Investigation on how dissolving chemicals in water changes the freezing point of a solution.
The higher amount of dissolving chemicals in water, the lower the freezing point of water.
Salt, also known as sodium chloride (CaCl2), lowers the freezing point of water. This process is called the freezing point depression. The freezing point depression can occur as long as there is a solution. Adding a solute, like salt, to a solvent, like water, lowers the freezing point of the solvent. However, the freezing point of the solvent varies in different conditions: the molality of the solution, the van't Hoff factor of the solute, and the molal freezing-point-depression constant of the solvent.
Molality, is defined as shown below:
Molality (moles/kg) = Moles (mol) of Solute/Kilograms (kg) of Solvent
Freezing point depression is a colligative property, a property that depends on the number of solute particles are in the solvent. The van't Hoff factor on the other hand, is how a molecule of a solute dissociates, or breaks apart, in the solvent. Covalent compounds, like sucrose (C12H22O11), do not dissociate in solution. These compounds have van't Hoff factors i = 1. Ionic compounds, like table salt (NaCl), dissociate when in solution. Table salt (NaCl) has a van't Hoff factor i = 2 because it dissociates into two ions in solution Na+ and Cl-. The third factor, the molal freezing-point-depression constant, Kf, is different for every solvent. It has units of (° C/m), and it tells indicates how much 1 mol of solute added to 1 kg of solvent will lower the solvent's freezing point. For pure water, Kf = 1.86° C/m. Combining these three factors—molality, m, van't Hoff factor, i, and molal freezing-point-depression constant, Kf—into an equation that predicts how much the freezing point of a solvent will decrease, ΔT, when a certain amount of solute is added.
The freezing point depression formula is defined as shown below:
Degrees Freezing Point is Depressed (° C) = Molal Freezing-Point-Depression Constant (° C/m)
× molality of solution (mol solute/kg solvent) × van't Hoff Factor (unitless)
In terms of symbols, the freezing point depression formula is stated below with what the symbols stands for:
ΔT = Kf m i
ΔT - freezing point in degrees Celsius (° C)
Kf - molal freezing point depression constant in degrees Celsius per molal (° C/m)
m - molality of the solution in moles per kilogram (mol/kg)
i - van't Hoff factor of the solute
The equation below shows the formula for the freezing point of a solution:
Solution Freezing Point (° C) = Solvent Freezing Point (° C) - Degrees Freezing Point is Depressed (° C)
In terms of symbols, the freezing point of a solution is as stated below:
Tn = Tf - ΔT
Tn - freezing point of the solution in degrees Celsius (° C)
Tf - freezing point of the solvent in degrees Celsius (° C)
ΔT i- freezing point depression in degrees Celsius (° C)
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MacQuade, J., et al. (1986). It's Getting Colder (Freezing Point Depression). Retrieved July 10, 2013, from http://www.woodrow.org/teachers/chemistry/institutes/1986/exp9.html.
Science Buddies Organization. (n.d.). Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water. Retrieved July 11, 2013 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/FoodSci_p013.shtml#summary.