The reactivity series is a list of elements placed in the order of their reactivity. The most reactive element is placed at the top and the reactivity decreases down the list. 

It would be ideal if you remember the following reactivity series. You would not be given any information of reactivity of elements in the exam. 

Lithium —————- Most reactive in this reactivity series
Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminium 
Zinc
Iron
Copper
Silver
Gold
Platinum —————- Least reactive in this reactivity series

Try making a sentence with the first letter of each element in the above reactivity series. It will help you remember the series. 


Displacement Reactions

A more reactive element will displace a less reactive element from a compound. 
More reactive elements have a greater tendency to lose electrons and form positive ions. 

Example:
Iron + Copper sulphate → Iron sulphate + Copper

You can see from the reactivity series that Iron is more reactive than Copper. If a compound containing Copper (such as copper sulphate) reacts with Iron, then the Iron will displace the Copper from the compound. 

This is known as a displacement reaction.