Every winter we have the same picture: trucks spreading sacks and sacks of salt on the road to prevent it from freezing, but for what reason?
What happens between salt (NaCl) and water (H2O) is pure chemistry . Popular belief says that salt melts ice, but that is not exactly what happens. To understand the process well, you have to know how water and salt molecules are formed.
Salt is made up of sodium chloride crystals , the same sodium atoms as chlorine, and they are held together by electrostatic force. Chlorine gives up an electron to sodium, leaving one with a positive and one negative charge. The same forces act on the water molecule, which although apparently neutral, its components do have a charge . Oxygen has a negative charge and the two hydrogen atoms are positive, therefore they are held together with oxygen. As is well known, opposite signs attract.
Hydrogen bonds
There is also another attractive force in water, known as ‘hydrogen bridges’, which are also the cause of some of its most interesting properties. It is the force of attraction between the oxygen in one molecule and the hydrogens in the other. When the temperature exceeds 0 degrees, the molecules move very fast, and these bonds do not have enough strength to join together completely, remaining in a liquid state. However, below zero, its speed decreases, forming hydrogen bonds and solidifying.
This is where the salt chemistry comes in. When it dissolves in water, sodium (positive) and chlorine (negative) ions remain floating, and one attracts hydrogen and the other one to oxygen, breaking the aforementioned hydrogen bonds . Thanks to this, the water does not solidify, and the resulting compound has a freezing temperature much lower than that of normal water, around -21ºC. In this way it is easier for the roads to remain ice-free during winter, as temperatures are not usually that low. This formula is more effective when there is still no ice, that is, it is convenient to anticipate frost, but if the forecast fails us, the remedy also works. It will take longer to take effect but the end will be exactly the same.
It would be an ideal remedy if it weren’t for the inconvenience to the environment . The salt that we throw on the road raises the levels of chlorine and sodium in the water, going to the earth, the substrate where plants live. In fact, in some countries it is forbidden to use salt to prevent freezing of the road, and it is even punished with significant fines. There are other, cleaner forms of antifreeze, such as potassium acetate or calcium-magnesium acetate, which are not harmful. Their problem lies in the high cost they represent compared to how cheap the use of salt is.
