What must water be combined with to effectively cool lighter petroleum distillates?

To effectively cool lighter petroleum distillates, water needs to be combined with foam additives. This combination enhances the water's ability to form a blanket of foam over the burning liquid, thereby preventing the release of flammable vapors and reducing the heat at the surface of the fire. Foam works by converting the water into a concentrated foam solution that can smother the flames and provide a cooling effect, making it particularly effective in fire suppression situations involving flammable liquids.

Foam additives are essential in this process as they help create stable bubbles that can maintain their structure when in contact with the burning liquid and provide a barrier that excludes oxygen—one of the elements needed for combustion. Therefore, combining water with foam additives specifically addresses the unique challenges posed by lighter petroleum distillates.