Boiling Point

CLAIMED FOR EDITING BY SHREENU SIVAKUMAR

The Main Idea

Boiling point is a key property of matter in which the vapor pressure of a liquid equals the pressure around the liquid and the liquid turns into a vapor. The boiling point of a substance is highly dependent on the environment around the substance. For example, at a high pressure a liquid has a higher boiling point than it would have at atmospheric pressure. Similarly, at low pressure a liquid has a lower boiling point. Another environmental factor that affects the boiling point of a liquid is whether the liquid is in a partial vacuum. In this state, the boiling point of a liquid will be lower than the boiling point of the same liquid at atmospheric pressure. In addition, different liquids boil at different temperatures for a set pressure.

A Mathematical Model

There are several equations that relate to boiling point, including the Clausius–Clapeyron equation and the boiling point elevation equation.

Clausius-Clapeyron Equation

{\displaystyle T_{\text{B}}={\Bigg (}{\frac {1}{T_{0}}}-{\frac {R\,\ln {\frac {P}{P_{0}}}}{\Delta H_{\text{vap}}}}{\Bigg )}^{-1},}

T_{B} is the boiling point at the pressure of interest, {\displaystyle R} R is the ideal gas constant, {\displaystyle P} P is the vapour pressure of the liquid at the pressure of interest, {\displaystyle P_{0}} P_{0} is some pressure where the corresponding {\displaystyle T_{0}} T_{0} is known (usually data available at 1 atm or 100 kPa), {\displaystyle \Delta H_{\text{vap}}} {\displaystyle \Delta H_{\text{vap}}} is the heat of vaporization of the liquid, {\displaystyle T_{0}} T_{0} is the boiling temperature, {\displaystyle \ln } \ln is the natural logarithm.

Boiling Point Elevation Equation

The main mathematical model for this property is the boiling point elevation equation. This equation takes into account the effect that adding a solute to a solvent has on it's boiling point. For example ΔT = i K_bm, where ΔT is the temperature difference that arises from adding the solute, i is the van 't Hoff factor which is equivalent to the number of substances a molecule ionizes into (i.e NaCl is 2, sugar is 1, MgCl₂ is 3), K_b is a thermodynamic constant relating to the solvent, and m is the molality.

A Computational Model

Creating a computational model for this equation would be pretty easy, you would first have to initialize the constants, which would be i, K_b, and either m or the information that goes into calculating molality.

K = ???

m = moles of solute/mass of solvent

i = ???

ΔT = i*K*m

Examples

An example of an easy, middling and difficult problem are included in the link below. An easy example would be problems 3-5, a middling example would be problems 6, 8, 9, and 10. A difficult example would be the bonus problems.

Boiling Point Elevation

Connectedness

Boiling point in itself is very important in many every day processes and especially in my major (chemical engineering). It is a very important property that often helps to solve many problems about a system. One universal use for boiling point elevation is in cooking. Adding a solute such as salt to water that you are trying to boil will cause it to be hotter than it would be otherwise when the boiling point has not been elevated. A large amount of solute would be necessary to acquire an appreciable increase, however there is a very small increase no matter how much you use. Boiling point elevation is also used in sugar refining; at some points during the process the syrup is boiled and the temperature at which it boils depends on the concentration of sugar at that time.

History

In 1741, Anders Celsius defined his temperature scale on the melting and boiling temperature of water. Although Celsius did not discover the thermometer – both Philo and Hero of Alexandria (who also mentioned steam power in 50 BC) described such a principle – his design was much more precise than any previous such invention. Celsius scaled his measurements as 0 for boiling point and 100 for freezing point but the order was later reversed.

See also

For information on melting point, a very similar property, see Melting Point

Further reading

An article from Purdue:

Boiling

An article out of the Britannica Online Encyclopedia:

Boiling Point

External links

See Below

References

Uses of Boiling Point Elevation Boiling Point Elevation Chemistry Basics Melting Point, Freezing Point, Boiling Point Boiling Point of Water