Physical and Chemical Changes Class 7 NCERT Notes & Guide

Understanding Physical and Chemical Changes

In Class 7 Science, we divide all changes around us into two broad categories based on whether a new substance is formed or not.

1. Physical Changes

A change in which a substance undergoes a change in its physical properties—such as shape, size, color, or physical state (solid, liquid, gas)—is called a physical change. Crucially, no new substance is formed during a physical change. Most of these changes are reversible.

• Examples: Melting of ice (changes state from solid to liquid, but remains water), dissolving salt in water, tearing a sheet of paper, or stretching a rubber band.

2. Chemical Changes

A change in which one or more new substances are formed is called a chemical change (also known as a chemical reaction). During a chemical change, the chemical composition of the original substance changes entirely, and it is usually irreversible.

• Examples: Burning of wood, digestion of food, souring of milk, and rusting of iron.
• Signs of a Chemical Change: Evolution of heat or light, production of gas (bubbles), change of color, or formation of a precipitate.

Physical Changes vs. Chemical Changes

Key Processes: Rusting and Crystallization

1. Understanding Rusting of Iron — Rusting occurs when iron objects are exposed to oxygen and moisture (water or water vapor) in the air. The process can be represented by the following chemical equation: Iron (Fe) + Oxygen (O2, from air) + Water (H2O) -> Rust (Iron Oxide, Fe2O3). Both oxygen and water are essential for rusting to happen.
2. Preventing Rusting (Galvanization) — To prevent rust, we must stop iron from coming in contact with oxygen and water. This is done by applying a coat of paint, grease, or by depositing a layer of a metal like chromium or zinc on iron. The process of depositing a layer of zinc on iron is called Galvanization.
3. Crystallization (Physical Process) — Crystallization is used to obtain large crystals of pure substances from their hot, saturated solutions. For instance, pure crystals of copper sulphate can be obtained by adding copper sulphate powder to hot water containing a few drops of dilute sulphuric acid until no more dissolves, followed by filtering and cooling.

Avoid These Common Mistakes in Exams!

1. Irreversible Physical Changes: Students often think all physical changes must be reversible. Remember, tearing a piece of paper or breaking a glass tumbler are physical changes because no new chemical substance is formed, even though they cannot be reversed to their original forms.

2. Slicing Fruits: Slicing an apple is a physical change, but the brown color that appears on the cut surface after some time is due to a chemical reaction between air and the chemical compounds in the apple. Read the exam question carefully to spot what is being asked!

3. Condensation vs. Rusting: Rusting requires both air and moisture. Simply dry air or pure, air-free water will not cause rusting.

Practice Questions with Solutions

• Q: Classify the following processes as physical or chemical changes: (a) Photosynthesis (b) Dissolving sugar in water (c) Burning of coal (d) Beating aluminium to make aluminium foil A: Step 1: Analyze process (a) Photosynthesis. Carbon dioxide and water combine in the presence of sunlight to form glucose and oxygen. Since new substances are formed, it is a Chemical Change. Step 2: Analyze process (b) Dissolving sugar in water. Sugar molecules disperse in water but do not react to create a new chemical. It can be reversed by evaporating water. This is a Physical Change. Step 3: Analyze process (c) Burning of coal. Coal (carbon) burns in oxygen to form carbon dioxide gas and ash. New substances are formed, making it a Chemical Change. Step 4: Analyze process (d) Beating aluminium. It only changes the shape and physical form of aluminium to make a foil, with no change in its chemical nature. This is a Physical Change. Final answer: Photosynthesis: Chemical; Dissolving sugar: Physical; Burning of coal: Chemical; Beating aluminium: Physical.
• Q: Explain why burning of a candle involves both physical and chemical changes. A: Step 1: Identify the physical change. When a candle burns, the solid wax melts into liquid wax due to heat. This melted wax can solidify back into solid wax. Since there is only a change in state and no new substance is formed, melting of wax is a physical change. Step 2: Identify the chemical change. The liquid wax rises up the wick and burns in the presence of oxygen. This combustion process produces entirely new substances, namely carbon dioxide, water vapor, soot (carbon particles), heat, and light. This change is permanent and irreversible. Final answer: Melting of the wax is a physical change (change of state), while the burning of the wax wick and vapor is a chemical change (producing carbon dioxide and soot).
• Q: What happens when baking soda is added to vinegar? Write the word equation and explain the observation. A: Step 1: Identify the reactants. Vinegar is weak acetic acid, and baking soda is sodium hydrogen carbonate. Step 2: Describe the reaction. When mixed, they undergo a chemical change producing sodium acetate, water, and carbon dioxide gas. Step 3: State the word equation: Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) -> Carbon dioxide + Other substances. Step 4: Note the observation. You will see bubbles of carbon dioxide gas escaping with a hissing sound. Final answer: A chemical reaction occurs with a hissing sound, producing carbon dioxide bubbles.
• Q: Why is rusting of iron objects faster in coastal areas than in deserts? A: Step 1: Recall the essential conditions for rusting, which are the presence of both oxygen and moisture (water vapor). Step 2: Compare moisture levels. In coastal areas, the air contains a high amount of humidity (moisture content). In contrast, desert air is dry and lacks humidity. Step 3: Connect humidity to reaction speed. Because moisture levels are extremely high in coastal regions, the chemical reaction of iron reacting with oxygen and water accelerates. Final answer: Rusting is faster in coastal areas due to high humidity and salt content in the air, which speeds up the oxidation of iron compared to dry desert conditions.

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