TutorForAll

Chemistry 11–12 · Year 12

Comparing properties along the alcohol homologous series: boiling point, evaporative cooling, miscibility and flame

Module 7: Organic Chemistry

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Members of a homologous series differ by one CH2 unit, so properties that depend on dispersion forces change steadily along the series while the functional group's chemistry stays the same.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

  • all four alcohols and hexane are highly flammable
  • methanol is toxic by inhalation and skin absorption
  • hexane vapour harmful

Controls

  • no flames on the learner bench; the combustion step is a teacher demonstration on a heat mat away from the stock bottles
  • stoppered tubes, well-ventilated room
  • gloves for methanol
  • eye protection

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

  • Methanol, ethanol, propan-1-ol, butan-1-ol and hexane, 5 mL each in stoppered tubes
  • Temperature probes with logger, four, and cotton wool sleeves; or thermometers to 0.1 degrees Celsius
  • Boiling tube, water bath at 90 degrees Celsius for the ethanol boiling test, anti-bumping granules and a thermometer in the vapour
  • Watch glasses, five; spirit burner caps; deionised water, test tubes, droppers

How to do it

  1. Evaporative cooling: wrap each probe tip in cotton wool, soak each in a different alcohol, start logging and record the lowest temperature reached and the time to reach it; hexane last.
  2. Boiling point (ethanol only, in the 90 degrees Celsius water bath): heat 2 mL with granules in a boiling tube and read the steady vapour temperature; record. Propan-1-ol boils at 97.2 degrees Celsius, above what the bath reaches, so its value comes from the data table.
  3. Miscibility: add 1 mL of each alcohol and of hexane to 3 mL of water; shake and record whether one layer or two forms.
  4. Combustion: the teacher lights three drops of each on separate watch glasses; record flame colour and any soot.
  5. Plot the boiling points from the data table (with the one you measured marked) against carbon number, and the evaporative temperature drop against carbon number; explain the trends in terms of dispersion forces and hydrogen bonding.
  6. In the simulation compare boiling point, energy per mole and energy per gram along the series.

What you should see

Boiling points rise steadily along the series: methanol 64.7, ethanol 78.4, propan-1-ol 97.2 and butan-1-ol 117.5 degrees Celsius (NIST WebBook), steps of 14, 19 and 20 degrees; the measured plateau for ethanol sits close to 78.4 degrees Celsius. Evaporative cooling is largest for methanol and falls at each step to butan-1-ol, in the same order as volatility; hexane cools strongly too because it has no hydrogen bonding to overcome. Methanol, ethanol and propan-1-ol mix fully with water; 1 mL of butan-1-ol in 3 mL of water is more than dissolves, so a second layer floats; hexane forms a separate layer at once. Flames go from nearly invisible blue (methanol) to yellow and sooty (butan-1-ol and hexane).

What changes

What you change
carbon number of the alcohol
What you measure
boiling point, evaporative temperature drop, miscibility, flame appearance
What you keep the same
  • volumes
  • room temperature and airflow
  • probe wrapping
  • same water bath

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Bigger molecules have stronger bonds so they boil higher (the covalent bonds are unchanged; the intermolecular dispersion forces grow with chain length).
  • Alcohols mix with water because they are liquids (they mix through hydrogen bonding at the OH group; hexane, also a liquid, does not).
  • The alcohol that cools the probe most has the highest boiling point (the reverse: the most volatile member evaporates fastest and cools most).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-14CH11/12-5CH11/12-6
  • Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-03CH-12WS-05CH-12WS-06
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/chemistry-stage-6-2017
  2. www.nsw.gov.au/sites/default/files/noindex/2025-03/chemistry-stage6-syllabus-word.docx
  3. curriculum.nsw.edu.au/learning-areas/science/chemistry-11-12-2025/outcomes
  4. edu.rsc.org/experiments/the-properties-of-alcohols/463.article
  5. webbook.nist.gov/cgi/cbook.cgi?ID=C64175&Mask=4
  6. webbook.nist.gov/chemistry

All Concept Studio activities