Exploring Mixtures and their Separation · Lesson 6 of 13
Distillation
“One liquid escapes as vapour, cools off, and arrives in a different flask.”
• Explain vaporisation and condensation during distillation. • Describe the function of each apparatus component. • Separate a solvent from a dissolved solid. • Separate miscible liquids with sufficiently different boiling points. • Distinguish simple and fractional distillation.
Evaporation can recover salt from salt water, but the water is lost to the surroundings. Suppose the water is the valuable part, or suppose two mixed liquids must both be collected. The escaping vapour must be captured, cooled and returned to liquid form.
Distillation is a separation process in which a liquid is vaporised by heating and the vapour is cooled to form a liquid called the distillate.
Simple distillation can separate a liquid from a solution containing a dissolved solid. It can also separate two miscible liquids when their boiling points differ by at least about 25 °C. The lower-boiling liquid forms vapour first in much greater proportion and is collected after condensation.
| Part | Function |
|---|---|
| Distillation flask | Holds the mixture and receives heat |
| Thermometer | Monitors the temperature of vapour reaching the condenser |
| Condenser | Provides a cool surface where vapour becomes liquid |
| Lower water inlet | Introduces cooling water from below |
| Upper water outlet | Allows warmed water to leave while keeping the jacket full |
| Receiving flask | Collects the condensed distillate |
Problem
Explain why acetone and water can be separated by simple distillation.
- 1.Acetone and water are miscible, so a separating funnel cannot be used.
- 2.Acetone boils at about 56 °C, while water boils at 100 °C.
- 3.The boiling-point difference is about 44 °C, which is sufficiently large.
- 4.On heating, acetone-rich vapour forms first.
- 5.The vapour passes through the condenser and is collected as liquid acetone.
- 6.Most water remains in the distillation flask during the early collection.
Problem
How can both water and salt be obtained from salt solution?
- 1.Place the salt solution in the distillation flask.
- 2.Heat it so that water vaporises while non-volatile salt remains behind.
- 3.Cool the water vapour in the condenser.
- 4.Collect liquid water as the distillate.
- 5.Recover the salt left in the flask after safe cooling.
A distillation system must never be sealed. Use proper supports, gentle heating and adult supervision. Cooling water should enter from below, and the apparatus should be allowed to cool before handling.
India’s Scientific Contributions
Traditional distillation has long been used to extract fragrances. In Kannauj, the earthy smell associated with the first rain is captured as Mitti ka Ittar using the Deg-Bhapka method. This craft applies the same sequence of heating, vapour transfer, cooling and collection.
Ready to Go Beyond
Fractional distillation separates miscible liquids whose boiling points differ by less than about 25 °C by providing repeated vaporisation and condensation inside a fractionating arrangement.
When boiling points are close, both liquids enter the vapour in significant amounts during simple distillation. A fractionating column improves separation by repeatedly condensing higher-boiling components and allowing lower-boiling components to continue upward.
Crude petroleum is separated into useful fractions such as petroleum gas, petrol, aviation fuel, diesel, lubricating oil and bitumen. The fractions have different boiling ranges and condense at different levels in a large fractionating column.
Problem
Liquid A boils at 60 °C and liquid B at 90 °C. Which method is suitable?
- 1.Find the difference: 90 °C − 60 °C = 30 °C.
- 2.The liquids are miscible, so layer separation is not possible.
- 3.The difference is greater than about 25 °C.
- 4.Simple distillation is therefore suitable.
Problem
Alcohol boils at 78 °C and benzene at 80 °C. Which method is more suitable?
- 1.The boiling-point difference is only 2 °C.
- 2.Both liquids would vaporise significantly over nearly the same temperature range.
- 3.Simple distillation would give poor separation.
- 4.Fractional distillation is the more suitable method.
Quiz
Which description best matches Distillation?
Which description best matches Fractional Distillation?
Which term matches this description: Distillation is a separation process in which a liquid is vaporised by heating and the vapour is cooled to form a liquid called the distillate.
Which term matches this description: Fractional distillation separates miscible liquids whose boiling points differ by less than about 25 °C by providing repeated vaporisation and condensation inside a fractionating arrangement.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why the condenser is supplied with water from the lower end.
- Choose a method for water and salt, acetone and water, and alcohol and benzene.
- What remains in the flask while water is distilled from salt solution?
- Compare evaporation with distillation when the solvent is valuable.
Key Takeaways
• Distillation combines vaporisation with condensation. • It can recover a liquid from a dissolved solid. • Simple distillation suits miscible liquids with a sufficiently large boiling-point difference. • Fractional distillation is used when boiling points are close. • Correct condenser-water flow keeps the cooling jacket effective.