🔬 Science — Class X
Chemistry · Biology · Physics. NCERT-based CBSE board preparation.
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✏️ Worked Examples
Step-by-step solved problems across Physics, Chemistry, Biology
📖 Core Theory
Ch 1: Chemical Reactions and Equations
A chemical reaction involves breaking and forming bonds to produce new substances. Reactions are classified by the change that occurs: combination, decomposition, displacement, double displacement, and redox reactions. Reactions must be balanced — atoms of each element must be equal on both sides (Law of Conservation of Mass).
🔑 Key Reactions — Class X Chemistry
Ch 2: Acids, Bases and Salts
Acids produce H+ ions in water (turn litmus red, react with metals to give H2). Bases produce OH- ions (turn litmus blue). Neutralisation: Acid + Base → Salt + Water. pH scale 0–14: below 7 acidic, 7 neutral, above 7 basic.
🔑 Key Reactions — Acids and Bases
Ch 3: Metals and Non-metals
Metals are lustrous, malleable, ductile, good conductors, and form basic oxides. Non-metals are brittle, poor conductors, and form acidic oxides. Reactivity series determines which metals displace others. Ionic bonding: metal donates electron to non-metal.
Ch 4: Carbon and its Compounds
Carbon forms 4 covalent bonds — tetravalent. Catenation: carbon chains with itself. Huge variety of organic compounds (hydrocarbons, alcohols, acids, esters). Homologous series: compounds with same functional group differing by -CH2-.
🔑 Key Facts — Carbon Chemistry
Ch 5: Periodic Classification
Mendeleev's Periodic Table arranged elements by atomic mass; left gaps for undiscovered elements. Modern Periodic Table arranged by atomic number (Moseley). Periods = 7 rows; Groups = 18 columns. Properties vary periodically.
✏️ Worked Examples
Example 1 — Balanced Equation (Board 2023, 2M)
Q: Balance: Fe + H2O → Fe3O4 + H2. Identify the type of reaction.
Example 2 — pH and Acids (Board 2022, 3M)
Q: A solution has pH = 2 and another has pH = 11. (a) Which is acid? (b) What colour will litmus show in each?
📝 Practice Questions — Chemistry
(A) Decomposition (B) Combination (C) Displacement (D) Double Displacement
Ans: (B) Combination
(A) 1–2 (B) 7 (C) 9 (D) 12
Ans: (A) 1–2 — stomach acid (HCl) is strongly acidic.
(A) Tetravalency (B) Catenation (C) Electronegativity (D) Ionisation)
Ans: (B) Catenation
(A) Number of valence electrons (B) Number of shells (C) Atomic mass (D) Group number)
Ans: (B) Number of shells
R: In electrolytic decomposition, electrical energy is used to break the compound — Na+ ions migrate to cathode to form Na, and Cl- ions to anode to form Cl2 gas.
Ans: (A)
R: Zinc, being more reactive than iron, forms a protective layer that preferentially oxidises, preventing oxygen and moisture from reaching the iron below.
Ans: (A)
Case — Acids and Bases: A student tests four solutions with universal indicator. Solution A turns red (pH 2), Solution B turns green (pH 7), Solution C turns blue (pH 10), Solution D turns yellow (pH 5). She then adds a few drops of Solution A to Solution C and observes the colour change. She also tests which solution reacts with a piece of zinc metal to release a gas that burns with a 'pop' sound.
Ans: A homologous series is a group of organic compounds that have the same functional group, same general formula, and similar chemical properties — and whose consecutive members differ by one CH2 unit (14 mass units). Features: (1) Each successive member differs by CH2 in molecular formula and 14 in molecular mass. (2) Physical properties (melting point, boiling point, solubility) show gradual variation. (3) All members have the same functional group and similar chemical properties. Example: Alkanes: Methane (CH4), Ethane (C2H6), Propane (C3H8) — general formula CnH2n+2. All undergo substitution and combustion reactions.
Ans: The Activity Series (Reactivity Series) is an arrangement of metals in decreasing order of their chemical reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au > Pt. Significance: (1) A more reactive metal displaces a less reactive metal from its salt solution. (2) Metals above hydrogen displace it from dilute acids; those below hydrogen cannot. (3) The position in the series determines the method of extraction of the metal from its ore. Applications: (1) Displacement Reactions: Zinc displaces copper from copper sulphate solution (Zn + CuSO4 → ZnSO4 + Cu) — explained by Zn being more reactive. (2) Metal Extraction: Highly reactive metals (Na, Ca, K) are extracted by electrolysis; moderately reactive (Zn, Fe) by reduction with carbon; least reactive (Cu, Ag, Au) found free in nature or easily reduced. (3) Thermite Reaction: Aluminium's high reactivity (above Fe in series) allows it to reduce iron oxide — used in welding railway tracks.
