Today I want to say something more about aerosols (fine droplets of a liquid or fine particles of a solid in a gas such as air). Aerosols can react very differently from solid lumps of matter – because of their large surface area.
Imagine a cube of metal, 10 centimetres along each side. Each face of the cube is 100 square centimetres, and there are 6 faces, for a total surface area of 600 square centimetres. If we cut the cube into smaller cubes, each 1 centimetre along each side, there will be 1,000 small cubes each of 6 square centimetres, for a total of 6,000 square centimetres – 10 times the surface area of the large cube.
If the large cube is divided into mini-cubes each 1 millimetre along each side, there will be 1,000,000 mini-cubes each of 6 square millimetres, for a total of 6,000,000 square millimetres (60,000 square centimetres) – 100 times the surface area of the large cube. Similarly, micro-cubes each 0.1 millimetre along each side will have a surface area 1,000 times the surface area of the large cube; micro-cubes each 0.01 millimetre along each side will have a surface area 10,000 times the surface area of the large cube; and micro-cubes each 0.001 millimetre (1 micrometre) along each side will have a surface area 100,000 times the surface area of the large cube.
The large surface area formed by the many, many tiny particles in an aerosol mean that chemical and physical reactions that happen at the surface can happen very fast. In particular, an aerosol formed from a flammable solid or liquid can burn so fast that it explodes. Coal dust can explode that way, and so can flour. Over the centuries, a number of flour mills have exploded when an aerosol of flour was ignited by a spark – flour is surprisingly dangerous! The 1878 explosion of the enormous Washburn “A” Mill in Minneapolis is a famous example. The image below (by Hans-Peter Scholz) shows a flour aerosol exploding: