Two drums are both liquids: one labelled Class 8 (acid) and one labelled as a cyanide (toxic, with a known acid incompatibility). What should the driver conclude?

1. All liquids may always be mixed because they share a physical state
2. Physical state is not enough; acids and cyanides can generate hydrogen cyanide and are a classic prohibited combination — check 7.5.2 and special provisions and refuse if prohibited
3. Mixing them cancels both labels
4. They may be mixed if the acid drum is placed on top of the cyanide drum