Absolute temperature: This is a temperature reading made relative to absolute zero. It used for the unit of Kelvins for these readings.
Absolute zero: This is the lowest temperature possible. If you remember that temperature is a measurement of how much atoms move around in a solid, you can guess that they stop moving entirely at absolute zero. In reality, bonds still vibrate a little bit, but for the most part you don’t see much happening.
Accuracy: When you measure something, the accuracy is how close your measured value is to the real value.
Acid: This is anything that gives off H+ ions in water. Acids have a pH less than 7 and are good at dissolving metals. They turn litmus paper red and phenolphthalein colorless.
Acid anhydride: This is an oxide that forms an acid when you stick it in water. An example is SO3 – when you add water it turns into sulfuric acid, H2SO4.
Acid dissociation constant (Ka): This is equal to the ratio of the concentrations of an acid’s conjugate base and the acid present when a weak acid dissociates in water. That is, if you have a solution of Acid X where the concentration of the conjugate base is 0.5 M and the concentration of the acid is 10 M, the acid dissociation constant is 0.5/10 = 0.05.
Activated complex: In a chemical reaction, the reagents have to join together into a great big blob before they can fall back apart into the products. This great big blob is called the activated complex (a.k.a. transition state)
Activation energy: The minimum amount of energy needed for a chemical reaction to take place. For some reactions this is very small (it only takes a spark to make gasoline burn). For others, it’s very high (when you burn magnesium, you need to hold it over a Bunsen burner for a minute or so).
Activity series: This is when you arrange elements in the order of how much they tend to react with water and acids.
Actual yield: When you do a chemical reaction, this is the amount of chemical that you actually make (i.e. the amount of stuff you can weigh).
Addition reaction: A reaction where atoms add to a carbon-carbon multiple bonds.
Adsorption: When one substance collects of the surface of another one.
Alcohol: An organic molecule containing an -OH group
Aldehyde: An organic molecule containing a -COH group
Alkali metals: Group I in the periodic table.
Alkaline earth metals: Group II in the periodic table.
Alkane: An organic molecule which contains only single carbon-carbon bonds.
Alkene: An organic molecule containing at least one C=C bond
Alkyne: An organic molecule containing at least one C-C triple bond.
Allotropes: When you have different forms of an element in the same state. The relationship that white phosphorus and red phosphorus have to each other is that they’re allotropes.
Alloy: A mixture of two metals. Usually, you add very small amounts of a different element to make the metal stronger and harder.
Alpha particle: A radioactive particle equivalent to a helium nucleus (2 protons, 2 neutrons)
Amine: An organic molecule which consists of an ammonia molecule where one or more of the hydrogen atoms has been replaced by organic groups.
Amino acid: The basic building blocks of proteins. They’re called “amino acids” because they’re both amines (they contain nitrogen) and acids (carboxylic acids, to be precise)
Amphiprotic: When something is both an acid and a base. Like amino acids, for example.
Amphoteric: When something is both an acid and a base. Sounds familiar, huh?
Anode: The electrode where oxidation occurs. In other words, this is where electrons are lost by a substance.
Aqueous: dissolved in water
Atomic mass unit (a.m.u.): This is the smallest unit of mass we use in chemistry, and is equivalent to 1/12 the mass of carbon-12. To all intents and purposes, protons and neutrons weigh 1 a.m.u.
Atomic radius: This is one half the distance between two bonded nuclei. Why don’t we just measure the distance from the nucleus to the outside of the atom – after all, isn’t that the same thing as a radius? It is, but atoms are also (theoretically) infinitely large (due to quantum mechanics), making this impossible to measure.
Atomic solid: A solid where there’s a bunch of atoms in the lattice. This is different from an ionic solid, where ions are the things that are sticking together.
Aufbau principle: When you add protons to the nucleus to build up the elements, electrons are added into orbitals.