Phase
One of the three states of matter: solid, liquid, or gas. (Plasma is often considered a fourth phase of
matter.)
Phase diagram
A plot, usually of pressure versus temperature, showing which phases a compound will exhibit under
any set of conditions.
Phase equilibrium
For a particular substance, any temperature and pressure at which two or three phases coexist in
equilibrium. See Triple point.
Photon
A quantum of energy in the form of light with a value of Planck’s constant multiplied by the
frequency of the light.
Physical property
A property of a substance related to its physical, not chemical, characteristics; e.g., density, smell,
color.
Pi (π) bond
A covalent bond formed by parallel overlap of two unhybridized atomic p orbitals.
pOH
A measure of the hydroxide (OH–) ion content of an aqueous solution, defined to be equal to the
negative log of the OH– concentration.
Polar covalent bond