On which of the following heated surfaces of a flash type evaporator would you be more likely to find soft scale formation?

Correct Answer: Option A — Feed water heater internal tube surfaces.

Flash evaporators utilize feed water heaters where temperatures are highest. Soft scale, primarily calcium carbonate, precipitates most readily on these heated surfaces due to the inverse solubility of scale-forming salts as temperature increases, adhering to the internal tube surfaces before the water reaches the flash chamber.