PW Class 7 Science Chapter 5 Notes: Changes Around Us – Physical and Chemical

Changes happen around us every day. Ice melts into water, fruits ripen, iron rusts, paper is cut into pieces, and food is cooked. Although all these examples involve change, they do not affect substances in the same way. Some changes only alter the appearance or state of a material, while others produce completely new substances with different properties.

PW Class 7 Science Chapter 5 Notes explain the complete chapter in a simple and descriptive manner. You can use these notes to understand the difference between physical and chemical changes, revise important experiments, learn natural processes such as weathering and erosion, and prepare confidently for textbook questions and school assessments.

What Are Changes Around Us?

A change is any process that alters the appearance, state, shape, size, or composition of a substance. Scientists broadly classify changes into physical changes and chemical changes based on whether a new substance is formed.

Understanding this difference helps explain why melting ice can be reversed, while rusting iron or burning paper cannot.

What Is a Physical Change?

A physical change is a change in which the physical properties of a substance are altered without changing its chemical composition. The substance remains the same, and in many cases the original material can be recovered.

Characteristics of a physical change

• No new substance is formed.

• The chemical identity remains unchanged.

• Only shape, size, state, or appearance changes.

• Most physical changes are reversible.

Examples of physical changes

• Melting of ice into water

• Boiling water into steam

• Crushing chalk into powder

• Cutting a sheet of paper

• Inflating a balloon

In each of these examples, the substance remains the same even though its appearance or form changes.

What Is a Chemical Change?

A chemical change is a change in which one or more new substances with different properties are formed. During a chemical change, the original substances lose their identity and are converted into new products through a chemical reaction.

Characteristics of a chemical change

• A new substance is always produced.

• The chemical composition changes completely.

• Heat, light, sound, colour change, or gas may be produced.

• Most chemical changes are irreversible.

Examples of chemical changes

• Rusting of iron

• Burning wood

• Formation of curd from milk

• Cooking food

• Ripening of fruits

These changes cannot usually be reversed because the original substance has been transformed into a different material.

Physical Change vs Chemical Change

Before identifying different types of changes, it is important to understand how physical and chemical changes differ. The table below compares their key characteristics, helping you recognise whether a change only alters a substance’s appearance or forms a completely new substance.

Activity 1: Blowing Air into Water and Lime Water

This activity helps distinguish between a physical and a chemical change.

Materials Required

• Two glass tumblers

• Tap water

• Lime water

• Two straws

Observations

• When air is blown into tap water, bubbles appear but the water remains unchanged.

• When air is blown into lime water, the solution turns milky and a white substance settles at the bottom.

Conclusion

Blowing air into tap water is a physical change because no new substance is produced.

Blowing air into lime water is a chemical change because the carbon dioxide present in exhaled air reacts with calcium hydroxide to form calcium carbonate, a white insoluble substance. The milky appearance of lime water is therefore a simple test for the presence of carbon dioxide.

Activity 2: Mixing Vinegar and Baking Soda

This experiment demonstrates the formation of a new substance through a chemical reaction.

Materials Required

• Vinegar or lemon juice

• Baking soda

• Test tube

Observations

• A fizzing sound is heard.

• Gas bubbles are produced.

• The released gas turns lime water milky.

Conclusion

The bubbling shows that carbon dioxide gas has been formed. Since a new substance is produced, the reaction between vinegar and baking soda is a chemical change.

What Is Rusting of Iron?

Rusting is a slow chemical process in which iron reacts with oxygen in the presence of water or moisture to form iron oxide, commonly called rust.

Conditions necessary for rusting

• Presence of oxygen

• Presence of water or moisture

Rust appears as a flaky reddish-brown layer that weakens iron objects over time. Bridges, gates, ships, and railway tracks are often protected from rust because corrosion reduces their strength.

