Materials of Daily Use — Metals, Acids, Fibres and Fuels

Metals and Non-metals

NCERT Class 8 distinguishes metals from non-metals by their physical and chemical properties. Metals are typically lustrous, malleable, ductile, good conductors of heat and electricity, and sonorous. Non-metals are dull, brittle, poor conductors (except graphite), and non-sonorous.

Key reactions of metals:

• With oxygen — form basic oxides (e.g., MgO is basic). Metal oxides are basic in nature — this is an assertion-reason favourite in CTET.
• With water — reactivity varies. Na and K react violently at room temperature; Ca reacts slowly; Mg reacts with hot water; Fe reacts with steam; Cu and Ag do not react with water.
• With dilute acids — reactive metals displace hydrogen: Zn + H2SO4 → ZnSO4 + H2↑. Non-metals generally do NOT react with dilute acids.

The reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au) is essential for predicting displacement reactions. A more reactive metal displaces a less reactive one from its salt solution. CTET Aug 2023 Q67 tested which reactions are NOT feasible based on this series.

Acids, Bases and Salts

NCERT Class 7 Chapter 5 covers acids and bases through the lens of indicators — observable, activity-based learning that CTET heavily tests in both content and pedagogy questions.

Acids turn blue litmus red; sour taste; pH < 7. Bases turn red litmus blue; bitter, soapy; pH > 7. Neutral substances: pH = 7 (e.g., pure water).

Natural indicators and their colours:

CTET Jul 2024 Q82 asked: A=turmeric in base (reddish brown), B=china rose in base (green), C=phenolphthalein in acid (colourless), D=methyl orange in acid (orange). Correct answer: reddish brown, green, colourless, orange.

Neutralisation: Acid + Base → Salt + Water. HCl + NaOH → NaCl + H2O. This is exothermic. Dilution of acids (adding acid to water, not water to acid) is also exothermic — CTET Jul 2024 Q63 tested this.

Physical and Chemical Changes

NCERT Class 7 distinguishes physical changes (reversible, no new substance) from chemical changes (new substance formed, often irreversible). Key chemical changes tested in CTET:

• Rusting of iron — requires oxygen AND water. Chemical reaction, exothermic, irreversible. Formula: 4Fe + 3O2 + 6H2O → 4Fe(OH)3 → Fe2O3·xH2O (rust). CTET Jan 2024 Q64: rusting is a, b, c true (chemical reaction with O2, exothermic, occurs at room temperature); d is false (NOT reversible).
• Burning of magnesium — forms magnesium oxide (white ash). Exothermic.
• Reaction of potassium with water — extremely vigorous, exothermic, releases hydrogen gas with pop sound. CTET Jul 2024 Q88 asked which observation is incorrect: answer = 'it is endothermic' (it is actually exothermic).

Observations vs inferences is a key pedagogy concept that CTET often frames using chemical reaction scenarios. Observations are directly seen/measured (lime water turns milky, air expands); inferences are conclusions drawn (metal oxides are basic).

Fibres, Coal, Petroleum and Combustion

Fibres: Natural fibres include cotton (cellulose), silk (protein), wool (protein), jute. Synthetic fibres include nylon, polyester, acrylic, rayon. Rayon is semi-synthetic — made from natural cellulose (wood pulp) but processed chemically. NCERT Class 8 categorises rayon as a man-made fibre. CTET Jan 2024 Q90: S1 (rayon is synthetic) may be debated; S2 (made without any natural raw material) is FALSE because cellulose is natural. Official answer: S1 true, S2 false.

Fossil Fuels: Coal, petroleum, and natural gas formed over millions of years from buried organic matter. They are non-renewable. Petroleum refining products: petrol, diesel, kerosene, paraffin wax, lubricating oil, bitumen (for road making). Coal distillation gives coke, coal tar (for road making), and coal gas. CTET Jan 2024 Q81: X = bitumen (from petroleum), Y = coal tar (from coal).

