salt spread on icy roads because

It makes the ice melt.

Salt is spread on icy roads because it helps to melt the ice and snow, making the roads safer for driving. When salt comes into contact with ice or snow, it lowers the freezing point of water, causing the ice to start melting. This process is called freezing point depression.

To understand why salt is effective in melting ice, it's important to know how it works. Salt is made up of sodium chloride (NaCl) crystals. When these crystals are spread on icy roads, they dissolve in the thin layer of liquid water formed by the heat produced by passing cars and sunlight.

The dissolved sodium and chloride ions in the liquid water disrupt the structure of ice crystals by preventing water molecules from bonding to each other efficiently. This makes it harder for the ice to stay in its solid form and it starts to melt.

In addition to melting the ice, salt also helps to prevent the formation of new ice by creating a brine solution with a lower freezing point than water. Even when the temperature drops below the freezing point of water, the saltwater mixture remains in its liquid form, reducing the likelihood of ice formation. This enables the salt to keep the roads relatively clear and safer for driving.

It should be noted that while salt is effective in melting ice, there are some drawbacks to its usage. The excessive use of salt can have negative environmental impacts, such as contaminating bodies of water and damaging vegetation along roadsides. Therefore, it is important to use salt in moderation and explore alternative de-icing methods when possible.