representation of a chemical reaction using chemical formulas, coefficients, and an arrow.
Balancing Equations
Adjusting the coefficients in a chemical equation so each type of atom is equal on both sides.
Conservation of Mass
In a chemical reaction, the total mass of reactants equals the total mass of products.
Exothermic Reaction
A reaction that releases energy (usually as heat) to the surroundings, making them warmer.
Endothermic Reaction
A reaction that absorbs energy (usually as heat) from the surroundings, making them cooler.
Activation Energy
The minimum energy that reactant particles must have to initiate a chemical reaction.
Catalyst
A substance that increases the rate of a chemical reaction without being consumed or altered.
Reaction Rate
How quickly reactants are converted into products, measured as the change in concentration per unit time.
Collision Theory
Reactions occur when particles collide with sufficient energy and in the correct orientation.
Chemical Equilibrium
A state where the forward and reverse reactions proceed at equal rates, so the net concentrations of reactants and products remain constant.
Le Chatelier's Principle
When a system at equilibrium is disturbed, it shifts to counteract the disturbance.
Acid
A substance that donates $\text{H}^+$ ions (protons) when dissolved in water, lowering the pH.
Base
A substance that accepts $\text{H}^+$ ions or donates $\text{OH}^-$ ions in solution, raising pH.
pH
A logarithmic scale from 0 to 14 that measures the concentration of hydrogen ions in a solution, indicating how acidic or basic it is.
Neutralization
A reaction between an acid and a base in which the H+ ions and OH- ions combine to form water and an ionic salt.
Oxidation
The loss of electrons by an atom, ion, or molecule, increasing its oxidation state.
Reduction
The gain of electrons by an atom, ion, or molecule, decreasing its oxidation state.
Redox Reaction
A reaction in which electrons are transferred from one substance to another — one is oxidized, one is reduced.
Electrolyte
A substance that dissociates into ions in solution, allowing the solution to conduct electricity.
Equilibrium Constant
A numerical value expressing the ratio of product concentrations to reactant concentrations at equilibrium.
Enthalpy
A thermodynamic quantity representing the total heat content of a system at constant pressure.