If a metal and metal solution react, the more reactive metal will displace the less reactive metal from solution.

If the metal in solution you start with is formed from a more reactive metal than the metal to be added, no reaction will occur.

You can now predict which metal will win by using the reactivity series of metals.

Note: This can be used as an alternative method of testing to put the reactivity series in the correct order.

Zinc nitrate   +   Lead   →   No reaction  

But

Lead nitrate   +   Zinc   →   Zinc nitrate   +   Lead  

Magnesium nitrate   +   Lead   →   No reaction  

But

Lead nitrate   +   Magnesium   →   Magnesium nitrate   +   Lead  

Lead nitrate   +   Silver   →   No reaction  
Pb(NO3)2   +   Ag   →   No reaction  

As no reaction occurs, lead must be more reactive than silver.

Lead nitrate   +   Copper   →   No reaction  
Pb(NO3)2   +   Cu   →   No reaction  

As no reaction occurs, lead must be more reactive than copper.

Lead nitrate   +   Zinc   →   Zinc nitrate   +   Lead  
Pb(NO3)2   +   Zn   →   Zn(NO3)2   +   Pb  

The zinc displaces the lead from solution to form zinc nitrate, so zinc must be more reactive than lead.

The zinc turns black as it becomes coated in a layer of lead that’s been displaced from the lead nitrate solution. 

Lead nitrate   +   Magnesium   →   Magnesium nitrate   +   Lead  
Pb(NO3)2   +   Mg   →   Mg(NO3)2   +   Pb  

The magnesium displaces the lead from solution to form magnesium nitrate, so magnesium must be more reactive than lead.

The magnesium turns black as it becomes coated in a layer of lead that’s been displaced from the lead nitrate solution.

Zinc sulphate   +   Magnesium   →   Magnesium sulphate   +   Zinc  
ZnSO4   +   Mg   →   MgSO4   +   Zn  

The more reactive magnesium displaces the less reactive zinc from the solution to form magnesium sulphate.

Magnesium sulphate   +   Zinc   →   No reaction  
MgSO4   +   Zn   →   No reaction  

The less reactive zinc will not react with the solution which contains the more reactive magnesium.