(Terms defined elsewhere in the glossary are shown in italics.)
Acid-base reaction: a chemical reaction in which an acid and a base react together, neutralizing their acidity and alkalinity (and usually forming a salt)
Alpha helix: a helical structure often found as part of the folded chain of proteins
Amphiphile: a molecule that has both hydrophilic and hydrophobic parts
Anthocyanin: a class of pigments responsible for the rosy red colors of some plants
Base: a chemical substance that forms alkaline solutions
Carotenoid: a class of pigments that produce red and yellow colors in plants
Catalyst: a molecule or substance that is able to speed up the rate of a chemical reaction
Chemical garden: a complex structure formed when a metal silicate compound precipitates from solution as a thin membrane. Typically these structures grow into organic-looking tubes and other forms
Chlorophyll: the pigment primarily responsible for absorbing sunlight in green plants to begin the process of photosynthesis
Chromosome: a large molecular structure in which some of the DNA of a cell is gathered together, along with proteins that help to “package” it
Compound: a chemical substance containing more than one kind of element
Corrosion: the degradation of a surface (generally of a metal) caused by chemical reactions with substances in its environment, such as air and water
Crystal structure: the way in which atoms are arranged in a crystal. This could refer to the way atoms are packed in an “extended” solid like a gem or a metal, or the way they are arranged in individual molecules when packed together into crystals
Denaturation: the process in which a protein unravels from its compact, folded form
Dendritic growth: the process by which crystals grow as snowflake-like branches
Displacement reaction: a chemical reaction in which one reactant is replaced by another: for example, when one metal displaces another in a salt
DNA: the molecule in all living cells that carries the genes, the “instructions” for making proteins that are inherited when a cell divides or an organism reproduces. It has a structure in which two chainlike strands wind around one another in a double helix
Electrodeposition: the growth of a metal on the surface from a solution of its ions, driven by an electric current
Electrolysis: the splitting apart of a chemical substance, generally into its constituent elements, using an electrical current
Electromagnetic spectrum: the gamut of possible types of electromagnetic radiation, ranging from radio waves (with long wavelengths), through visible light, to X-rays and gamma rays
Electron: one of the fundamental particles that make up atoms. Electrons are much lighter than the two other particles (which sit in the atom's nucleus), and they form clouds of negative charge around that nucleus
Element: one of the basic building blocks of all chemical matter: a chemical substance that can't be split into a simpler one. Each element is constituted from a different kind of atom
Endothermic reaction: a chemical reaction that takes in heat from its surroundings, making them colder
Entropy: a measure of how much disorder there is in a system—more strictly, how many indistinguishable ways its constituent parts can be arranged
Enzyme: a protein that acts as a catalyst, helping a biochemical reaction to occur in living cells
Exothermic reaction: a chemical reaction that produces heat
Flocculation: the clumping together of fine particles within a liquid
Fractal: a shape that looks essentially the same at increasing scales of magnification
Free radical: an atom or molecule containing an electron that is not paired with another either in a chemical bond or a lone pair. Such entities are generally very chemically reactive
Fullerene: a molecule made entirely from carbon atoms, joined into a closed spherical or ovoid cage built up from hexagonal and pentagonal rings of atoms
Hydrocarbon: a compound containing just the elements carbon and hydrogen, joined into molecules
Hydrogen bond: a kind of weak chemical bond in which hydrogen atoms on one molecule (or part of a molecule) are attracted to electron lone pairs on another molecule (or part thereof)
Hydrophobic and hydrophilic: adjectives describing, respectively, molecules (or parts thereof) that repel or attract water
Indicator: a chemical substance that shows the pH of a solution by changing color in acidic and alkaline conditions
Ion: an atom or molecule that bears an overall electrical charge, either positive or negative, because of an imbalance of protons and electrons
Le Chatelier's Principle: the precept that if some change (in temperature or pressure, say) is made to the conditions of a chemical system that has reach equilibrium, the system will adjust to a new equilibrium state that counteracts the change
Lone pair: a pair of electrons attached to an atom (generally as part of a molecule) that are not involved in forming strong chemical bonds
Melanin: a class of pigments that cause black, brown, and tawny coloration in mammal hair and skin
Molecule: a group of two or more atoms joined together by chemical bonds
Neutron: one of the two subatomic particles that make up an atomic nucleus. It has no electrical charge
Nucleation: the process by which crystals start to grow through the formation of a tiny “seed.” The word can also refer to the growth of other particles, for example liquid droplets in air, from a small initial “nucleus.”
Nucleus (atomic): the dense blob of matter at the center of an atom, where most of its mass resides. This consists of the subatomic particles protons and neutrons. (Not to be confused with the “nucleus” of nucleation!)
Oxidation: initially this meant the reaction of a substance with oxygen. But more generally it refers to the loss of electrons from the substance
Periodic table: a way of arranging and organizing the chemical elements to show the similarities in their chemical properties
pH: A measure of the acidity of a solution. The lower the pH, the more acidic the solution is. Pure water, which is neither acidic nor alkaline, has a pH of 7 units
Photon: the “particle” of light—a sort of package of light energy, with a characteristic wavelength that determines the light's color
Photosynthesis: the process in which plants (and some bacteria) convert carbon dioxide in the air into sugar molecules, using the energy of sunlight to drive the reactions
Pigment: a compound that imparts color because it absorbs some wavelengths of visible light strongly
Polar group: a part of a molecule that bears an electrical charge. Molecules in which there is more positive charge in one part and more negative charge in another are said to be polar
Precipitation: the formation of a solid, insoluble substance in a chemical solution
Protein: a molecule consisting of chains of amino acids, generally folded into a compact shape. Proteins are one of the key molecules of life
Proton: along with the neutron, the subatomic particle that makes up an atomic nucleus. It has a positive electrical charge
Redox reaction: a chemical reaction involving the transfer of electrons from one reactant (which is then oxidized) to another (which is reduced)
Reduction: the process in which an atom or molecule gains electrons
Salt: a generic term for an ionic compound composed of positively charged ions of a metal and negatively charged ions composed of (or containing) nonmetals. Table salt (sodium chloride) is just one example
Solubility product: a mathematical way of writing how soluble an ionic substance is
Solute: a substance dissolved in a solvent
Solution: a liquid (typically, but not necessarily, water) containing a dissolved chemical substance
Solvent: a fluid that will dissolve another substance
Supersaturation: the state of a solution that contains more dissolved substance than it could hold under normal circumstances
Surface tension: a kind of force that pulls liquid surfaces into the smallest area possible in the circumstances. It can be thought of as giving the liquid something like a “surface skin.”
Thermodynamics: the branch of science concerned with how energy is transformed from one form to another and how heat moves around
Voltaic pile: an early type of battery, consisting of plates or disks of two different metals stacked on top of one another, and alternatingly separated by sheets of material soaked in an electrolyte solution
X-ray crystallography: a technique for deducing the structure of crystalline chemical substances—finding out how their atoms are arranged—by bouncing X-rays off them