Water i think would be the popular choice.
a book
The most needed chemical in the world is water.
give examples of physical and chemical change
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.
Helium.
acids
Yes,burning fuel is a useful chemical reaction as energy is released and can be converted into different forms to do useful work
spitfire
Chemical energy in gasoline becomes useful when we run our vehicles. In wood, it becomes useful when we burn it to heat our homes. In foods, it becomes useful when we eat the food.