Increasing boiling point without change in physical conditions

Question

Recently while going through the chapter calorimetry of class 10 I came across a passage saying "if we add impurities to water such as salt in proper proportion the boiling point of water increases."

How is this happening?

We know when a substance reaches its boiling point it evaporates. But why does it happen when salt is added to water or any other impurities are added to some other liquid, the boiling point of the liquid in mixture increases than the original boiling point of the liquid.

And had it been true then fractional distillation wouldn't have worked, when supplying different temperatures to a mixture, different constituent substances evaporate from the mixture and get separated out.

I want to know the explanation of the above told phenomenon within double quotes.

Thank you!!

Answer 1

This is called a colligative property of liquids, and the boiling point increase is dependent on the number of particles in the liquid per unit volume and independent of the molecular or ionic species of the particles. Much more information can be found at https://en.wikipedia.org/wiki/Colligative_properties. To answer the question of how a boiling point increase is seen, my best guess is either that the extra non-volatile particles in the mixture are impeding the evaporation of water at the surface by physically getting in the way, or those particles are hydrogen bonding with water molecules and making it harder for them to leave the surface.

Regarding fractional distillation, you can only supply one temperature to a liquid mixture. Each component in the mixture will boil out of that mixture such that the components with higher vapor pressure will be selectively enriched in the resulting vapor relative to components with lower vapor pressures. For mixtures of straight chain hydrocarbons, the relationship between the vapor composition and liquid composition is practically "ideal", allowing the use of K values, shown in https://en.wikipedia.org/wiki/Relative_volatility. For mixtures of things like non-polar molecules mixed with polar molecules, that relationship is much more complicated, and fugacity coefficients are used, as shown in https://en.wikipedia.org/wiki/Fugacity. Note that if colligative properties didn't exist, it's a "stretch" to think that fractional distillation wouldn't work, as fractional distillation depends on component vapor pressures and not on whether or not those vapor pressures are depressed by some non-volatile mixture component.

Answer 2

The key to boiling point elevation (and freezing point depression) is that the impurity stays in the liquid phase, where it happily remains dissolved, instead of boiling or freezing.

When you boil salty water, the salt doesn't boil (put another way, it's nonvolatile). Similarly, when you freeze salty water, almost none of the salt is trapped in the ice but is instead segregated from the growing crystal structure, which doesn't have room for it.

We now have the basis for a simplified explanation: We know that the concentration of water in impure water must be less than in pure water (where it's 100%, as it is in water vapor and ice). As a result, we have to increase the driving force for boiling by increasing the temperature to make up for this lower concentration. In other words, because there are fewer water molecules per unit, we have to give them an extra thermal kick for them to reach a vapor pressure of 1 atm, the threshold for boiling. (In contrast, we have to increase the driving force for freezing by decreasing the temperature.)

Here's a more technical explanation: A phase transformation occurs where the so-called chemical potentials of two phases are equal. (The chemical potential is like a generalized concentration that takes bonding into account—and just as with concentration, matter tends to shift to regions where its chemical potential is relatively low.) The chemical potential of water in impure water is lower than in pure water, so the liquid phase is favored over the solid and gas phases. This is equivalent to saying that the boiling point has been elevated and the freezing point depressed.

We can also show this effect schematically. Below is a schematic of the chemical potential vs. temperature for solid, liquid, and gas. The slope of each curve is the entropy of that phase (high for gases, low for solids), and the absolute-zero intercept is the enthalpy (slightly higher for liquids than solids because of the poor intermolecular bonding, much higher for gases because of the lack of such bonding). The lowest curve at any given temperature is always the equilibrium phase at that temperature. Note that if the liquid curve is dropped (because the water concentration decreases when you add impurities), the intersection with the pure solid curve moves to a lower temperature, and the intersection with the pure gas curve moves to a higher temperature. This is illustrating the same phenomenon of boiling point elevation and freezing point depression.