Melting-Point Methods and Apparatus

These flashcards cover key concepts and procedures related to melting-point methods and apparatus as discussed in the lecture notes.

Melting Point (mp)

A small, commercially available glass tube with one sealed end, used to hold a small solid sample for melting-point determination.

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Term: Sample Preparation (Melting Point)
Definition: Placing 2–3 mm of solid into a capillary tube and packing it firmly by dropping the tube through a larger 6- to 8-mm tube.

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Term: Heating Rate for Melting Point Measurement
Definition: Approximately 1−2°C/min1−2°C/min for accurate measurements, allowing thermal equilibrium between the sample and thermometer. A rapid assessment can be done at a higher rate initially.

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Term: Onset of Melting
Definition: The temperature at which the first minute droplet of liquid forms in the capillary tube.

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Term: Melting Range
Definition: The temperature range from the appearance of the first liquid droplet to the complete liquefaction of the last solid.

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Term: Thiele Tube
Definition: A basic melting-point apparatus that uses convection currents in a heating liquid (e.g., silicone oil) to distribute heat evenly without stirring. Heat is applied via a microburner.

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Term: Electric Melting-Point Devices
Definition: Modern apparatus that use either silicone oil baths with motor-driven stirrers or heated metal blocks to provide uniform heat, often with magnifying lenses and digital temperature readouts.

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Term: Physical Constants
Definition: Precise numerical values associated with measurable properties of compounds (e.g., melting point, boiling point, density) crucial for identification and characterization.

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Term: Effect of Impurities on Melting Point
Definition: Impurities cause a depression of the melting point and broaden the observed temperature range. This phenomenon is used for identification (mixed melting point determination).

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Term: Mixed Melting Point Determination x
Definition: Mixing an unknown compound with a known standard. If the melting point remains unchanged, the compounds are identical; if it's lowered and broadened, they are different.

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Term: Safety Precautions (Melting Point)
Definition: Wear safety glasses/goggles and gloves, keep flammable solvents away from flames, ensure heating oils are water-free, avoid skin contact with chemicals, and handle hot Thiele tubes with care.

The temperature at which a substance transitions from a solid to a liquid phase. For pure substances, it's a narrow range where solid and liquid phases exist in equilibrium.

Capillary Tube

A small, commercially available glass tube with one sealed end, used to hold a small solid sample for melting-point determination.

Sample Preparation (Melting Point)

The melting point was determined using a calibrated thermometer.

What safety precautions should be taken when using a burner in melting point experiments?

Keep flammable solvents away, turn off the burner when not in use, and monitor for oil contamination.