Melting point

Melting point

Definition of a melting point

No physical constant of solids is more widely used by organic chemists than the melting point (MP). In a practical sense, the melting point of a crystalline solid is simply the temperature at which the solid changes to a liquid under the pressure of one atmosphere. For a pure substance, the change is usually quite sharp and the temperature is characteristic and not significantly affected by moderate changes in pressure. Hence, the melting point is a convenient constant commonly used in the identification of solids. In addition, because the melting point is almost always markedly altered by the presence of impurities, it is a valuable criterion of purity.

The liquid form of a pure substance, when cooled in such a way that supercooling is prevented, solidifies at the same temperature at which the pure solid phase melts. For a pure substance, then, the melting and freezing points are identical. The melting and freezing points of a substance are best defined as the temperature at which the liquid and solid phases can exist in equilibrium with each other under a total pressure of one atmosphere.

To measure the melting point of a substance, it is necessary to gradually heat a small sample of the substance while monitoring its temperature with a thermometer. The temperature at which liquid is first seen is the lower end of the melting point range. The temperature at which the last solid disappears is the upper end of the melting point range. A pure substance normally has a melting point range no larger than 1-1.5 ^oC. There are several methods by which melting points can be determined, and the choice of method depends mainly upon how much material is available. One of the most common ways of determining the melting point of a substance is a capillary method.

VOCABULARY:

affect - wywierać wpływ                                        rough - nierówny, szorstki, pobieżny

alter – zmienić                                                                                       solid – ciało stałe

convenient - wygodny, dogodny                              bath - kąpiel

criterion (Sg) criteria (Pl) – kryterium                                         purity – czystość

determination - oznaczanie                                    change - przemiana

freezing point – temperatura/punkt krzepnięcia            capillary - kapilara

impurities - zanieczyszczenia                            insert -wkładać, wstawiać

in addition - na dodatek, w dodatku                    cool - chłodzić, stygnąć

liquid - ciecz                                                                                          range - zakres

markedly - wyraźnie, znacząco                          accurate - dokładny

melting point – temperatura/punkt topnienia                liquid - ciecz

moderate - umiarkowany                                  fill - napełniać

one atmosphere - jedna atmosfera                            melt - topić, topnieć

phase - faza, okres                                            crystalline - krystaliczny

pressure - ciśnienie                                          solidify - krzepnąć, tężeć

prevent - zapobiegać                                            freeze - zamarzać, krzepnąc

rapid - szybki                                                     identification - identyfikacja

sample – próbka                                                                                   constant – stała

sharp – ostry, nagły                                          exist - istnieć, znajdować się

significant - znaczący, ważny                                  equilibrium - równowaga

supercool – przechłodzić                                                                 gradualny - stopniowo

valuable - wartościowy, cenny                                pure - czysty