why does ice melt when salt is added
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. Salt is the primary component in most of today s ice melter products. Rock Salt, Calcium Chloride, Magnesium Chloride, and Potassium Chloride all contain a considerable amount of salt in them. If you live in a cold climate б with frequent snowfalls, then you have probably seen people spreading pure salt on sidewalks to melt the snow.
This practice has become quite common and you might wonder how the salt is able to melt ice and snow. Why does salt melt ice? In a nutshell, salt is a great ice melter because it causes freezing point depression. This means that salt helps in lowering the freezing point and, consequently, the melting point of water (the main component of snow and ice). In its pure state, water freezes at 0бC or 32бF. By using salt, that freezing point can be lowered which forces the ice to melt and prevents the water from freezing or re-freezing. It must be noted, however, that salt alone can t melt ice. It must first be combined with water to start the melting process. Fortunately, ice and snow are generally covered with a thin film of water. As salt touches this water, it starts to dissolve subsequently lowering the freezing point and melting the ice surrounding it. б It is because of this process б thatб most, if not all, ice melters today are made with some quantity of salt.
Perhaps you are also curious why ice melters also make use of different substances like calcium, potassium, and magnesium. These ingredients help in further decreasing freezing points. They also help in reducing tracking and the corrosive characteristics that salt naturally possesses. To give you an idea of how salt solutions work, here is a very short list: With 10% salt solution, water freezes at 20бF (-6бC) With 20% salt solution, water freezes at 2бF (-16бC) The reason why salt melts ice is quite simple. This melting power is the main reason salt will continue to be used for producing the majority of ice melters on the market today.
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