Introduction
Water is termed the ‘universal solvent’ since it is capable of dissolving a large variety of substances. These solutes can be broadly classified as ionic and molecular solutes, based on their behaviour in solution. Ionic solutes dissociate into their constituent ions when dissolved in water. An example of this class is sodium chloride (common salt). Molecular solutes are dissolved as uncharged molecules, like sucrose (sugar). The effect of ionic solutes on the boiling point of water is the focus of this project.
Boiling occurs when the vapour pressure above a liquid equals the pressure over it. Hence, the normal boiling point is the temperature at which the vapour pressure of the liquid equals the normal atmospheric pressure of 1 atm . Use of the standard atmospheric pressure (1 bar) yields the standard boiling point.
When a substance dissolves in water, its concentration is the same throughout the solution. The object of this study is to investigate the behaviour of solutions of non-volatile solutes. Hence, the vapour pressure above the water is only the vapour pressure of water (the solute does not contribute to vapour pressure). It is therefore expected that the vapour pressure of the solution is lower than that of pure water. Comparing this with the definition of the boiling point, it is expected that the boiling temperature of water would increase when something is dissolved in it. Moreover,
As mentioned earlier, ionic solutes dissociate in solution. When an ionic solute dissociates, the number of particles formed depends on the number of ionic sub-units within the species. Therefore, it is predicted that solutes that have a larger number of sub-units would have a higher boiling temperature than those with a few sub-units.
Hypotheses
The boiling point of a solution is higher than the boiling point of pure water.
Increasing the amount of solute increases the boiling point of the solution.
The boiling point depends on the number of sub-units in the molecular formula.
Experimental design
In Phase I of the experiment, samples of water (approximately 100 g each) are obtained and the following amounts of sodium chloride are dissolved: 0 g, 1 g, 5g, 10 g and 15 g. The concentration of sodium chloride in each sample is computed. The five samples of water are then heated to boiling (with an immersed thermometer/thermal probe) to determine the boiling temperature. The experiment is performed in triplicate to correct for experimental errors. The boiling temperature is plotted against the concentration of sodium chloride.
In Phase II of the experiment, the behaviours of three solutes with different numbers of sub-units are studied. The three solutes chosen for this study are sodium chloride, calcium chloride, and tri-sodium phosphate. Solutions are prepared as in Phase I (sodium phosphate solutions must be prepared at elevated temperatures) and their boiling points are measured in triplicate. The boiling temperatures are plotted against the concentration of solute.
Reference
