Which class includes materials that can cause spontaneous combustion?

The correct answer is Class five: oxidizers and organic peroxides. This classification is vital because oxidizers can provide additional oxygen to a reaction, which enhances the potential for combustion. Organic peroxides are particularly sensitive and can decompose violently under certain conditions, leading to the possibility of spontaneous combustion.

It is essential to understand that materials classified under this category may not only support combustion but can also engage in a highly exothermic reaction that can ignite nearby flammable materials without an external flame or spark. This characteristic makes handling and storage particularly hazardous.

In contrast, other classes do not typically include materials that can spontaneously combust. For instance, Class four focuses on solid materials that require an external ignition source but do not inherently possess the reactive properties that lead to spontaneous combustion. Class three encompasses flammable liquids, which need to be heated or exposed to a spark to ignite. Class two is concerned with gases, which can be flammable but do not directly cause spontaneous combustion as described in Class five materials.