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Home :Glossary

Glossary: P


paraffin. paraffin wax.
1. a waxy substance that is a mixture of alkanes* with chains containing 18 to 36 carbon atoms. 2. An alkane.

paramagnetism. paramagnetic. Compare with diamagnetism* and ferromagnetism*.
Paramagnetic materials are attracted to a magnetic field due to the presence of least one unpaired spin* in their atoms or molecules.

partial miscibility. partially miscible. Compare with miscible* and immiscible*.
Two liquids are considered partially miscible if shaking equal volumes of the liquids together results in a meniscus* visible between two layers of liquid, but the volumes of the layers are not identical to the volumes of the liquids originally added.

partial vaccuum. Compare with absolute vaccuum*.
A volume that contains traces of gas at very low pressure.

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

parts per million. (ppm)
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 solution, 10 µg NaCl/g solution, 10 ng NaCl/mg solution. In very dilute aqueous solutions, ppm is approximately equal to mg solute per liter of solution.

pascal. (Pa)
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. (b)
In absorption spectroscopy*, the length of a path taken by radiation through a sample.

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 alloys over time.

pattern recognition.
A computational technique used to find patterns and develop classification schemes for data in very large data sets.

Pauli principle. exclusion principle; Pauli exclusion; Pauli exclusion 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 two possible values, the Pauli principle says that a maximum of two electrons can occupy an atomic orbital- and these electrons must have opposite spins.

penetration. Compare with shielding*.
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 value means the orbital is small. A low ell value means the orbital has fewer nuclear nodes (planes that pass through the nucleus where the probability of locating the electron is zero).

In order of decreasing penetration, the subshells are s > p > d > f. A 1s orbital penetrates better than a 2s orbital.

A short polymer* made by linking together amino acid* molecules.

percent error. percentage error.
The relative error* times 100%.

percent yield. percentage yield. Compare with theoretical yield* and actual 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 exactly the same contribution to the total energy of the crystal.

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.

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 seven periods.

permanent hardness. permanent water hardness. Compare with temporary hardness* and water hardness*.
Water hardness* that remains after boiling the water, mainly due to dissolved calcium sulfate. Chlorides also contribute to permanent hardness.

permanganate. (MnO4-) permanganate ion.
Permanganate ion (MnO4-) is a powerful oxidizing agent used in chemical analysis and water treatment. The ion has an intense purple color.

pH is a measure of effective concentration of hydrogen ions in a solution. It is approximately related to the molarity of H+ by pH = - log [H+]

The study of drugs, which includes determination of biological activity, biological effects, breakdown and synthesis, and delivery.

Identification, isolation, and characterization of biologically active substances in living things.

phase. in phase; out of phase; wave phase.
1.A phase is a part of a sample of matter that is in contact with other parts but is separate from them. Properties within a phase are homogeneous (uniform). For example, oil and vinegar salad dressing contains two phases: an oil-rich liquid, and a vinegar-rich liquid. Shaking the bottle breaks the phases up into tiny droplets, but there are still two distinct phases. 2. In wave motion, phase is the fraction of a complete cycle that has passed a fixed point since the current cycle began. The phase is often expressed as an angle, since a full cycle is 360^deg; (2 pi). Two waves are "in phase" if the peaks of one wave align with the peaks of the other; they are "out of phase" if the peaks of one wave align with the troughs of the other.

phase boundary.
A phase boundary is a surface where two samples of matter with different properties are in contact. The surface of a gas bubble in water or the surface of a crystal are examples of phase boundaries.

phase change. phase transition.
A change in the state of a sample of matter; for example, solid to liquid or liquid to gas. Phase changes are considered physical rather than chemical changes.

phase diagram. phase map.
A map that shows which phases* of a sample are most stable for a given set of conditions. Phases are depicted as regions on the map; the borderlines between regions correspond to conditions where the phases can coexist in equilibrium*.

A group or molecule containing a benzene* ring that has a hydroxyl* group substituted for a ring hydrogen.

A common misspelling of phenolphthalein*.

