Simple distillation of salt water: boiling-point plateau and purity of the distillate

The idea

A homogeneous mixture is separated by boiling point: the thermometer reads a plateau at the boiling point of the volatile component while the dissolved solid stays behind.

Safety card

What you need

• Quickfit distillation set: 100 mL round-bottom flask, still head with thermometer pocket, Liebig condenser, receiver adapter, 50 mL receiving flask
• Thermometer or temperature probe, minus 10 to 110 degrees Celsius
• Heating mantle or Bunsen burner with gauze
• Anti-bumping granules, 3 or 4
• 50 mL of 5 percent sodium chloride solution (about 2.5 g NaCl in 50 mL water)
• Conductivity meter or two evaporating basins for a residue test
• Rubber tubing to a cold tap and to the sink

How to do it

1. Assemble the apparatus with the thermometer bulb level with the side arm of the still head and cooling water entering the condenser at the lower end.
2. Add the salt solution and anti-bumping granules to the flask; heat gently.
3. Record the thermometer reading every 30 seconds from first boiling until about 20 mL has distilled.
4. Test the distillate and the original solution with the conductivity meter, or evaporate 2 mL of each in separate basins and compare the residue.
5. Plot temperature against time and mark the plateau.
6. Repeat once with a fresh charge if time allows and compare plateau temperatures.

What you should see

The temperature climbs and then holds close to 100 degrees Celsius at the side arm while water distils (the boiling point of water at standard atmospheric pressure; a lower air pressure gives a slightly lower plateau); the plateau is the boiling point of water. The distillate gives near-zero conductivity and no residue on evaporation; the original solution conducts strongly and gives white salt on evaporation. The flask liquid becomes more concentrated as the water distils off.

What changes

What you change

time of heating

What you measure

temperature at the still head (degrees Celsius)

What you keep the same

• heating rate
• thermometer position
• starting volume

Common misconceptions

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

• The thermometer measures the liquid in the flask (it measures the vapour passing to the condenser, which is why it sits at the side arm).
• The thermometer reading at the side arm rises as the water boils off (the vapour reaching the side arm is pure water, so the reading holds at the boiling point of water; only the salt solution left in the flask boils a little above that, and a little higher still as it concentrates).

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)CH11-8CH11/12-3CH11/12-4
• Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-01CH-11WS-03
• 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/separating-mixtures/simple-distillation-practical-videos-14-16-years/4018538.article
5. edu.rsc.org/cpd/practical-distillation/3008222.article
6. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/chemistry/Chemistry-module-1-guide.docx