Chemistry IA · Structure 2 · Models of bonding and structure · topic 10 of 25

Plasticised starch films: how much glycerol? · IB Chemistry IA idea: guide, sample graph and step-by-step plan

make it, break it, explain it

Make a bioplastic in the lab, then find out how much glycerol turns a brittle film into a stretchy one.

Difficulty●●○ Medium
KitStandard: hot plate, balance, micrometer, hanging-mass rig
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like making things
  • ✓ you enjoy a bit of materials testing
  • ✓ you care about plastic alternatives

The big idea

← this is your RQ hook!

Starch films are held together by hydrogen bonds between chains. Glycerol is a small molecule with three –OH groups. It sits between the chains, breaks up some of those bonds and makes the film more flexible but weaker. So as glycerol rises, the breaking stress should fall. Measure stress, not just load: films vary in thickness, and a thicker film holds more mass for no chemical reason. Make film thickness part of your calculation, and batch-to-batch variation becomes something you can show and discuss.

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.

Swipe for more twists →

Measure the stretch too

Mark the strip and measure its extension at the break.

→ A second dependent variable. You can plot stress against strain and compare brittle and flexible films.
Change the acid or starch

Vary the vinegar volume, or try potato, maize or cassava starch.

→ A different independent variable with the glycerol held fixed. Acid affects the starch chains, so there is chemistry to explain.
Test the water content

Condition films at different humidities, or weigh them before and after drying.

→ Water is also a plasticiser, so it competes with glycerol. A control you must justify.
Use a better rig

Replace hanging masses with a force meter, or a balance pulled steadily.

→ Smoother loading and a clearer peak force. The evaluation compares the two methods.
Other plasticisers

Compare glycerol with sorbitol, or with plain water.

→ Same measurement, new chemistry: molecule size and number of –OH groups.
Give it a purpose

Frame it around replacing single-use plastic film.

→ A real reason for the introduction. Discuss how strong a real film must be.

Your Chemistry IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable (glycerol volume per fixed mixture), its range and how many values.
  • Names the dependent variable (breaking stress) and says how load and cross-section turn into it.
  • Names the controls (starch mass, water, acid, heating, drying, strip width) and how you hold each constant.
  • Says how and where you measure thickness, and how many strips and films you test at each value.
  • Is narrow enough: five or six glycerol values, at least three strips each.

Get your RQ checked · S$29

The set-up

One way to set it up. Draw your own version, with your real strip size and loading method, for the Method section.

mLfilm strip: fixed width w,thickness t measuredat the break pointclamp stand,well weightedadd mass in small equalsteps; note the last oneheld and the first to failcatch box underthe masses

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.

Research question and context~250
Your question; why plasticisers matter for bioplastics; what you predict about glycerol and strength.
Background chemistry~450
Starch structure and hydrogen bonding; what a plasticiser does at the molecular level; stress against load; prediction of your graph.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant, and why it matters.
Method~500
The recipe with quantities and why; heating; casting and drying; cutting strips; thickness measurement; the loading rig and how you add mass; repeats.
Safety, ethics, environment~80
Hot plate and hot gel burn: stir, tie hair back, let it cool. Goggles. Hanging masses can fall: catch box, feet clear. Never taste anything made in the lab. Films go in general waste.
Raw datatable
One clear table: glycerol volume, strip width, thickness, breaking mass, with units and instrument resolution.
Processing~300
Breaking force, cross-section, breaking stress, mean and spread, uncertainty. One worked example.
Graphs and analysis~350
Stress against glycerol with error bars; whether the shape fits your prediction; what the batch spread tells you.
Conclusion~300
Answer with numbers, within your range, and compare with a cited published range for starch films, saying how comparable the recipe is.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on stress (thickness, drying, loading steps, bubbles); improvements linked to each.
Referencesnot counted
Every source for the recipe, theory and comparison data.

What your Chemistry IA graph could look like

stress, not load ✓

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

0.00.51.01.52.02.53.03.50123456Glycerol / cm³ per 100 cm³ mixture (± 0.1)Breaking stress / MPa (± 0.4)a smooth fit through the datayour films, with error barsis the drop from 0 to 0.5 biggerthan the error bars?ILLUSTRATIVE · MADE-UP DATA
  • Stress, not load, on the vertical axis. Say why in the text.
  • Error bars on every point, from the spread between strips and the thickness uncertainty.
  • Check whether the fit is flat anywhere. A plateau needs an explanation.
  • Do not extend the fit beyond your range.
  • Another graph to consider: stress against strain for two or three compositions, if you measure extension too.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result, the breaking stress, not on raw readings. Here is one made-up strip, 20 mm wide, 0.15 mm thick, breaking at about 600 g. Work out yours.

Thickness (±0.02 mm of 0.15 mm)13.3%Mass added in 50 g steps4.2%Width (±0.5 mm of 20 mm)2.5%Balance (±1 g of 600 g)0.2%biggest effect: discuss this first!% uncertainty in breaking stress for one stripILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

break a spare strip first

Tick steps off as you go. We'll remember on this device.

0 of 11 done

  1. Read and predictRead about starch structure and plasticisers. Predict how breaking stress changes with glycerol, and what you expect to happen to how stretchy the film is.
  2. Trial run (1 lesson)Make one batch and cast a film. Does it peel off cleanly? Is the thickness even? How long does it take to dry? Break one strip to find the load range.
  3. Decide recipe, range and repeatsFix a recipe (starch, water, acid), pick at least five glycerol values and the number of strips per value. Write the reason for each choice.
  4. Fix the casting and drying routineDecide the heating time, the pouring method, the tray and the drying conditions, and use them every time.
  5. Measure and loadCut strips to a fixed width, measure thickness at several points, and add mass in small equal steps. Record the mass at which each strip breaks.
  6. ProcessWork out the breaking force and stress, mean and spread. Decide how thickness, width and mass uncertainties combine.
  7. Graph and interpretPlot with error bars. Does the shape fit your hydrogen-bonding prediction? Is any step bigger than the error bars?
  8. Compare with literatureFind a cited range for similar films. How comparable are the recipes?
  9. EvaluateRank the weaknesses with numbers. Make each improvement specific.
  10. Write, then cutWrite in the order of the layout above, within 3,000 words. Tables, calculations and graphs are not counted.
  11. 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

  • Reporting load instead of stress, so a thicker film looks stronger for no chemical reason.
  • Measuring thickness once, at an edge, rather than at several points including where it broke.
  • Films not dried equally. Water content changes strength as much as glycerol does.
  • Bubbles and uneven pouring, so every strip tears at the same weak spot and it is not a property of the recipe.
  • Adding mass in big jumps, so the breaking load is only known to the nearest big step.
  • Clamps that cut the film, so it always tears at the clip.
  • One film per composition and then claiming a trend.

More Chemistry IA ideas like this

Make sure it's yours

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.

Get your RQ checked · S$29