In winter, icy roads and sidewalks are sprinkled with salt to get rid of icy conditions and provide safe movement for pedestrians and various vehicles. It is salt that is used for a reason - there is a scientific explanation for this from the point of view of physics.
What is melting?
Melting is a physical process during which a substance passes from one state of aggregation to another, that is, from crystalline solid to liquid. For melting, certain conditions must be met. Each substance has its own melting point or a certain temperature range. In this case, ice or snow turns into water with the participation of salt. In a normal state and without external influence, the melting temperature of snow is 0 ℃.
Of great importance is the fact that a mixture of substances usually melts at a lower temperature than the same substances separately. For example, you can take a conventional metal A with a melting point of 500 ℃. If you add another metal to it to make some kind of alloy, then the melting point of this alloy will be already lower - for example, 480 ℃. A similar situation occurs with snow and salt.
Interesting fact: salt in an unusual way affects water and in the process of boiling. Fresh water boils at 100 ℃, but if salt is added and dissolved in it, the boiling point will increase. The exact degree depends on the concentration of salt in the water.For example, if the solution contains 20% salt, then it will boil at a temperature of 105 ℃.
How does snow melt with salt?
To understand this process, we can give as an example a simple experiment. It is necessary to collect half a glass of snow and put another handful on the plate, wait until the snow melts on the plate. If you put a glass on the formed water and measure the temperature inside it, then it will be close to 0 ℃.
Now you should pour a little salt into a glass, mix and immediately measure the temperature again. Surprisingly, the thermometer will show approximately -18 ℃. After a few minutes, the temperature will begin to rise, but the bottom of the glass will freeze to the plate. Why it happens?
When salt was added to the glass, the melting point of the ice dropped sharply. It can be about -21 ℃. To melt ice, a large amount of energy is required. It is taken from the environment and the mixture. In this example, a pool of water on the plank quickly froze. Thus, the temperature of the mixture of snow and salt is reduced.
It is this phenomenon that is used in ice. When ice is sprinkled with salt, a special layer is formed. The mixture of snow and salt melts as the freezing temperature of this layer decreases. Gradually, a film of water-salt solution appears, which expands and expands, thereby increasing the area of contact between ice and salt. This process lasts until all the ice has melted. Energy is taken from the surrounding air, therefore it is colder to stand on the resulting slurry of snow and salt.
If we consider this process at a more complex physical level, then the ice is covered with a thin layer of quasi-liquid water. It is formed due to very fast molecules breaking away from the main ice crystal. Quasi-liquid water hardens quickly at low temperatures. Salt helps to reduce the concentration of these molecules and prevents water from freezing.
Why is salt treatment dangerous for roads?
Despite the prevalence of this method in the fight against ice, it has a lot of disadvantages. When the snow, combined with salt, melts, a dirty liquid appears with a large number of chlorine compounds. These substances negatively affect surrounding plants, soil, as well as pedestrian shoes and vehicle tires.
For this reason, a set of rules has been established that must be followed when processing the coating with salt. In particular, it is compliance with the dosage. A maximum of 450 g of salt must be used per square meter of land. In industrial anti-icing products, sodium chloride is also used, but in addition to it, there are special additives in the composition that reduce the harm from salt.
A mixture of several substances melts at a lower temperature than the same substances taken separately. This principle works with salt and snow. Under normal conditions, snow begins to melt at 0 ℃. If mixed with salt, this process starts already at -18 ℃. The effect of a mixture of salt and ice is used to combat ice on the roads. But no more than 450 g of salt can be used per square meter of coating, since the resulting chloride compounds are harmful to the environment, shoes and car tires.
