Introduction to Changes Around Us

Changes are a constant part of our world. Many changes are taking place around us all the time, often without us even noticing. These changes can be natural processes or actions performed by humans. We are surrounded by a vast variety of changes, from the food we cook to the clothes we wear.

Some changes happen on their own, as part of the natural cycle:

• Crops in the fields grow and change from season to season.
• Leaves fall from trees, change colour, and dry out.
• Flowers bloom and then wither away.
• In our own bodies, our nails and hair grow, we grow taller, and our weight increases.

We can also bring about many changes ourselves, without any magical powers. For instance, we can blow a balloon, fold paper, or cook food. To understand these countless changes better, it is helpful to group them based on their similarities and differences. One of the most fundamental ways to group changes is to determine if they can be reversed.

Reversible Changes

A reversible change is a change that can be undone, meaning the substance can be brought back to its original form or state without creating any new material. In these changes, the properties of the substance, such as its shape or size, may change temporarily.

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Examples of Reversible Changes

1. Inflating a Balloon

When we blow air into a balloon, its shape and size change. However, this is a temporary change. If we let the air escape, the balloon returns to its original shape and size. Since we can get the original balloon back, this change is reversible.

2. Folding a Piece of Paper

Taking a sheet of paper and folding it into a toy aeroplane changes its shape. We can have fun flying it, and afterwards, we can simply unfold the paper. The paper returns to its original sheet form. This is a reversible change.

3. Making a Ball from Dough

When we take some dough and roll it out to make a roti, we change its shape. If we are not happy with the shape, we can easily gather the rolled dough and form it back into a ball. The material (dough) remains the same, so this is a reversible change.

In all these examples, it is possible to get back to the original material with which we started. This is the key characteristic of a reversible change.

Irreversible Changes

An irreversible change is a permanent change that cannot be undone. In such changes, we cannot get the original substance back in its original form. Often, an irreversible change results in the formation of a new substance.

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Examples of Irreversible Changes

1. Bursting a Balloon

If we take an inflated balloon and prick it with a pointed tip, it bursts. The balloon is torn into pieces, and we cannot get the original balloon back from these pieces. Therefore, bursting a balloon is an irreversible change.

2. Cutting Paper

If we draw an aeroplane on a sheet of paper and then cut it out along its outline, we change the paper. We cannot join the cut pieces back together to form the original sheet of paper. This is an irreversible change.

3. Baking a Roti

After rolling a roti from a ball of dough, if we bake it on a tawa (griddle), the dough gets cooked. The baked roti cannot be changed back into the raw dough. A new substance has been formed through cooking, making it an irreversible change.

4. A Potter Baking a Pot

A potter shapes a lump of clay into a pot on a wheel. This shaping is a reversible change, as the pot can be turned back into a lump of clay. However, once the potter bakes the pot in an oven, the clay hardens and undergoes a permanent change. The baked pot cannot be converted back into the original soft clay. This baking process is irreversible.

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Table of Common Changes

The following table provides some common changes and classifies them as reversible or irreversible.

Change	Can be reversed? (Yes/No)
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 a coiled string	Yes
Bud to flower	No
Milk to paneer	No

Other Ways to Bring About Change

Besides folding, cutting, or blowing, there are other common ways to bring about changes in substances. One of the most significant ways is by heating or cooling them.

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Change by Expansion and Contraction

Many materials, especially metals, change their size when heated or cooled. This property is used in various practical applications.

• Expansion: When materials are heated, they become slightly larger in size. This is called expansion.
• Contraction: When materials are cooled, they shrink and become slightly smaller in size. This is called contraction.

Application: Fixing Handles and Rims

This principle is used by blacksmiths to fix wooden handles onto iron tools like those used for digging. The iron blade has a ring that is slightly smaller than the handle.

1. The ring is heated until it becomes red-hot. Due to heating, the ring expands and becomes larger.
2. The wooden handle is then easily fitted into the expanded ring.
3. As the ring cools down, it contracts and fits tightly onto the handle.

A similar process is used for fixing a metal rim onto the wooden wheel of a cart. The metal rim is heated to expand, fitted onto the wheel, and then cooled with water to contract and create a tight fit.

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Change of State

Heating and cooling can also cause a substance to change its state (solid, liquid, gas). These changes are generally reversible.

• Melting: The change from a solid to a liquid state upon heating. For example, ice (solid) melts to become water (liquid) when heated. This is a reversible change, as the water can be frozen back into ice by cooling.
• Evaporation/Boiling: The change from a liquid to a gaseous state upon heating. For example, water (liquid) boils to become water vapour (gas). This is reversible through condensation.
• Condensation: The change from a gaseous to a liquid state upon cooling. For example, water vapour (gas) turns back into liquid water when it is cooled.

Example: Burning a Candle

Burning a candle involves two types of changes simultaneously:

• Reversible Change: The heat from the flame melts the wax. This molten wax can solidify back into solid wax upon cooling. So, melting of wax is a reversible change.
• Irreversible Change: The wax near the wick burns to produce new substances like soot (black ash), carbon dioxide gas, and water vapour, along with heat and light. This burning is an irreversible change.

Changes Caused by Mixing Substances

Changes can also occur when two or more substances are mixed together. The nature of these changes can be either reversible or irreversible.

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Reversible Change by Mixing

Dissolving Salt in Water

In Chapter 4, we learned that salt dissolves in water. This is a change where the salt seems to disappear. However, this is a reversible change. As we learned in Chapter 5, we can recover the salt from the water by the process of evaporation, and we can recover the water by condensation. Since we can get both the salt and water back, this is a reversible change.

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Irreversible Changes by Mixing (Chemical Changes)

Sometimes, mixing substances or initiating a process leads to the formation of entirely new materials. These are irreversible changes, also known as chemical changes.

1. Burning an Incense Stick (Agarbatti)

When we light an incense stick, it burns to produce new materials: ash (a solid) and some gases (which we can identify by their pleasant smell). We cannot get the original incense stick back from the ash and gases. This is an irreversible change. The same applies to the matchstick used to light it.

2. Setting of Curd from Milk

To make curd (dahi), a small amount of existing curd is added to warm milk. The mixture is then left undisturbed for a few hours in a warm place. During this time, the milk transforms into curd. The curd is a new substance with a different taste, smell, and texture compared to milk. This change cannot be reversed; we cannot get the milk back from the curd. This is an irreversible change.

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Conclusion on Grouping Changes

We find that there are many ways to bring about a change in a substance, such as by heating it or mixing it with something else. The most fundamental way to group these changes is based on whether they can be reversed or not.

• Reversible Changes: Changes that can be undone (e.g., melting ice, dissolving salt).
• Irreversible Changes: Changes that are permanent and cannot be undone (e.g., burning paper, setting of curd).

Understanding this distinction is a key step towards learning more about physical and chemical changes in higher classes.

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