Changes Around Us: Reversible, Irreversible, Physical, Chemical

Every second, everything in our environment changes. These changes can be seen at home, school, the park, or anywhere else. Changes Around Us that we commonly witness include our physical growth, changes in weather, changes in day and night, bursting of crackers, cooking of food, germination of seeds, growing plants, drying of garments, rusting of iron, and so on. All changes are brought about by a variety of factors. Because of the earth’s rotation, day turns into night, and weather changes as a result of the sun’s heat. Changes in shape, size, position and colour of items are caused by a variety of factors. Read the entire article to learn more about changes around us.

Classification of Changes

You’ll notice that not all changes are the same. Some changes happen slowly, while others happen quickly. Some are desired, while others are not; some are periodic, while others are not; some are reversible, while others are irreversible; some are physical, while others are chemical.

Reversible Change

The change in which the substances after the change can be reversed back to the original substance is called reversible change. To understand such a change, let us do the following activity:

Take a balloon and blow it. The shape of the balloon changes. Now, allow the air to escape from the balloon. The balloon goes back to its original shape.

We can conclude from the above explanation that the changes can be undone or reversed. Therefore, we can say that a reversible change is a temporary change.

Irreversible Change

When cement is mixed with water, we cannot get it back. A change that cannot be reversed is called an irreversible change. Examples of irreversible changes are burning of paper and wood, weathering of rocks, rusting of iron, etc. An irreversible change is a permanent change.

When rice is cooked, it becomes soft and cannot be reversed to its original state. In the same way, the wood cannot be brought back once it is burnt.

In these cases, the change is irreversible and cannot obtain the substances.

Physical Change

A change in which shape, size, appearance or state of the substance alter or change is called a physical change. In this change, no new substance is formed. A physical change is usually reversible and temporary.

Here are some more examples of physical changes.

Kicking a football: When you kick a football, its position changes, but it remains a football.

Crumpling of paper: On crumpling, a piece of paper changes its shape and size, but it can be changed back to its original shape by unfolding. So, no new substance is formed.

Some physical changes, such as tearing of paper or changing the size of a pencil with use, cannot be reversed.

However, most physical changes are temporary and can be reversed like melting ice, blowing of balloon orzing of water. So, most physical changes are reversible.

Chemical Change

The change in which new substances are formed is called a chemical change. In this change, a new substance with different properties is formed. The molecules of the original substance undergo changes to form molecules of new substances. Here are some examples of chemical changes.

Burning of paper: When we burn a piece of paper, smoke, carbon dioxide, water vapour and ash are given out. We cannot get back the original form of paper.

Slow and Fast Changes

We can also classify changes according to the time taken by the change to take place.

Slow Change

A change that takes much time to complete is called slow change. For example, turning of milk to curds, rusting of iron, digestion of food in our body, growth of living organisms, photosynthesis, i.e., preparation of food by plants, etc., are slow changes.

Fast Change

A change that takes less time to occur is called fast change. For example, burning of paper, bursting of crackers, melting ice, etc., are fast changes.

Desirable Change

A change that is useful to us is called desirable change. It may be natural or human-made. For example, change of season is a desirable change. This is very important for the growth of crops. Germination of seeds, ripening of fruits, formation of curds, preservation of fruits and vegetables by adding chemicals called preservatives are some examples of desirable changes.

Undesirable Change

A change that is harmful to humanity or the environment is called undesirable change. For example, rusting of iron—when iron articles are exposed to moisture and air, they get rusted. It is an undesirable change. Some more examples of undesirable changes are:

1. Flooding of rivers: This causes great harm to people, cattle, property and erodes the topsoil layer.
2. Spoilage of items: This is due to the action of microorganisms on food items.

Periodic Changes

Changes that repeat themselves after a regular period of time are called periodic changes. For example, change of season, changing phases of the moon, swinging of pendulum in a clock, etc., are periodic changes.

Non-periodic Changes

Changes that do not occur at regular intervals of time are called non-periodic changes. For example, the eruption of volcanoes, earthquakes, forest fires, etc., are non-periodic changes.

