Physical and Chemical Changes: CBSE Class 7 Science Notes with Examples and Signs

Every day, matter around us changes in many ways—ice melts, milk turns sour, paper burns. These changes can be grouped into physical changes and chemical changes depending on whether new substances are formed. This note explains the core idea, the signs to look for, and how to apply this knowledge in exams and real life.

What Is a Physical Change?

A physical change is a change in which the substance's size, shape, state, or appearance may change, but its chemical composition stays the same. No new substance is formed. For example, when ice melts to water, the molecules remain H2O; only the arrangement and energy of molecules change.

Most physical changes can be reversed easily. Freezing the water gives ice back; evaporating dissolved salt water leaves salt behind. However, reversibility is only a helpful clue, not a rule. Breaking a glass sheet is a physical change because the pieces are still glass, even though you cannot easily join them back.

• Common examples: melting wax, boiling water, dissolving sugar in water, cutting fruit, folding paper, and stretching a rubber band.
• Exam tip: In a physical change, the original substance can usually be recovered by a simple physical method such as evaporation, filtration, or changing temperature.

What Is a Chemical Change?

A chemical change is a change in which one or more new substances are formed. The original substances break apart and their atoms rearrange to form new compounds with different properties. Burning paper produces ash, carbon dioxide, and water vapour—none of these is paper, so the change is chemical.

Chemical changes are usually difficult to reverse because the original substances are not present anymore. For instance, when milk turns into curd, you cannot get fresh milk back from the curd by any simple physical method. Digestion, cooking, rusting, and fermentation are all chemical changes.

• Common examples: burning of wood, rusting of iron, souring of milk, cooking of food, and respiration in living beings.
• Exam tip: Look for the phrase “new substance formed” in the definition. If a new substance with a new composition appears, the change is chemical.

Physical vs Chemical Change: How to Tell Them Apart

The easiest way to classify a change is to ask: Is the original substance still present with the same chemical composition? If yes, it is physical; if no, it is chemical. A physical change may look dramatic, like a glowing electric bulb, but no new substance is made. A chemical change may be quiet, like iron rusting, but a new reddish-brown substance forms.

Compare these pairs: melting ice vs burning candle wax. Ice and water are the same substance, so melting is physical. Burning wax produces carbon dioxide and water vapour, so it is chemical. Similarly, dissolving sugar in water is physical because sugar can be recovered by evaporation; cooking sugar to make caramel is chemical because a new brown substance forms.

• Physical: no new substance, composition unchanged, usually reversible, mass remains same.
• Chemical: new substance formed, composition changed, usually irreversible, atoms rearrange but total mass is conserved.

Signs That Suggest a Chemical Change

Certain observable clues often accompany chemical changes: a change in colour, the release of gas bubbles, the formation of a solid precipitate, a change in temperature, or the emission of light or sound. For example, when vinegar reacts with baking soda, bubbles of carbon dioxide appear, and the mixture becomes cooler.

But these signs are not proof by themselves. A colour change can occur in a physical change too—adding ink to water changes its colour, yet no new substance forms. Therefore, always check whether new substances are produced, not just whether something looks different.

• Gas bubbles: e.g., zinc metal reacting with dilute acid produces hydrogen gas.
• Precipitate: e.g., passing carbon dioxide through lime water forms a white precipitate of calcium carbonate.
• Heat/light: e.g., burning magnesium ribbon gives a brilliant white light and white ash.

Rusting of Iron and How to Prevent It

Rusting is a slow chemical change in which iron reacts with oxygen and moisture from air to form hydrated iron oxide, a reddish-brown flaky substance. Unlike burning, rusting happens gradually, but it is still a chemical change because the rust is a new substance with properties different from iron.

Rusting damages iron objects, so prevention focuses on keeping air and moisture away from iron. Painting, oiling, greasing, coating with zinc (galvanisation), and alloying iron with chromium to make stainless steel are common methods. If an iron object is already rusted, the rust can be removed, but the iron lost to rust cannot be converted back easily.

• Conditions for rusting: both air (oxygen) and water/moisture are needed.
• Exam tip: Rusting is a chemical change because rust is a new substance; the total mass of a rusted iron object increases because oxygen from air combines with iron.

Conservation of Mass in Chemical Changes

In every chemical change, atoms are only rearranged—they are neither created nor destroyed. This is why the total mass of the reactants equals the total mass of the products. It may look like a burning candle loses mass, but the wax reacts with oxygen to form gases and water vapour that escape into the air.

Worked example: when carbon burns completely in oxygen, 12 grams of carbon combine with 32 grams of oxygen to form 44 grams of carbon dioxide. The mass of the product equals the sum of the masses of the reactants (12 + 32 = 44). This idea is called the law of conservation of mass.

• Take care: In an open container, gases may escape, so the remaining solid seems lighter. In a closed container, the total mass before and after a chemical change remains exactly the same.
• Exam tip: If a question gives masses of reactants and asks for product mass, add the reactant masses; if a product is missing, subtract the known product masses from the total reactant mass.

Key takeaways

• In a physical change, no new substance is formed and the chemical composition remains the same.
• In a chemical change, new substances with different properties are formed, and the change is usually irreversible.
• Signs like colour change, gas bubbles, precipitate, heat, or light are useful clues, but they are not proof of a chemical change.
• Melting wax is a physical change, but burning wax is a chemical change.
• Rusting of iron needs both air and moisture; painting, oiling, and galvanisation help prevent it.
• Mass is conserved in chemical changes because atoms are rearranged, not created or destroyed.

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