Boiling Point Elevation

A solution will boil at a higher temperature than the pure solvent. This is the colligative property called boiling point elevation.

The more solute dissolved, the greater the effect. An equation has been developed for this behavior. It is:

DT = K_b m

The temperature change from the pure to the solution is equal to two constants times the molality of the solution. The constant K_bis actually derived from several other constants and its derivation is covered in textbooks of introductory thermodynamics. The constant i will be discussed below. Here are some "molal boiling point elevation constants":

The units on the constant are degrees Celsius per molal (°C m¯¹). There are some variations on the theme you should also know:

1) K m¯¹ - the distance between a single Celsius degree and a Kelvin are the same.
2) °C kg mol¯¹ - this one takes molal (mol/kg) and brings the kg (which is in the denominator of the denominator) and brings it to the numerator.

This last one is very useful because it splits out the mol unit. We will be using this equation (or the freezing point) to calculate molecular weights. Keep in mind that the molecular weight unit is grams / mol. Another reminder: molal is moles solute over kg solvent.

Go to some Boiling Point Elevation problems

Freezing Point Depression

A solution will solidfy (freeze) at a lower temperature than the pure solvent. This is the colligative property called freezing point depression.

The more solute dissolved, the greater the effect. An equation has been developed for this behavior. It is:

DT = K_f m

The temperature change from the pure to the solution is equal to two constants times the molality of the solution. The constant K_f is actually derived from several other constants and its derivation is covered in textbooks of introductory thermodynamics. Here are some "molal freezing point depression constants":

The units discussion pertaining to K_b applies to K_f also.

Go to some Freezing Point Depression problems

An Important Point About the Boiling Point and Freezing Point of a Solution

Pure substances have true boiling points and freezing points, but solutions do not. For example, pure water has a boiling point of 100 °C and a freezing point of 0 °C. In boiling for example, as pure water vapor leaves the liquid, only pure water is left behind. Not so with a solution.

As a solution boils, if the solute is non-volatile, then only pure solvent enters the vapor phase. The solute stays behind (this is the meaning of non-volatile). However, the consequence is that the solution becomes more concentrated, hence its boiling point increases. If you were to plot the temperature change of a pure substance boiling versus time, the line would stay flat. With a solution, the line would tend to drift upward as the solution became more concentrated.

A non-volatile solute is one which stays in solution. The vapor that boils away is the pure solvent only. A volatile solute, on the other hand, boils away with the solvent.

Salt in water is an example of a non-volatile solute. Only water will boil away and, when dry, a white solid (the NaCl) remains. Hexane dissolved in pentane is an example of a volatile solute. The vapor will be a hexane-pentane mixture. However, here is something very interesting. The hexane-pentane percentages in the vapor will be DIFFERENT that the percentages of each in the solution. We will get into that.

One last thing that deserves a small mention is the concept of an azeotrope. This is a constant boiling mixture. What this means is that the mixture of the vapor coming from the boiling solution is the same as the mixture of the solution. The first occurence was reported by Dalton in 1802, but the word was not coined until 1911.

One example of a binary azeotrope is 4% (by weight) water and 96% ethyl alcohol. By the way, what this means is that you cannot produce pure, 100% alcohol (called absolute alcohol) by boiling. You must use some other means to get the last 4% out. It also means that absolute alcohol is hygroscopic - it absorbs water from the atmosphere.

The Handbook of Chemistry and Physics for 1992 lists the following:

Here is the composition of one quinary system. It boils at 76.5 ° C

Pretty exciting, eh?

Oh, by the way, the same lowering of the freezing (sometimes called solidification) point also happens with metal alloys such as solders. An alloy actually has a melting point below that of either of its parent metals. The ratio with the lowest point is called a "eutectic" alloy; a 63 parts tin to 37 parts lead electrical solder is one such eutectic mixture.