Science 7–10 · Year 7
Simple distillation: recovering pure water from salt water
Solutions and mixtures
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
Boiling turns only the water into vapour, so condensing that vapour gives water without the dissolved salt.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- boiling liquid and hot glass
- a sealed still can build pressure if the condenser outlet is blocked
- condenser hoses can slip
Controls
- adult checks the apparatus before heating; the receiving beaker is open to the air
- anti-bumping granules added cold
- never heat to dryness
- eye protection
Note
Salt water is low hazard; the risk is heat and pressure. Basic risk assessment per CSIS Section 1.7.
What you need
- round-bottomed or conical flask with 100 mL of salt water (about 3 g salt per 100 mL, coloured with a drop of food dye)
- still head with thermometer, Liebig condenser with cold-water hoses, receiving beaker
- anti-bumping granules, tripod, gauze, Bunsen burner or electric heating mantle, clamp stands
- two watch glasses and a conductivity meter or a simple two-electrode circuit with a bulb
How to do it
- Assemble the still with the thermometer bulb level with the side arm and the condenser water entering at the lower end.
- Add anti-bumping granules and heat gently; record the thermometer reading every minute from the first drop of distillate.
- Collect about 20 mL of distillate, then stop heating before the flask runs dry.
- Put a drop of the original salt water and a drop of distillate on separate watch glasses and let them evaporate; compare the residue.
- Test the conductivity of tap water, salt water and distillate with the same meter or bulb circuit.
What you should see
The thermometer settles near 100 °C at sea-level pressure while distillate is collected; the distillate is colourless (the dye stays behind), dries without residue when a drop evaporates, and gives a far lower conductivity reading than the salt water on the same meter or bulb circuit. The residue in the flask is more concentrated and more strongly coloured than the start. The learner knows it worked when the distillate drop dries with no white ring on the watch glass.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Boiling removes the salt with the steam.
- Distilled water still tastes salty because the salt is invisible.
- The condenser water mixes with the distillate.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-SOL-01SC4-WS-01
- Australian Curriculum v9AC9S7U06AC9S7I03
Sources
The pages the author read to write this activity.