Understanding Mixtures and Methods of Separation

Separation of Mixtures

1. Simple Distillation

Definition:
Simple distillation is a method used to separate a solvent from a solution based on differences in boiling points. It is most suitable for homogeneous solid–liquid mixtures where the solid does not evaporate. It is also suitable for homogeneous liquid-liquid mixtures for liquids of different boiling points.

Purpose / Used to separate:
- Homogeneous liquid-liquid mixtures with different boiling points.
Example: Separating acetone from water (acetone boils at 56°C, water at 100°C).

Apparatus:
- Distillation flask (contains the mixture)
- Heat source (Bunsen burner or electric heater)
- Condenser (to cool and condense the vapor)
- Receiving flask (to collect the distilled liquid)
- Thermometer
- Broken porcelain or boiling chips

Procedure:
i. Pour the liquid mixture (acetone + water) into the distillation flask.
ii. Add broken porcelain / boiling chips to the flask. Prevent sudden vigorous boiling (“bumping”) which can cause splashing.
iii. Heat the flask slowly.
iv. Acetone evaporates first because it has a lower boiling point (56°C).
v. The vapor passes into the condenser, where it cools and condenses into a liquid.
vi. Collect the condensed liquid (acetone) in the receiving flask.
vii. This liquid is pure acetone, separated from water.

💡 Fun Fact: Simple distillation works because acetone has a lower boiling point than water!

Key Points / Observations:
- The separation works because acetone has a lower boiling point than water.
- Broken porcelain prevents bumping during heating.
- The remaining liquid in the distillation flask is mostly water.
- This method can be applied to any mixture of liquids with significantly different boiling points.

Applications:
- Obtaining drinking water from salt water.
- Purifying water in laboratories.
- Separating alcohol from aqueous solutions (when using fractional distillation for closer boiling points).

2. Crystallisation

Definition:
Crystallisation is a method used to obtain pure solid crystals from a saturated solution. It is commonly used for homogeneous solid–liquid mixtures where the solute can form crystals on cooling or evaporation of the solvent.

Purpose / Used to separate:
- Homogeneous solid–liquid mixtures
Example: Obtaining copper (II) sulphate crystals from its solution.

Apparatus:
- Beaker or crystallisation dish
- Glass rod
- Heat source (Bunsen burner or water bath)
- Filter paper and funnel (optional, for removing impurities)

Procedure:
i. Prepare a saturated solution by dissolving the solute (e.g., copper (II) sulphate) in the minimum amount of hot solvent (water).
ii. Filter the solution if necessary to remove insoluble impurities.
iii. Pour the solution into a crystallisation dish.
iv. Allow the solution to cool slowly.
v. Dip a glass rod into the solution to check if it is saturated and to initiate crystal formation (seeding).
vi. As the solution cools, the solute crystallises out, forming pure crystals on the surface or at the bottom.
vii. Carefully collect the crystals and dry them using filter paper.

💡 Fun Fact: Slow cooling allows the formation of larger, well-formed crystals!

Key Points / Observations:
- Seeding: Dipping a glass rod provides a nucleation point for crystal growth.
- Purity: Only pure solute forms crystals; impurities remain in the solution (mother liquor).
- Slow cooling produces larger, well-formed crystals.
- Type of mixture separated: Homogeneous solid–liquid mixture.
- Result: Pure crystals of the solute, separated from the solvent.

Applications:
- Obtaining pure copper (II) sulphate crystals in the lab.
- Purifying sugar or salt.
- Producing pure chemical compounds in chemistry experiments.

3. Sublimation

Definition:
Sublimation is a method used to separate a solid–solid mixture in which one solid sublimes (changes directly from solid to gas) while the other does not.

Purpose / Used to separate:
- Solid–solid mixtures where one solid is volatile and can sublime.
Example: Separating iodine from salt.

Apparatus:
- Conical flask or beaker (contains the mixture)
- Round-bottomed flask or inverted funnel (to collect sublimed solid)
- Heat source (Bunsen burner or water bath)
- Stand and clamp (to hold the apparatus)

Procedure:
i. Place the mixture (e.g., iodine + salt) in a beaker.
ii. Set up a round-bottomed flask or inverted funnel above the mixture to collect the sublimed solid.
iii. Heat the mixture gently.
iv. Iodine sublimes (solid → gas) because it is volatile.
v. Salt does not sublimate and remains as a white solid in the beaker.
vi. The iodine gas rises and cools on the surface of the flask or funnel, undergoing deposition (gas → solid).
vii. Collect the purple/black iodine crystals formed on the cooler surface.

💡 Fun Fact: Sublimation is the process where a solid changes directly into a gas, skipping the liquid phase!

Key Points / Observations:
- Sublimation: Direct conversion of solid to gas without becoming liquid.
- Deposition: Gas to solid upon cooling.
- Separation:
- White solid left in beaker → salt
- Coloured solid on flask → iodine
- Useful for purifying solids that sublime.

