Biology IA · D · Continuity and change · topic 23 of 25

Potato cores: does salt or sugar pull more water? · IB Biology IA idea: guide, sample graph and step-by-step plan

classic, but done with a real prediction. good.

Same concentration of salt and sugar. Does the potato care? Predict where each one balances, then see if you're right.

Difficulty●○○ Gentle
KitBasic: potato, cork borer, balance, beakers, salt and sucrose solutions
TimeAbout 10 hours
FitsSL and HL (the water-potential twist is HL content)
  • ✓ you want the classic practical with a proper question
  • ✓ you like predictions you can test with numbers
  • ✓ you enjoy careful, repeatable technique

The big idea

← predict BEFORE you measure

Water moves by osmosis towards the region of higher solute concentration. A potato core gains mass in dilute solution and loses it in strong solution; somewhere in between, there is no change. That concentration is a measurement of what the tissue contains. Salt splits into two ions, sugar does not, so for the same molarity the pull on water should differ. Your question is to find where each solute gives zero change, and whether the two differ by the factor theory suggests. Make the prediction before you collect the data.

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 →

Different salts

Compare NaCl with CaCl₂ or KCl, using the same molarity range.

→ The number of ions per formula unit changes; your prediction for the zero-change concentration changes with it, and the safety notes differ.
Change the tissue

Use sweet potato, carrot, beetroot or apple instead of potato.

→ The zero-change concentration reflects a different tissue, so you can compare tissues; beetroot also lets you test pigment leakage.
Use length, not mass

Measure the change in core length instead of mass.

→ A different dependent variable, with different uncertainty; you must explain why it is a good measure and compare it with mass.
Follow it through time

Weigh cores at several times in one solution, not just once.

→ You get mass against time; you can see how long equilibrium takes, which helps you justify your chosen time.
Convert to water potential

Use a cited relationship to convert your zero-change concentrations to water potential.

→ Your conclusion is in kilopascals and compares with published values; you must show the conversion and its assumptions.
Change the context

Link it to road salt on plants, preserving food with salt or sugar, or drip solutions in hospitals.

→ Gives the introduction a real purpose; the method and the graph stay the same.

Your Biology IA research question: run the check

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

  • Names the two solutes and the concentration range, with units (mol dm⁻³) and how many values.
  • Names the dependent variable as a percentage change in mass and gives the formula.
  • Says what you will find from the graph (the concentration at zero change) and how you will get it with an uncertainty.
  • Says what stays the same: core size and shape, volume of solution, time, temperature and how the cores are blotted.
  • Says what you will compare the result with, and states your prediction before the data.
  • Is small enough for at least three cores per concentration for both solutes in the time you have.

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The set-up

One way to arrange it. Draw yours with your real volumes and times for the Method section.

NaCl00.20.40.6sucrose00.20.40.6mol dm⁻³gsame volume in each,cover to stop evaporationblot every corethe same waycut with a borer,trim to equal length

How your Biology 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 osmosis matters; your prediction for each solute, and why the two should differ.
Background biology~450
Osmosis and water potential; why NaCl and sucrose differ; what zero mass change means; how the two solutions should compare in theory, with citations.
Variablestable
Independent, dependent and controlled variables with how each is held or measured.
Method~450
How cores are cut and trimmed; how solutions are made up by dilution; volume, time, temperature, blotting, weighing and number of cores per concentration.
Safety, ethics, environment~50
Short and specific: sharp borer, wet floors, disposal of the potato and solutions.
Raw datatable
Initial and final mass of every core for both solutes, with units and balance resolution.
Processing~350
Percentage change in mass for each core; mean and spread per concentration; one worked example; how the zero-change concentration is found with an uncertainty.
Graphs and analysis~400
Percentage change against concentration for both solutes on one graph, with error bars; the line or curve chosen and why; the zero crossing and its uncertainty.
Conclusion~400
Answer with the two concentrations and their uncertainties; compare the ratio with your prediction and with published values; say what range the claim covers.
Evaluation~600, the biggest part!
Weaknesses ranked by their effect on the zero-change concentration: blotting, differences between cores, time and solution concentration.
Referencesnot counted
Theory, osmosis background and any published values.

What your Biology IA graph could look like

both solutes, one graph ✓

Made-up data, to show you the form of a strong graph, not the answer. The data points are for one solute; yours needs both on the graph. example only

0.00.10.20.30.40.50.6-25-20-15-10-5051015202530Concentration of solute / mol dm⁻³ (± 0.01)Change in mass / % (mean ± SD, n = 3 cores)sucrose: fitted curveNaCl: fitted curve (no points drawn)sucrose cores, with SDwhere each curve crosseszero is your resultILLUSTRATIVE · MADE-UP DATA
  • The zero crossing is read from the curve or from a straight line fitted to the points near zero. State which, and give an uncertainty.
  • Error bars are the spread of the replicate cores, and the caption says n.
  • Plot both solutes on the same axes, in different symbols, so the shift in the crossing is visible.
  • Percentage change allows cores of different starting masses to be compared.
  • Another graph to consider: the same data plotted against the concentration of ions (mol of particles per dm³), to test whether the two curves overlap.

Which uncertainty matters most?

Rank by the effect on your result, the percentage change in mass, not on the raw weighings. Numbers here are for a made-up core of about 2.0 g.

Blotting (±0.05 g)2.5 ppSpread between cores (SD)2 ppBalance (±0.01 g, two weighings)0.7 ppSolution strength (±0.01)0.4 ppbiggest effect: discuss this first!uncertainty in % change in mass, in percentage points (pp)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

the trial run tells you how long to soak

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

0 of 10 done

  1. Read and predictLearn osmosis and water potential. Work out what your two solutes should do and where the zero-change concentration should be for each. Write the prediction before you collect data.
  2. Trial run (1 lesson)Try a few concentrations and a few times. How long until the change stops? How much does blotting vary? Are your cores really the same size?
  3. Decide range, time and replicatesChoose at least five concentrations that cross zero for both solutes, a time, and at least three cores per concentration. Write down why.
  4. Make up the solutionsDilute from a stock with a clear method. Record the volume and measuring equipment, since the concentration is your independent variable.
  5. Cut and weigh the coresSame borer, same length, no skin. Blot and weigh each core before it goes in and record the order.
  6. Soak and re-weighSame time, same temperature, covered beakers. Blot each core in exactly the same way and weigh quickly.
  7. ProcessPercentage change for each core, then mean and SD for each concentration. Decide how you will find the zero point and its uncertainty.
  8. Graph and comparePlot both solutes with error bars. Compare the zero points with your prediction, with uncertainty, and with published values.
  9. EvaluateRank the weaknesses by their effect on the zero-change concentration. Tie each improvement to a problem in the data.
  10. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Biology you'll need

Traps to avoid in this Biology IA

  • Blotting differently each time: a wet core weighs more, and the error is as large as the effect you are measuring.
  • Cores of different length or from different potatoes, which differ in how much solute they hold.
  • Only three or four concentrations, none close to the zero point.
  • Using final mass instead of percentage change, so cores of different sizes cannot be compared.
  • Assuming NaCl should give exactly half the sucrose value and calling any other result an error. Test the prediction, and say what the ions do in real solutions.
  • Leaving the beakers uncovered so the solution evaporates and becomes stronger.
  • Reading the zero point by eye with no uncertainty.

More Biology 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.

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