Question

A certain chemistry instructor felt that there were some "dry-labbers" in the chemistry course. ["Dry-labbers" think they know the answer to a lab experiment, and give their expected answer without doing the experiment--which isn't fair to the others in the lab!] So, all students in the lab were asked to determine the boiling point of pure water contained in a flask labeled "pure water." The "pure water" was actually a very concentrated calcium dichloride solution. Three students were caught not actually doing the experiment and given F's for their experiment. Explain how the chemistry instructor was able to catch the dishonest "dry-labbers."

Answer 1

For pure water, the boiling point is 100 degrees Celsius (212 Fahrenheit) at one atmosphere of pressure, and the melting point is 0 degrees Celsius (32 degrees Fahrenheit) at one atmosphere of pressure.

All students in the lab were asked to determine the boiling point of pure water contained in a flask labeled "pure water."

dry-labbers assume that ,they have to determine the boiling point of The "pure water".Being an chemistry student they knows the exact boiling point of the pure water. so they mention their answer as 100 degrees Celsius without doing an actual experiment.

But chemistry instructor put an very concentrated calcium dichloride solution in the flask which is labeled as "pure water".But due to addition of calcium dichloride boiling point of that solution is increased or elevated which is greater than 100 degrees Celsius.so that these Three students were caught not actually doing the experiment and the chemistry instructor was able to catch the dishonest "dry-labbers."

Explaination for boiling point elevation as follow :

Elevation in BP: This can be explained in terms of Vapour pressure and Chemical potential.

In vapor pressure terms, a liquid boils at the temperature when its vapor pressure equals the surrounding pressure. For the solvent, the presence of the solute decreases its vapor pressure by dilution. A non-volatile solute has a vapor pressure of zero, so the vapor pressure of the solution is less than the vapor pressure of the solvent. Thus, a higher temperature is needed for the vapor pressure to reach the surrounding pressure, and the boiling point is elevated.

In chemical potential terms, at the boiling point, the liquid phase and the gas (or vapor) phase have the same chemical potential (or vapor pressure) meaning that they are energetically equivalent. The chemical potential is dependent on the temperature, and at other temperatures either the liquid or the gas phase has a lower chemical potential and is more energetically favorable than the other phase. This means that when a non-volatile solute is added, the chemical potential of the solvent in the liquid phase is decreased by dilution, but the chemical potential of the solvent in the gas phase is not affected. This means in turn that the equilibrium between the liquid and gas phase is established at another temperature for a solution than a pure liquid, i.e., the boiling point is elevated.(Boiling-point elevation )