Chemistry IA · Structure 2 · Models of bonding and structure · topic 7 of 25
Same molar mass, different boiling points · IB Chemistry IA idea: guide, sample graph and step-by-step plan
a good first database IAMolecules with almost the same molar mass can boil more than 100 °C apart. Find out what each functional group adds, and why.
- ✓ you like intermolecular forces
- ✓ you want a gentle first database IA
- ✓ you can match compounds carefully
The big idea
← this is your RQ hook!Boiling needs the molecules to overcome the forces between them, so the boiling point tells you how strong those forces are. If you hold molar mass almost constant, dispersion forces are about equal and the differences come from the functional group: dipoles, then hydrogen bonding. Match your compounds carefully, or the effect you are hunting hides behind molar mass. Then go beyond ranking: use the enthalpy of vaporisation to put a number on each group's contribution.
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 →
Compare compounds with the same molecular formula, such as isomers of C₃H₈O, so molar mass is identical.
→ The control is perfect. Differences now come only from structure, and your explanation is sharper.Collect the enthalpy of vaporisation for each compound from the same source and compare it with the boiling point.
→ Adds a second variable and a graph. You can test the idea that boiling point is about the energy needed to separate molecules.Calculate the entropy of vaporisation and compare it across the groups.
→ Hydrogen-bonded liquids should break the rule. The size of the deviation is a clear, measurable result. Entropy is HL (Reactivity 1.4), so SL students introduce it themselves.Compare a series (for example alkane, alcohol and amine) at two or three chain lengths.
→ Two variables at once. You can separate the effect of the group from the effect of size, and see whether the group's contribution is constant.Include a straight-chain and a branched isomer in each family.
→ Adds a shape effect that works through contact area, a good test of whether your 'same forces' assumption holds.Your Chemistry IA research question: run the check
Write your own question first. Then make sure it ticks every box:
- Names the independent variable (the functional group) and the dependent variable (boiling point, with unit and pressure).
- Says how closely molar mass will be matched (give a range or tolerance) and why that range is tight enough.
- Lists the compounds chosen from each group, with enough per group to see a spread.
- Names the single source for all the boiling points and states the pressure they apply to.
- Is narrow enough to finish in about 10 hours and has an explanation you can test with a number.
The set-up
Your 'apparatus' is a data table, and the chemistry sketch is what you explain with. Draw the forces for your own compounds.
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 intermolecular forces matter (fuels, solvents, separations); the order of boiling points you predict and why.
Dispersion, dipole-dipole and hydrogen bonding; what each depends on; where a carboxylic acid dimer fits. Cite sources.
Independent, dependent and controlled variables, with how molar mass and source were matched and checked.
Which source, which compounds, and the rule for choosing them; how close in molar mass; the pressure of the values.
No chemicals, so keep it short. Mention source terms of use.
Compound, formula, molar mass, functional group, boiling point, with units and the pressure.
Calculation of molar mass and mean per group; any enthalpy or entropy calculation with one worked example; units.
A graph that shows groups side by side; what the ordering says about forces; where one group does not fit.
Answer with values; say how big each group's effect is compared with the spread inside a group.
Limits of the sample, of molar-mass matching, of the source, and of your explanation; what you would add.
The source of each value and every text used.
What your Chemistry IA graph could look like
the reference line does the thinkingMade-up data to show the form of a strong graph, not the answer. Here, boiling point is plotted against enthalpy of vaporisation. example only
- Both quantities come from tables, so the 'error bar' is the rounding in the source. Say so.
- The reference line gives the reader something to compare each point against, not just a trend.
- Label or colour the points by functional group so the grouping can be seen.
- Boiling point in kelvin here because ΔS = ΔH/T needs it. If you plot °C, say why.
- Another graph to consider: boiling point by group with every compound shown, so the spread inside a group is clear.
Your game plan
write the molar-mass rule down FIRSTTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictRevise the three kinds of intermolecular force. Rank the groups you plan to compare by the forces they allow, and write that order down before collecting data.
- Choose and test a sourcePick one source for every value. Check the pressure the boiling points refer to and how it lists isomers.
- Trial run (1 lesson)Find one compound per group and check molar masses. Can you match them closely? If not, relax the tolerance or change the groups.
- Decide your sampleSet the molar mass range and list the compounds in each group. Write the rule before you see all the boiling points.
- Collect the dataRecord each value with formula, molar mass, source and pressure. Keep the raw sheet untouched.
- ProcessCalculate means and spread for each group. Add any enthalpy or entropy calculation on a separate sheet with units.
- Explain with evidenceFor each group, say which force explains its position and test the claim with a number, not only a statement.
- EvaluateWhat limits the conclusion most: the molar-mass tolerance, the small sample in a group, or the single source? Rank them.
- Write, then cutWrite in the order of the layout. Tables, graphs and references are not counted.
- Check before your teacher's draftRun the checklist below and get your RQ or draft checked.
Chemistry you'll need
- IB Chemistry data booklet →
- Uncertainties and error bars (physics examples, same rules) →
- IB command terms explained →
Traps to avoid in this Chemistry IA
- Not matching molar mass closely enough: if one compound is 20 g mol⁻¹ heavier, the group effect is mixed up with size.
- Treating carboxylic acids like other hydrogen-bonded compounds. They pair up as dimers, which changes both the boiling point and the vapour, so comment on it.
- Calling every difference 'hydrogen bonding'. A ketone has no O–H, so say what it does have.
- Mixing boiling points measured at different pressures, or from different sources.
- One compound per group, so a single odd result cannot be told from a real group effect.
- An evaluation that says 'the source might be wrong' without a specific test or comparison.
- Taking enthalpies of vaporisation at 25 °C for some compounds and at the boiling point for others. Trouton's rule needs the value at the boiling point.
More Chemistry IA ideas like this
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- Plasticised starch films: how much glycerol?●●○
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.