Salt melts ice on roads in winter due to a few interrelated phenomena:

1. Freezing Point Depression: When salt (usually sodium chloride or rock salt) is spread onto ice or snow, it lowers the freezing point of the water. Water normally freezes at 0°C (32°F). However, when salt dissolves in water (or brine forms when salt comes into contact with ice or snow), it disrupts the bonding of water molecules, making it harder for them to form the crystalline structure necessary for ice. As a result, the mixture needs to reach a lower temperature before it can freeze again. The freezing point depression depends on the type and concentration of salt used, but typical road salt can lower the freezing point to around -10°C to -15°C (14°F to 5°F).
2. Chemical Reaction: Sodium chloride dissociates into Na+ and Cl- ions when it dissolves. These ions interfere with the arrangement of water molecules as they form ice crystals. Each dissolved salt ion disrupts the lattice structure of ice, requiring more energy (lower temperatures) for the water to freeze.
3. Melting and Deicing: As the salt begins to dissolve and create brine, it starts to melt the ice beneath it. The melted water then carries more salt to the surrounding ice, continuing the melting process. This is particularly effective in clearing thin layers of ice or preventing ice from forming.
4. Preventative Action: Salt is often used preemptively before snowfall or frost to prevent ice from bonding to the road surface. The salty layer prevents ice from adhering to the pavement, making it easier to remove snow and ice later.

However, it’s important to note that salt becomes less effective at extremely low temperatures. Road crews may use other materials like calcium chloride or magnesium chloride, which are more effective at lower temperatures, or a mix of salts and abrasives to combat ice and snow on roads. Also, excessive salt use can lead to environmental concerns such as soil and water contamination, so it’s crucial to apply it judiciously.