why do we use salt to melt ice

[ Editor's note: In his answer to this question, the late John Margrave argued that salt dissolves in water as ions of sodium and chlorine, and these ions hydrate, or join to, the water molecules. This process gives off heat, which thaws ice. A number of readers alerted us to problems with this explanation. Chemical engineering professor Arthur Pelton of the University of Montreal provided a representative correction. His explanation follows, and Margrave's original answer appears below that. Although the hydration process gives off heat, this is more than compensated for by the heat absorbed during the initial decomposition of the salt into ions. In other words, the total process of dissolution--decomposition into ions plus hydration--absorbs heat. This can easily be demonstrated: pour some water into a glass and test its temperature with your finger. Add some salt, stir, and test it again. The temperature will have decreased. The actual reason that the application of salt causes ice to melt is that a solution of water and dissolved salt has a lower freezing point than pure water. When added to ice, salt first dissolves in the film of liquid water that is always present on the surface, thereby lowering its freezing point below the ices temperature. Ice in contact with salty water therefore melts, creating more liquid water, which dissolves more salt, thereby causing more ice to melt, and so on.

The higher the concentration of dissolved salt, the lower its overall freezing point. There is a limit, however, to the amount of salt that can be dissolved in water. Water containing a maximum amount of dissolved salt has a freezing point of about zero degrees Fahrenheit. Therefore, the application of salt will not melt the ice on a sidewalk if the temperature is below zero degrees F. To understand why water containing dissolved salt has a lower freezing point than pure water, consider that when ice and water are in contact there is a dynamic exchange at the interface of the two phase states. Because of thermal vibrations in the ice, a large number of molecules per second become detached from its surface and enter into the water. During the same period of time, a large number of water molecules attach themselves to the surface of the ice and become part of the solid phase. At higher temperatures, the former rate is faster than the latter and the ice melts. At lower temperatures the reverse is true. At the freezing point the two rates are equal. If salt is dissolved in the water, the rate of detachment of the ice molecules is unaffected but the rate at which water molecules attach to the ice surface is decreased, mainly because the concentration of water molecules in the liquid (molecules per cubic centimeter) is lower.

Hence, the melting point is lower. John Margrave, a chemistry professor at Rice University, explains. All icy surfaces in fact contain small puddles of water. Because salt is soluble in water, salt applied to such surfaces dissolves. Liquid water has what is known as a high dielectric constant, which allows the ions in the salt (positively charged sodium and negatively charged chlorine) to separate. These ions, in turn, react with water molecules and hydratethat is, form hydrated ions (charged ions joined to water molecules). This process gives off heat, because hydrates are more stable than the individual ions. That energy then melts microscopic parts of the ice surface. Thus a substantial amount of salt spread over a large surface can actually thaw the ice. In addition, if you drive over the ice in your automobile, the pressure helps force the salt into the ice and more of this hydration occurs. The rock salt applied to icy roads in the winter is the same substance that comes out of your salt shaker. The only difference is the size. Rock salt is the material that has crystalized in larger pieces, whereas table salt has been ground up and pulverized to a more or less uniform size distribution.

Calcium chloride is just as commonly used to melt ice on the streets as sodium chloride is. In fact, it's cheaper than sodium chloride. Companies manufacture large amounts of calcium chloride from brines and other natural materials that can be used for the same purpose. Originally published on December 8, 2003.
More than 20 million tons of are used every year to melt snow and ice in cold northern regions. But how does salt do it? First, itБs important to understand a bit about H O in the winter. Thirty-two degrees Fahrenheit (0 degrees Celsius) is its Бthat is, when water reaches 32 бF, it turns into ice. At this temperature, your icy road generally has a thin layer of water on top of the ice, and the ice molecules and water molecules are interacting. This water is constantly melting some of the ice, while the ice beneath it is freezing some of the water. At this temperature, the exchange rate is pretty constant, meaning the amount of water and the amount of ice stay the same. If it gets colder, more water becomes ice. If it gets warmer, more ice becomes water. When the salt is added to the equation, it lowers the freezing point of the water, which means the ice on the ground canБt freeze that layer of water at 32 бF anymore.

The water, however, can still melt the ice at that temperature, which results in less ice on the roads. But you may be asking how salt lowers the freezing point of water. This concept is called Бfreezing point depression. Б Essentially, the salt makes it harder for the water molecules to bond together in their rigid structure. In water, salt is a solute, and it will break into its elements. So, if youБre using table salt, also known as (NaCl), to melt ice, the salt will dissolve into separate sodium and chloride ions. Often, however, cities use calcium chloride (CaCl ), another type of salt, on their icy streets. Calcium chloride is more effective at melting ice because it can break down into three ions instead of two: one calcium ion and two chloride ions. More ions mean more ions getting in the way of those rigid ice bonds. Unfortunately, chloride is superbad for the environment. It can kill aquatic animals, and that can thereby affect other animal populations in their food web. Chloride also dehydrates and kills plants and can alter soil composition, making it harder for vegetation to grow. While some other compounds that can melt ice and snow donБt include chloride, they are much more expensive than sodium chloride or calcium chloride.

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