An organic compound used as an acid-base indicator. The compound is colorless in acidic solution and pink in basic solution (with the transition occuring around pH 8.3). Phenolpthalein was used for many years as a laxative in very low concentrations- high concentrations are toxic!

phenyl. (phi)
A molecular group or fragment formed by abstracting or substituting one of the hydrogen atoms attached to a benzene* ring.

-amino-ß%3b-phenylpropionic%20acid]">phenylalanine. Phe; alpha-amino-ß-phenylpropionic acid].
A naturally occuring amino acid* with a nonpolar side chain terminated by a phenyl* group.

phosphate. (PO4-3)
1. The PO4-3 ion. 2. A compound containing the PO4-3 ion.

An ester* of glycerol* with two fatty acids* and phosphoric acid (H3PO4) or a derivative of phosphoric acid group (like H2PO4CH2CH2N(CH3)3). Phospholipids have a hydrophilic* head (the phosphate group) and a lipophilic* tail (the fatty acids).

phospholipid bilayer. bilayer; phospholipid membrane.
In an aqueous environment phospholipids* can form a two-layered "sandwich" with the hydrophobic* lipid* tails on the inside and the hydrophilic* phosphate heads facing outward. These bilayers are the essential structure for building cell membranes.

The study of chemical changes caused by light. For example, many of the key reactions that generate smog are photochemical.

photoelectric effect.
Ejection of electrons from an atom or molecule that has absorbed a photon* of sufficient energy. The photoelectric effect is the operating principle behind "electric eyes"; it is experimental evidence for particle-like behavior of electromagnetic radiation*.

An electron* ejected from an atom or molecule that has absorbed a photon*.

photon. (hnu) quantum; quanta.
A discrete packet of energy associated with electromagnetic radiation. Each photon carries energy E proportional to the frequency* nu of the radiation: E = hnu, where h is Planck's constant*.

A complex process used by many plants and bacteria to build carbohydrates* from carbon dioxide and water, using energy derived from light.

physical change. Compare with chemical change*.
A change which does not transform one substance into another. For example, freezing water is a physical change because both water and ice are H2O. However, electrolysis* of water would not be a physical change because passing a strong electric current through water can decompose it into H2 and O2.

physical chemistry. chemical physics.
A branch of chemistry that studies chemical phenomena from a physical and mathematical perspective. Physical chemistry includes chemical thermodynamics, kinetics, spectroscopy, quantum chemistry, and statistical mechanics.

physical property. physical properties. Compare with chemical property*.
Measurement of a physical property may change the arrangement but not the structure of the molecules of a material. Examples of physical properties are density, color, boiling point, volume, temperature, and mass.

phytochemistry. phytochemical.
The study of substances found in plants. "Phytochemicals" are materials extracted from plant tissue.

pi bond. (pi bond) Compare with sigma bond*.
In the valence bond theory*, a pi bond is a valence bond* formed by side-by-side overlap of p orbitals on two bonded atoms. In most multiple bonds, the first bond is a sigma bond and all of the others are pi bonds.

pico-. (p)
Prefix used in the SI* system meaning "multiply by 10-12". For example, 3 pm means 3× 10-12 meters.

picogram. pg.
One picogram is 10-12 grams.

picoliter. pL.
One picoliter is 10-12 liters.

picometer. pm.
One picometer is 10-12 meters.

pKa. (pKa) Compare with acid dissociation constant*.
The pKa of an acid is minus the base-10 log of its acid dissociation constant*, pKa = - log Ka. For example, a pKa of 5 is equivalent to an acid dissociation constant of 10-5.

pKb. (pKb) Compare with base hydrolysis constant*.
The pKb of an base is minus the base-10 log of its base hydrolysis constant*, pKb = - log Kb. For example, a pKb of 5 is equivalent to an base hydrolysis constant of 10-5.

Planck's constant. (h)
A proportionality constant that relates the energy carried by a photon* to its frequency*. Planck's constant has a value of 6.6262 × 10-34 J s.

