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03 Exp 2 Melting Point DeterminationSunday, July 01, 2012

10:41 AM

Discussiono 9.1 Physical Properties

Commonly recognized physical properties of a compound Color Melting Point Boiling Point Density Refractive Index Molecular Weight Optical Rotation Spectra

Infrared Nuclear magnetic resonance Mass Spectroscopy Ultraviolet-Visible

o 9.2 Melting Point Uses

Determine identity of a compound Establish purity

Temperatures noted while observing melting point First drop of liquid forms among the crystals Point at which the whole mass of crystals turns to a clear liquid.

Range of melting How Melting Point indicates purity

Purer material, closer to true melting point Increase in impurity, decrease from true melting point Increase in impurity, freezing point depression

Purer material, narrower its melting point range Melting point behavior of mixtures

Upper curves indicates all the sample has melted

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Lower curves indicate the temperature at which melting is observed tobegin.

Left hand and right hand edges: Pure compounds, sharp melting point, norange.

Minimum melting point temperature, Eutectic pointNot all mixtures form eutectic points. Some have multiple eutectic

points.

Vertical distances between the upper and lower curves represent themelting point range.

o 9.5 Packing the Melting Point Tube Melting points are usually determined by heating a capillary tubing (1mm x 100

mm) sealed at one end. Pack the tube, press the open end gently into a pulverized sample. Amount of solid pressed into the tube should be 1 to 2 mm high. Transfer crystals to the closed end of the tube, drop the capillary tube, closed end

first, down a meter length glass tubing, held upright on the desktop. Tapping with fingers is not recommended Commercial melting-point instruments have built-in vibrating device that is

designed to pack capillary tubes.o 9.6 Determining the Melting Point - The Thiele Tube

Shape allows convection currents to form and maintains a uniform temperaturethrough the oil in the mixture

Rubber band should be above the level of the oil (Allowing for oil expansion whenheating) Use a triangular wedge if a rubber stopper is used to hold a thermometer for

pressure equalization. Move burner slowly back and forth along the bottom of the sidearm. If near the suspected melting point, 1C per minute

o 9.7 Determining the Melting Point - Electrical Instruments

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Save time by preparing 2 samples. Measuring above 150C needs stem corrections.

o 9.8 Decomposition, Discoloration, Softening, Shrinkage, and Sublimation Unusual behavior before melting

Decomposition and Discoloration Usually evidenced by discoloration Can also be used as a reliable physical property Decomposition temperature are indicated in melting points with a

symbol "d" Decomposition is a result of reaction with oxygen in air, avoided with

a sealed, evacuated melting point tube.

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Some substances decompose below their melting points. Softening and Shrinkage

Change in crystal structure or mixing with impurities. Sweat or release solvent of crystallization before melting Not start of melting. First drop of liquid is visible until all solids are converted to liquids. May include softening in melting point range

211 C (softens), 223-225 C (melts) Sublimation

High vapor pressure, they sublime at or below their melting points. Symbols sub, subl, and sometiems s are used to designate

sublimation temps. Avoided by sealing a packed tube.

o 9.9 Thermometer Calibration