I believe the following reactions will occur and will result in the following manner:
Magnesium carbonate and dilute sulfuric acid.
MgCO3(s) + H2SO4(aq) --> MgSO4(aq) + H2O(l) + CO2(g)
Nickel sulfate solution and hydrogen sulfide gas.
NiSO4(aq) + H2S(g) --> NiS(s) + H2SO4(aq)
Mainly because metal sulphides are insoluble
Aluminium chloride solution and aqueous ammonia solution.
AlCl3(aq) + 3NH3(aq) + 3H2O(l) --> Al(OH)3(s) + 3NH4Cl(aq)
or AlCl3(aq) + 3NH3(aq) + 3H2O(l) --> Al(OH)4(aq) + NH4Cl(aq)
But I don't know which one would occur and if there will be a precipitate produced.
Lithium chloride solution and potassium nitrate solution.
LiCl(aq) + KNO3(aq) --> LiNO3(aq) + KCl(aq)
I am not sure if this would proceed since they are both aqueous, how could you tell the difference?