Chapter 6: Physical and Chemical Changes
भौतिक एवं रासायनिक परिवर्तन (जंग लगना, यशदलेपन, कॉपर सल्फेट एवं मैग्नीशियम फीता)
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- Physical vs Chemical Change: Physical change alters only physical properties (size, shape, state) without forming new chemical substances (reversible e.g. melting ice, tearing paper). Chemical change creates one or more completely NEW substances with different properties (irreversible e.g. burning candle, rusting iron).
- Burning of Candle: Involves BOTH physical and chemical changes (melting of wax is physical; burning of wax vapor forming CO2 and water vapor is chemical).
- Rusting of Iron: Iron reacts with BOTH Oxygen and Water/Moisture to form hydrated iron oxide (Rust: Fe2O3.xH2O). If either oxygen or moisture is absent, iron will NOT rust.
- Galvanization (यशदलेपन): The industrial process of depositing a protective layer of **Zinc (जस्ता)** on iron sheets to prevent rust.
- Displacement Reaction: Iron nail placed in blue Copper Sulfate solution -> Solution turns green (formation of Iron Sulfate / Ferrous sulfate) and a brown layer of metallic Copper deposits on the iron nail.
- Trap: Does burning a candle involve only a chemical change? NO. It involves **BOTH physical (wax melting) and chemical (wax vapor combustion) changes**.
- Trap: What are the two essential conditions required for iron to rust? Correct: **Both Oxygen AND Water (moisture)**.
- Trap: What metal is coated on iron in galvanization? Correct: **Zinc (जस्ता)**.
NCERT Core Facts: Reaction Dynamics, Rusting & Galvanization
Class 7 Chapter 6 tests fundamental thermodynamic and reaction transformations:
1. Comparing Physical vs. Chemical Changes
| Characteristic | Physical Change (भौतिक परिवर्तन) | Chemical Change (रासायनिक परिवर्तन) |
|---|---|---|
| New Substance | NO new substance is formed. | One or more NEW substances are created. |
| Reversibility | Usually reversible. | Generally irreversible. |
| Examples | Melting ice, boiling water, dissolving sugar, tearing cloth. | Rusting of iron, digestion of food, burning of coal, curdling of milk. |
2. Landmark NCERT Chemical Experiments
A. Rusting of Iron (लोहे में जंग लगना)
- Essential Prerequisite: Both atmospheric O_2 and liquid/vapor H_2O must be present.
- In coastal areas with high humidity, iron rusts far more rapidly than in dry deserts.
- Galvanization: Coating steel/iron water pipes with a layer of Zinc blocks contact with air and moisture.
B. Copper Sulfate and Iron Nail Displacement
- The blue color fades to green due to formation of Ferrous Sulfate.
Test Your Mastery: Physical and Chemical Changes
Authentic CBSE exam questions tested directly from this NCERT chapter.
An iron nail is dipped into a blue copper sulfate solution. After 30 minutes, what changes in the color of the solution and the iron nail are observed?
एक लोहे की कील को कॉपर सल्फेट के नीले विलयन में डुबोया जाता है। 30 मिनट बाद विलयन और कील के रंग में क्या परिवर्तन देखा जाता है?
Which of the following processes represents BOTH a physical and a chemical change occurring simultaneously?
निम्नलिखित में से किस प्रक्रिया में भौतिक और रासायनिक दोनों परिवर्तन एक साथ होते हैं?
Why does the burning of a wax candle involve both physical and chemical changes?
Melting and solidification of wax is a physical change, while combustion of vaporized wax into carbon dioxide and soot is a chemical change.
What metal is used in galvanization to protect iron from rusting?
Zinc (Zn).