Introduction
Freezing temperatures during winter months greatly contributes to hazardous conditions on American roadways. According to the Salt Institute (2012) 70% of roadways in the United States experience congestion due to winter weather. Icy roadways greatly contribute to hazardous winter conditions and can increase the chances of personal injury or accidental death for motorists. Currently, across the nation, icy roadways are addressed with the application of sodium chloride (salt) to deice icy roadways and to prevent the formation of ice on dry roadways.
The mechanism by which salt causes ice to melt and prevents ice from forming has been extensively debated. John Margrave, a chemistry professor at Rice University attributed salt’s ability to melt ice to the fact that when salt dissolves in water, the NaCl molecule of salt separates into positively charged sodium and negatively charged chlorine ions (2005). These ions, then, react with water molecules to form hydrated ions in a chemical reaction that releases heat in the process. It is this heat, Margrave argues, that causes ice to melt on roads after salt is applied. Arthur Pelton, a chemical engineering professor at the University of Montreal, agrees with the idea that heat is given off as chloride and sodium ions react with water, however, he argues that this heat is used and essentially reabsorbed when the salt is broken down into ions (2005). Overall, the process absorbs heat and has a net cooling effect which does not contribute to salt’s deicing properties. Instead, Pelton ascribes salt deicing properties to the idea that when salt dissolves in water, it lowers the freezing point of water below the temperature of the frozen ice and, thus, melting the ice. As the ice melts, more water is available to dissolve salt and more ice melts. This argument indicates that the concentration of salt directly influences the freezing point of water and a higher concentration of salt will result in a lower freezing point. A third explanation of how salt reduces icy conditions is provided by Chemical engineering professor Bob Wolke (2008). Wolke states that salt does not melt ice or reduce icy conditions through a freezing point phenomenon but instead he argues that salt shifts the equilibrium of solid state H20 (ice) to liquid state H2O (water). When salt dissolves in water the ions disrupt the ability of liquid state H20 to become part of the solid lattice needed for ice formation. This shifts the reaction towards liquid state H20. This argument indicates that the concentration of salt may not be as important as the mere present of salt.
While salt application is an effective way to deal with icy road conditions, there are a few negative effects of applying salt to roadways. One major problem with salt on the roadways is that salt can cause corrosive damage to vehicles. Salt related corrosion manifests as rust on automobiles that regularly come into contact with salt on the roadways. Rust on a car can be very expensive to repair. Another issue with salt application on the roadways is that high salt concentrations can hurt roadside vegetation and change the salt concentration of the rivers and streams into which melting ice and snow flows into. Too much salt in the soil can kill plants and too much salt in fresh waterways can be negatively affect aquatic life. It is clear that salt application is crucial to reducing hazardous conditions on roadways in the winter, however, it is important to examine whether low concentrations of salt which is better for cars and the environment is just as effective at reducing ice as high concentrations of salt. If low concentrations of salt are equally effective in deicing roadways, perhaps, transportation agencies throughout the nation can reduce the amount of salt they apply on roads in the winter and, in doing so, the damage to cars and the environment can be minimized.
The experiment in this study examines whether the concentration of salt (NaCl) in water affects the freezing rate of water. Two previous experiments clearly demonstrate that salt in water lowers the freezing point of the water. In one experiment conducted in 2005, Nadim Islam examined whether different substances including salt, sugar, vinegar and lemon juice effected the freezing point of water. He found that salt was the most effective in lowering the freezing point of water. In another experiment highlighted in All Science Fair Projects.com, the experimenter examined whether salt and sugar solutions could lower the freezing point of water. The experiments showed that both salt and sugar were effective at lowering the freezing point of water. In both experiments, the more salt that was added the lower the freezing point of water was, indicating that the freezing point is directly related to the salt concentration of the solution. We hypothesize that low salt concentrations are less effective as high concentrations of salt in preventing the freezing of water and that both low and high concentrations of salt prevent the freezing of water more effectively than no salt at all.
Methods
Three concentrations of salt water solutions were prepared. One solution contained only 100 ml of water (0% NaCl). The second solution contained low concentration of salt consisting of 10g of NaCl in 100ml of H2O (10% NaCl). The third solution contained a high concentration of salt consisting of 80g NaCl in 100 ml of H20 (80% NaCl). 100 milliliters of each solution was placed in 3 distinct Styrofoam cups. Cups were placed in the freezer. Every hour, solutions were observed in order to determine if any freezing occurred. The time (in hours) that it took for each solution to freeze into completely solid pieces of ice was recorded. This experiment was repeated 3 times. The average time it took to freeze the three solutions was calculated and reported in the results section. In this experiment, the independent variable is the salt concentration while the dependent variable is the time it takes for the solution to freeze and the controlled variable is the freezer temperature.
Results
All three solutions froze completely by the end of the experiment of each experiment. Table 1 shows the time it took for each solution to freeze during each experiment.
Discussion/Conclusion
The results of this experiment clearly demonstrated that the concentration of salt is directly related to the freezing properties of the solutions. Water alone froze first while the low concentration froze second and the high concentration of salt froze lastly indicating that higher salt concentrations are more effective at reducing ice formation. These results support my initial hypothesis that low and high salt concentrations are equally effective in reducing ice formation. In addition, both the low and high concentration of salt prevented ice formation better than no salt. As such, applying high concentrations of salt to roadways may be the most effective means of dealing with icy conditions in the winter despite the negative effects of high salt concentrations. In this experiment, I used very small and very large concentrations of salt and perhaps there are more intermediate salt concentrations that would freeze at a comparable rate to the high concentration salt solution. Future experiments should explore other salt solution concentrations. Experimental design is a key factor in the success of the scientific inquiry because it ensures that a scientific question is explored in a manner in which quantifiable data can be collected and valid comparisons between experimental groups can be made. If the experimental design does not include a means of collecting quantifiable data and only qualitative data is obtained, then clear comparisons between experimental groups cannot be made. A solid experimental design can be replicated easily by other researchers and their ability to reproduce similar results validates the experimental design as well as the experimental findings.
References
The effect of salt and water on the freezing point of water. Retrieved 01/16/2013, from http://www.all-science-fair-projects.com/print_project_1151_38
Pelton, A., Margrave, J. (2005). Why do we put salt on icy sidewalks in the winter? Scientific American, 293(6).
Salt Institute. (2012) Winter Road Safety. Retrieved 1/16/13 from http://www.saltinstitute.org/Uses-benefits/Winter-road-safety
Stop Freezing! Effects of Salt on Different Substances on the Freezing Point of Water. Retrieved 01/16/ 2013, from http://www.usc.edu/CSSF/History/2005/Projects/J0516.pdf
Wolke, B. (2008). Newscripts. Chemical and Engineering News, 86(13), 56-57.
