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Goa Board of Secondary and Higher Secondary Education
Class 7
Science
Ch 5: Physical and Chemical Changes

GBSHSE • Class 7 • Science • Ch 5

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Physical and Chemical Changes

In Class 7 Science, Chapter 5 "Changes Around Us: Physical and Chemical" investigates the fundamental transformations of matter. Aligned with the 2026–27 NCERT Curiosity curriculum, this master material distinguishes between physical changes (where no new substance is formed) and chemical changes (where bonds break and new chemical entities emerge), analyzes classic reaction proofs (magnesium burning, lime water milky test), investigates the electro-chemical destruction of iron rusting, and explores purification via crystallization.

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🕯️ Have You Ever Wondered?

Is burning a candle a physical change or a chemical change?

If you watch a burning candle closely, you see two completely different changes happening at the same time. The solid candle wax near the flame melts into a clear liquid, drips down, and solidifies again into wax. That is a physical change—the wax only changed its state of matter.

Meanwhile, liquid wax rises up the wick and burns with oxygen to produce light, heat, carbon dioxide, and water vapor. That melted wax is gone forever; you can never turn the smoke and gas back into a candle! That is a chemical change.

From rust slowly devouring an iron gate to the dough in an oven rising into fragrant bread, chemical changes are the irreversible engines of our universe.

Why This Chapter Matters

Before You Begin (Prerequisites)

States of matter: solid, liquid, gas, and phase changes (melting, freezing, evaporation, condensation).
Basic elements and compounds: iron, copper, magnesium, oxygen, and carbon dioxide.
Safety rule: burning magnesium produces intense UV light; never stare directly at the flame.

What You Will Learn (Core Objectives)

Differentiate between physical and chemical changes based on whether a new substance is formed.
Identify observable indicators of chemical reactions: gas evolution, color changes, temperature shifts, and light emission.
Explain the chemical reactions of burning magnesium, copper sulfate displacement, and the lime-water test for carbon dioxide.
State the essential conditions for rusting of iron and evaluate prevention methods (painting, galvanization, stainless steel).
Demonstrate the process of crystallization as a physical purification technique.

Chapter Roadmap & Progression

1 1. Physical vs. Chemical Changes: T...

2 2. Classic Laboratory Demonstration...

3 3. Rusting of Iron & Purification b...

Complete Concept Guide (100% Curriculum Coverage)

1. Physical vs. Chemical Changes: The Golden Line

1. The Defining Difference

Feature	Physical Change	Chemical Change
New Substance	NO new substance is formed. Molecular composition remains identical.	One or more NEW substances with entirely new properties are formed.
Reversibility	Usually reversible (e.g. melting ice can be refrozen).	Generally irreversible (e.g. ashes cannot become wood).
Energy Change	Small amount of energy absorbed or released.	Substantial energy exchange (heat, light, sound often emitted).
Examples	Melting wax, tearing paper, boiling water, stretching rubber band.	Rusting iron, curdling milk, cooking rice, burning coal.

2. Pitfall & Examiner Trap

⚠️ Trap: Tearing Paper vs. Burning Paper
Students often think tearing paper into tiny pieces is chemical because "you cannot paste it back into the original sheet".
Fact: Tearing is purely a physical change! Each tiny piece is still chemically cellulose paper. But burning paper produces ash and carbon dioxide—a brand new chemical substance—making it a chemical change.

2. Classic Laboratory Demonstrations of Chemical Change

1. Burning Magnesium Ribbon

When a clean strip of magnesium ribbon is held in a Bunsen flame, it burns with a dazzling white light and leaves behind a powdery white ash called magnesium oxide:

$$2\text{Mg} + \text{O}_2 \longrightarrow 2\text{MgO} \quad (\text{White ash})$$

Dissolving this ash in water creates magnesium hydroxide: $$\text{MgO} + \text{H}_2\text{O} \longrightarrow \text{Mg(OH)}_2$$ which turns red litmus blue, proving the new product is a chemical base!

2. The Carbon Dioxide & Lime Water Test

When baking soda (sodium hydrogen carbonate) is mixed with vinegar (acetic acid), vigorous effervescence (bubbling) occurs, releasing carbon dioxide gas:

$$\text{Vinegar} + \text{Baking Soda} \longrightarrow \text{CO}_2 \uparrow + \text{Sodium acetate} + \text{Water}$$

Passing this gas through freshly prepared lime water [$\text{Ca(OH)}_2$] turns it milky:

$$\text{CO}_2 + \text{Ca(OH)}_2 \longrightarrow \mathbf{\text{CaCO}_3 \downarrow} + \text{H}_2\text{O}$$

The milky appearance is due to the formation of insoluble calcium carbonate ($\text{CaCO}_3$). This is the standard diagnostic test for $\text{CO}_2$ gas!

