Describe single-replacement and double-replacement reactions.

When a single replacement reaction takes place, the element that is separate to the compound would combine with the opposite element of the compound that its getting added to. 

AX + Y = AY +X

On the reactant side, y is the opposite of x. Let's suppose that the equation is 

2Ag + CuSO4 = Ag2SO4 +CU

Ag has a positive charge because it is a metal

SO4 is a negative ion, it's sulfate. They combine because they are opposite and combine to make a compound. Cu becomes separate. 

Double Replacement:

Double replacement based on the same concept. Except that the reactant has two compounds instead of one. The same concept applies, the positive element combines with the negative element on the seocnd compound. And the negative compound combines with the positive of the second.

For example: 

2NaI + CaCL2 = 2NaCl + CaI 

Na (positive) would combine with cl (the negative element in CaCL2)

I (negative) would combine with Ca (positive)