📖 Core Theory
Ch 6: Life Processes
Life processes maintain living state: nutrition (autotrophic/heterotrophic), respiration (aerobic/anaerobic), transportation, and excretion. Photosynthesis converts CO2 + H2O to glucose using light energy in chloroplasts.
🔑 Key Facts — Life Processes
Ch 8: How do Organisms Reproduce?
Reproduction is essential for species survival. Asexual: binary fission (Amoeba), budding (Hydra), regeneration (Planaria), vegetative propagation (plants), spore formation (Rhizopus). Sexual: involves gametes, fertilisation, and offspring with genetic variation.
Ch 9: Heredity and Evolution
Mendel's laws: Law of Dominance, Law of Segregation. Traits inherited through genes on chromosomes. Acquired traits (Lamarck) NOT inherited — Weismann's experiment with mice. Evolution: gradual change in heritable traits over generations.
Ch 15: Our Environment / Ch 16: Management of Natural Resources
✏️ Worked Examples
Example 1 — Monohybrid Cross (Board 2023, 3M)
Q: A tall plant (TT) is crossed with a short plant (tt). What is the F1 phenotype? On self-crossing F1, what is the F2 ratio?
📝 Practice Questions — Biology
(A) Liver (B) Pancreas (C) Spleen (D) Kidney)
Ans: (B) Pancreas
(A) Meiosis (B) Binary fission (C) Mitosis (D) Budding)
Ans: (C) Mitosis
(A) Respiration (B) Digestion (C) Photosynthesis (D) Transpiration)
Ans: (C) Photosynthesis
(A) Binary fission (B) Vegetative propagation (C) Budding (D) Spore formation)
Ans: (B) Vegetative propagation
R: Chloroplasts contain the green pigment chlorophyll, which absorbs light energy to drive the conversion of CO2 and water into glucose and oxygen.
Ans: (A)
R: The small intestine has a very large surface area due to villi and microvilli, receives enzymes from the pancreas and bile from the liver, and absorbs nutrients into blood vessels and lacteals.
Ans: (A)
Case — Reproduction in Plants: A biology teacher grows new rose plants from stem cuttings and grafts a high-quality apple branch onto a hardier rootstock. She also cultivates Bryophyllum by placing its leaves in soil, observing new plants form at the leaf edges. Simultaneously, she observes a fern releasing spores from its underside.
Ans: Stem cuttings — vegetative propagation (artificial). Grafting — vegetative propagation (artificial). Bryophyllum leaf — vegetative propagation (natural). Spore formation — asexual reproduction.
Ans: Grafting combines the desirable qualities of two plants — the rootstock (hardy, disease-resistant) and the scion (high quality fruit/flower). The resulting plant is stronger than either parent alone. Also, plants grown by grafting often produce fruit/flowers much earlier than seed-grown plants, and the offspring is genetically identical to the desirable parent (no genetic variation that seeds would introduce).
Ans: Arteries: Carry oxygenated blood (except pulmonary artery) away from the heart. Have thick, elastic walls to withstand high pressure. Blood flows in pulses. No valves (except at origin from heart). Lie deep within body. Veins: Carry deoxygenated blood (except pulmonary vein) towards the heart. Have thinner, less elastic walls — blood flows at lower pressure. Have valves to prevent backflow of blood. Often lie closer to skin surface. Exception: Pulmonary artery carries deoxygenated blood; pulmonary vein carries oxygenated blood.
Ans: Photosynthesis is the process by which green plants synthesise food (glucose) from carbon dioxide and water in the presence of sunlight and chlorophyll. Equation: 6CO2 + 6H2O → C6H12O6 + 6O2 (chlorophyll + light energy). Process: It occurs in two stages. (1) Light reactions (in thylakoid membrane): Light energy is absorbed by chlorophyll. Water is split (photolysis) releasing O2 and energising electrons. ATP and NADPH are produced. (2) Dark reactions / Calvin Cycle (in stroma): CO2 is fixed using ATP and NADPH to produce glucose. Factors affecting rate: (1) Light intensity: Rate increases with light up to a saturation point. (2) CO2 concentration: Higher CO2 increases rate (up to a limit). (3) Temperature: Increases rate up to ~35°C, decreases beyond (enzyme denaturation). (4) Water availability: Shortage leads to stomatal closure, reducing CO2 entry. (5) Chlorophyll content: More chlorophyll = more absorption = higher rate.
