Freezing Point Temperatures of Solutions

Problem Water freezes in normal conditions at exactly 0°C. Dissolved substances lower the freezing temperature of the solution based on the concentration of the solvent 1-molar glucose solution freezes at — 1.9°C, 2-molar glucose solution freezes at —3.8°C. One can state that the higher the concentration of a solution, the lower the freezing temperature. This fact can be used as an experimental introduction to the concept of ions. A one molar table salt solution does not deliver the expected value of— 1.9° C, but rather doubles the value of approximately —3.8°C. The analysis shows, that 11 of 1-molar NaCl solution contains 2 mol particles lmol sodium ions and lmol chloride ions. One can state that 1-molar sodium chloride solution is 2 molar with respect to the total number of ions. 

Material Glass bowl, test tubes, thermometer glucose, sodium chloride, ice. 

Procedure Prepare 1 molar solutions of glucose and sodium chloride, and add several milliliters in two test tubes. Place the solutions in a cold mixture of ice and table salt (— 15°C) and stir with a thermometer. For observational purposes, quickly remove the test tubes from the mixture and as soon as the first crystals appear, measure the temperature. 

Observation The glucose solution freezes at approximately —2°C, the sodium chloride solution at approximately —4°C. 

Tip 1 -molar calcium chloride solution (CaCl2) could be tested as a further example it freezes at approximately —6°C. 11 of the 1-molar solution contains 3 mol of ions, 1 mol calcium ions and 2 mol chloride ions. 

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