Chemistry IA · Structure 2 · Models of bonding and structure · topic 8 of 25
Chromatography with a changing solvent polarity · IB Chemistry IA idea: guide, sample graph and step-by-step plan
colourful and easy to startDye chromatography, but now you choose how polar the solvent is. Does polarity change who wins the race?
- ✓ you like colourful practicals
- ✓ you can be patient with a ruler
- ✓ you enjoy explaining with polarity
The big idea
← this is your RQ hook!In paper chromatography each dye is pulled two ways: dissolved in the moving solvent, or held by the paper, which is polar and holds water. The Rf value, the spot distance divided by the solvent-front distance, measures that balance. Change the ethanol-to-water ratio and you change the polarity of the solvent. Predict from polarity first: which way should Rf move for a very polar dye? Then test it, and see whether two dyes respond differently, which is what a good separation needs.
Make it yours: pick one twist
Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.
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Choose dyes that differ in size or number of charged groups, for example a red and a blue.
→ The curves of Rf against composition change, and the link between structure and behaviour gets stronger.Use propanone or propan-2-ol with water instead. Check hazards first.
→ A different polarity scale, so you compare solvents as well as compositions.Work out the distance between two dyes' spots at each composition.
→ One quantity that tells you the best mix, so your conclusion can name an optimum.Change the pH of the water part with a small, controlled amount of acid or alkali.
→ Some dyes change charge or colour with pH, so you can discuss ionisation. Check hazards.Photograph each paper beside a ruler and measure distances on screen.
→ Reading uncertainty drops and you can quote a smaller uncertainty on Rf.Frame it around checking food colourings, or ink in a forensic case.
→ A real reason for the introduction. An unknown sample can be matched by its Rf.Your Chemistry IA research question: run the check
Write your own question first. Then make sure it ticks every box:
- Names the independent variable (ethanol per cent by volume), its range, step and number of values, and which dyes.
- Names the dependent variable, Rf, and says which point of each spot you measure to.
- Names the controls (paper, run distance or time, spot size, temperature, tank saturation) and how you hold each constant.
- Says how each solvent mixture is made accurately, and how many separate runs you do at each composition.
- Says how the uncertainty in Rf is estimated from your distance measurements.
The set-up
One way to set it up. Draw your own version, with your real paper size and distances, for the Method section.
How your Chemistry IA could be laid out
The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.
Your question; why separation matters (food colours, inks, forensics); what you predict about polarity and Rf.
Stationary and mobile phase; polarity of ethanol, water and the dyes; what Rf means; prediction of your graph for each dye.
Independent, dependent and controlled variables, each with how you set, measure or keep it constant, and why it matters.
How each solvent is made; paper, spotting and tank; baseline and front; how distances are read; repeats; a diagram.
Ethanol is flammable and its vapour builds up in a closed tank: no flames, ventilate. Goggles; gloves for dyes, which stain. Pencil baseline. Dispose of dilute waste as your teacher directs.
One clear table: composition, distances moved by each spot and the front, with units and ruler resolution.
Rf for each spot and run, mean and spread, uncertainty. One worked example.
Rf against composition for each dye with error bars; where the dyes separate best; what the shape says about polarity.
Answer with numbers, inside your range, and compare with a cited explanation of dye behaviour on paper.
Weaknesses ranked by effect on Rf (spot reading, front, tank saturation, mixing accuracy); improvements linked to each.
Every source for method, dye information and theory.
What your Chemistry IA graph could look like
two dyes, one graph ✓Made-up data, to show you the form of a strong graph, not the answer. The direction of your curve is for you to predict and test. example only
- Rf has no unit. Say how you measure the distances that give it.
- Error bars on every point, and on both dyes if you plot both.
- Two curves show whether polarity helps separate the dyes. Where they come together, it does not.
- The curve direction here is invented. Predict yours from polarity before you test.
- Another graph to consider: the distance between the two spots against composition, to find the best solvent.
Which uncertainty matters most?
rank them: don't just list themRank by the effect on your result, Rf, not on raw readings. Here is one made-up spot that moved 35 mm while the front moved 80 mm. Work out yours.
Your game plan
mark the front the moment it's outTick steps off as you go. We'll remember on this device.
0 of 11 done
- Read and predictRead about paper chromatography and polarity. Predict which way Rf should move for your dyes as ethanol rises, and whether the dyes should separate more or less.
- Trial run (1 lesson)Run two dyes in one mixture. Does the front run straight? Do the spots stay round? How long does a run take? Does the baseline stay above the solvent?
- Decide range, dyes and repeatsChoose at least six compositions, your dyes and the run distance. Decide how many runs per composition, with reasons.
- Make the solventsMake each mixture accurately, with a pipette or burette, and keep it covered. Make it fresh for each set of runs.
- Control the runDecide spot size, baseline height, tank saturation and run distance, and use them for every run.
- CollectMark the front as soon as you remove the paper. Record every distance. Photograph the paper with a ruler.
- ProcessWork out Rf for each spot, then mean and spread. Decide how the reading uncertainty carries through.
- Graph and interpretPlot with error bars. Does the direction match your polarity prediction? Where are the dyes best separated?
- EvaluateRank the weaknesses with numbers. Make each improvement specific.
- Write, then cutWrite in the order of the layout above, within 3,000 words. Tables, calculations and graphs are not counted.
- Check before your teacher's draftRun the checklist, then get your RQ or draft checked. Your teacher reads one draft only.
Chemistry you'll need
Traps to avoid in this Chemistry IA
- Letting the solvent front run off the paper, or removing it late. You cannot find the front, so Rf is a guess.
- Drawing the baseline in pen. The ink itself separates.
- Reading the centre of a spread-out spot differently each time. Decide a rule, and photograph the paper.
- A baseline below the solvent level, so the spots wash into the tank.
- Mixing solvents in a measuring cylinder and leaving them open: ethanol evaporates and the composition drifts.
- A tank that is not saturated with vapour, so the front curves and the solvent evaporates from the paper.
- Drawing a smooth curve through a few compositions and then claiming an optimum between them.
More Chemistry IA ideas like this
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This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.