When an atom, ion, or molecule of a reactant switches places with an unpaired reactant, the type of reaction is called
decomposition reaction
double replacement reaction
single replacement reaction
synthesis reaction

When an atom, ion or molecule of a reactant switches places with an unpaired reactant, this type of reaction is called single replacement/displacement reaction. A single displacement reaction is one in which an element react with a compound and take the place of another element in that compound. For instance: A + XY = X + AY. In this chemical reaction, the element A has displaced X from XY forming a new compound AY.

Sorry i dont understand the question can you rephase it?

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged