Melting point & Boiling point

Melting point :

The melting point of a substance is the specific temperature at which it changes from a solid state to a liquid state. At this temperature, the solid and liquid phases of the substance can coexist in equilibrium.

Key characteristics of the melting point:

• Characteristic Property: For pure crystalline substances, the melting point is a distinct and characteristic physical property. This means a pure sample of a given substance will always melt at the same temperature under standard pressure.

• Equilibrium: At the melting point, the solid and liquid forms of the substance are in dynamic equilibrium, meaning that melting and freezing are occurring at equal rates.                                                                                                                                                                                                                 Examples of Melting Points:

• Water (Ice): 0∘C (32∘F)

• Mercury (Hg): −38.83∘C 

• Ethanol (Alcohol): −114∘C

• Table Salt (Sodium Chloride, NaCl): 801∘C

• Aluminum (Al): 660.32∘C
• Copper (Cu): 1084.62∘C
• Gold (Au): 1064.18∘C
• Iron (Fe): 1538∘C
• Tungsten (W): 3422∘C (one of the highest melting points among metals)
• Carbon (Graphite): $>3527^{\circ }\text{C}$ 
• Naphthalene: 80∘CSucrose (Table Sugar): 186∘C

Boiling point:

The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, allowing the liquid to change into a gas (vapor) throughout its bulk, forming bubbles.

Key characteristics of the boiling point:

• Vapor Pressure Equivalence: Boiling occurs when the internal pressure created by the vapor of the liquid (vapor pressure) becomes equal to the external pressure acting on the liquid (usually atmospheric pressure).

• Bubble Formation: Unlike evaporation, which is a surface phenomenon, boiling involves the formation of vapor bubbles throughout the entire liquid. These bubbles then rise to the surface and release the vapor.

• Constant Temperature during Phase Change: Similar to melting, once a liquid reaches its boiling point, its temperature remains constant as long as boiling continues, even with the continued addition of heat. The added energy is used to overcome the intermolecular forces and convert the liquid into a gas, rather than increasing the temperature. This energy is known as the latent heat of vaporization.

Examples of Boiling Points:

• Water (H2O): 100∘C (212∘F)

• Ethanol (Ethyl Alcohol, C2H5OH): 78.37∘C

• Methanol (Methyl Alcohol, CH3OH): 64.7∘C

• Acetone (CH3COCH3): 56.08∘C

• Mercury (Hg): 356.7∘C

• Ammonia (NH3): −33.3∘C 

• Nitrogen (N2): −195.8∘C 

• Oxygen (O2): −183∘C

• Hydrogen (H2): −252.87∘C

• Chlorine (Cl2): −34.04∘C

• Benzene (C6H6): 80.1∘C

• Sulfuric Acid (H2SO4): 337∘C