IB Chemistry HLTopic 2 — Models of Bonding & StructurePaper 1 & 2Practical skill~8 min read
Chromatography
Chromatography looks like a practical technique dropped into the middle of a bonding topic. It is actually a direct application of it: the whole separation is a competition between two sets of intermolecular forces, and whichever set wins for a given substance decides how far up the paper it travels.
📘 What you need to know
Chromatography separates the components of a mixture so they can be identified.
There are always two phases: a mobile phase (a liquid solvent that moves) and a stationary phase (a solid that stays put).
Each component moves at its own rate depending on its solubility in the mobile phase and its attraction to the stationary phase.
The separation happens because of differences in intermolecular forces.
Paper chromatography uses cellulose, which carries polar –OH groups. TLC uses a thin layer of silica or alumina on glass or plastic.
The retardation factor is Rf = distance travelled by the component ÷ distance travelled by the solvent.
Rf values have no units and are always less than 1.
The tug of war that does the separating
Picture one molecule of your mixture sitting on the paper as the solvent creeps past. It has a choice at every moment: stick to the paper, or dissolve and travel with the solvent.
A molecule that is strongly attracted to the stationary phase spends most of its time stuck. It travels a short distance and has a low Rf.
A molecule that is more soluble in the mobile phase spends most of its time moving. It travels far and has a high Rf.
Because paper is made of cellulose, which is covered in polar –OH groups, it forms hydrogen bonds with polar molecules. So with a nonpolar solvent, polar substances cling to the paper and lag behind while nonpolar substances race ahead.
Watch the wording in questions. If the solvent is polar rather than nonpolar, the pattern can reverse. Reason it out from the forces each time instead of memorising “polar goes slower”.
A paper chromatography run
The baseline is drawn in pencil for a reason: pencil is graphite, which is insoluble and stays put. Ink would dissolve and run up the paper along with your sample.
🧩 The method, step by step
Draw a baseline in pencil near the bottom of the paper.
Spot the mixture on the baseline. Keep the spots small and let each application dry before adding the next.
Suspend the paper in the solvent with the baseline above the solvent level.
Cover the tank so the solvent does not evaporate and the atmosphere stays saturated.
Let the solvent rise by capillary action, carrying the components at different rates.
Remove the paper before the solvent reaches the top and immediately mark the solvent front.
Dry the chromatogram and measure from the baseline to the centre of each spot.
Thin layer chromatography
TLC works on exactly the same principle. The only change is the stationary phase: instead of paper you use a thin layer of an inert solid — usually silica or alumina — coated onto a sheet of glass or plastic.
Those surfaces also carry –OH groups, so they form hydrogen bonds with polar components in the same way. Here components are said to be adsorbed onto the surface to different extents, which is what holds some back more than others. TLC plates give sharper spots and run faster than paper, which is why they are used to follow the progress of a reaction.
Calculating Rf
Retardation factor
Rf = distance travelled by the componentdistance travelled by the solvent
Measuring the two distances
Rf only means anything alongside the conditions: the same substance gives a different value with a different solvent, so always state which solvent was used.
🧠 Why Rf can never reach 1
The solvent is what carries the component up the paper. A spot cannot possibly travel further than the thing pushing it, so x < y and the ratio must be below 1. If you calculate an Rf greater than 1, you have measured to the wrong line or divided the wrong way round.
Worked examples
WE 1
A spot travels 3.6 cm while the solvent front travels 8.0 cm. Calculate the Rf value.
Step 1: write the formula
Rf = distance by component ÷ distance by solvent
Step 2: substituteRₜ = 3.68.0Step 3: divide and check it is sensible3.6 ÷ 8.0 = 0.45
Less than 1, so the answer is reasonable.
Rₜ = 0.45 (no units)writing “0.45 cm” loses the mark — the units cancel
WE 2
Two substances give Rf values of 0.18 and 0.76 in the same nonpolar solvent. Which is more polar, and why?
Step 1: what does a low Rf tell you?
It travelled a short distance, so it spent most of its time stuck to the paper.
Step 2: why would it stick?
Paper is cellulose with polar –OH groups, so it hydrogen bonds to polar molecules.
Step 3: apply it to the numbers
The one with Rf 0.76 dissolved better in the nonpolar solvent, so it must be the less polar of the two.
The Rₜ = 0.18 substance is more polarthis reasoning flips if the solvent is polar — always check which solvent you were given
WE 3
A student’s spots do not move from the baseline at all. Explain what went wrong and what to do.
Step 1: what does no movement mean?
The components did not dissolve in the mobile phase, so they were never carried anywhere.
Step 2: name the cause
The sample is insoluble in that solvent.
Step 3: give a practical fix
Repeat with a different solvent, and try several to find one that separates all the components clearly.
Insoluble in the mobile phase — change the solventif all the spots run together at the top, the solvent is too good — try a weaker one
💡 Exam tips
Baseline in pencil, above the solvent. Both details are commonly worth a mark on their own.
Explain separations with intermolecular forces, naming hydrogen bonding where it applies.
Give Rf as a decimal with no units, usually to 2 significant figures.
Measure to the centre of each spot, and take both measurements from the baseline.
Say why the tank is covered: to stop the solvent evaporating and keep the atmosphere saturated.
Identify unknowns by running reference compounds alongside on the same paper, in the same solvent.
⚠ Common mix-ups
Starting the paper below the solvent line. The sample dissolves off into the tank and the run is ruined.
Drawing the baseline in pen. The ink separates too and contaminates the chromatogram.
Dividing the wrong way round. Component distance goes on top, solvent on the bottom.
Giving Rf units. It is a ratio of two lengths, so the units cancel.
Comparing Rf values from different solvents. They are only comparable under identical conditions.
Mixing up the phases. Mobile is the moving solvent; stationary is the paper or plate.
Up next: Resonance Structures — the first of the higher level topics, where you meet molecules that a single Lewis formula simply cannot describe properly.
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