Chemistry & Environmental Dictionary

Half-life - Hyaluronic acid


The time it takes for one-half of the atoms of an unstable element or nuclide to decay radioactively into another element or nuclide


The reactive nonmetals that are in Group 17 of the periodic table. All of these elements are electronegative.

Hazardous Material (HazMat)

Any material or substance, which even in normal use, poses a risk to health, safety, property or the environment. Hazardous materials are broken down into nine primary classes by the United Nations, the USDOT, IATA and other regulatory bodies for the purposes of the placarding and identification the hazards of shipments.

HDPE (high density polyethylene)

The structure is similar to PVC except that there is no chlorine (Cl) associated with the molecule. The tight structure makes it very dense.


An energy form proportional to and associated with molecular motion. Conduction, convection or radiation can transfer heat from one mass of matter to another.

Heat of Fusion

The heat required to convert a solid into a liquid with no temperature change. This is also called the latent heat of fusion or melting.

Heat of Vaporization

The heat required to convert a substance from the liquid to the gaseous state with no temperature change. This is also called the latent heat of vaporization.

Heisenberg's Uncertainty Principle

the conclusion that it is impossible to know simultaneously the absolute exact position and the absolute exact speed of a particle such as an electron. The more precisely the position is known of a particle at a moment in time, the less precisely can its speed be determined. Conversely the more precisely the exact speed of a particle is known, the less precisely its position can be known. Werner P. Heisenberg, a German physicist, developed this principle from his work with quantum mechanics. He was awarded the Nobel Prize in physics in 1932 for his brilliant work.

As a side note, Star Trek fans will every now and then hear the crew refer to the "Heisenberg compensator" in the transporter pads. This is the writer's nod to this principle and the problems it would create for actually being able to teleport something they way they do in the Star Trek series.


A mixture or material consisting of more than one substance. The earth's atmosphere is heterogeneous in that it typically consists of 20.7% oxygen, 78% nitrogen, 0.93% argon and 0.03% CO2 with other gasses making up the remaining percentage. Heterogeneous is Latin for "different kinds".


A substance or material that contains only one kind of compound or one element. Homogeneous is Latin for "the same kind". An example of a homogeneous substance would be pure water, which only contains the compound H2O or pure table salt that only contains the compound NaCl.

Hyaluronic acid

A viscous polysaccharide found in the connective tissue space and the synovial fluid of movable joints and the humors of the eye; it protects tissue.

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