For what purpose are melting points routinely used? (a) To determine whether a compound will decompose (b) To separate mixtures of a compound (c) To assign its chemical properties (d) For identity and purity determination
The correct answer and explanation is:
Correct Answer: (d) For identity and purity determination
Melting points are routinely used in chemistry for determining the identity and purity of a compound. Each pure compound has a specific, sharp melting point at which it transitions from solid to liquid. If a compound is pure, this change happens at a very narrow temperature range, typically within 1 to 2 degrees Celsius. When impurities are present, the melting point becomes lower and occurs over a wider temperature range. This happens because impurities disrupt the orderly arrangement of molecules in the solid state, which weakens the forces holding the structure together.
For identification purposes, chemists often compare the observed melting point of an unknown substance with the known melting point of a suspected compound. If they match, and especially if a mixed melting point test (combining the unknown with the known sample) also yields a sharp melting point, then it confirms the identity of the compound.
For purity analysis, a sharp and consistent melting point indicates that the substance is pure. If the sample contains even a small amount of impurity, its melting point will be depressed and broadened. This makes the melting point a quick and efficient way to assess purity without advanced instruments.
The other answer choices are incorrect. Melting points are not used to determine whether a compound will decompose; decomposition is usually observed separately and often at a different temperature. Melting points are not used to separate mixtures; separation methods like distillation, filtration, or chromatography serve that purpose. Also, melting points do not directly assign chemical properties such as reactivity or solubility.
In summary, melting point analysis is a fundamental tool in organic and analytical chemistry used primarily to confirm a compound’s identity and assess its purity.