JEE doubts
700 doubts found
Why must a Grignard reagent be made and used in perfectly dry conditions?
A Grignard reagent (R–MgX) has a strongly nucleophilic, basic carbon.
How does the Lucas test tell primary, secondary and tertiary alcohols apart?
The Lucas reagent (conc.
Why is acetic acid (vinegar) a stronger acid than phenol or ethanol?
When a carboxylic acid loses its O–H proton, the negative charge on the carboxylate is shared equally over two oxygen atoms by resonance, giving a very stable ion — so the acid readily gives up its proton.
How are diazonium salts used to make the bright dyes in clothes?
A primary aromatic amine treated with nitrous acid at low temperature (0–5°C) forms a diazonium salt.
I keep getting “Molar mass” questions wrong in Some Basic Concepts (Mole Concept). What's the trap?
using atomic number instead of atomic mass to find molar mass.
I keep getting “Orbitals” questions wrong in Structure of Atom. What's the trap?
drawing fixed circular orbits (Bohr) instead of probability clouds.
I keep getting “VSEPR theory” questions wrong in Chemical Bonding & Molecular Structure. What's the trap?
counting only bonding pairs and ignoring lone pairs.
I keep getting “Entropy (S)” questions wrong in Chemical Thermodynamics. What's the trap?
thinking entropy always increases for the system alone.
I keep getting “Le Chatelier's principle” questions wrong in Equilibrium. What's the trap?
predicting the wrong direction of shift.
I keep getting “Oxidation number” questions wrong in Redox Reactions. What's the trap?
forgetting that oxidation number can be fractional or zero (e.g. in O₂, it's 0).
I keep getting “Ionization energy” questions wrong in Periodic Table & Periodicity. What's the trap?
ignoring small dips (e.g. B<Be, O<N) caused by sub-shell stability.
I keep getting “Rate constant” questions wrong in Chemical Kinetics. What's the trap?
thinking k depends on concentration.
I keep getting “Anode and cathode” questions wrong in Electrochemistry. What's the trap?
assuming the anode is always the positive terminal.
I keep getting “Resonance” questions wrong in Organic Chemistry — Basic Concepts. What's the trap?
believing the separate resonance structures actually exist and interconvert.
I keep getting “Markovnikov's rule” questions wrong in Hydrocarbons. What's the trap?
reversing it, except in the peroxide (anti-Markovnikov) effect.
Why must the saline drip given to a patient be 'isotonic' and not plain water?
Red blood cells sit in blood plasma of a certain solute concentration.
How does adding a tiny amount of another element (doping) turn silicon into a useful semiconductor?
Pure silicon has few free carriers.
Why do real gases stop obeying PV = nRT at very high pressure or very low temperature?
The ideal-gas law assumes molecules take up no space and don't attract each other.
Why are coordination compounds like chlorophyll and vitamin B12 essential to life?
Both are metal complexes doing specific jobs.
Why does a tertiary halide give a racemic (mixed left/right) product while a primary one gives a single mirror image?
A tertiary halide reacts by SN1 through a flat, planar carbocation.
Why does oxidising a primary alcohol give a different product than a secondary one?
Oxidation removes hydrogens from the C–OH carbon.
Why does a carboxylic acid have a much higher boiling point than an alcohol of similar mass?
Carboxylic acids form especially strong hydrogen bonds — in fact two molecules pair up into a cyclic 'dimer' held by two hydrogen bonds, effectively doubling the particle size.
Why does glucose give a positive Fehling's/Tollens' test but table sugar (sucrose) doesn't?
These tests need a free aldehyde (or α-hydroxy ketone) group that can be oxidised.
What does “Limiting reagent” actually mean in Some Basic Concepts (Mole Concept), and why does it matter for JEE?
the reactant that runs out first and therefore caps how much product forms.