Melting/Freezing Point

Detailed Description

The melting point and freezing point are closely related physical properties that describe the temperature at which a substance changes state between solid and liquid phases under standard pressure conditions (typically 101.3 kPa or 1 atmosphere).

Melting point: The temperature at which a solid substance changes to a liquid state. At this temperature, the solid and liquid phases exist in equilibrium.
Freezing point: The temperature at which a liquid substance changes to a solid state. For pure substances, the freezing point is theoretically identical to the melting point, though in practice, supercooling effects can cause differences.

These properties are influenced by several factors:

Molecular structure: The arrangement and bonding of molecules affect the energy required to break the solid structure.
Intermolecular forces: Stronger forces between molecules generally result in higher melting/freezing points.
Purity: Impurities typically lower the melting point and create a melting range rather than a sharp melting point.
Pressure: Changes in pressure can affect the melting/freezing point, though this effect is usually small for solids and liquids.
Crystalline structure: Different polymorphs of the same substance can have different melting points.

Importance in Safety Data Sheets

The melting/freezing point of a substance is significant in an SDS for several reasons:

Physical state determination: Helps predict the physical state of a substance at ambient and process temperatures.
Storage requirements: Informs appropriate storage temperature conditions to maintain the desired physical state.
Handling procedures: Affects handling requirements, especially for substances that may solidify or melt during normal use.
Transportation considerations: Important for determining appropriate transportation conditions and potential phase changes during shipping.
Process safety: Critical for designing safe processes, especially those involving heating or cooling of materials.
Identification: Can serve as a characteristic property for substance identification and purity assessment.

Measurement Methods

Several techniques are used to determine melting and freezing points:

Method	Description	Typical Applications
Capillary Method	Sample is placed in a thin glass capillary tube and heated slowly until melting is observed	Pure crystalline solids, pharmaceuticals, organic compounds
Differential Scanning Calorimetry (DSC)	Measures heat flow associated with phase transitions as a function of temperature	Complex mixtures, polymers, amorphous materials
Thiele Tube Method	Sample in a capillary is heated in an oil bath until melting occurs	Laboratory determination for pure compounds
Cooling Curve Method	Temperature is monitored as a liquid cools, with a plateau indicating the freezing point	Freezing point determination, cryoscopic measurements
Automated Melting Point Apparatus	Digital instruments that heat samples at controlled rates and detect melting optically	Quality control, pharmaceutical analysis
Kofler Hot Stage Microscopy	Sample is observed under a microscope while being heated on a temperature-controlled stage	Visual observation of phase transitions, polymorphism studies

Melting/Freezing Point Ranges for Common Chemical Classes

Chemical Class	Typical Melting/Freezing Point Range	Examples
Inorganic Salts	Generally high (300-800°C)	NaCl (801°C), CaCl₂ (772°C)
Metals	Varies widely (-39 to 3400°C)	Mercury (-39°C), Tungsten (3422°C)
Small Organic Molecules	Low to moderate (-100 to 300°C)	Benzene (5.5°C), Naphthalene (80°C)
Carboxylic Acids	Moderate (0 to 150°C)	Acetic acid (16.6°C), Benzoic acid (122°C)
Alcohols	Low to moderate (-130 to 150°C)	Ethanol (-114°C), 1-Octanol (-16°C)
Polymers	Often exhibit glass transition rather than sharp melting	Polyethylene (115-135°C), Nylon-6,6 (250-260°C)
Ionic Liquids	Very low (below 100°C by definition)	[BMIM][PF₆] (10°C), [EMIM][BF₄] (15°C)
Eutectic Mixtures	Lower than constituent components	NaCl/H₂O eutectic (-21.1°C)

Special Considerations

Melting Range vs. Melting Point

Many substances, especially impure compounds or mixtures, exhibit a melting range rather than a sharp melting point. The melting range is typically reported as:

Initial melting temperature: Temperature at which the first liquid appears
Final melting temperature: Temperature at which the entire sample has melted

A narrow melting range generally indicates high purity, while a broad range suggests impurities or a mixture.

Depression of Freezing Point

The addition of solutes to a pure liquid typically lowers its freezing point. This colligative property is described by the equation:

ΔTf = Kf × m

Where:

ΔTf = freezing point depression
Kf = cryoscopic constant (specific to the solvent)
m = molality of the solution (moles of solute per kg of solvent)

Supercooling

Supercooling occurs when a liquid is cooled below its freezing point without solidification. This metastable state can persist until disturbed by agitation, seeding with crystals, or further cooling. Supercooling can affect the accurate determination of freezing points.

Polymorphism

Some substances can exist in multiple crystalline forms (polymorphs) with different melting points. The most stable polymorph typically has the highest melting point. Polymorphism is particularly important in pharmaceuticals and can affect bioavailability and stability.

Safety Considerations

Understanding melting and freezing points is important for safety for several reasons:

Phase changes can affect containment: Expansion or contraction during freezing/melting can damage containers or create pressure.
Energy absorption/release: Melting and freezing involve latent heat, which can affect thermal stability during processing.
Viscosity changes: Substances near their melting/freezing point may have unpredictable flow properties.
Crystallization hazards: Sudden crystallization of supercooled liquids can be rapid and potentially hazardous.
Storage considerations: Materials stored near their melting/freezing point may undergo unintended phase changes with temperature fluctuations.

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), the melting point/freezing point should be indicated in Section 9 of the Safety Data Sheet as part of the description of basic physical and chemical properties. This information is considered mandatory for solids and liquids, though it may be reported as "not applicable" for gases or liquids with very low freezing points.

Best Practices

When reporting melting/freezing points in an SDS:

Specify the temperature in degrees Celsius (°C) and optionally in other units (e.g., Kelvin or Fahrenheit)
For substances with a melting range, provide both the initial and final temperatures
Indicate the pressure at which the measurement was made if different from standard pressure
Note if decomposition occurs before or during melting
For mixtures, consider reporting the freezing range or solidification range
Specify the test method used, especially for non-standard materials
For substances that do not exhibit a clear melting point (e.g., amorphous materials), report the glass transition temperature if relevant
Use appropriate qualifiers for approximate values (e.g., "approximately", "ca.", "~")