A chemically distinct form of an element. For example, ozone (O3) contains three oxygen atoms and is chemically distinct from the oxygen (O2) we all need to survive.
The smallest unit of an element that can exist as a stable, independent entity.
A compound containing only two elements.
A substance or substances used to facilitate a chemical reaction but which is not consumed, or changed by that chemical reaction.
Chlorofluorocarbons. Also known as Freons, these compounds were developed as nonreactive substances for use as refrigerants. It subsequently was found that the presence of these manmade compounds were almost certainly implicated in the decrease of stratospheric ozone concentrations.
Shorthand used to represent the atom ratio of the elements contained in a compound.
The rapid reaction of a fuel such as oil, wood, gas, paper, etc. with oxygen resulting in the evolution of heat and product formation.
A pure substance made up of two or more elements, chemically combined, in a fixed, or constant, composition.
Specification for the amount of a pure substance either contained in the total mass of a mixture or in a volume of space. For example, 100 g of air contains 78 g of the substance nitrogen; hence, the air is 78% nitrogen. Or stated as a mass contained in a certain volume, the concentration of nitrogen in air (at sea level!) is 1.25 g/L.
The relative amounts, often stated as percentages, of the elements contained in a compound. For example, methane is 25% hydrogen and 75% carbon.
A pure substance that cannot be broken down into simpler substances by chemical means.
The amount or mass of a substance which is taken into the body. For example, if the concentration of arsenic is 1 mg/gallon in a water suppy, a person who drank 1 gallon would have an exposure to arsenic of 1 mg.
Hydrochlorofluorocarbons. Substances developed as replacement refrigerants for CFCs banned by the Monbtreal Protocol in 1987. Their advantage lies in a higher reactivity, causing them to decompose more readily before diffusing up to the altitude of maximum ozone concentration at 75-80,000 feet.
A mixture of two or more substances wherein the components are randomly distributed down to the atomic/molecular level.
Realm of reality open to observation and measurement.
ANYTHING that has mass and takes up space, i.e. has volume.
One millionth of a gram, abbreviation μg.
Realm of reality unavailable to direct observation and measurment. Scientific knowledge in this sphere is constructed from observations made in the macroscopic realm.
A physical combination of two or more substances that may be present in variable amounts.
Shorthand to represent the number and type of atoms contained in a molecule.
A chemical change in which the input of light energy is required.
Unit of concentration represented as a fraction over a billion, generally used for very low concentrations. For example, 1.35 ppm may be represented as 1.35 parts/1,000,000,000 parts in which "parts" may represent any unit of mass or weight.
Unit of concentration represented as a fraction over a million. For example, 12.5 ppm may be represented as 12.5 parts/1,000,000 parts in which "parts" may represent any unit of mass or weight.
Phenomenon in which the concentration of a substance remains constant due to equal rates of formation and destruction of the substance, as exemplified by stratospheric ozone.
Matter with fixed composition and properties which may be altered ONLY through the process of chemical change.
Intrinsic hazard of a substance. For example, plutonium has a high toxicity (less than a millionth of a gram can kill a person) whereas ethyl alcohol is considerably less toxic (400-500g will kill approximately half those so intoxicated).
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