{"id":23782,"date":"2017-12-06T08:00:16","date_gmt":"2017-12-06T07:00:16","guid":{"rendered":"https:\/\/www.ferrovial.com\/blog\/?p=23782"},"modified":"2025-12-11T23:54:16","modified_gmt":"2025-12-11T22:54:16","slug":"why-is-salt-spread-on-roads-to-melt-ice","status":"publish","type":"post","link":"https:\/\/www.ferrovial.com\/blog\/en\/2017\/12\/why-is-salt-spread-on-roads-to-melt-ice\/","title":{"rendered":"Why is salt spread on roads to melt ice?"},"content":{"rendered":"

Although we have seen that there are roads of ice<\/a> on which it is possible to drive, for the most part drivers will try to avoid these improvised ice rinks. Therefore, when it snows<\/strong>, in addition to the snow ploughs opening a route over the white ground, we may quite often find workers shovelling around what from a distance might look like more snow.<\/p>\n

\"Man<\/a><\/p>\n

In fact, it is salt, or sodium chloride<\/strong>, that is spread over roads and paths. Lorries truck it in from nearby storage depots and, shortly after covering the road with it, the snow and ice starts to disappear, turning into water which drains away along the side of the road. Why is salt used to melt ice?<\/strong> How does salt melt the snow?<\/p>\n

What are salt and water made up of?<\/h2>\n

Some people think that salt, spread as dust or brine<\/strong> over the ice, is what melts it. As if the reaction between the two was exothermic (i.e., giving off heat), and thus melted the snow or ice to water again. Nothing of the sort happens, which is where the magic of chemistry lies: the temperature doesn\u2019t rise when you mix salt and water.<\/p>\n

On the one hand, we have salt or sodium chloride (NaCl)<\/strong>, a crystalline structure not very different from the stuff we keep at home for seasoning food, made up of sodium and chlorine atoms<\/strong>. Although we see it as dust (it\u2019s a crystal, but it fractures easily), seen through a microscope it has a grid-like structure in which sodium (Na) and chlorine (Cl) atoms are arranged in such a way that each is surrounded by six atoms of the other. Here\u2019s what it looks like in a mine, with impurities mixed in of course:<\/p>\n

\"Salt<\/a><\/p>\n

At an atomic level, the chlorine in salt has a negative charge (Cl–<\/sup>), while sodium has a positive charge (Na+<\/sup>). As there are the same number of atoms of both, the crystal or dust is neutral.<\/p>\n

And then we have our well-known water molecule<\/strong> (H2<\/sub>O)<\/strong>, which forms both ice and snow (less compacted ice). Although we are familiar with water, we\u2019re not too aware of the hydrogen bridges<\/strong> which form links between the oxygen of one water molecule and the hydrogen of a nearby water molecule, as shown below:<\/p>\n

\"Hydrogen<\/a><\/p>\n

And these are important, because it is precisely these hydrogen links that give water interesting properties, such as high boiling and melting points<\/strong> (100\u00baC and 0\u00baC respectively). The key to how salt melts ice is in this last figure, and we can test this out at home.<\/p>\n

Salt doesn\u2019t melt ice, but rather combines with it to lower its melting point<\/h2>\n

There\u2019s a very interesting experiment<\/strong> we can do at home. Take three bowls or similar sized containers (it\u2019s best if they are made of plastic or flexible), fill them with water and put them in the freezer. Once they have completely frozen, take them out and do the following:<\/p>\n