Physical and Chemical Changes — Study Notes

1. Introduction

Every day many changes take place around us. Ice melts into water, paper is cut into pieces, milk turns into curd, and iron rusts. Some of these changes can be easily reversed, while others cannot. To understand such differences, scientists classify changes into physical changes and chemical changes. In this chapter we study the characteristics of both types of changes and learn to identify them.

A physical change is a change in which no new substance is formed and the change is usually easy to reverse. Melting of ice, cutting of paper and stretching of a rubber band are physical changes. A chemical change, on the other hand, is a change in which one or more new substances are formed, and the change is generally difficult to reverse. Rusting of iron, burning of paper and turning of milk into curd are chemical changes. Let us study both types in detail.

2. Physical Changes

A physical change is a change in which the physical properties of a substance, such as shape, size, state or colour, may change, but no new substance is formed. The identity of the substance remains the same. For example, when ice is heated, it melts to form water, and water can be frozen again to form ice. When wax is melted, it changes from solid to liquid, but it is still wax; on cooling it becomes solid wax again.

Physical changes are generally reversible. Some common examples include cutting a piece of paper into small pieces (the pieces are still paper), stretching a rubber band, dissolving sugar in water (sugar can be recovered by evaporation), and magnetising a piece of iron. In a physical change, the molecules of the substance remain the same; only the arrangement or state changes. No new substance with different chemical properties is produced.

3. Chemical Changes

A chemical change is a change in which new substances with different properties are formed. Such changes are usually irreversible. For example, when milk is left at room temperature in summer, it turns into curd; curd cannot be changed back into milk. When a piece of paper is burnt, it turns into ash and smoke; the ash cannot be changed back into paper. Similarly, rusting of iron, fermentation of food and cooking of food are chemical changes.

A chemical change may be accompanied by certain observations. There may be a change in colour, evolution of gas, change in temperature (heat is given out or absorbed), formation of a precipitate, or production of light or sound. For example, when iron nails are kept in a copper sulphate solution, the blue solution slowly turns green and a brown coating of copper is deposited on the nails - this indicates a chemical change. When vinegar reacts with baking soda, bubbles of carbon dioxide gas are given out. When magnesium ribbon is burnt, it produces a brilliant white light and forms a white powder of magnesium oxide, which is a new substance. In a chemical change, the original substance is consumed and cannot be obtained back easily.

4. Rusting of Iron

Rusting is a common chemical change that occurs when iron objects are exposed to moisture and air for some time. A reddish-brown flaky substance called rust forms on the surface of the iron. The formula of rust is approximately Fe2O3.xH2O. Rusting requires both air (oxygen) and moisture (water) together. If iron is kept in dry air, it does not rust, and if it is kept in boiled water without dissolved air, it does not rust either.

The rusting of iron is a slow process that can be prevented by several methods. Painting iron objects, applying oil or grease, galvanising (coating with a layer of zinc), tinning (coating with tin), and alloying iron with other metals such as chromium and nickel to make stainless steel are common methods. Galvanisation is one of the most effective methods and is used for iron pipes and gates. Rusting causes huge loss to iron articles, so preventing it is important in daily life and industry.

5. Crystallisation and Other Examples

Crystallisation is the process of forming crystals from a solution. When a saturated solution of a salt is allowed to cool slowly, the salt separates out in the form of crystals. This process is a physical change because the salt can be obtained back and no new substance is formed. Common examples include the formation of crystals of copper sulphate and sugar. Crystallisation is used to purify substances and to obtain large crystals in industry.

Some other examples help us distinguish the two types of changes. Melting of wax in a candle is a physical change, but the burning of the candle wick is a chemical change; both happen together in a burning candle. When a candle burns, wax melts (physical change) and the wax vapour burns to form carbon dioxide and water vapour (chemical change). Cutting of wood is a physical change, but burning of wood is a chemical change. Photosynthesis in plants, digestion of food and formation of curd from milk are all chemical changes occurring in living organisms.

Quick Revision Tables

Mind Map

Important Diagrams (SVG)

Diagram 1: Iron Nails in Copper Sulphate Solution

Diagram 2: Rusting of Iron

Common Mistakes

1. Thinking that melting of wax and burning of a candle are the same; melting is physical, burning is chemical.
2. Believing that all changes are reversible; chemical changes like rusting and burning are irreversible.
3. Confusing crystallisation as a chemical change; it is a physical change because no new substance is formed.
4. Thinking that rusting needs only water or only air; it requires both air (oxygen) and moisture together.
5. Believing that cutting wood is a chemical change; it is physical since no new substance is formed.
6. Forgetting that dissolving sugar in water is reversible; sugar can be recovered by evaporation.
7. Thinking that heating an iron nail changes it chemically; it becomes hot but remains iron (physical change).
8. Believing that galvanisation means painting; it is coating iron with zinc.
9. Confusing the observation of light with a physical change; burning of magnesium gives light and forms a new substance (chemical change).
10. Thinking that formation of curd from milk is a physical change; it is a chemical change and cannot be reversed.

Exam Tips

1. Memorise at least five examples each of physical and chemical changes with a reason for classification.
2. Define both types of changes in one precise sentence each and compare them in a table.
3. Explain the rusting experiment with three test tubes showing air-only, water-only and both conditions.
4. Learn all methods of preventing rusting: painting, oiling, galvanising, tinning and alloying.
5. Describe the iron nail in copper sulphate experiment with colour changes and conclusions.
6. Define crystallisation with an example and state why it is a physical change.
7. Remember the conditions for rusting: both air and moisture must be present.
8. Practise identifying physical and chemical changes in a burning candle (both occur simultaneously).
9. Use the keywords 'reversible', 'irreversible', 'new substance' while answering.
10. Know the uses of crystallisation in purifying substances and obtaining large crystals.

Conclusion

Changes are taking place all around us continuously, and understanding their nature helps us make sense of the world. Physical changes alter only the form or state of a substance without creating anything new, and they are reversible. Chemical changes produce new substances with new properties and are generally irreversible. Rusting of iron is an important chemical change that causes great loss, and it can be prevented by methods like painting, oiling, galvanising and alloying. Crystallisation is an interesting physical change used in purification. By distinguishing physical and chemical changes, we develop a deeper understanding of matter and its behaviour.