Why is brine solution consisting of a small fraction of salt plus water more effective in melting ice than just pure salt on an ice surface?
Because the brine solution is already in solution, whereas granular salt has to dissolve first. If Ice is present instead of water, this is difficult, as there is no liquid water for the salt to dissolve in. secondally, salt dissolution is endothermic, so will further reduce the temperature of the surroundings. For example, if you have an ice and water solution and add salt crystals you can bring the temperature of the ice/water combination to a much lower temperature; below -10°C!