Causes of Changes

We have studied many examples of changes. Let us know how these changes take place. Some ways by which we can bring about changes are heating, cooling, applying pressure and mixing. Let us study them in detail.

1. Changes Caused by Heating or Cooling

When we heat an object, it is affected in one or another way. Heat affects various things in the following ways:

(a) Certain Things Become Hot on Heating and Cold on Cooling:

1. When we heat water, the water temperature rises, and it becomes hot. Similarly, when we keep water in a refrigerator for some time, the water temperature falls, and it becomes cold.

The process of heating and cooling goes on in nature as well. The days are warm due to the heat of the sun. The nights are cool as there is no heat. Land breeze and sea breeze are also caused by the heating and cooling of land and seawater/ocean water.

(b) Certain Things Expand on Heating and Contract on Cooling:

Many materials expand (i.e., increase in size) on heating and contract (i.e., decrease in size) on cooling. For example, metals like aluminium and iron expand on heating and contract on cooling. Let us understand this by some examples

(i) Fixing of the Iron Rim on a Bullock-cart Wheel:

Bullock-carts have wooden wheels. The heating and cooling process is also used in fixing the iron rim on the bullock cartwheel. Bullock carts have wooden wheels which are fitted with iron rims. The iron rim is made slightly smaller than the diameter of the wooden wheel. It is heated so that it becomes slightly larger in size, i.e., expands. The hot iron rim is then fixed on the wooden wheel and then cooled quickly by pouring water on it. On cooling, the iron rim contracts and fixes firmly on the wooden wheel.

(ii) Inserting of Iron Blade:

Tools such as spades and ploughs are used for digging the soil. The iron blade of these tools has a ring (oval-shaped hole) in which the wooden handle is fixed. Normally, the ring is made slightly smaller in size than the wooden handle. The ironsmith heats this ring of the iron blade over dry dung cake fire. On heating, the ring expands and becomes somewhat bigger in size. Now, the wooden handle easily fits into the hot ring. Then, water is poured over the hot ring to cool it. When the ring cools down, it contracts and fits tightly on the wooden handle.

(c) Certain Things Change their State:

We know that heating and cooling can change the state of matter. For example, on heating, ice (solid-state) changes to water (liquid state) and water (liquid state) changes to steam (gaseous state). Similarly, on cooling, steam (gaseous state) changes to water (liquid state), and water (liquid state) changes to ice (solid-state).

(d) Certain Things begin to Burn on Heating:

When a combustible substance (for example, candle, paper, coal and kerosene) is heated, it starts burning. When we burn paper, it changes into ash.

2. Changes Caused by Pressure

Pressure can cause many kinds of changes. Some important ones are discussed below:

a. Change in Shape and Size:

Blow up a balloon. Observe the change in shape and size of the balloon. Now, let the air escape from the balloon. The balloon deflates, and the shape and size change again. Similarly, when we press soft solids (like a cushion, a loaf, a ball of wool or a sponge), their shape changes. When the pressure is released, they get back to their original shape.

b. Breaking Objects:

When pressure is applied to brittle substances (for example, glass, biscuits and coal), they break into pieces. It is an irreversible change.

3.  Changes Caused by Mixing

Many changes occur due to the mixing of substances. For example, dissolving salt in the water to obtain a salt solution. We know that how to separate salt from its solution in water. So, we can say that it is a reversible change. When quicklime is mixed with water, it is converted into slaked lime. It is an irreversible change.

Summary

Every day, changes take place all around us. Every change occurs due to some reason. Changes are important for our life. There are some factors, like, heating, pressure or mixing with other materials, that are responsible for the changes around us.

A physical change in which the physical property of a substance changes and no new substance is formed. A chemical change is which the chemical properties of a substance or substances change and form a new substance. In this article, we learned about the slow changes, fast changes, reversible and irreversible changes, periodic and non-periodic, desirable and undesirable changes and some causes of changes.

FAQs on Changes Around Us

The commonly asked questions about changes around us are answered here: