Science / Chemistry Glossary

Partial Vaccuum: A volume that contains traces of gas at very low pressure.

Particulate: Composed of distinct particles. Smoke is particulate; pure gases are not.

Parts Per Million: Concentration expressed as parts of solute per million parts of solution. Usually refers to parts per million by mass. For example, a 10 ppm nacl solution can be written as: 10 mg nacl/kg so . . . View Full Definition

Pascal: The SI unit of pressure, equal to a force of one newton per square meter. 101325 pascals = 1 atmosphere; 105 pascals = 1 bar.

Path Length: In absorption spectroscopy, the length of a path taken by radiation through a sample.

Patina: A thin layer of corrosion products with a distinctive coloration that forms on a metal surface exposed to air and water. Patina usually refers to the greenish coating that forms on copper al . . . View Full Definition

Pattern Recognition: A computational technique used to find patterns and develop classification schemes for data in very large data sets.

Pauli Principle: No two electrons in an atom can have the same set of 4 quantum numbers. Because the n, ell, and mell quantum numbers address a particular orbital, and because the ms quantum number has only . . . View Full Definition

Penetration: Electrons in penetrating orbitals can reach the nucleus. The n and ell quantum numbers determine how well an orbital penetrates. Lower n and lower ell values mean better penetration. A low n . . . View Full Definition

Peptide: A short polymer made by linking together amino acid molecules.

Percent Error: The relative error times 100%.

Percent Yield: Percent yield equals experimental yield divided by theoretical yield times 100%.

Perfect Crystal: A crystal with no defects or impurities, made of completely identical repeating subunits. Further, a perfect crystal has only one possible arrangement of subunits, with every subunit making . . . View Full Definition

Period: Rows in the periodic table are called periods. For example, all of the elements in the second row are referred to as 'second period elements'. All elements currently known fall in the first . . . View Full Definition

Periodic Law: The periodic law states that physical and chemical properties of the elements recur in a regular way when the elements are arranged in order of increasing atomic number.

Periodic Table: An arrangement of the elements according to increasing atomic number that shows relationships between element properties.

Periodic Trend: A regular variation in element properties with increasing atomic number that is ultimately due to regular variations in atomic structure.

Permanent Hardness: Water hardness that remains after boiling the water, mainly due to dissolved calcium sulfate. Chlorides also contribute to permanent hardness.

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