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ChemistryClass 10CBSE

Why is NaOH's molar mass exactly 40?

Atomic masses: Na=23, O=16, H=1 → 23+16+1 = 40 g/mol exactly. This means 1 mole of NaOH weighs 40 g, making it easy to prepare 1M solutions (40 g per litre).

ChemistryClass 10CBSE

Molecular Mass Calculations

NaOH molecular mass = 40 g/mol:Na(23) + O(16) + H(1) = 40 g/mol FeSO₄·7H₂O (Green Vitriol) = 278 g/mol:Fe(56) + S(32) + 4×O(16) + 7×H₂O(18) = 56 + 32 + 64 + 126 = 278 g/mol Na₂SO₄·10H₂O (Washing Soda / Glauber's Salt) = 322 g/mol:2×Na(23) + S(32) + 4×O(16) + 10×H₂O(18) = 46 + 32 + 64 + 180 = 322 g/mol

ScienceClass 10CBSE

Lactobacillus benefits

Makes curd/yoghurt/cheese, produces lactic acid, maintains gut microbiome, boosts immunity, supports vaginal health, reduces antibiotic-associated diarrhoea. Classified as GRAS (Generally Recognised As Safe).

ScienceClass 10CBSE

Are all bacteria harmful?

False most are neutral or beneficial

ScienceClass 10CBSE

Is it a type of bacteria?

True Gram-positive, rod-shaped, Firmicutes phylum

ScienceClass 10CBSE

Is Lactobacillus harmful?

False it is beneficial (probiotic)

GeneralClass 10CBSE

Natural vs Synthetic 

Natural = plant-based (litmus from lichens, turmeric, china rose); Synthetic = lab-made chemicals (phenolphthalein, methyl orange). Both detect acids/bases by colour change.Type 2 indicator = Reversible indicator returns to original colour when pH conditions change (e.g., phenolphthalein reverts to colourless when pH drops below 8.2).

ChemistryClass 10CBSE

Synthetic Indicators 

IndicatorTypeIn AcidIn BaseLitmusNaturalRedBlueTurmericNaturalYellowRed-brownPhenolphthaleinSyntheticColourlessPinkMethyl OrangeSyntheticRedYellow

ChemistryClass 10CBSE

Why polar aprotic → SN2

No H-bonding with nucleophile → nucleophile stays "naked" and highly reactive → faster backside attack.Best example: DMSO (dimethyl sulfoxide) most commonly cited polar aprotic solvent in exams.

ChemistryClass 10CBSE

Polar Aprotic Solvents

FeatureProtic SolventsPolar Aprotic SolventsO–H / N–H bondsYesNoH-bond donationYesNoExamplesWater, ethanolDMSO, DMF, acetoneFavoursSN1 reactionsSN2 reactions

GeneralClass 10CBSE

Favourable factors for cation formation

Low ionisation energy, large atomic radius, high electron shielding, stable noble-gas configuration after electron loss.

ChemistryClass 10CBSE

Conditions for ionic bond formation

Large electronegativity difference (>1.7 Pauling units)Low ionisation energy of the metal (easy electron loss)High electron affinity of the non-metal (eager electron gain)High lattice energy of the product (stabilises the ionic crystal)Both ions achieve noble gas / stable configuration (octet rule)Class 10 example — NaCl:Na (2,8,1) → Na⁺ (2,8) + e⁻ [loses 1 electron] Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8) [gains 1 electron] Na⁺ + Cl⁻ → NaCl (ionic bond via electrostatic attraction)

HistoryClass 10CBSE

Jacobin Club

Full name: Society of the Jacobins, Friends of Freedom and EqualityFounded: 1789, Paris (named after St. Jacob's convent)Members mainly from: Lower/lower-middle class small shopkeepers, artisans, petty traders, daily-wage workers, domestic servants = the sans-culottesLed by: Maximilien Robespierre (lawyer), also Marat, Danton, Saint-JustCame to power: 1793 - Launched Reign of Terror (1793–94): ~16,000–40,000 executedFell: July 1794 Robespierre arrested and guillotined (Thermidorian Reaction)

GeneralClass 10CBSE

Why is 7 lucky?

Cultural (7 days, 7 colours, 7 notes, 7 seas), religious (7 heavens, 7 sacred rivers), mathematical (prime number), and psychological (most chosen number between 1–10 in studies).

MathematicsClass 10CBSE

7 & 8 Digit Numbers

QuestionAnswerGreatest 7-digit number9,99,99,999 — wait, 99,99,999 (Seven 9s)Greatest 7-digit number with 3 in hundreds place99,99,399Greatest 8-digit number9,99,99,999 (Nine crore ninety-nine lakh…)

ScienceClass 10CBSE

10 Examples of Shrubs

Rose, Hibiscus (Gudhal), Jasmine, Tulsi (Holy Basil), Henna (Mehndi), Bougainvillea, Lantana, Croton, Azalea, OleanderA shrub is identified by: woody stem + multiple stems from base + height 0.5–6 m + perennial lifespanShrub-tree examples: Pomegranate, Oleander, Curry Leaf Plant, Bottle Brush (Callistemon) plants that can grow as large shrubs or small trees.

ChemistryClass 10CBSE

Salt and diabetes

Excess salt is NOT recommended it raises blood pressure, worsening diabetic kidney and heart complications. WHO recommends <5 g/day; diabetic patients should follow a low-sodium diet.

ChemistryClass 10CBSE

Na-22 vs Na-23

Na-23 is the naturally occurring stable form; Na-22 is a radioactive isotope used in PET scans. For all school chemistry: atomic mass of Na = 23 u.

ChemistryClass 10CBSE

Why sodium's mass is 23

Na has 11 protons + 12 neutrons = 23 u. It has only one stable isotope (Na-23) unlike most elements, no weighted average is needed.

PhysicsClass 10CBSE

Causes of friction

Interlocking of microscopic surface irregularities (asperities) — primary causeMolecular adhesive forces (van der Waals forces between contact surfaces)Normal force — greater pressing force = greater interlocking = more friction (f = μN)Nature of materials — rubber/concrete = high friction; ice/ice = very lowSurface deformation — soft materials deform to increase contact areaLubricants — reduce friction by creating a separating film

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