Methods of preventing rusting

• Painting the surface

• Applying grease or oil

• Galvanisation (coating iron with zinc)

Galvanisation protects iron because zinc corrodes before the iron underneath is affected.

What Is Combustion?

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. It is commonly observed when fuels burn.

Three requirements for combustion

1. Fuel – a combustible substance

2. Oxygen – supports burning

3. Heat – raises the fuel to its ignition temperature

What is ignition temperature?

The ignition temperature is the minimum temperature at which a substance catches fire. Different fuels have different ignition temperatures.

Examples of combustible substances

• LPG

• Kerosene

• Paper

• Wood

Why Is a Burning Candle Both a Physical and Chemical Change?

A burning candle is one of the best examples of two different changes happening together.

Physical change

• Solid wax melts into liquid wax.

• Liquid wax evaporates into wax vapour.

• No new substance is formed during melting or evaporation.

Chemical change

• Wax vapour burns in the presence of oxygen.

• New substances such as carbon dioxide and water vapour are produced.

• Heat and light are released.

Thus, the same process includes both physical and chemical changes.

Reversible and Irreversible Changes

Changes can also be classified according to whether they can be reversed.

Reversible changes

These are changes in which the original substance can be obtained again.

Examples:

• Melting and freezing ice

• Boiling and condensing water

• Stretching a rubber band

Irreversible changes

These are changes in which the original material cannot be recovered.

Examples:

• Cooking food

• Chopping vegetables

• Making popcorn

• Burning paper

What Is Crystallisation?

Crystallisation is a physical method used to obtain a pure solid substance from its solution in the form of large crystals. Unlike evaporation, crystallisation produces cleaner and purer solids.

Why is crystallisation important?

• Produces large, pure crystals

• Removes many dissolved impurities

• Prevents decomposition of heat-sensitive substances

Example: Copper sulphate crystals obtained from its solution.

Natural Changes: Weathering and Erosion

Nature continuously changes the Earth's surface through weathering and erosion.

Weathering

Weathering is the gradual breakdown of rocks into smaller pieces. It may occur through both physical and chemical processes.

Causes include:

• Temperature changes

• Freezing water

• Plant roots

• Chemical reactions with water

Erosion

Erosion is the movement of rocks and sediments from one place to another by natural agents such as wind and water.

Class 7 Science Chapter 5 Notes PDF

Download the CBSE Class 7 Science Chapter 5 Notes PDF to revise the properties, uses, and reactions of metals and non-metals in a concise format.

Why Choose Physics Wallah Class 7 Science Chapter 5 Notes?

Physics Wallah Class 7 Science Chapter 5 Notes present the complete chapter in a descriptive and easy-to-revise format. They help you understand the scientific reasoning behind everyday changes while strengthening conceptual learning.

• Understand Physical and Chemical Changes: Learn how to identify whether a new substance is formed.

• Revise Important Activities: Recall the observations, results, and conclusions of classroom experiments.

• Master Rusting and Combustion: Understand their causes, conditions, prevention methods, and applications.

• Compare Different Types of Changes: Differentiate physical, chemical, reversible, and irreversible changes with clarity.

• Learn Natural Processes: Revise weathering, erosion, and crystallisation through simple explanations and examples.

• Connect Science with Daily Life: Relate chapter concepts to familiar situations such as melting ice, cooking food, burning candles, and rusting iron.

• Prepare for School Exams: Quickly revise definitions, comparison tables, activities, and important scientific facts before assessments.

Understanding the difference between physical and chemical changes helps explain many natural and everyday processes around us. From rusting and combustion to crystallisation and weathering, this chapter builds a strong foundation in chemistry by connecting scientific concepts with real-life observations. These notes provide a structured revision resource for effective exam preparation.

Related Resources

• CBSE Class 7 Science Syllabus

• CBSE Class 7 Science Notes

• CBSE Class 7 Science NCERT Solutions

• CBSE Class 7 Science Important Questions

• CBSE Class 7 Science Sample Papers