3. Rusting of Iron & Purification by Crystallization

1. The Chemistry of Rusting

Rusting is the slow chemical destruction of iron when exposed to atmospheric elements. For rusting to occur, BOTH Oxygen and Water (moisture) are strictly required:

$$\mathbf{4\text{Fe} + 3\text{O}_2 + 2x\text{H}_2\text{O} \longrightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}} \quad (\text{Hydrated Iron Oxide / Rust})$$

If either oxygen or water is absent (e.g. iron nail submerged in boiled water covered with an oil layer), zero rusting occurs! Salt water accelerates rusting because dissolved ions speed up electro-chemical corrosion.

2. Methods to Prevent Rusting

Barrier Protection: Coating iron surfaces with paint or grease prevents direct contact with air and moisture.
Galvanization: The process of depositing a protective layer of zinc metal over iron. Zinc sacrifices itself because it oxidizes to form an impermeable protective crust. (Used in water pipes and roofing sheets).
Alloying: Mixing molten iron with carbon and metals like chromium and nickel produces stainless steel, which never rusts.

3. Crystallization: Pure Physical Separation

Crystallization is a physical process used to obtain large, beautifully shaped crystals of a pure substance from an impure solution.

By dissolving copper sulfate powder in boiling water containing a few drops of dilute sulfuric acid until no more dissolves (saturated solution), filtering it, and allowing it to cool undisturbed, brilliant blue needle-like crystals of pure $\text{CuSO}_4$ separate out!

Visual Learning & Conceptual Map

The Dual Conditions of Rusting

Both Oxygen AND Moisture must be present simultaneously

OXYGEN ($\text{O}_2$)

From Air

WATER ($\text{H}_2\text{O}$)

Moisture / Humidity

→

RUST

$\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$

Chapter Summary & 10 Key Takeaways

Takeaway 1

Physical Change: Alters physical properties (shape, size, state) without creating any new chemical substance (usually reversible).

Takeaway 2

Chemical Change: Produces one or more new substances with altered chemical identities; accompanied by heat, light, color shift, or gas.

Takeaway 3

Burning of Magnesium: Produces basic magnesium oxide ($\text{MgO}$), which dissolves in water to form alkaline $\text{Mg(OH)}_2$.

Takeaway 4

Lime Water Test: Carbon dioxide reacting with lime water produces insoluble calcium carbonate ($\text{CaCO}_3$), turning it milky.

Takeaway 5

Rusting of Iron: Requires BOTH oxygen and water; prevented by painting, galvanization (zinc coating), and stainless steel alloying.

Takeaway 6

Crystallization: A physical separation technique to obtain large, pure solid crystals from a saturated hot solution.

Check Your Understanding (Diagnostic Practice Questions)

Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.

Classify the following as physical or chemical changes: (a) Digestion of food (b) Melting of butter (c) Ripening of a mango (d) Dissolving salt in water.

Reveal Answer & Explanation

Answer: (a) Chemical (b) Physical (c) Chemical (d) Physical.
Ask: Is a brand new substance formed that cannot be un-done?

Why does lime water turn milky when exhaled breath is bubbled through it using a straw?

Reveal Answer & Explanation

Answer: Exhaled breath contains carbon dioxide gas ($\text{CO}_2$). When bubbled into lime water [$\text{Ca(OH)}_2$], it reacts chemically to form insoluble white calcium carbonate ($\text{CaCO}_3$) precipitate, making the solution look milky.
Write down the word equation for the lime water test.

Why do iron objects rust much faster in coastal cities (like Mumbai or Chennai) than in desert cities (like Jaisalmer)?

Reveal Answer & Explanation

Answer: Coastal areas have high atmospheric humidity (water vapor) and salty air. Salt accelerates electro-chemical corrosion. Desert areas have extremely low humidity, depriving the rusting reaction of essential moisture.
Remember the dual conditions required for rusting.

What is galvanization, and which metal is used in the process?

Reveal Answer & Explanation

Answer: Galvanization is the process of applying a protective coating of Zinc metal onto iron or steel to prevent rusting.
A transition metal that forms a protective white oxide film.

Is crystallization a physical change or a chemical change? Give a reason for your answer.

Reveal Answer & Explanation

Answer: Crystallization is a physical change because no new chemical substance is formed. Dissolved copper sulfate simply organizes its molecules into solid crystal geometry and can be redissolved.
Check if the chemical formula changes during crystallization.

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