Melting point

The melting temperature is defined as the temperature at which a substance will melt, i.e. from the solid to the liquid aggregate state. The melting temperature is dependent on the material, in contrast to the boiling point, but very little on the pressure ( fusion pressure). Melting temperature and pressure are referred to as melting point, this describes the state of a pure substance, and is part of the melting curve in the phase diagram of the substance. Some fabrics can not melt because they disintegrate previously chemically, and others can only sublimate at atmospheric conditions.

Pure chemical elements for the melting point is identical to the freezing point and remains constant during the entire melting operation. Impurities or in mixtures, the melting temperature is generally lowered ( freezing point depression ), also may increase during the melting process, whereby one has to do with a melting range of the temperature. The melting point depression ( cryoscopic ) by dissolved substances is a reason why ice can be melted by salt.

In contrast to chemical elements, there may be discrepancies between the melting point and freezing point even in pure chemical compounds. If the freezing temperature is below the melting point temperature, it is called a thermal hysteresis. This is for example the case for pure water; without nuclei, and under a pressure of 1 bar, water freezes at about -40 ° C and melts at about 0 ° C. In amorphous materials such as glasses and some plastics is called the transition temperature. The definition of a softening temperature is possible.

The melting temperature is one of density, fracture toughness, strength, ductility, hardness, and stiffness of the material properties of a material.

Pressure dependence

Although the melting point depends on the pressure, but only slightly: To change the melting point by only 1 K, the pressure must be on average increased by about 100 bar. It follows that changes in atmospheric pressure - can cause noticeable changes to the boiling point - virtually no effect on the melting point.

For melting applies as for other phase transitions, the Clausius- Clapeyron equation, the following temperature change? T gives a good approximation for the melting at different pressures:

Where TM is the melting point, the volume change upon melting DELTA.V, Ap, the difference of the observed pressure and the enthalpy of fusion HM. However, since the volume change upon melting DELTA.V are relatively small, the pressure dependence of the melting point is relatively small. For example, changes in a pressure increase of 100 bar, the melting point of ice at -0.76 K - ice melts under pressure so easily - while increasing the melting point of carbon tetrachloride to 3.7 K. The fact that lowers the melting point of ice or for example also of bismuth with pressure increasing, it follows that their volume is reduced during melting: Then is negative in the above equation? V and? T.

Analysis

The determination of the melting point of a substance is also of great importance in the qualitative analysis, including identity verification, as many substances can be identified by their melting point. The purity of substances can be qualitatively also measured above the melting point. Impurities have lower melting points result. Liquid substances or those with a low melting point can be converted to a slightly crystallized derivatives: alcohols can be identified, for example by measuring the melting points of its esters of 4-nitrobenzoic acid, or 3,5- dinitrobenzoic. For this purpose the substance to be analyzed is reacted in the presence of small amounts of sulfuric acid. The melting points of these derivatives are usually sharp.

The derivatives of 3,5- dinitrobenzoic generally have higher melting points than that of 4-nitrobenzoic acid. You are then preferably selected when the melting point of 4-nitrobenzoic acid is too low and no more accurate determination is possible.

Extensive table works with details of melting points of organic compounds, as an important tool for analysts before lie. Melting points of derivatives of individual classes of substances are listed in the textbooks of organic analysis.

Determination

An approximate measurement is of course easy with a thermometer by melting the sample and reading the melting temperature possible.

For the accurate measurement of the melting point of different methods are available:

• Apparatus according to Thiele, in which the sample is melted in a stirred oil bath, or by convection flowing
• An apparatus according to DAB, with standard ground joint 29/32, consisting of flask of about 100 ml and insert pipe with vent hole
• Apparatus according to Dr. CF Linström ( often written erroneously also Lindström ) Here, the sample is heated in a copper block to the melting point

• Heiztischapparatur Kofler (see also Kofler hot stage ), Tottoli
• Differential scanning calorimetry ( DSC)

In most cases the measured values ​​are indicated with the fact that they are not corrected. This refers to the ( small ) error caused by the fact that only the reservoir of the thermometer dips into the measuring medium, but not the ascending thread so that a different temperature and expansion has.

In the practical laboratory work today often find automatic melting point measuring devices use that provide the result digitally in a short time.