Melting Points

What are the effects of impurities?

• depresses (lower) and broadens the MP range.

How do you identify an unknown using mixed melting points?

1. Do a mixed melting point with known sample and unknown

2. Take MP values as normal

3. If the range is low and wide, then the sample is impure and there are two different compounds present in our melting point. If the range is within 1-2 C, then the compound is pure.

Decomposition

• solid darkens to a black solid without liquefying; usually a sharp range is observed.

Polymorphs

• different crystalline forms of a solid that have different mp ranges.

  • ex: diamond and graphite

Hydrates

• anhydrous solid will have a different MP range compared to its hydrated form

Sublimation

• sample will undergo phase change from solid directly to a gas

  • no real MP observed

eutectic point

• the lowest temperature at which a mixture of two or more substances will completely melt, forming a eutectic mixture.

eutectic mixture

• a specific combination of two or more substances that, when mixed together in certain proportions, forms a mixture with a lower melting point than any of the individual component’s original melting points.

the different regions of a melting point diagram

What is the heating rate for a proper melting point?

• 1-2 C per minute when near melting point

  • for quick melting point, 10 °C per minute

Describe the preparation of a melting point sample.

• grind sample on spot plate

• pack 1-2 mm of sample into capillary tube by dropping it down a glass tube

• insert in mp apparatus and take melting point

  • For mixed MP, mix small, evenn amount of two different components to make a homogeneous mixture

Why take a melting point?

• To aid in determining purity of a solid

• To aid in determining identity of a solid