Combustion: Exothermic reaction with oxygen producing heat and light. Three conditions: fuel, oxygen, ignition temperature. Matchbox rubbing surface contains red phosphorus and glass powder — CTET Aug 2023 Q78. Colour of flame depends on nature of combustible substance AND oxygen supply — CTET Jul 2024 Q83.

Atomic Structure and Chemical Formulas

NCERT Class 8 Chapter 4 covers the structure of the atom. Key points for CTET:

• Atomic number = number of protons = number of electrons in neutral atom.
• Mass number = protons + neutrons. Neutrons = mass number − atomic number.
• Ions — atoms that have gained or lost electrons. X³⁺ means the atom has lost 3 electrons. CTET Aug 2023 Q75: element with atomic mass 27 and atomic number 13 → X³⁺ has 13 protons, 10 electrons (13−3), 14 neutrons (27−13). Answer: 13, 10, 14.

For CTET Jul 2024 Q85: A (atomic number 13 = Al, valency 3) and B (atomic number 17 = Cl, valency 1). Al needs 3 Cl atoms to satisfy its valency. Formula = AlCl3 = AB3.

Chemical naming: CTET Jan 2024 Q65 required matching common names to chemical names: slaked lime = Ca(OH)2 (calcium hydroxide); quick lime = CaO (calcium oxide); limestone = CaCO3 (calcium carbonate); table salt = NaCl (sodium chloride); baking soda = NaHCO3 (sodium bicarbonate); caustic soda = NaOH (sodium hydroxide).

Separation of Mixtures

NCERT Class 6 Chapter 5 covers methods of separating components of mixtures. These are tested both as content and as practical skills for CTET pedagogy questions.

• Winnowing — separates lighter husk from heavier grain using wind. Used by farmers after threshing.
• Sieving — separates particles of different sizes.
• Sedimentation and decantation — heavier particles settle; clear liquid poured off.
• Filtration — separates insoluble solids from liquid using filter paper.
• Evaporation — separates dissolved solid (e.g., salt) from liquid.
• Distillation — separates miscible liquids with different boiling points. Used in petroleum refining.
• Magnetic separation — separates magnetic material (iron filings) from non-magnetic.
• Sublimation — separates a sublimable solid (naphthalene, camphor, ammonium chloride) from non-sublimable impurities.

Pedagogical Approaches for Materials Topics

Several CTET questions on pedagogy of Materials topics require understanding of how to introduce concepts contextually:

CTET Jul 2024 Q73 (science pedagogy): Scientific laws describe relationships between observable phenomena — relevant when teaching Ohm's Law or Newton's Laws through observation before formal statement.

For teaching chemical reactions (CTET Jan 2024 Q73), the best strategy at upper primary level is to start with chemical reactions students have observed in their lives — rusting, cooking, burning — rather than beginning with symbolic equations or definitions. This is the constructivist approach recommended by NCF 2005.

For teaching acids and bases, using natural indicators (turmeric, china rose, litmus) involves students directly in observation and inference. The teacher's role shifts from explainer to facilitator of inquiry. Students develop process skills: observing (colour changes), classifying (acid/base), inferring (what the indicator tells us), and communicating (recording tables).

Observation vs Inference in Chemistry

A recurring CTET theme is distinguishing between observations and inferences in the context of chemical experiments. NCERT stresses this distinction in science learning.

CTET Jul 2024 Q61 used four statements about chemistry: Air expands on heating (B) and lime water turns milky on passing CO2 (C) are observations — they are directly perceivable. Metal oxides are basic (A) and neutralisation of HCl and NaOH (D) are inferences — conclusions derived from evidence.

CTET Jan 2024 Q67 used electroscope and charged rod: leaves moving apart (a) and leaves acquiring similar charge (b) are observations; negative charges passing on to electroscope (c) is an inference about the mechanism.