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Reversible changes are those which can be reverted back by external means like change in temperature, pressure or some manual process or separation from a mixture.

These are also called physical changes.  Examples are melting of ice, water becoming vapour on heating, dissolving of salt in water, change in colour when light falls on substances, expansion of gas in a cylinder in to a room etc.  Some batteries are rechargeable, the chemical reaction taking place inside is reversible.   Fluorescent materials shine when light falls  on them.  The filament of a bulb turns red and radiates light when it is hot. 

Irreversible changes are those which cannot be reverted back. The original state and condition of substances cannot be obtained again.  These are also called chemical changes. 

Examples are burning of coal and production of CO2.  Mixing of hydrogen and oxygen to form water.  Often these are the changes involving formation of different compounds or substances from those we started with.