The world around us is always changing. The sun rises and sets, the seasons change, milk turns into curd, ice melts into water, and a seed grows into a plant. Some changes happen so slowly that we barely notice them, while others happen quickly. In this chapter, we study the changes that take place around us and try to understand their nature.
Changes can be classified in many ways. Some changes can be reversed, and some cannot. Some changes form new substances, and some do not. Understanding these classifications helps us predict and control changes. For example, we know that iron rusts in the presence of moisture, so we protect iron objects by painting them.
This chapter introduces two important ideas: reversible and irreversible changes, and physical and chemical changes. These ideas form the foundation for understanding matter and reactions in higher classes. By observing changes around us, we learn to think like scientists.
A change is an event in which a substance is altered in some way. When we observe the world, we see that everything around us is undergoing change all the time. A change may involve a change in shape, size, colour, state, or composition of a substance.
Examples of changes all around us: - Paper is torn into pieces. - Water boils and becomes steam. - A candle burns and produces heat and light. - Food is cooked and becomes soft. - A seed grows into a seedling. - Wood is cut into pieces.
Some changes are useful to us, like cooking food and making paper from wood. Some changes are harmful, like rusting of iron and spoiling of food. Science helps us understand the reasons behind changes so that we can use the useful ones and prevent the harmful ones.
A change that can be reversed, or that can be changed back to its original condition, is called a reversible change. In a reversible change, no new substance is formed. The substance simply changes its shape, size, or state, and can be brought back to its original form.
Examples of reversible changes: - Melting of ice: Ice melts to form water, and water can be frozen back into ice. - Melting of wax: Wax melts when heated and becomes solid again when cooled. - Folding of paper: A folded paper can be unfolded back to its original shape. - Stretching of a rubber band: A stretched rubber band returns to its original shape when released. - Filling a balloon with air: The balloon can be deflated back to its original size.
In reversible changes, the composition of the substance remains the same. Only the arrangement or state of the substance changes. These changes are usually easy to bring back.
A change that cannot be reversed, or that cannot be changed back to its original condition, is called an irreversible change. In an irreversible change, a new substance is formed, and we cannot get back the original substance.
Examples of irreversible changes: - Burning of paper: When paper burns, it turns into ash and smoke. We cannot get the paper back from the ash. - Cooking of food: When raw food is cooked, it becomes soft and changes taste. We cannot make it raw again. - Curdling of milk: Milk turns into curd, and we cannot get back the original milk. - Growth of a plant: A seedling grows into a plant; it cannot become a seed again. - Rusting of iron: Iron reacts with moisture and oxygen to form rust, which cannot be changed back to pure iron easily.
Irreversible changes usually involve the formation of new substances. Once the change happens, the original substance is lost.
A physical change is a change in which the substance changes its physical state or appearance, but no new substance is formed. Physical changes are usually reversible.
Examples of physical changes: - Melting of ice - Boiling of water - Cutting of paper or cloth - Crushing of a can - Dissolving sugar in water - Changing of shape of clay
In a physical change: - No new substance is formed. - The change is usually reversible. - The composition of the substance remains the same. - Only physical properties like shape, size, and state change.
For example, when ice melts into water, the water can be frozen back into ice. The chemical composition remains H2O in both states. When sugar dissolves in water, the sugar can be recovered by evaporation.
A chemical change is a change in which a new substance is formed, with new properties. Chemical changes are usually irreversible.
Examples of chemical changes: - Burning of wood or paper - Cooking of food - Ripening of fruits - Rusting of iron - Curdling of milk - Souring of milk
In a chemical change: - A new substance is formed. - The change is usually irreversible. - The composition of the substance changes. - Heat, light, or sound may be produced. - The change may be accompanied by a change in colour, smell, or taste.
For example, when we burn wood, it turns into charcoal, ash, and gases. The ash and gases are new substances that cannot be turned back into wood. Similarly, when milk turns sour, its taste and smell change, and a new substance is formed.
| Physical Change | Chemical Change |
|---|---|
| No new substance is formed | New substance is formed |
| Usually reversible | Usually irreversible |
| Composition remains the same | Composition changes |
| Only physical properties change | Chemical properties change |
| Examples: melting, boiling, cutting | Examples: burning, cooking, rusting |
Some changes, however, cannot be clearly classified as physical or chemical. For example, when a candle burns, the wax melts (physical change) and the wick burns (chemical change). Both changes happen together. Similarly, when we cut a log of wood, it is a physical change, but when the wood burns, it is a chemical change.
Matter exists in three states: solid, liquid, and gas. Changes in temperature can change matter from one state to another. These state changes are examples of physical changes because no new substance is formed.
These changes are reversible. By heating or cooling, we can go back and forth between the states of a substance. For example, water can change from ice to water to steam and back again.
Changes can also be classified according to their speed.
The speed of a change depends on many factors. Some changes are slow because they need a long time to complete, while others are fast because they release energy quickly. Classifying changes as slow or fast is useful in daily life, for example, in knowing when fruits will ripen or how quickly food will spoil.
Some changes are helpful to us, and some are harmful. We should try to promote helpful changes and prevent harmful ones.
Helpful changes: - Cooking of food makes it tasty and easy to digest. - Ripening of fruits makes them sweet and nutritious. - Burning of fuels gives us heat and light. - Boiling of water kills germs and makes it safe to drink.
Harmful changes: - Rusting of iron damages tools, machines, and bridges. - Spoiling of food wastes valuable food items. - Rotting of wood weakens buildings and furniture. - Souring of milk makes it unfit for drinking.
We can prevent some harmful changes. For example, we can prevent rusting by painting iron objects or applying grease, and we can prevent food spoilage by storing food in airtight containers or refrigerators.
| Type of Change | Reversible or Not | Example |
|---|---|---|
| Melting of ice | Reversible | Water can be frozen back into ice |
| Burning of paper | Irreversible | Paper cannot be recovered from ash |
| Folding of paper | Reversible | Paper can be unfolded |
| Cooking of food | Irreversible | Raw food cannot be recovered |
| Rusting of iron | Irreversible | Rust cannot be changed back to iron |
| Property | Physical Change | Chemical Change |
|---|---|---|
| New substance formed | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Composition | Same | Changes |
| Examples | Melting, boiling, cutting | Burning, cooking, rusting |
In this chapter, we learned that change is the very nature of our world. Everything around us is constantly changing, whether quickly or slowly. We classified changes into reversible and irreversible, and physical and chemical, based on whether new substances are formed and whether the change can be undone. We saw that state changes like melting, freezing, evaporation, and condensation are reversible physical changes, while burning, cooking, rusting, and curdling are irreversible chemical changes. We also understood that changes can be slow or fast, and helpful or harmful. By understanding the nature of changes, we can use useful changes, prevent harmful ones, and appreciate the ever-changing world around us.