Hey, im a student from australia, i need to name 2 substances that can become hazardous if their chemical or physical state change and i need to know health risks associated with them. if someone can help i would appreciate it.

A good one is oxygen - if converted into ozone (thats O3 instead of O2) it becomes quite toxic to in inhale for humans.

an easy one, the incomplete combustion of CO2 in an engine gives off CO, a very harmful gas.

Try looking up HCN. Reactions with CN anion and a acidic substance is performed within a ventilated hood.

Carbon (14) this dosent cause (much) problems untill it breaks down into N and completly changes the properties of a cell

Nitro Glycerine would be the obvious one, change it`s chemical or physical state and you have a whole heap of trouble on your plate

 

you may also try a mix of Hydrogen and Chlorine, it`s perfectly safe as a mixed gas if kept in the dark, any sunlight (UV rays) and it will react quite violently.

 

there are many more such reactions, but here`s another 2 for you to work on

 

{edit} I hit the wrong key so had to come back and finish the post DOH!