Water i think would be the popular choice.
water
The most needed chemical in the world is water.
What the most useful machine is, is a matter of opinion. Some would argue that the wheel is the most useful machine, and others would argue that the lever is the most useful.
The most useful object in the world is a screw. not kidding
Google is very useful because it finds webpages for you.
spitfire
Sensors of varying nature can measure the physical and chemical changes in the Environment. At present, there are no instruments that can measure both physical and chemical factors.
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.
Most useful and more useful.
more useful, the most useful
For each useful and expensive chemical a specific method exist.
kid