Can all changes always be reversed?

TYPES OF CHANGES 

(i)    Reversible and irreversible changes
    (a)  Reversible  change
    A change which can be reversed by the conditions is called a reversible change. 
    eg. Melting of ice, dissolving of salt in water,  drying of clothes etc.

(b)  Irreversible  change
 A change that cannot be reversed by reversing the condition is called an irreversible change.
 eg. Burning of paper, curdling of milk 

Activity–1
AIM : To find out weather the change can be reversed or not.
                                  Some common changes                 
                           Change             Can be Reversed          

        Raw egg to boiled egg                 Yes/No
        Batter to idli
        Wet clothes to dry clothes
        Woollen yarn to knitted sweater
        Grain to its flour
        Cold milk to hot milk
        Straight string to coil string
        Bud to flower
        Milk to paneer
        Cow dung to biogas
        Stretched rubber band to its normal size
        Solid ice cream to molten
    Some common changes are given in above table. We have to find that the changes given can be reversed or not. 

The answer of changes which can be reversed or not is given in Yes/no in the table below. 

                           Some common changes           
             Change                              Can be Reversed        

        Raw egg to boiled egg                                 No
        Batter to idli                                                 No
        Wet clothes to dry clothes                          Yes
        Woollen yarn to knitted sweater                 Yes
        Grain to its flour                                          No
        Cold milk to hot milk                                   Yes
        Straight string to coil string                         Yes
        Bud to flower                                               No
        Milk to paneer                                             No
        Cow dung to biogas                                    No
        Stretched rubber band to its normal size    Yes
        Solid ice cream to molten                           Yes

 

INTRODUCTION

Everything around us is undergoing a process of change. Our hair and nails keep growing. Leaves die and new leaves take their place. While some changes in our environment are temporary and can change back to their original positions, other changes are relatively permanent.

We can bring about a change in a substance by doing one or more of the following processes:

• Heating.
• Applying force.
• Mixing it with something else.

CHANGE

• Change is anything that becomes different.
• If something new is formed that means a change has occurred.
• For Example: - When we burn a piece of paper it will turn to ashes. This shows a big change has occurred as ashes are very different in colour, size or texture.

Changes caused by heating: When an object is heated, it gets affected in one or many possible ways.

• Some objects get hot but do not change in any other way.
• Some objects get hot and also expand in size.
• Some objects get hot and begin to bum.
• Some objects get hot and change their state.

Changes by applying pressure: When we apply force to an object,

• We can change its shape and size.
• Air can be compressed.
• Metals can be hammered into thin sheets.
• Elastic can be stretched.
• Cotton can be spun into thin threads.

Changes by mixing a substance with other: We can bring about a change in a substance by mixing it with another. For example, making solution by mixing water soluble substances in water.

Metals expand on heating and contract on cooling.

Chemical changes: These are the changes in which chemical properties of a substance change, and a new substance is formed. For example, cooking of food.

Physical changes: These are the changes in which only physical property of a substance changes and no new substance is formed.

Characteristics of physical changes:

• No new substances are formed.
• Products are identical to the reactants.
• These changes are reversible.

Characteristics of chemical changes:

• Properties of products are different from the properties of reactants.
• Most of the chemical changes are irreversible.
• These changes always result in energy changes.

Reversible changes: These are the changes that can be reversed. For example, stretching of rubber.

Irreversible changes: These are the changes which cannot be brought back to its original state. For example, burning of paper.

Natural changes: The changes which occur in nature on their own are called natural changes. For example, change of day and night, change of season.

Slow changes: The changes which take longer time to occur are called slow changes. For example, rusting of iron, tooth decay.

Contraction: A process in which an object becomes smaller or shrinks is called contraction.

Evaporation: A process in which liquid changes into vapour is called evaporation.

Expansion: A process in which an object becomes bigger in size, e.g., metals expand on heating.

Melting: A process in which a solid melts to become a liquid on heating is called melting.

Can all changes always be Reversed?

On this basis, changes around us can be classified into two broad categories:

1. Reversible changes
2. Irreversible changes

Reversible changes can be described as changes that can be reversed by reversing the action or changing the conditions. Example: freezing of water, rolling of a chapati from dough etc.

Irreversible changes can be described as changes that cannot be reversed even after bringing about changes in the conditions. Example: rusting of iron, cooking of vegetables etc.

Figure 1 Closing and opening of mimosa leaves represent a reversible change

Figure 2 Cooking of an egg into an omelette represents an irreversible change

Substances and materials usually undergo two major types of changes:

• Physical change: This represents a change not in the chemical identity but the physical form of a substance. When substances undergo a physical change, there is no formation of a new substance and more or less these changes can be reversed. Example: boiling of water and melting of ice represent reversible physical changes while growing of height is an irreversible physical change.

Figure 3 Physical Change

• Chemical change: This represents a change in the chemical identity of a substance. These are irreversible changes because the original substance gets converted into a new substance and cannot be brought back. Example: cooking of rice, burning of matchstick etc.

Figure 4 Chemical Change

Difference between physical and chemical changes:

Physical Change

Chemical Change

A change in matter which occurs without causing any change in the composition of the matter is known as physical change

While a chemical change is defined as the change in the chemical composition of matter

Usually, physical changes are reversible in nature

While chemical changes are often irreversible

No new products are formed when an object undergoes physical change

Chemical changes often lead to formation of new products

These changes have no impact on the molecular composition of the substance

Chemical changes have a direct impact on the chemical bonds and molecular composition of a substance