Acombustion reaction occurs when a substance, usually containing carbon, reacts with to produce energy in the form of heat and light.

The answer to this question is OXYGEN. Combustion is also called burning. Burning something can produce light, example of this is when we burn pile of wood it will give light in the form of flames. There are two types of combustion, this is the complete combustion and incomplete combustion. Complete combustion is when there is plenty of air that caused enough burning to a specific thing while incomplete combustion is when there is limited air. 

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !

clouds form

solid ice forms on leaves.

water droplets form on grass.