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Ideal gas: a theoretical idea, an ideal gas is one that obeys perfectly the ideal gas law, PV = nRT, at all temperature and pressure conditions.  Real gases (nitrogen, oxygen, etc.) behave almost exactly like ideal gases when the temperature and pressure are close to room conditions.  An ideal gas would have no attractive forces between its molecules, and its molecules would take up no space (they would be mathematical points).  This is not really true, since:

• all gases do have attractive forces between their molecules (they can all be condensed to liquids by cooling them enough that the attractive forces are able to make the molecules clump together as liquids)
• all gases molecules have some size, or you would be able to infinitely compress them (and you cannot)

The ideal gas law can be derived from the assumptions that are made in the kinetic-molecular theory.  When the temperature is quite a way above the molecule's boiling point, then attractive forces are almost meaningless.  When the pressure is not too high, molecules have so much space between them that their small size is not relevant.   Under these conditions, real gases obey the ideal gas law very closely.

Ideal gas constant: the value of R in the ideal gas law, PV = nRT.  R is often expressed in different units, depending on its purpose.  In addition to its use in calculating gas pressures, R is also a fundmental constant that is used to derive the kinetic molecular theory of gases.   Because of this it finds its way into many calculations involving gases.

Immiscible: two liquids that will not mix with each other, and that are therefore heterogeneous (as for example oil and water).

In-situ: in-place.  When referring to coal gassification, it means carrying out the reaction underground, without first mining the coal.

Ion: an ion is a positively or negatively charged atom or molecule.  An ion will be positive (a cation) if it has less electrons than protons.  It will be negative (an anion) if it has more electrons than protons.

Isotope: atoms of an element may contain different numbers of neutrons. The number of protons must be the same--it is the protons which give an atom its identity--but the neutrons are variable. Isotopes differ slightly in atomic mass, but have virtually identical chemical properties with other atoms of the same element. Some, but not all, isotopes are radioactive.

Isotopes are often used as tracers to follow the progress of molecules through a reaction. Radioactive isotopes (eg. Iodine-131, Carbon-14) can be tracked by following the movement of radioactivity through a reaction. Non-radioactive isotopes (eg. Oxygen-18, Deuterium) can be followed by using mass-spectroscopy to follow the different atomic masses.