Chemistry IA · Reactivity 2 · How much, how fast, how far? · topic 21 of 25
Does lime water get less soluble when warm? · IB Chemistry IA idea: guide, sample graph and step-by-step plan
the odd one out! good choiceSugar dissolves better in hot tea. Calcium hydroxide does the opposite. A titration will show you how much, and the reason takes you into energetics.
- ✓ you like titrations and clean technique
- ✓ you enjoy a result that goes against expectation
- ✓ you like energy ideas explaining a trend
The big idea
← your hook: why does heat *reduce* it?Calcium hydroxide is sparingly soluble, and unlike most solids it dissolves less as the water gets warmer. A saturated solution is an equilibrium, Ca(OH)₂(s) ⇌ Ca²⁺(aq) + 2OH⁻(aq), so you can find its concentration by titrating the hydroxide with standard acid. The prediction is simple. The challenge is getting a truly saturated, truly filtered sample at each temperature. Do that well and your data can be linked to the sign of the enthalpy change of dissolving, and, if you go further, to an equilibrium constant for dissolving (Ksp is not in the course, so introduce it yourself).
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 →
Convert each titre into [Ca²⁺] and [OH⁻], then calculate Ksp at each temperature (Ksp is not in the IB course: introduce it yourself).
→ Your analysis moves from solubility to an equilibrium constant. You must say what you assume (for example ignoring ionic activity) and your graph changes to ln Ksp against 1/T.Use the temperature trend of Ksp to estimate the enthalpy change of dissolving, as an HL extension (it builds on ΔG = −RT ln K).
→ You derive the link yourself and compare with a literature value. A strong conclusion with a quantitative comparison.Repeat at one temperature in solutions with extra calcium ions or hydroxide ions.
→ A second, smaller study on the common-ion effect, with the same titration method.Compare calcium hydroxide with another sparingly soluble salt whose solubility rises with temperature.
→ A comparison of two trends, explained by different signs of the enthalpy of solution. Needs a different analysis method for the second salt.Measure the pH of each filtered sample and calculate [OH⁻] from it.
→ A quick cross-check on the titration. Your evaluation compares the two methods' uncertainty.Link to limewater tests for carbon dioxide, hard water, or lime in soil and cement.
→ Gives your introduction a purpose; the method and graph stay the same.Your Chemistry IA research question: run the check
write it FIRST, then check itWrite your own question first. Then make sure it ticks every box:
- Names the independent variable and its range (which temperatures in °C between 10 and 60, and how many values).
- Names the dependent variable as a concentration in mol dm⁻³, and says it is found by titration with which acid.
- Says how you will make sure each sample is saturated, and how long you leave it at the target temperature.
- Says what you hold the same (volume of sample, acid concentration, indicator, contact time).
- Is narrow enough for about 10 hours, with repeat titrations (concordant titres) at each temperature.
The set-up
One way to set it up. Draw your own with your real temperatures and volumes 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; where sparingly soluble hydroxides matter (limewater, cement, soil); your prediction and the reason.
The solubility equilibrium; the link between the sign of ΔH and the effect of temperature; the neutralisation equation used in the titration; the link to Ksp if you use it (beyond the course, so explain it).
Independent, dependent and controlled variables, each with how you set, measure or keep it constant.
How you saturated each sample and held its temperature; how you filtered without it cooling; how you took an aliquot; the titration; repeats. Repeatable by someone else.
Calcium hydroxide (solid and saturated solution) causes serious eye damage: goggles, not glasses. Hydrochloric acid at your concentration (about 0.05 to 0.1 mol dm⁻³ is low hazard); water bath below 60 °C. Neutralised waste and the filtered solid go where your school directs.
Initial and final burette readings and titres for each temperature, with units and uncertainties in the headers.
From mean titre to mol of acid, mol of hydroxide, then concentration of calcium hydroxide. One worked example with the uncertainty.
Concentration against temperature with error bars, and what the trend and its shape suggest. A second graph if you take the Ksp route.
Answer with numbers: how much the concentration changed over your range, what that says about ΔH, and how your values compare with a published solubility.
Strengths, weaknesses ranked by their effect on the concentration, and improvements linked to each.
Your published solubility data and any method you adapted.
What your Chemistry IA graph could look like
error bars on EVERY point ✓Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only
- Axis labels name the quantity and its unit, with the uncertainty of the temperature you set.
- Error bars on every point. The uncertainty in concentration comes from the titre, the pipette and the acid, not from the thermometer.
- The trend goes down as temperature rises. Say so, then explain it with the sign of ΔH.
- The scale does not start at zero, so the change is visible. State this in your text.
- Another graph to consider: ln Ksp against 1/T (HL extension; Ksp itself you introduce). If it is a straight line, its gradient is linked to the enthalpy of solution.
Which uncertainty matters most?
Rank by the effect on your result, here the concentration from a 60 °C sample (about 8 cm³ of acid). A sample that cools as it is filtered can matter more than the burette.
Your game plan
Tick steps off as you go. We'll remember on this device.
0 of 11 done
- Read and predictWrite the dissolving equilibrium and the titration equation. Predict whether solubility rises or falls with temperature and what that tells you about ΔH.
- Trial run (1 lesson)Make a saturated solution, take one sample at room temperature and one near 60 °C, and titrate both. Does a surface film form? Does the warm sample cool while filtering? How big is a titre?
- Decide range, contact time and repeatsChoose at least five temperatures, how long each mixture sits at temperature, the acid concentration and how many concordant titres. Write the reason for each.
- Standardise your acidDecide whether you will use the stated concentration or check it, and say why. This is a decision examiners like to see explained.
- Collect the dataKeep the container covered. Record readings straight into your table. Note the sample temperature when you filter, not only the bath temperature.
- ProcessConvert titres into concentrations, with uncertainties. Decide how you will combine the uncertainties of the burette, pipette and temperature.
- Graph and fitPlot concentration against temperature with error bars. Decide whether a line, a curve or a transformation tells the story best.
- Compare with published valuesHow close are your values to a data-book solubility at the same temperatures? Is any difference bigger than your uncertainty?
- EvaluateWhich uncertainty or systematic error moved the concentration most? Rank them and make every improvement specific.
- Write, then cutWrite in the order of the layout above. Keep within 3,000 words: tables, graphs, equations and references are not counted, so let them carry detail.
- 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.
Chemistry you'll need
Traps to avoid in this Chemistry IA
CO₂ gets everyone- Carbon dioxide from the air turns dissolved hydroxide into calcium carbonate, lowering the concentration you titrate.
- A sample that cools on the filter paper, so the concentration belongs to a lower temperature than the one you wrote.
- A solution that is not saturated: not enough solid, or too little time at temperature.
- Taking the sample with solid particles in it, which dissolve in the acid and raise the titre.
- Using a different indicator volume or end-point colour each time.
- Reporting only the bath temperature with no uncertainty, and ignoring the uncertainty of the titre.
More Chemistry IA ideas like this
- The vitamin C clock: finding activation energy●●○
- Does the rate really double every 10 °C?●●○
- Iodine and propanone: which reactant sets the rate?●●○
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.