Based on this, which of the reactions below are double-replacement reactions?
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
2 AgNO3(aq) + Zn(8) + 2Ag(s) + Zn(NO3)2(aq)
Pb(NO3)2(aq) + 2KI(aq) + 2KNO3(aq) + Pb12(s)
BaCl2(aq) + Na2SO4 (aq) + BaSO4(s) + 2NaCl(aq)

BaCl2(aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl(aq)

Explanation:

Chemical reactions are of different types namely; combination reaction, displacement reaction, double displacement reaction etc. Double displacement/replacement reaction is that reaction in which an exchange of ions occur between two reacting ionic compounds to form new products.

In this chemical reaction/equation;

BaCl2(aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl(aq)

2Cl- and SO42- are the ions exchanged in this reaction. Barium (Ba) displaces SO42-, while Sodium (Na) displaces 2Cl-, hence it is called DOUBLE DISPLACEMENT OR REPLACEMENT because the displacement involves two compounds/ions.

one negative thing it that it can kill water animals. another negative thing is it can cause the destruction of ecosystem. one way water pollution can be reduced is to conserve energy.