How would you know if the material is harmful or useful to you ?

If the material is harful to you, you would know that it can bring danger to people and the environment. If the material is useful, it serves a purpose you see everyday.

Explanation:

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.

no starving yourself won't it will make you lose more weight what you need to do is find out what foods make you gain weight like bread and meats with fat on