Melting Point

FIELDS OF STUDY: Physical Chemistry; Metallurgy; Analytical Chemistry

ABSTRACT

The physical property of the melting point is discussed. Melting is the result of the energetic motions of atoms and molecules exceeding the ability of intermolecular attractive forces to maintain a rigid formation, allowing the component atoms or molecules to move freely about each other while maintaining contact.

Melting and Freezing Explained

All matter is composed of atoms, which can combine with other atoms to form molecules of compounds. Each element consists of just a single kind of atom, while compounds consist of atoms of two or more different elements. On Earth, matter typically exists at any one time in one of three physical phases: solid, liquid, or gas.

The phase in which matter exists is temperature dependent. When a material is in the solid phase, its atoms or molecules are held so rigidly in place that the material cannot alter its shape to conform to the shape of its container, as a liquid or gas can; the intermolecular attraction between the component particles prevents them from moving freely about relative to each other. As the temperature is raised, however, the atoms or molecules vibrate more and more energetically until, at a certain energy level, the motion is sufficient to overcome the intermolecular forces, allowing the particles to move relative to each other. At that point the material goes through a phase transition as it changes from solid to liquid. The melting point of a material is an intensive property and is characteristic of the material; this means a sample of a pure compound weighing just a few milligrams melts at the same temperature as thousands of kilograms of the same material—though it would take a much longer time for the larger mass of material to achieve that temperature, as greater mass requires the input of more heat energy to reach the same temperature.

The opposite mechanism takes place as the temperature of the material is reduced and heat energy is removed from the material. Atomic or molecular motion decreases to the point where it is no longer sufficient to defeat the intermolecular forces that act to bind the particles in place, and the material passes from the liquid phase to the solid phase. The temperature at which this occurs is the freezing point of the material. In theory, the law of conservation of energy requires the melting point and the freezing point of a pure element or compound to be equal, although in practice there are certain exceptions.

Impurities and the Melting Point

The particles that make up certain solids, especially solid ionic compounds, are arranged in a regular, rigidly structured array called a crystal lattice. Such solids are said to be crystalline, as opposed to amorphous, which describes solids with irregular particle structures.

The presence of a foreign material in a quantity of a crystalline compound will disrupt the regularity of the crystal structure, and any impurity can alter the melting behavior of the compound. An impurity is typically an undesired component that is present only in very small proportions. The presence of any amount of foreign material in an otherwise pure compound has the effect of lowering the melting point of the mixture relative to the melting point of each component when pure. This phenomenon is the basis of the mixed melting-point test, which can be used to determine the identity of an unknown compound. In this test, the melting point of the unknown compound is determined, a known compound with a similar melting point is obtained, and the two compounds are combined. If the melting point stays the same, they are both the same compound; if the melting point drops, then they are not the same, and further tests are necessary to determine the identity of the unknown compound.

The amount by which the melting point of a mixture is lowered is directly related to the proportions of the materials in the mixture, which gives rise to an analytical procedure called freezing-point depression analysis. In a freezing-point depression analysis, a precisely weighed sample of a compound is mixed with a certain mass of another material used as a standard. The standard material must be of very high purity and have a very sharp melting point—that is, a melting point that exists within a very narrow range of temperatures. The amount by which the freezing point of the mixture is lowered, or depressed, relative to the melting point of the standard material can then be used to calculate the molecular mass of the unknown material from the mass of the sample.

Mixtures of different compounds often have broad melting points, meaning that one compound in the mixture melts sooner than another, causing the overall melting process to take place over a broad temperature range. In some cases, however, specific proportions of compounds form what is termed a "eutectic" mixture, characterized by a sharp melting point. For example, an alloy (a mixture of a metal and some other element) is said to be eutectic when it is formed from a precise proportion of components that gives it a narrowly defined melting point; different proportions of the same components would cause it to have a much broader melting point, at which point it would no longer be eutectic.

The Effects of Molecular Structure

A crystal lattice is an orderly arrangement that can be compared to the arrangement of bricks in a wall; all of the component pieces have the same size, shape, and electronic properties, including polarity (the distribution of electric charge). These electronic properties are the source of the intermolecular attraction that holds the material together in the solid phase. Typically, the more polar the molecules of a compound, the more tightly they combine in the solid phase and the higher the melting point of the solid. In metals, the strength of the atomic interactions that maintain the material’s solid phase is due to the metallic bonds between the atoms, which in effect make the entire mass of the metal behave as a single molecule; as a result, metals tend to have very high melting points. Conversely, molecules that are nonpolar or of very low polarity also have lower melting points.

PRINCIPAL TERMS

• freezing point: the temperature at which a liquid undergoes a phase change to become a solid.
• intensive properties: the properties of a substance that do not depend on the amount of the substance present, such as density, hardness, and melting and boiling point.
• liquid: a state of matter in which material is fluid, has definite volume but indefinite shape, and maintains a relatively constant density.
• phase transition: the change of matter from one state to another, such as from solid to liquid or liquid to gas, due to the transfer of thermal energy.
• solid: a state of matter in which material is non-fluid, has definite volume and shape, and maintains a near-constant density.

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