Boiling point of liquids

The temperature at which a liquid’s vapor pressure is equal to the surrounding vapor pressure (exerted on a liquid equal to 760 torr at sea level) is called the boiling point (Bp). Under this condition, the liquid is converted to vapor. The reason behind it is that the bubbles formed have more internal pressure than the surface environmental pressure of liquids. The BP of every liquid is different because of different vapor pressures, which are due to the varying intermolecular forces between the liquid molecules.

The boiling point also varies within the same liquid depending on the external pressure. As the external pressure decreases, the boiling point decreases. For instance, water reaches its Bp at sea level at 100 oC but drops to 98 oC at Murree Hills because of the external pressure of 700 torr.

Mechanism of vapor formation at boiling point:

When heat raises the temperature of the liquid, the kinetic energy also increases. At the BP, the kinetic energy is at its maximum, and further heating does not increase the temperature. The heat is utilized to make vapors by breaking the intermolecular forces of liquids.

Molar heat of vaporization:

At the boiling point, the required heat to vaporize one mole of the liquid is termed the molar heat of vaporization. For instance, the molar heat of vaporization of water is 40.6 kJmol1.

Vapor pressure:

The substance’s tendency to convert into vapors with the rise of temperature is called vapor pressure, and the boiling point at the liquid surface is reached when the vapor pressure is equal to the pressure applied by the surrounding environment.

Saturated temperature and pressure:

Saturated temperature is known as the temperature at which liquids transfer into the vapor phase, and the other name for this process is boiling point. On the other hand, saturated pressure is known as the pressure at which liquids start to boil and convert into vapors. Also, a direct relationship is found between saturated temperature and pressure.

So, the boiling occurs at a saturated temperature and pressure.

Boiling and Evaporation:

Evaporation is different from boiling. Any temperature can experience evaporation, Which is a surface phenomenon in which molecules at the liquid boundary leave as vapor as there is not sufficient liquid pressure to keep them on all sides. Boiling, in contrast, impacts all the liquid’s molecules, not just those on the surface. Bubbles develop when the liquid’s molecules transform into vapor.

Liquids having different boiling points:

Liquids Boiling points

(oC)

Liquids Boiling points (oC)
Water 100 Ethanol 78.3
Pentane 36 Hexane 69
Heptane 99 Octane 126
Decane 174 Iso octane 99.3
Dodecane 216.4 Acetic acid anhydride 139
Alcohol 97-117 Aniline 184
Butyric acid 162 Carbonic acid 182
Ethylene bromide 131 Ethylene glycol 197
Dowtherm 258 Glycerin 290
Iodine 184 Jet fuel 163
Nitrobenzene 210 Nonane-n 150
Kerosene 150-300 Mercury (Hg) 359
Naphthalene 218 Nitric acid 120
Petroleum 210 Propionic acid 141
Phenol 182 Acetol 147
Octane-n 125 Olive oil 300
Glycolaldehyde 131 Acetic acid 118
Xylene-o 142 Turpentine 160
Toluene 110 Formic acid 101
Propanal 49 Propylene glycol 187

FAQs:

What is the boiling point for water?

100 °C at 760 °F.

What is the boiling point in Fahrenheit?

The water BP in Fahrenheit is 212 F at 760 Torr pressure.

What is the boiling point?

The temperature at which a liquid’s vapor pressure is equal to the surrounding vapor pressure (exerted on liquid equal to 760 torr at sea level) is called the boiling point (Bp).

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