Science / Chemistry Glossary

Power: The rate at which energy is supplied. Power has define[SI] units of J/s, sometimes called 'Watts' (W).

Precipitate: An insoluble substance that has been formed from substances dissolved in a solution. For example, mixing silver nitrate and sodium chloride solutions produces a precipitate, insoluble silver . . . View Full Definition

Precipitation: Precipitation is the conversion of a dissolved substance into insoluble form by chemical or physical means.

Precision: Precision is reproducibility. Saying 'These measurements are precise' is the same as saying, 'The same measurement was repeated several times, and the measurements were all very close to one . . . View Full Definition

Pressure: Force per unit area. The SI unit of pressure is the pascal, defined as one newton per square meter. Other common pressure units are the atmosphere, the bar, and the Torr.

Primary Standard: A stable, high-purity material used in titrations and other chemical analyses to prepare solutions of precisely known concentration, or to determine the concentrations of substances which re . . . View Full Definition

Principal Quantum Number: The quantum number that determines the size and (in hydrogen atoms) the energy of an orbital. N is used to label electron shells. N may take on integer values from 1 to infinity.

Product: A substance that is produced during a chemical change.

Proline: A naturally occurring amino acid with a heterocyclic ring that is classified as nonessential in the diet.

Propane: A colorless, odorless, flammable gas, found in petroleum and natural gas. It is used as a fuel and as a raw material for building more complex organic molecules. Propane is the third member . . . View Full Definition

Propellant: 1. A mixture of fuel and oxidizing agent that reacts to produce a high-energy stream of product gases that can produce thrust. For examples, see What makes a good rocket fuel? 2. A compresse . . . View Full Definition

Protein: A complex polymer made by linking together amino acid molecules. Proteins sometimes contain non-amino acid components such as metal ions or porphyrin rings embedded within.

Proton: An elementary particle found the atomic nucleus with a positive charge equal and opposite that of the electron. Protons have a mass of 1.007276 daltons.

Proton Donor: Because a free H+ ion is technically a bare proton, acids are sometimes referred to as 'proton donors' because they release hydrogen ions in solution. The term 'proton donor' is misleading, . . . View Full Definition

Pseudocore: Electrons in d or f subshells which are outside the noble gas core.

Pure Substance: A sample of matter that cannot be separated into simpler components without chemical change. Physical changes can alter the state of matter but not the chemical identity of a pure substance. . . . View Full Definition

Pyrophoric: Catches fire spontaneously when exposed to air at normal room temperature. For example, powdered potassium metal is pyrophoric.

Qualitative Analysis: A chemical analysis that detects the presence of a substance in a sample.

Quantitative Analysis: A chemical analysis that determines the concentration of a substance in a sample.

Quantitative Structure-Activity Relationship: A mathematical connection between chemical structure and biological activity, established by statistical analysis or pattern recognition techniques.

Quantum: A discrete packet of energy.

Quantum Mechanics: A branch of physics that describes the behavior of objects of atomic and subatomic size.

Quantum Number: Indices that label quantized energy states. Quantum numbers are used to describe the state of a confined electron, e. G. An electron in an atom.

Racemic: A mixture of equal parts of the levorotatory and dextrorotatory isomers of the the same substance. Racemic mixtures are not optically active.

Radian: An angle with vertex at the center of a circle of radius r that encompasses an arc of length r.

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