Question

suppose you need to build a raft to cross a fast-moving river. describe the physical and chemical properties of the raft that world be important to ensure your safety

Answer 1

Physical: the raft needs to float, which means what you make the raft out of must be less dense than water. 
The raft would need sturdiness- if the raft is frail, the water might break it
Chemical: It can not react with water and it can not be made of a completely soluable substance (something that would dissolve in it like salt)