Ice melting is a physical process that occurs when heat energy breaks the hydrogen bonds holding water molecules in a solid state, allowing them to move more freely. This transition occurs at a temperature known as the melting point, which is 32 degrees Fahrenheit or 0 degrees Celsius (temperatures are measured differently on different scales). As water melts it cools the surrounding air and absorbs energy from its environment.
The process of ice melting is driven by the Second Law of Thermodynamics. As the ice absorbs heat energy it decreases the temperature of its surroundings, and this cooling helps to break the hydrogen bonds that hold water molecules in a solid structure. As the ice continues to melt, it releases that energy back into its surroundings, causing the temperature of its surroundings to increase again (see below diagram).
Various chemical compounds are used to help remove ice from roads and sidewalks during cold weather. Typically, they are applied before a snow or ice storm to prevent the formation of ice. These products differ in their melting temperatures, cost, and effects on natural (plants, animals, waterways) and manmade (doorways, grates) surfaces they come into contact with.
Rock salt is an economical choice but can corrode concrete and metal. Magnesium chloride is safer for the environment but is slow acting. Urea is slower still, but contains nitrogen and overuse can cause it to pollute water sources, contributing to algal blooms. Blended ice melts often combine the best attributes of each product to provide optimal performance without raising costs.