Reversible and Irreversible Reaction

Our Objective

To examine the nature of the following changes and whether they can be reversed or not; 

  1.  Disappearance of common salt on dissolving in water.  
  2. Cutting of potato 

 

The Theory 

Some changes are slow, while others are rapid; some are natural, while others are man-made. You will notice that many changes are occurring around us, such as the growth of plants on your balcony, a change in colour, and the drying of leaves. There are two types of changes, reversible changes, and irreversible changes. 

Reversible Process 

Reversible process is defined as the original form of a substance can be retained by any physical process. Reversible changes include melting, boiling, evaporation, freezing, condensation, and disintegration. Melting wax, freezing ice, and boiling water that evaporates as steam and condenses back to water are a few examples. 

 

Examples: 

  • Folding and unfolding a paper: Paper can be folded into various shapes and unfolded to its original form. 

 

  • Magnetizing and demagnetizing a magnet: A magnet can be magnetized by rubbing it with another magnet and demagnetized by heating it. 

  • Stretching of a rubber band: A rubber band can be stretched and return to its original shape when the force is removed. 

Reactions occur when two or more substances known as reactants interact to form a product (s). A double headed arrow (⇌) connects the reactants and products created in a reversible process. This indicates that reactants can be recovered from the products. 

Consider the reaction below, 

                                              W +X ⇌ Y + Z 

Here, W and X are the reactants which react to give Y and Z. The two-headed arrow indicates that the reaction is reversible and the reactants, W and X can be obtained from Y and Z.

Irreversible Process 

An irreversible process is one in which the system and its surroundings do not return to their original state once the process is initiated. For example, when we burn a piece of paper or wood, it turns to ash and smoke. We cannot obtain paper and wood from the ash again.  

Examples: 

  • Cutting a piece of fruit: Slicing a fruit changes its original structure, and it cannot return to its whole form. 

  • Rusting of iron: Once iron rusts, it undergoes a chemical change that cannot be reversed without significant intervention. 

  • Breaking a glass bottle: Once a glass bottle is broken, it cannot be restored to its original shape. 

When a reaction occurs in a single direction, it is referred to be an irreversible reaction. In such reactions, reactants totally react over time to generate a product. A one-way arrow (→) denotes response here. 

Consider the reaction below, 

                                                    X → Y +Z 

Here, X is the reactant which is completely converted into products Y and Z which do not react to form X.

Disappearance of common salt on dissolving in water 

Dissolving common salt in water is a physical change because the process can be reversed. If we boil the saltwater solution, the water will be evaporated, and the salt can be obtained. The evaporated water can be condensed to liquid form again. Hence, it is a reversible process. 

Cutting of potato 

On cutting the potato it was converted into pieces. However, there was no way to get back the material (potato) to its original shape and size. Hence, it is an irreversible process. 

 

Learning Outcomes  

  • Students will understand the concept of reversible or irreversible processes.
  • Students will be able to do the experiment in a real lab.