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The effects of salts on the freezing point of liquids
Introduction
Freezing refers to the process of substance changing from liquid to solid. This occurs when liquid molecules slows down, making their attraction occur in a way that they arrange themselves in a fixed position- solid (University, 2014). Therefore, this activity explores the concept of freezing point of liquids and how it is affected by impurities (salts). This helps in focusing on the interactions that takes place at molecular level of a solution as it freezes (Pedersen and Myers, 2010).
Problem:
What are effects of salts on freezing point of liquids?
Hypothesis:
Salts affect lowers the freezing point of liquids
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Data/ Discussion
In this experiment, the following materials are used; tape, marker; 2 cups; 2 ice cubes, timer and half tea spoon of salt. The tape and the marker were used to label the cups, “No salt” and “salt.” The ice cube is placed in each of the cup; salt was added in the cup marked “salt”. The procedure was done three times. The temperature of each cup and other physical observation was made after every 10 minutes for thirty minutes. The data obtained was recorded as shown in the table below (VanCleave, 2002):
| Time (minutes | Temperature recorded in salt cup | Temperature recorded in non-salt cup |
| Starting | 0.1º C | 0.1º C |
| 10 min | -3 C | 0.1º C |
| 20 min | – 4º C | 0.0º C |
| 30 min | – 4º C | 0.0º C |
| Average | – 3.5º C | 0.01º C |
After thirty minutes mark, it was observed that the cup containing salt begun to melt; whereas the other cup containing no salt consisted of completely frozen solid. The temperatures recorded indicated that the temperatures of the frozen ice reduced in cup containing salt. Whereas the average temperature of the frozen ice without salt is 0.1º C; the temperature for the cup containing salt lowered by 3-4 degrees Celsius lower that of the control experiment (VanCleave, 2002).
This can be concluded that salts lower the freezing point of water. The differences in observed temperature with theoretical temperature are attributable to differences in atmospheric pressure. However, the difference is negligible (Pedersen and Myers, 2010). This is because in low temperatures, water molecules slow down. These molecules move slow making the intermolecular attractions between the molecules become very strong, forming a lattice of water molecules which becomes ice. During the freezing temperature, the rate at which the molecules enter and leave is same (University, 2014).
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However, on addition of salt, the equilibrium is disrupted. This is because with the addition of salt, there is less water molecules, due to the interface of solute and solution. This implies that the solute particles block the interaction of the water molecules, thus, more water are leaving and less water are re-entering the solid phase. As the temperature continues to lower even further, the water molecules leaving the solid state slow down even further. This describes the concept of freezing point depression (Rawn and Ouellette, n.d.).
Freezing point depression refers to the process where addition of a salute to a solvent causes a decrease in the solvent freezing point. Other examples include alcohol in water and mixing of any impurity in a solvent. This is the phenomenon that causes the sea water (presence of impurities) to remain liquid even during temperatures below 0ºC (University, 2014).
This concept has many uses. For example, it is used to ensure that automobile radiator fluid does not freeze during winter. This is because the radiator fluid is a mixture of alcohol and water. Road salting during winter is also an application of freeze-point depression, as the salt lowers the freezing point of snow, making it melt; thus preventing the accumulation of snow during winter (Pedersen and Myers, 2010).
However, it is imperative to note that the maximum depression of freezing water of sodium chloride (commonly used) is -21ºC; therefore, for ambient temperatures that are lower, sodium chloride is ineffective. However, other salts such as calcium chloride, mixture of many and magnesium chloride can be used, as the phenomenon concept is the sale.
However, these other salts are corrosive, especially on iron, and in some places, safer salts should be used (Rawn and Ouellette, n.d.). Additionally, it is imperative to note that the exact depression of the freezing point depends on the amount of salt used. Literature indicates that 0.5 mol of NaCl lowers freezing point by 1.65º C, but the maximum depression temperature is -21º F, which is a approximately -6º C (Pedersen and Myers, 2010).
Conclusion
The study hypothesis is proven; salts lowers the temperature at which the fluid (water) freezes. The reason behind this is probably because the molecule interaction of pure water is different from the molecule interaction in presence of salt. This makes the salt water to have different chemical properties as the salt joins in the interface between the Hydrogen and Oxygen bond, making the interaction of these two molecules less strong (Rawn and Ouellette, n.d.).
References
Pedersen, S. and Myers, A. (2010). Understanding the principles of organic chemistry. Belmont, CA: Brooks/Cole Cengage Learning.
Rawn, J. and Ouellette, R. (n.d.). Organic chemistry.
University, O. (2014). Seawater. Jordan Hill: Elsevier Science.
VanCleave, J. (2002). Janice VanCleave’s help! my science project is due tomorrow!. New York: Wiley.
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