📖 Core Theory
Ch 12: Electricity
Electric current (I) is the rate of flow of charge. Potential difference (V) drives current through a conductor. Ohm's Law: V = IR. Resistance depends on material, length, area of cross-section, and temperature.
🔑 Key Formulae — Electricity
Ch 13: Magnetic Effects of Electric Current
A current-carrying conductor produces a magnetic field around it (Oersted, 1820). The direction is given by the Right-Hand Thumb Rule. Electromagnets and electric motors use this principle. Electromagnetic induction (Faraday) — changing magnetic field induces EMF.
Ch 10 & 11: Light — Reflection and Refraction
🔑 Key Formulae — Light
✏️ Worked Examples
Example 1 — Ohm's Law / Resistance (Board 2023, 3M)
Q: Three resistors of 3Ω, 6Ω and 2Ω are connected in parallel. Find the equivalent resistance and total current if the voltage is 12V.
Example 2 — Mirror Formula (Board 2022, 3M)
Q: An object is placed 30cm in front of a concave mirror of focal length 20cm. Find image distance and magnification.
📝 Practice Questions — Physics
(A) Volt (B) Ohm (C) Ampere (D) Watt)
Ans: (C) Ampere
(A) Current halves (B) Current doubles (C) Current stays same (D) Power halves)
Ans: (B) Current doubles — V = IR; I = V/R.
(A) Diverging lens (B) Concave lens (C) Converging lens (D) Plano-concave lens)
Ans: (C) Converging lens
(A) Real, inverted (B) Virtual, erect, laterally inverted (C) Virtual, inverted (D) Real, erect)
Ans: (B) Virtual, erect, laterally inverted
R: In a series circuit, there is only one path for current to flow — so the same current must pass through every component in the loop.
Ans: (A)
R: Short wavelength blue light is scattered much more (Rayleigh scattering — inversely proportional to λ⁴) by atmospheric molecules than long wavelength red light, so blue light reaches our eyes from all directions.
Ans: (A)
Case — Electricity: A circuit has a battery of 12V connected to two resistors R1 = 4Ω and R2 = 8Ω. First, the circuit is connected in series. Then, the same components are reconnected in parallel.
Ans: R_total = R1 + R2 = 4 + 8 = 12Ω. Current I = V/R = 12/12 = 1A.
Ans: 1/R_eq = 1/4 + 1/8 = 2/8 + 1/8 = 3/8. R_eq = 8/3 = 2.67Ω. Total current = 12 / (8/3) = 12 × 3/8 = 4.5A.
Ans: Parallel — lower total resistance → more total current → greater power (P = VI = V²/R). In parallel, each resistor also receives the full 12V.
Ans: (1) Object at infinity: Image at F, real, inverted, point-sized. (2) Object beyond 2F: Image between F and 2F, real, inverted, diminished. (3) Object at 2F: Image at 2F, real, inverted, same size. (4) Object between F and 2F: Image beyond 2F, real, inverted, magnified. (5) Object at F: Image at infinity. (6) Object between F and lens: Image same side as object, virtual, erect, magnified (used in magnifying glass).
Ans: Ohm's Law: The current through a conductor is directly proportional to the potential difference across it, provided temperature remains constant: V = IR. R = resistance (Ω), V = voltage (V), I = current (A). (a) Series: Same current I flows through all. Voltage divides: V = V1 + V2 + V3 = IR1 + IR2 + IR3 = I(R1+R2+R3). So R_series = R1 + R2 + R3. Equivalent resistance is greater than any individual resistance. (b) Parallel: Same voltage V across all. Current divides: I = I1 + I2 + I3 = V/R1 + V/R2 + V/R3 = V(1/R1 + 1/R2 + 1/R3). So 1/R_parallel = 1/R1 + 1/R2 + 1/R3. Equivalent resistance is less than the smallest individual resistance. Advantage of parallel: Each device gets full voltage; failure of one does not affect others (household wiring uses parallel).
📝 Practice Papers
Full-length 80-mark board-pattern papers · Science X
SCIENCE X — PRACTICE PAPER 1
Time: 3 Hours | Max Marks: 80 | CBSE 2024-25
SCIENCE X — PRACTICE PAPER 2
Different chapter emphasis
SCIENCE X — PRACTICE PAPER 3 (Challenge)
Pre-Board application level
Oxidation: Gain of oxygen / Loss of hydrogen / Loss of electrons
Example: 2Mg + O₂ → 2MgO (Mg is oxidised — gains oxygen)
Reduction: Loss of oxygen / Gain of hydrogen / Gain of electrons
Example: CuO + H₂ → Cu + H₂O (CuO is reduced — loses oxygen)
In the reaction CuO + H₂ → Cu + H₂O: CuO = oxidising agent | H₂ = reducing agent
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
Test for CO₂: Pass gas through lime water (Ca(OH)₂) → turns milky due to formation of CaCO₃.
