EDISI 17/XII/2016-2017 - (Grade 12) Water boils at 100oC at 1 atmospheric pressure. However, when we try to boil sea water at the same atmospheric pressure, it turns out that the boiling point is higher than 100oC. This happens because sea water contains salt, a solute which can increase the boiling point of water. This boiling point elevation is one of colligative properties.
Colligative properties are properties of solutions that depend on the concentration of solutes. There are 4 colligative properties that the students learnt this week through literature study, discussions, and experiments in the laboratory.
Solution is a mixture that is formed when solute dissolves in solvent. Water is the most common solvent. For example, NaCl solution means that NaCl solid dissolves in water. Adding non-volatile solute into a solvent will affect some properties of the solution. It will lower the vapor pressure, increase the boiling point, lower the freezing point, and increase the osmotic pressure.
During the lab session, the students learnt that adding salt into water will make the freezing point decrease. The more salt added, the lower the freezing point of the solution is and the colder it gets. By applying this phenomenon the students use a mixture of ice (H2Oin solid), water (H2O in liquid), and salt to make a freezer to make ice cream without using refrigerator. The students worked really careful in handling the freezer because it is so cold when it touches our skin for only couple of minutes that it might cause frostbite.
Besides using this phenomenon to make ice cream in the lab, they also used this phenomenon to identify the unknown sample given by determining its molar mass using the data taken during the experiments which are the freezing point of water and the freezing point of its solution (water plus sample). (Paulina)