Physical chemistry is the part of chemistry that seeks to account for the properties and transformations of matter in terms of concepts, principles, and laws drawn from physics. This glossary is a compilation of definitions, descriptions, formulae, and illustrations of concepts that are encountered throughout the subject. This section describes the concepts that begin with the letter Z; where appropriate, the entries also describe subsidiary but related concepts. Refer to the Directory for a full list of all the concepts treated.
Zeeman Effect
The Zeeman effect is the splitting of spectral lines observed in the presence of a magnetic field (Figure Z.1). In the normal Zeeman effect, which is characteristic of atoms and ions with S = 0, spectral lines are split into three. It arises from the removal of the degeneracy of states 1 with different values of MJ in conjunction with the selection rules ΔMJ = 0, ±1. In the anomalous Zeeman effect, splitting into more than three lines are observed. The effect arises on account of the different magnetic properties of terms as expressed by the Landé g-factor in combination with the same selection rules. The two effects are illustrated below for the transitions 1P → 1S (normal) and 3P → 3S (anomalous).
The normal (left) and anomalous (right) Zeeman effect.
Zeroth-order Reaction
Zeroth-order reactions are often encountered in surface chemistry where the rate-determining step takes place after fast adsorption has occurred. A pseudozeroth-order reaction is encountered in solution when a reactant is in such great excess that its concentration remains constant in the course of the reaction.
Zero-point Energy
A rotor of any geometry has no zero-point energy. In general, the magnitude of the zero-point energy, when it is nonzero, decreases with increasing mass.
Zeroth Law
The Zeroth law of thermodynamics states that if A is in thermal equilibrium with B when they are in contact through diathermic walls, and B is in thermal equilibrium with C when they are likewise in contact, then C would be in thermal equilibrium with A when they are brought into contact (Figure Z.2). The law establishes the concept of temperature, for A, B, and C are all in mutual thermal equilibrium if they have the same value of a property that is called ‘temperature’. Moreover, the law establishes the concept of a thermometer, a device for measuring temperature, with B playing that role; for if B displays the same temperature as A when it is in thermal contact with it, and if B also displays the same temperature when it is in contact with C, then it is known that A and C have the same temperature and would therefore be in thermal equilibrium if they were placed in thermal contact.
The mutual thermal equilibrium of three systems.
Zone Refining
In the technique of zone refining, which is used to purify a solid, a train of narrow molten bands is passed along a bar of the impure material; impurities collect in the molten region and are carried to the end of the sample. In the modification known as zone levelling, the pure material is doped uniformly by dragging a molten zone containing the dopant repeatedly in alternate directions through the sample.