If excess CO₂ passed: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (milkiness disappears)
• Mouth: Salivary amylase → starch to maltose
• Stomach: HCl + Pepsin → proteins to peptides | HCl kills bacteria, activates pepsin
• Small intestine (Duodenum): Bile (from liver) emulsifies fats | Pancreatic juice: amylase, trypsin, lipase
• Small intestine (Ileum): Intestinal juice (succus entericus) → final breakdown, absorption through villi
• Large intestine: Water absorption, formation of faeces
Diagram must show: mouth → oesophagus → stomach → small intestine (duodenum + ileum) → large intestine → anus, with liver and pancreas labelled.
F1: All Tt → all TALL (100% tall phenotype; tall is dominant)
F1 × F1: Tt × Tt
F2 genotypes: TT : Tt : tt = 1:2:1
F2 phenotypes: Tall : Dwarf = 3:1
Mendel's ratio of 3:1 in F2 is the phenotypic ratio for monohybrid cross.
V = V₁ + V₂ + V₃ = IR₁ + IR₂ + IR₃ = I(R₁+R₂+R₃)
∴ Rs = R₁ + R₂ + R₃
Parallel: Same voltage V across R₁, R₂, R₃.
I = I₁ + I₂ + I₃ = V/R₁ + V/R₂ + V/R₃ = V(1/R₁ + 1/R₂ + 1/R₃)
∴ 1/Rp = 1/R₁ + 1/R₂ + 1/R₃
Parallel Rp < smallest individual resistance; Series Rs > largest individual resistance.
Balancing Chemical Equations — Not Balancing
Students write unbalanced equations and lose 1 mark per equation.
Photosynthesis Equation — Wrong Products
Students write CO₂ or H₂ instead of O₂ as a product.
Ohm's Law Formula Inversion
Students write I = VR instead of I = V/R.
Series vs Parallel: Equivalent Resistance Rule
Students write smaller resistance for series and larger for parallel.
Heredity: Dominant vs Recessive Mix-up
Students identify the wrong trait as dominant in Mendel's experiments.
Reflection: Laws of Reflection Application
Students confuse normal, incident ray, and reflected ray.
Lens Formula Sign Convention
Students forget to apply sign convention for object/image distances.
Acid/Base: Confusing Indicators
Students mix up litmus, phenolphthalein, and methyl orange colours.
🎯 Tip 1 — Label Diagrams Completely
In Biology and Physics, diagrams carry 2 marks: 1 for the drawing + 1 for correct labels. Always label EVERY structure asked. A beautifully drawn neuron with no labels scores 0 on the label mark. Write neatly in pencil, use a scale for circuits.
🎯 Tip 2 — Chemistry: Always Balance the Equation
Every chemical reaction question awards 1 mark for a balanced equation. Write the word equation first, then symbol equation, then balance. Unbalanced equations lose this mark automatically. Check atom count on both sides before moving to the next step.
🎯 Tip 3 — Physics Numericals: Units in Every Step
Write units alongside every quantity in every step of a numerical. "V = 12 V, R = 4 Ω, I = V/R = 12/4 = 3 A." Missing units in the final answer costs 0.5 marks. Examiners check units explicitly in Section D numericals.
🎯 Tip 4 — Heredity: Always Draw the Cross Table
For Mendel's cross questions, draw the Punnett Square. Even for monohybrid crosses, a correct Punnett Square earns 1 mark for process (even if the final ratio is wrong). F1 genotypes → F1 × F1 → F2 Punnett Square → phenotypic ratio.
🎯 Tip 5 — Electricity: Show the Circuit and Formula
For resistance combination numericals, draw a quick circuit diagram showing the arrangement, then write the formula for equivalent resistance. This earns the diagram + formula marks before you even calculate. Many students lose 2 marks by jumping straight to numbers.
🎯 Tip 6 — Life Processes: Name the Enzyme, Not Just the Organ
In digestion questions, naming the organ (stomach) earns partial marks. Naming the enzyme (pepsin) earns full marks. For nutrition: name gland + enzyme + substrate + product. "Pancreas secretes lipase which breaks down fats into fatty acids and glycerol."
