Question

which one of the following is not an example of a combination reaction a. 2H2O(I)---> 2H2(g)+O2(g) b. 2K+CI2--->2KCI c. 2H2+O2--->2H2O d. 2Mg+O2--->2MgO

Answer 1

The answer would be A.

Combination reaction, otherwise known as synthesis reaction, occurs when two or more substances react and form a single, more complex substance.

A combination reaction looks like this in an equation:

                                      A + B         →       AB
                                   reactants              product

So from two reactants they become one product. 
Let's take a look at choice A.

                                   2H₂O(l)       →      2H₂(g) + O₂(g)
                                  reactant                     product(s)

Notice that from one substance, it broke down into two substances. So this is not a combination reaction. This is what you call a decomposition reaction.