Question bank — Chromatography — TLC, column, GC, HPLC (principles)

False — large $K$ means it prefers the stationary phase, so $k$ is large, $v=v_m/(1+k)$ is small, and it elutes last.

False — polar silica grabs polar molecules, so they lag near the baseline and $R_f$ is small.

False — $R_f$ also depends on solvent polarity, plate, and temperature; it is only reproducible for fixed conditions, which is why knowns are run alongside.

False — separation needs different $k$; identical $k$ means identical average speed, so they never pull apart no matter the length.

True — the stationary phase is non-polar (C₁₈), so polar molecules barely stick, ride the polar mobile phase, and elute early.

True — $k=0$ means no time spent stuck, so $v=v_m/(1+0)=v_m$; it marks the "dead time" of an unretained marker.

False — order is set by relative $k$ values; faster flow shortens all retention times but keeps the sequence (least-sticky-first) unchanged.

False — sugars char before they vaporise; GC needs volatile, thermally stable samples, so you'd use UV-detected HPLC instead.

True — $k=K{V}_s/V_m$ grows with $V_s$, so all $k$ rise and everything slows (though separation may improve).

The pressure does not separate; separation comes from differing $K$/$k$. Pressure merely forces solvent through the tiny (3–5 µm) particles, whose large surface area is what improves resolution.

Backwards — high boiling point means it stays a liquid on the film longer, spends less time as vapour, has larger $k$, and elutes last; nothing "falls out".

Impossible — $R_f$ = spot distance ÷ solvent-front distance, and a spot cannot pass the front, so $R_f$ is always between 0 and 1. A value above 1 signals a measurement error.

No — they are opposite. Normal phase (polar silica) retains polar analytes; reverse phase (non-polar C₁₈) retains non-polar analytes.

Wrong — stickiest means largest $k$, slowest velocity, so it elutes last; the least-retained (least polar) component comes out first.

Not necessarily — $K$ compares concentrations, not total amounts; the actual fraction stuck also depends on the phase volumes $V_s$ and $V_m$ (that's why $k$, not $K$, sets the speed).

Gravity opposes the rise; the solvent climbs by Capillary Action — surface tension pulling liquid into the fine silica pores against gravity.

Because a ratio cancels how long you ran the plate; the same compound gives the same $R_f$ whether the front rose 5 cm or 8 cm, making it a reproducible fingerprint.

The mobile phase is the only thing physically flowing; while stuck on the stationary phase the molecule is anchored and contributes zero forward displacement, so its average speed is diluted by its stuck-time fraction.

Gases diffuse and re-equilibrate between phases very fast, so bands stay narrow; less spreading means better-resolved peaks — the core advantage of GC.

Because solvent polarity, plate material, and temperature all shift how strongly a compound partitions (Distribution / Partition Equilibrium), so an $R_f$ is only meaningful with its run conditions attached.

Finer particles pack more tightly and resist flow (higher back-pressure), so a high-pressure pump is needed to keep solvent moving at a usable speed while gaining the resolution the fine particles offer.

Area accumulates signal over the whole band, so it scales with how much analyte passed; retention time reflects how the compound partitions, which is characteristic of its identity under fixed conditions.

Because affinity depends on the specific intermolecular forces (H-bonding, shape, functional groups), not a single "polarity" number, so their $K$ values — and hence speeds — can still differ.

$R_f=1$ — it travels with the solvent front because it spends no time on the stationary phase ($k=0$).

$R_f=0$ — it is so strongly held ($K\to \infty$, $k\to \infty$) that $v_{\text{analyte}}=v_m/(1+k)\to 0$ and it doesn't move.

$k=K{V}_s/V_m\to 0$ for all, so every compound approaches $v_m$ and nothing separates — you need some stationary phase to create differential lag.

Change the mobile-phase polarity (a different solvent) so their $K$ values diverge more, spreading the spots apart; separation quality is not fixed by the molecules alone.

No — the protein would decompose on vaporisation, so GC misreads or destroys it; you'd need HPLC (or split methods) to keep the protein in solution.

It reduces to $v_{\text{analyte}}=v_m$; an unretained "dead-time" marker (e.g. solvent front) travels at full mobile speed and defines the zero-retention reference.

The two-phase tug-of-war collapses — with no stable stationary phase there is no differential retention, so no separation occurs (a real hazard when choosing incompatible solvents).