As we know a chemical reaction is a process by which one or more substances (chemical elements or their compounds) are converted into one or more different substances. The original substances are called reactants and the resulting substances or compounds are called products. A chemical reaction involves rearrangement of the atoms of the reactants. As a result of this chemical changes, the resulting products have different properties than the original reactants. It might be noted that many chemical reactions are non-reversible that is once the reactants have undergone the change to the products, we cannot reconvert the products back to the original reactant form.

Single Displacement reactions
Now that we understand chemical reactions, let us take a look at displacement reactions (also called replacement reactions). A single displacement reaction is a type of chemical reaction in which an atom or a group of atoms in a molecule are replaced by an atom from another element. The less reactive element in the compound is displaced by a more reactive element in a displacement reaction. One example of a displacement reaction is the reaction that occurs between iron nails and copper sulphate solution.

Single displacement reactions can be represented by the following equation:
A + BC → AC +B
Example: Magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas.
Mg + 2H2O → Mg(OH)2 + H2

Single displacement reaction examples
Let us take a look at some examples of single displacement reactions:

Experiment 1:
Take some copper sulphate solution. Copper sulphate or CuSO4 is a blue coloured solution. Now place some iron nails in this solution.  You will find that the color of the solution has changed from blue to light green. The ferrum in the iron nail reacts with the copper sulphate solution and copper is replaced by ferrous. As a result Ferrous sulphate or FeSO4 is formed. Ferrous sulphate is light green in colour.

You should note that in the above experiment, the iron can be in solid or aqueous state for the reaction to take place but the copper sulphate solution must be in an aqueous state.

Types of single replacement/ displacement reactions
Single replacement reactions are of two types namely
Single replacement reactions involving metals
Single replacement reactions involving nonmetals

Single replacement reactions involving metals
The products in a single-replacement reactions involving metals follow the rule,
A reactive metal in the activity series will displace the ion of any metal lower on the list in the activity series.

Single replacement reaction of Copper wire in a solution of silver nitrate. The chemical equation can be represented as:

Cu (s) + 2 AgNO3 (aq)  Cu(NO3)2 (aq) + 2 Ag (s)

Note that copper (Cu) is higher on the activity series than Silver (Ag) and therefore copper replaces silver ions in the above reaction and you will notice that silver is displaced on the copper wire.

Single replacement reactions involving nonmetals
Reactivity levels of different nonmetals is different. The products in a single-replacement reactions involving nonmetals follow the rule,
The more reactive non metal will displace the less reactive nonmetal in the chemical reaction.

Chlorine gas reacts with  aqueous sodium bromide, NaBr to produce Bromine. The chemical equation can be represented as:
Cl2 (g) + 2 NaBr (aq) 2 NaCl (aq) + Br2 (aq)

Note that in the above reaction, Cl2 replaces the bromide ion, Br- in NaBr to release Bromine. This is because chlorine is more reactive than Bromine.


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