Applications:
- Purifying iodine from salt.
- Producing mothballs (naphthalene) from mixtures.
- Separating volatile solids from non-volatile solids in labs.

4. Solvent Extraction

Definition:
Solvent extraction is a method used to separate a substance from a mixture based on its solubility in a particular solvent. It is often used to extract desired substances from solids or liquids.

Purpose / Used to separate:
- Separates substances that are soluble in a particular solvent but not in others.
Example: Extracting oil from groundnuts using propanone.

Apparatus:
- Beaker or container (contains the solid mixture, e.g., groundnuts)
- Solvent (e.g., propanone)
- Filter paper (optional, to remove insoluble solid particles)
- Evaporating dish or tray (to recover the extracted substance)

Procedure:
i. Place the solid mixture (e.g., groundnuts) in a beaker.
ii. Add a suitable solvent (e.g., propanone) that dissolves the desired substance (oil).
iii. Stir or shake the mixture to allow maximum dissolution of the oil in the solvent.
iv. Filter the mixture if necessary to remove insoluble solid residue.
v. Allow the solvent to evaporate naturally or gently heat it in an evaporating dish.
vi. The oil remains after the solvent has evaporated.

💡 Fun Fact: Solvent extraction is commonly used to extract oils from seeds such as groundnuts and sunflower seeds!

Key Points / Observations:
- The solvent selectively dissolves the substance to be separated.
- Evaporation of the solvent leaves behind the desired substance.
- Paper test: When oil is applied to paper, it becomes translucent, confirming the presence of oil.

Applications:
- Extracting edible oils from seeds (groundnuts, sunflower, etc.)
- Making perfumes and essential oils
- Extracting plant chemicals for medicines or dyes

5. Paper Chromatography

Definition:
Paper chromatography is a method used to separate the components of a mixture based on their solubility and movement on a special paper. It is commonly used for analyzing coloured substances like inks, dyes, and plant pigments.

Purpose:
- Used to separate components of a homogeneous mixture based on solubility and adsorption.
Example: Separating ink dyes into individual colours.

Apparatus:
- Chromatography paper (filter paper or special strip)
- Beaker or container
- Solvent (e.g., water, alcohol)
- Pencil (to draw baseline)
- Capillary tube or dropper (to place sample)

Procedure:
i. Draw a baseline near the bottom of the chromatography paper using a pencil.
ii. Place a small drop of the mixture (e.g., black ink) on the baseline.
iii. Place the paper in a beaker containing a small amount of solvent, ensuring the baseline is above the solvent level.
iv. Allow the solvent to rise up the paper by capillary action, carrying the components of the mixture with it.
v. Observe the separation of components:
vi. Most soluble component moves furthest up the paper.
vii. Least soluble component remains near the baseline.
viii. Remove the paper once the solvent front reaches a desired height and allow it to dry.

💡 Did you know? The distance travelled by each component in chromatography depends on its solubility in the solvent used!

Key Points / Observations:
- The method works because different components have different solubility and attraction to the paper.
- Separation is visible as different coloured spots on the paper.
- Distance travelled by a component depends on its solubility in the solvent.

Applications:
- Detecting illegal drugs in athletes
- Separating and identifying food colourings
- Forensic investigations (e.g., analysing inks, blood pigments)
- Studying plant pigments and chemical composition

6. Evaporation

Meaning:
Evaporation is a method used to separate a solvent from a solution by heating or leaving it exposed to air. It is used to separate a solvent from a solution.
Example: Obtaining salt from sea water. When sea water is heated or left in shallow ponds under the sun, the water evaporates and salt crystals remain.

💡 Did you know? Evaporation occurs naturally under the sun, which is why salt is often harvested from salt pans by evaporating sea water!

Key Points:
- The liquid (solvent) changes into vapor.
- The solid (solute) remains behind as crystals.
- Evaporation occurs faster when:
- Temperature is high
- Surface area is large
- Wind is blowing

Other Applications:
- Drying grains after harvesting.
- Drying clothes under the sun.
- Making salt in salt pans.

7. Fractional Distillation

Meaning:
Fractional distillation is a method used to separate two or more liquids that are miscible (mixed together) and have close boiling points. It is used to separate liquids with close boiling points.
Example: Separating ethanol from water and refining crude oil into useful products such as petrol, diesel, and kerosene.

Key Points:
- Separation is based on differences in boiling points.
- Ethanol boils at about 78°C, while water boils at 100°C.
- The liquid with the lower boiling point evaporates first.
- A fractionating column is used to improve separation.
- In oil refining, different fractions are collected at different temperatures.

Products from Crude Oil:
- Petrol (gasoline)
- Kerosene
- Diesel
- Lubricating oil
- Bitumen