Cobalt (II) chloride hydrated and dehydrated
Thermal decomposition of hydrated cobalt (II) chloride (CoCl2·6H2O) which has a pink color will produce the dehydrated form of cobalt (II) chloride which has a sky blue color (CoCl2). This reaction is reversible because if we add water to the dehydrated sky blue colored cobalt(II) chloride, it would return to its original pink color which is the hydrated cobalt(II) chloride (CoCl2·6H2O).
Iron (III) nitrate and potassium thiocyanate
When iron (III) nitrate [Fe(NO3)3] is mixed with potassium thiocyanate [KSCN] in solution, an equilibrium mixture of Fe+3, SCN– , and the complex ion FeSCN+2 is formed. This reaction is also an example of a reversible reaction in which addition of the ions involved would result in a color change of the solution.
Burning magnesium
Combustion reactions are generally irreversible like the burning of magnesium metal as it reacts with oxygen found in the air to form magnesium oxide. After it burns, it forms a white powder is very different from the magnesium metal.
Burning steel wool (ACSCH090)
Combustion or burning of steel wool is also an example of an irreversible reaction. Steel wool is actually mostly iron (Fe and if it reacts with oxygen in the air it will form iron oxide which cannot be transformed back to the original material.
