Chapter 2: Sorting Materials into Groups
पदार्थों का समूहन (चमक, विलेयता, घनत्व, पारदर्शी, पारभासी एवं अपारदर्शी पदार्थ)
Don't Let a 20-Minute Bubbling Delay Cost You CTET Qualification
In offline CTET exams, bubbling 150 bubbles takes 18 to 22 minutes of mechanical time. A single misaligned row can trigger an 8-question domino error. Practice bubble-filling this chapter's solved MCQs inside Mentora's official OMR simulator.
- Lustre (चमक): Materials having natural shine when freshly cut or polished are lustrous (metals like gold, silver, copper, iron). Non-metals and oxidized surfaces appear dull.
- Hardness (कठोरता): Materials easily compressed or scratched are soft (chalk, cotton, wax); materials difficult to scratch or compress are hard (iron, diamond, granite).
- Solubility in Water: Soluble solids (sugar, common salt) dissolve completely forming a homogeneous phase. Insoluble substances (sand, sawdust, chalk powder) do not disappear even after prolonged stirring.
- Miscible vs Immiscible Liquids: Liquids like lemon juice and vinegar mix completely with water (miscible); mustard oil, kerosene, and coconut oil do not mix and form a distinct upper layer (immiscible, lower density than water). Glycerine mixes completely with water.
- Flotation vs Sinking: Objects having lower density than water float (dry leaves, plastic ball, cork, oil layer); objects with higher density sink to the bottom (iron nail, coin, stone, key).
- Optical Transmission: 1. Transparent: Light passes through completely; objects clearly visible (clear glass, clean water, air). 2. Translucent: Light passes partially; objects seen vaguely without sharp details (oiled paper, frosted glass, tracing paper). 3. Opaque: No light passes; cannot see through at all (wood, cardboard, sheet iron).
- Trap: CBSE often asks if glycerine forms a separate layer like mustard oil in water. Correct: Glycerine is completely MISCIBLE with water, unlike vegetable oil or kerosene.
- Trap: CBSE asks whether frosted glass or butter paper is transparent or translucent. Correct: It is TRANSLUCENT (पारभासी), because light penetrates partially but images cannot be resolved clearly.
- Trap: Freshly cut iron has lustre; tarnished or rusted iron appears non-lustrous because of chemical action of moist air.
NCERT Core Facts: Physical Classification of Everyday Materials
In CTET Paper 2 Science, questions frequently assess the ability to classify substances based on physical parameters:
1. Classification Matrix of Materials
| Property | Scientific Definition | Example Materials | CTET MCQ Focus |
|---|---|---|---|
| Lustre (द्युति / चमक) | Metallic sheen when freshly sliced or polished | Copper, Aluminum, Brass, Iron | Tarnishing occurs due to moist air oxidation |
| Solubility (विलेयता) | Complete dissolution in water forming a clear solution | Salt, Sugar, Copper Sulfate, Glycerine | Sand, chalk powder, sawdust are insoluble |
| Flotation (प्लवन) | Floating or sinking governed by relative density | Floats: Cork, wood, ice, kerosene; Sinks: Marble, iron pin | Kerosene floats on water because its density is lower |
| Transparency (पारदर्शिता) | Degree of light transmission through material | Transparent: Clear glass, air; Translucent: Oiled paper; Opaque: Wood, iron sheet | Butter paper and greasy paper are translucent |
2. Optical Classification of Media
- 1.Transparent (पारदर्शी): Allows light to pass straight through with minimal scattering. We can see through clearly (e.g., spectacles glass, pure water).
- 2.Translucent (पारभासी): Allows light to pass partially; light rays scatter in multiple directions. You know light is behind it, but cannot distinguish shapes clearly (e.g., tissue paper, frosted glass, thin wax paper).
- 3.Opaque (अपारदर्शी): Blocks electromagnetic radiation in the visible spectrum completely (e.g., wooden plank, brick wall, steel plate). Shadows are formed exclusively by opaque and translucent obstacles.
Test Your Mastery: Sorting Materials into Groups
Authentic CBSE exam questions tested directly from this NCERT chapter.
A teacher provides four liquids to students: Vinegar, Mustard oil, Lemon juice, and Glycerine. Which of these will NOT dissolve in water and will instead float as a separate layer?
एक शिक्षक छात्रों को चार द्रव देता है: सिरका, सरसों का तेल, नींबू का रस और ग्लिसरीन। इनमें से कौन पानी में नहीं घुलेगा और एक अलग परत के रूप में ऊपर तैरेगा?
Consider three objects: X (Clear cellophane sheet), Y (Frosted glass window), and Z (Plywood board). Categorize X, Y, and Z based on optical transmission:
तीन वस्तुओं पर विचार करें: X (पारदर्शी सेलोफेन शीट), Y (धुंधला काँच/फ्रॉस्टेड ग्लास), और Z (प्लाईवुड बोर्ड)। प्रकाश संचरण के आधार पर X, Y और Z का वर्गीकरण करें:
Why does an iron nail sink in water while an iron bowl or wood block floats?
An iron nail sinks because the density of solid iron is greater than water. A wooden block floats because its overall density is less than water.
Is paper with a drop of cooking oil transparent, translucent, or opaque?
It becomes translucent because the oil fills the fibrous air spaces in the paper, allowing partial light transmission with internal scattering.