What type of deposit does scale leave on heating surfaces?

Scale typically leaves a porous deposit on heating surfaces. This is particularly relevant in the context of boilers and heat exchangers, where the high temperatures and the presence of minerals in water can lead to the formation of scale as a byproduct of evaporation and concentration of dissolved solids.

The porous nature of scale means it can trap air and moisture, which can adversely affect heat transfer efficiency by creating an insulating layer. This reduces the effectiveness of the heating surface, often leading to overheating or further damage if not addressed. In contrast, the other options imply different characteristics: a rigid surface suggests a hard, brittle formation unsuitable for the typical properties of scale, a smooth film would not effectively describe the bulkier presence of scale buildup, and a clear coating does not reflect the opaque and often cloudy appearance of mineral deposits seen in scaling phenomena. Therefore, the accurate depiction of scale on heating surfaces is as a porous deposit due to its impact on heat transfer and efficiency.