The reason why some water and heat are added after Mg ribbon is reacted completely with air is to remove Mg3N2 as MgO.
Combustion reaction is a reaction in which a substance is completely burnt in oxygen. Magnesium ribbon can be converted to magnesium oxide by combustion in air.
The reaction is as follows; 2Mg(s) + O2 ----> 2MgO(s)
Since air contains nitrogen, Mg3N2 is also formed. Heat and water can be added to the reaction thereby converting magnesium nitride to magnesium oxide.
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A food web is a system of interlocking and interdependent food chains.
A food chain is a hierarchical series of organisms each dependent on the next as a source of food.
Basically, the difference of the two is that a food web contains multiple different species in the same level that eat the animal below it's status. A food chain is one after the other.
I think the answer is codominant??