Salt lowers the freezing point and melting point of water/ice. Ice forms when water freezes and becomes 0 degree Celsius. When salt is added to ice, it dissolves into the liquid water within the ice and lowers that temperature and freezing point. “A 10-percent salt solution freezes at -6 Celsius, and a 20-percent solution freezes at -16 C.”
If one has ever watched salt melting ice, you can see the dissolving process happen, as the salt dissolves in the water and spreads out from the point first melted. Although, if the temperature of the roadway is lower than -9 Celsius or so, then the salt really won't have any affect the solid salt cannot get into the structure of the solid water to start the dissolving process.
Effect of Sand on Ice
Sand can melt ice if it is heated naturally from the sun or a warm temperature, just like all other substances that is at a higher temperature than ice. The actual texture of sand does not cause ice to melt. Sand, unless it is at a higher temperature than the ice, it would not melt.
Effect of Sugar on Ice
Fact: Sugar, and anything else that is able to dissolve in water, will melt ice. Like salt, sugar melts ice by lowering water's melting and freezing points. When sugar is added onto ice, it dissolves and interferes with the water molecules, which need to bind to ice crystals in order to freeze. Sugar’s presence needs the temperature to be much colder before the water molecules come into contact with ice crystals.
While sugar will melt ice, it still isn’t as effective as salt, which breaks down into sodium and chloride ions. When one molecule of salt dissolves, it adds two components to the solution, providing more interference in preventing water molecules from freezing.
Effect of Flour on Ice
“If you put flour in water, you make pancake batter which freezes at about the same temperature as water. So it won't melt ice at all.”
Conclusion: Sugar and salt lowers the freezing/melting pointing of water/ice,therefore making the ice melt. Although salt is more efficient when one molecule of salt dissolves,
it adds two components to the solution, providing more interference in preventing water molecules from freezing. Sand melts ice by providing traction and absorbing heat and being at a higher temperature than the ice, hence, melting it. Flour has no effect on ice, and may even serve as an insulator from the higher temperature that surrounds the ice.