in the bottom of a melting point tube. Put a small amount of the compound in the open end of the melting point tube. Turn over and tape the closed end on the desk
top until the compound falls to the bottom. Sample in the melting point tube. Use a Thiele tube filled with mineral oil to heat your sample.
Attach the melting point tube to a thermometer. Heat about 5o per minute until within about 10o of the melting point
Near the melting point heat at 1 - 2o per minute Mel-Temp in the lab Temperature
Starts to melt Finished melting m.p. = Start - Finish Why is salt put on snow covered roads?
Ice melts at 0oC What happens to the melting point if salt is added? Ice melts! Impurities such as salt lowers the melting
point of water. Putting salt on icy roads causes the ice to melt because it lowers the melting point of water. Impure compounds usually melt lower than pure compounds so the melting point may be used as a measure of the purity of a compound
Acetic Acid, CH3COOH, is a colorless liquid that melts at 16.6oC. Lets look at the melting point of mixtures of water and acetic acid.
Plot of melting point vs. mole fraction water for mixtures of water and acetic acid. Eutectic point Cool a acetic acid - water solution with a mole fraction water of 0.9
Heat a acetic acid - water solution with a mole fraction water of 0.9 Heat a acetic acid - water solution with a mole fraction water of 0.2 Pure compounds usually melt over a narrow temperature range, often
1o or less. Impure compounds melt lower than pure compounds and over a wider temperature range. Melting points are a measure of purity
m.p. = 115o - 119o m.p. = 118o - 120o m.p. = 121o - 122o Two of these bottles contain benzoic acid and one m-nitrophenylacetic acid.
m.p. = 120o - 122o m.p. = 120o - 122o m.p. = 120o - 122o How do you tell what is in each bottle?
Mixed Melting Points Grind samples together to be sure they are mixed and then measure the melting point. Results
1 2 3 Mixed 1 and 2
m.p. = 120o - 122o Mixed 1 and 3 m.p. = 114o - 117o Mixed 2 and 3
m.p. = 115o - 118o 1. Measure the melting point of your unknown 2. Run mixed melting points to confirm identification
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