Melting point

some melting points Θ in °C
material Θ
[°C]
ethanol (C 2 H 5 OH) -114
mercury -38.36
water 0
benzene 5.5
Naphthalin 80
sulfur (rhombic) 113
sulfur (monoclinic) 119
tin 231
lead 327.4
aluminum 658
common salt 801
Silver 960.8
gold 1,064
copper 1,084
iron 1,536
platinum 1,773, 5
tungsten 3,422

as fusing temperature one designates the temperature, at which a material melts, i.e. changes from the firm into the liquid state of aggregation. The fusing temperature depends on the material,contrary to the boiling temperature however only very few of the pressure (fusion pressure). Fusing temperature and pressure are called together melting point, whereby this describes the condition of a pure material and part of the fusion curve is in the phase diagram of the material. For pure materials is thatMelting point identically to the freezing point and remains constant during the entire Schmelzvorganges. By impurities, solvent remainders of etc. the fusing temperature is usually degraded (melting point degradation).

Amorphous materials do not possess a bloom point, but a bloom range or a transition temperaturelike z. B.Glasses and some plastics. Also the determination of a softening point is possible. </br> The fusing temperature ranks with the density tear tenacity, firmness, ductility, hardness, and the rigidity among the material properties of a material.

Table of contents

pressure dependence

the melting point depends on the pressure , however only slightly: In order to change the melting point around only 1 K, the pressure must by approximately 100 at on the average increasedbecome. Hence it follows that itself changes of the atmospheric pressure - which can cause noticeable changes of the boiling point - affect practically not on the melting point.

To melting as to other phase conversions the Clausius Clapeyron equation, those applies in good approximationfor melting with different pressures the following change of temperature Δ T results in:

<math> \ delta T = \ frac {T_M \ delta V \ delta p} {H_M}< /math>

T M is the melting point, Δ V the variation in volume when melting, Δ p the difference of the regarded pressures, and HM the fusion enthalpy. Since however the variations in volume are relatively small Δ V when melting, also the pressure dependence of the melting point is relatively small. For example the melting point of ice changes around -0.76 with an increase of the pressure by 100 atK - Ice melts thus under pressure more easily -, while the melting point of carbon tetrachloride increases by +3.7 K. The fact that the melting point of ice degrades itself or for example also from bismuth during increase in pressure, follows from the fact that itVolume at melting is reduced: Then Δ T is negative in the above equation Δ V and.

meaning

the melting point of water has great importance in biology and for the weather. The determination of the melting pointa substance is also of great importance in qualitative analytics, including the Identitätsprüfung, since many substances can be identified over their melting point. Liquid substances or such with low melting point are converted to it into easily crystallizing derivatives: Alcohols can for example by the measurement of the melting points of their esters of the Nitrobenzoesäure or the Dinitrobenzoesäure to be identified.

The melting point degradation (cryoskopy) by solved substances is the reason, why ice can be melted by salt.

regulation

an approximate measurement is naturalsimply with a thermometer through melt opens of the sample and reading off the fusing temperature possible.

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

see also

Phase diagram, molecular melting point degradation, boiling temperature

 

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