1. In biology, the fluid in which blood cells or lymph cells are suspended. 2. A gaslike state of matter consisting of positively charged ions, free electrons, and neutral particles. Plasma is found in stars, the sun, the solar wind, lightning, and fire.

poise. (P)
A cgs* unit of resistance to fluid flow (viscosity*). If a force of 1 dyne* is needed to force two fluid layers with 1 cm2 area that are 1 cm apart past each other at a speed of 1 cm/s, the liquid has a viscosity of 1 poise.

polar bond. Compare with covalent bond* and ionic bond*.
A bond* involving electrons that are unequally shared. Polar bonds can be thought of as intermediate between the extremes represented by covalent bonds* and ionic bonds*.

polar molecule. polar. Compare with covalent compound*, ionic compound* and polar bond*.
An asymmetric molecule containing polar bonds*. H2O, NH3, and HCl are examples of polar molecules. Non-examples are CO2, CCl4, and BCl3 which contain polar bonds but are nonpolar because they have symmetric shapes. Alkanes* are usually asymmetric but are nonpolar because they contain no polar bonds. Polar molecules are electric dipoles* and they attract each other via dipole-dipole forces*.

A property associated with molecules when the center of positive charge and the center of negative charge don't coincide. See also polar molecule* and polar bond*.

polyatomic ion. Compare with molecule*, ion* and polyatomic molecule*.
A polyatomic ion is a charged particle that contains more than two covalently bound* atoms. See Polyatomic Ions for more.

polyatomic molecule. Compare with polyatomic ion* and diatomic molecule*.
A polyatomic molecule is an uncharged particle that contains more than two atoms.

polydentate. polydentate ligand.
A ligand* that has more than one atom that coordinates directly to the central atom in a complex. Polydentate ligands are called chelating agents when two or more coordinating atoms are attached to the same metal ion in a complex. For example, EDTA* or ethylenediaminotetracetic acid is a hexadentate ligand of calcium ion.

A large molecule made by linking smaller molecules ("monomers") together.

A process that links smaller molecules together to form a larger molecule.

To link smaller molecules together to form a larger molecule.

polymorph. polymorphism; polymorphic. Compare with isotope* and allotrope*.
Solid substances that occur in several distinct forms. Polymorphs have different chemical and physical properties. allotropes* are polymorphs of elements.

polysaccharide. Compare with oligosaccharide* and monosaccharide*.
A carbohydrate* consisting of a large number of linked monosaccharide* units. Examples of polysaccharides are cellulose and starch.

position of equilibrium. Compare with dynamic equilibrium*.
When a reaction's equilibrium "lies to the right", the concentrations of products will be greater than the concentrations of reactants when equilibrium is established. Conversely, an equilibrium that lies to the left will have a relatively small fraction of products.

potential difference. electrical potential difference.
Work that must be done to move an electric charge between specified points. Electric potential differences are measured in volts*.

potential energy. Compare with kinetic energy*.
energy* an object possesses by virtue of its position. For example, lifting a mass mby h meters increases its potential energy by mgh, where g is the acceleration due to gravity.

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

precipitate. (downarrow) ppt.
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 chloride (along with soluble sodium nitrate.

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

precision. reproducibility. Compare with accuracy*.
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 another". Don't confuse precision with accuracy*.

pressure. (P)
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 react with the primary standard material. For example, NaCl is a primary standard that can be used to determine the concentration of AgNO3 in a solution by titration: NaCl(aq) + AgNO3(aq) = AgCl(s) + NaNO3(aq).

principal quantum number. (n)
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. Compare with reactant*.
A substance that is produced during a chemical change*.

proline. Pro.
A naturally occurring amino acid* with a heterocyclic ring* that is classified as nonessential in the diet.

propane. (C3H8) Compare with alkane* and hydrocarbon*.
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 of the alkane* series.

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 compressed gas used to push a material through a nozzle, forming an aerosol* or a foam*. For example, nitrogen or propane are used as propellants for shaving cream; nitrous oxide is used as a propellant for whipped cream.

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. (p+) Compare with electron* and neutron*.
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. acid. Compare with base*.
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, since in aqueous solution, the hydrogen ion is never a bare proton- it's covalently bound* to a water molecule as an H3O+ ion. Further, acids don't "donate" protons; they yield them to bases with a stronger affinity for them.

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

pure substance. 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. Pure substances have fixed, characteristic elemental compositions and properties.

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


General Chemistry Online! Glossary: P

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/glossary/p.shtml