Melting point measurement (lab 2)

What is the melting point (MP) of a pure compound?

The specific temperature at which a pure solid changes to liquid at 1 atm pressure.

What two things happen to melting point when impurities are present?

1. MP decreases

2. MP range broadens

Why do impurities lower the melting point?

Impurities disrupt crystal lattice structure, making it easier for the solid to melt.

What does a pure compound's melting point range look like?

Very narrow, typically 1-2 °C

What does an impure compound's melting point range look like?

Lower temperature and wide range.

What are the three main reasons to record melting point?

1. Assess purity

2. Characterize a new compound

3. Identify an unknown compound

Rule of thumb for purity based on MP range?

Range > 2 °C = likely impure.

How is melting point used to identify an unknown?

Compare observed MP to known literature values.

What is a mixed melting point test?

Mixing unknown with a known compound to confirm identity.

What does it mean if the mixed melting point stays the same?

The compounds are identical.

What does it mean if the mixed melting point is lower and broad?

The two compounds are different; the mixture behaves like an impure sample

What is a eutectic point?

The lowest possible melting temperature for a specific ratio of two solids.

What is a eutectic mixture?

: A mixture of two solids that melts sharply at a temperature lower than either pure component.

What are the applications of melting point determination?

Identify an unknown

Confirm identity

Evaluate purity

Calibrate thermometers

What equipment is used for melting point measurement?

Melting point capillary tubes

Mel-Temp device

Watch glass

Glass stirring rod

Why must a new capillary tube and clean spatula be used?

To avoid contamination → contamination affects MP and MP range.

What is sublimation?

Transition of a solid directly to gas without passing through liquid.

What is the heating rate rule for accurate melting point?

Heat slowly near the MP to avoid overshooting and inaccurate readings.

Which sample in the lab sublimates if overheated?

Naphthalene (common sublimation compound).

What were the melting point ranges of the known compounds?

Cinnamic acid: 131-136 °C

Naphthalene: 80-82 °C

Phthalic anhydride: 131-132 °C

Why can phthalic anhydride and cinnamic acid be confusing during identification?

They have similar melting point ranges, so mixed MP must be used.

What is the purpose of preparing two different mixing ratios?

To strengthen confirmation of identity and avoid false positives.

What indicates perfect identity in a mixed melting point test?

Sharp, unchanged melting point with both mixing ratios.

What happens if literature MP is 132 °C and your unknown melts at 131-132 °C?

The unknown may be that compound (pure or nearly pure).

What happens if an unknown melts at 117-120 °C when literature says 121-123 °C?

Sample is contaminated or impure.