Final answer:
A dust fire can occur when fine dust particles are exposed to water due to their large surface area and ability to react quickly. An example is seen in coal mines where fine coal dust can explode when exposed to a spark.
Step-by-step explanation:
A dust fire can occur when fine dust particles are exposed to water. This is true because fine dust particles have a large surface area, which allows them to react more quickly with water and produce heat. When the heat reaches a certain temperature, it can ignite the surrounding dust particles, causing a fire. An example of this is seen in coal mines, where the fine coal dust in the air can explode when exposed to a spark. In modern coal mines, water sprinklers are used to reduce the coal dust and prevent explosions.