Based on your knowledge of factors affecting the rate of reaction, why is there a danger of explosions in places such as flour mills and coal mines where there are large quantities of powdered, combustible materials? 

The large quantities of powdered, combustible materials in silos and coal mines would equate to a higher concentration of materials and technically, a higher surface area. Coincidentally, these places often harbor higher temperatures, increasing the collision frequency. With these three factors combined, there would be an increased reaction rate and this case, a quicker rate for the combustible materials to be released in the form explosions.

Cr(oh)3 -> cr(3+) + 3oh-