🎯 Tip 7 — Light: Draw Ray Diagrams with Arrow Directions
All rays in optics diagrams must have arrows showing direction of light travel. Examiners deduct 0.5 marks for missing arrow directions on incident, refracted, or reflected rays. Also mark the normal (dotted line) at the point of incidence.
🎯 Tip 8 — Assertion-Reason: Eliminate Systematically
For Assertion-Reason questions: (a) Is A true? (b) Is R true? (c) If both true, does R correctly explain A? Don't guess. Eliminate false options first. The most common trap: both A and R are true but R does NOT explain A → correct answer is (b) not (a).
Every CBSE Science paper carries a 5-mark numerical on equivalent resistance (series, parallel, or mixed), current, voltage, or power. Master all formulas: V=IR, Rs=R₁+R₂+R₃, 1/Rp=1/R₁+1/R₂+1/R₃, P=VI=I²R=V²/R. Draw circuit diagram first.
A labelled diagram of the human digestive system OR the nephron/respiratory system appears in almost every paper. Practice drawing these from memory in under 3 minutes. Label all organs + name enzymes/functions for full marks. Diagram = 2 marks, written part = 3 marks.
Questions combining identification of reaction types (combination, decomposition, displacement, double displacement, redox) with balanced equations appear in every session. Know examples of each and be able to write and balance the equation. Oxidising vs reducing agent identification is high-frequency.
Ray diagram for concave/convex mirror or lens with object at various positions, + mirror/lens formula numerical appear every year. New Cartesian Sign Convention (u always negative), 1/v + 1/u = 1/f for mirrors, 1/v − 1/u = 1/f for lenses. Magnification m = −v/u (mirror) or v/u (lens).
Monohybrid cross with F1 and F2 Punnett squares, laws of inheritance, and sex determination in humans (XX/XY) appear every other year. Know 3:1 phenotypic ratio in F2, 1:2:1 genotypic ratio. Prepare both monohybrid and dihybrid cross patterns.
Reactions of acids with metals/carbonates/bases, indicators (litmus, phenolphthalein, methyl orange), and importance of pH in everyday life (digestion, soil, toothpaste) appear in 3–5 mark questions. Know all four types of acid-base reactions with balanced equations.
Fleming's Left Hand Rule (motor effect) vs Right Hand Rule (generator effect), electromagnetic induction, and AC vs DC appear as 3-mark questions. Know which rule is used when and state all three fingers' directions (thumb=force, index=field, middle=current).
Identifying functional groups (−OH, −CHO, −COOH, −CO−), naming compounds (IUPAC), and properties of homologous series appear in Section C. Know alkane/alkene/alkyne differences, properties of ethanol and ethanoic acid, and saponification reaction.
- Acid + Metal → Salt + H₂ | Acid + Base → Salt + H₂O (neutralisation)
- Acid + Carbonate → Salt + H₂O + CO₂ | CO₂ turns lime water milky
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in light, chlorophyll)
- Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (38 ATP)
- Redox: Oxidation = loss of e⁻ / gain of O₂ | Reduction = gain of e⁻ / loss of O₂
- Indicator colours: Litmus (Acid=Red, Base=Blue) | Phenolphthalein (Base=Pink) | Methyl orange (Acid=Red)
- Digestive enzymes: Salivary amylase (starch) | Pepsin (protein) | Bile (fat emulsification) | Lipase (fats) | Trypsin (proteins)
- Excretion: Kidney → nephron → Bowman's capsule → tubule → collecting duct → ureter → bladder
- Neuron: Dendrite → Cell body → Axon → Synapse → Effector
- Mendel: F1 all dominant phenotype | F2 phenotypic ratio 3:1 | genotypic ratio 1:2:1
- Sex determination: XX = female, XY = male. Father determines sex of child.
- Vegetative propagation: Potato (tuber) | Ginger (rhizome) | Rose (stem cutting) | Onion (bulb)
- Ohm's Law: V = IR | Power P = VI = I²R = V²/R | Unit: V=volt, I=ampere, R=ohm
- Series: Rs = R₁+R₂+R₃ (same current, different voltage)
- Parallel: 1/Rp = 1/R₁+1/R₂+1/R₃ (same voltage, different current)
- Mirror formula: 1/v + 1/u = 1/f | Lens formula: 1/v − 1/u = 1/f
- Sign convention: All distances from pole/optical centre | incident direction = positive
- u always negative (object on left) | Concave mirror f negative | Convex lens f positive
- Fleming's LHR: Motor (force direction) | RHR/Dynamo rule: Generator (current direction)