Chemistry Terms and Reactions

Definitions of Key Terms in Chemistry

Chemical Equation: A statement using chemical formulas to describe the identities and relative amounts of the reactants and products involved in a chemical reaction.
Chemical Reaction: The process by which the atoms of one or more substances are rearranged to form different substances; occurrence can be indicated by changes in temperature, color, odor, and physical state.
Precipitate: A solid produced during a chemical reaction in a solution.
Complete Ionic Equation: An ionic equation that shows all the particles in a solution as they realistically exist.
Net Ionic Equation: An ionic equation that includes only the particles that participate in the reaction.
Spectator Ion: An ion that does not participate in a reaction.
Product: A substance formed during a chemical reaction.
Reactant: The starting substance in a chemical reaction.
Coefficient: In a chemical equation, the number written in front of a reactant or product; in a balanced equation describes the lowest whole-number ratio of the amounts of all reactants and products.
Liquid: A form of matter that flows, has a constant volume, and takes the shape of its container.
Aqueous Solution: A solution in which the solvent is water.
Solution: A uniform mixture that can contain solids, liquids, or gases; also called a homogeneous mixture.
Solvent: The substance that dissolves a solute to form a solution; the most plentiful substance in the solution.
Solute: One or more substances dissolved in a solution.
Soluble: Describes a substance that can be dissolved in a given solvent.
Insoluble: Describes a substance that cannot be dissolved in a given solvent.
Miscible: Describes two liquids that are soluble in each other.
Immiscible: Describes two liquids that can be mixed together but separate shortly after you cease mixing them.

Types of Solutions and Reactions

Unsaturated Solution: A solution that contains less dissolved solute than it has the capacity to hold at a given temperature; more solute can still dissolve.
Single Replacement Reaction: A reaction in which one element replaces a similar element in a compound, represented as:
$A + BC → AC + B$
Double Replacement Reaction: A reaction where ions in two compounds exchange partners to form two new compounds, represented as:
$AB + CD → AD + CB$
Dilute Solution: A solution in which there are more solvent (water) molecules compared to solute particles; to make (a liquid) thinner or weaker by adding water or another solvent (e.g., adding water to juice).
Saturated Solution: Contains the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure.
Supersaturated Solution: An unstable solution that contains more dissolved solute than the usual saturation point at a given temperature; excess solute can crystallize out.
Combustion Reaction: A chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light.
Synthesis Reaction: A chemical reaction in which two or more substances react to yield a single product.
Decomposition Reaction: A chemical reaction that occurs when a single compound breaks down into two or more elements or new compounds.

Important Definitions in Chemistry

Molar Mass: The mass in grams of one mole of any pure substance.
Molecular Formula: A formula that specifies the actual number of atoms of each element in one molecule of a substance.
Empirical Formula: A formula that shows the smallest whole-number mole ratio of the elements of a compound; it might or might not be the same as the actual molecular formula.
Mole: The SI base unit used to measure the amount of a substance, abbreviated mol; the number of carbon atoms in exactly 12 g of pure carbon; one mole is the amount of a pure substance that contains $6.02 × 10^{23}$ representative particles.
Hydrate: A compound that has a specific number of water molecules bound to its atoms.
Avogadro’s Number: The number $6.0221367 × 10^{23}$ , which is the number of representative particles in a mole and can be rounded to three significant digits as $6.02 × 10^{23}$ .

Thermochemistry and Gas Laws

Thermochemistry: The study of energy changes (heat transfer) that accompany chemical reactions and physical changes.
Heat of Reaction: The enthalpy change associated with a chemical reaction, usually reported as $riangle H$ (positive for endothermic, negative for exothermic).
Boyle’s Law: States that the volume of a fixed amount of gas held at a constant temperature varies inversely with the pressure:
$PV = k$ (where P is pressure, V is volume, and k is a constant).
Charles’ Law: States that the volume of a given mass of gas is directly proportional to its Kelvin temperature at constant pressure:
$V ext{ (volume)} = kT$ .
Gay-Lussac’s Law: States that the pressure of a fixed mass of gas varies directly with the Kelvin temperature when the volume remains constant:
$P/T = k$ .
Avogadro’s Law (Principle): States that equal volumes of gases at the same temperature and pressure contain equal numbers of particles:
$V/n = k$ (where n is the number of moles).
Combined Gas Law: A single law combining Boyle's, Charles's, and Gay-Lussac's laws that states the relationship among pressure, volume, and temperature of a fixed amount of gas:
$\frac{PV}{T} = k$ .
Ideal Gas Law: Describes the physical behavior of an ideal gas in terms of pressure, volume, temperature, and number of moles of gas:
$PV = nRT$ (where R is the ideal gas constant).
Ideal Gas Constant: An experimentally determined constant whose value in the ideal gas equation depends on the units that are used for pressure.
Molar Volume: For a gas, the volume that one mole occupies at $0.00°C$ and $1.00 atm$ pressure; generally about $22.4 L$ .
Enthalpy: The heat content of a system at constant pressure.
Calorimeter: An insulated device that is used to measure the amount of heat released or absorbed during a physical or chemical process.
System: In thermochemistry, the specific part of the universe containing the reaction or process being studied.
Surroundings: In thermochemistry, includes everything in the universe except the system.
Universe: In thermochemistry, is the system plus the surroundings.