This depends on which chemical you are referring to because different chemicals are beneficial in there own ways.
yes some chemical reactions are harmful to us. as digestion of food, photosynthesis, decomposition of organic waste are some examples of the useful chemical reactions.
For each useful and expensive chemical a specific method exist.
water
kid
making cake
Chemical stains make light microscopes more useful by showing specific structures in a cell.
So they can see if you are missing a chemical and to see if you are retainingthe chemical in your body.
energy
Chemical energy becomes useful to you when it is converted into a form of energy that your body can use, such as through the process of cellular respiration. This energy is then used to fuel various functions in your body, such as movement, digestion, and brain function.
chemical potential energy
Please provide supplier list of inhabiter chemical in mumbai only.
Some useful chemical changes are when a solid changes into a liquid. Such as when ice melts it is turned into water.Sorry but I am afraid that is wrong.Freezing, melting and evaporation including sublimation, i.e., Changes of State, are all physical, not chemical, changes.As for "useful" chemical changes.... Almost all biological processes, the conversion of fuel to its oxides etc with consequent release of heat &/or light, the list goes on!