How far apart must boiling points be?
Distillation needs a large enough difference.
Two pairs of liquids
Pair 1 boils at 56 °C and 100 °C. Pair 2 boils at 95 °C and 100 °C. You heat each pair.
Which pair is easier to separate by distillation?
What this lesson covers
The idea
Distillation separates liquids whose boiling points differ by at least about 25 °C, because a large difference lets one liquid vaporise before the other forms vapour in significant amount.
Two pairs of liquids
Pair 1 boils at 56 °C and 100 °C. Pair 2 boils at 95 °C and 100 °C. You heat each pair.
Which pair is easier to separate by distillation?
- Pair 1, with the big difference
- Pair 2, with the small difference
- Both are equally easy
Move the boiling point
Change B’s boiling point and the heater. Look for a window where only A vaporises.
The difference must be large enough
Distillation can separate liquids whose boiling points differ by at least about 25 °C. A large difference lets one liquid vaporise before the vapours of the other form in significant amount.
Example. Acetone boils at about 56 °C and water at 100 °C: a difference of 44 °C, so they can be separated by distillation.
Notes
Distillation works when boiling points differ by at least about 25 °C: one liquid then vaporises first, before the other forms significant vapour.
Check yourself
Acetone boils at 56 °C and water at 100 °C. What is the difference in boiling points?
Answer: 44 °C
100 − 56 = 44 °C, more than 25 °C, so distillation works.
Two liquids boil at 90 °C and 100 °C. Is distillation a good method?
Why does a large difference in boiling points help?
Distillation separates liquids whose boiling points differ by about…
- No. The difference is much less than about 25 °C — correct. Yes. Both would form vapour in significant amount.
- Yes. Any two liquids can be distilled. The boiling points must differ enough.
- Yes, because both boil above 80 °C. The size of the difference matters, not the height.
- Both vaporise together and mix. That is what a small difference causes.
- One liquid vaporises before the other forms vapour in significant amount — correct. Yes.
- The mixture does not need heating. It still needs heating.
- 1 °C. Too close to separate.
- 2 °C or less. Too close to separate.
- 25 °C or more — correct. Yes.