IB MYP 4-5 Chemistry -Distillation and fractional distillation- Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Distillation and fractional distillation- Study Notes
Key Concepts
- Distillation and Fractional Distillation
Distillation and Fractional Distillation
Distillation and Fractional Distillation
Distillation is a physical separation technique used to separate and purify liquids or liquid mixtures based on differences in their boiling points. It involves heating a liquid to form vapor and then cooling the vapor to obtain the pure liquid again.
Simple Distillation
Simple distillation is a method used to separate a liquid from a solution when the liquid has a much lower boiling point than the dissolved solid or impurity.
- Used for separating a solvent from a solute.
- Also used when two liquids have boiling points differing by more than 25°C–30°C.
Principle:
Different substances have different boiling points. When a mixture is heated, the component with the lowest boiling point vaporizes first. The vapor is then cooled and condensed back into a liquid using a condenser.
Apparatus Setup for Simple Distillation
| Apparatus | Function |
|---|---|
| Round-bottom flask | Contains the solution to be distilled |
| Thermometer | Monitors the temperature of vapor |
| Condenser | Cools and condenses vapor into liquid |
| Receiver / Beaker | Collects the condensed pure liquid (distillate) |
Example: Separation of Salt and Water
- Salt water is heated in a distillation flask.
- Water (boiling point 100°C) evaporates first, forming steam.
- The steam passes through the condenser and cools to liquid water.
- Salt (non-volatile) remains in the flask as residue.
Result: Pure water (distillate) is collected, and salt remains behind.
Key Points about Simple Distillation
- Used to purify a liquid by removing non-volatile impurities.
- Solvent can be recovered and reused.
- Effective only when the difference in boiling points is large.
Fractional Distillation
Fractional distillation is a method used to separate a mixture of two or more liquids that have close boiling points (difference less than 25°C).
Principle:
Each liquid in a mixture boils and vaporizes at its own characteristic temperature. When the mixture is heated, the vapor composition changes gradually, and liquids can be separated based on their boiling points using a fractionating column.
Apparatus Setup for Fractional Distillation
The apparatus is similar to simple distillation but includes a fractionating column between the flask and the condenser.
- The column is filled with glass beads or plates to provide surface area for condensation and vaporization.
- It allows repeated condensation and vaporization, enriching the vapor of the liquid with the lower boiling point.
Example:Separation of Alcohol and Water
- Alcohol (boiling point 78°C) and water (boiling point 100°C) are mixed.
- On heating, alcohol vaporizes first and rises through the column.
- Water vapor condenses back into the flask due to its higher boiling point.
- Alcohol vapor passes through the condenser, cools, and is collected separately.
Result: Alcohol is obtained as the first distillate, followed by water at a higher temperature.
Key Points about Fractional Distillation
- Used when boiling points of liquids are close together.
- Fractionating column allows better separation by repeated condensation-vaporization cycles.
- Used in both laboratory and industry (e.g., petroleum refining).
Applications of Distillation
| Application | Type of Distillation Used |
|---|---|
| Purification of drinking water | Simple distillation |
| Separation of alcohol from water | Fractional distillation |
| Refining crude oil into petrol, diesel, etc. | Fractional distillation (industrial scale) |
| Liquefied air separation (oxygen, nitrogen, argon) | Fractional distillation of air |
Comparison Table: Simple vs. Fractional Distillation
| Feature | Simple Distillation | Fractional Distillation |
|---|---|---|
| Purpose | Separates liquid from dissolved solid or widely different liquids | Separates two or more liquids with close boiling points |
| Column Used | No fractionating column | Uses fractionating column |
| Separation Efficiency | Less efficient for close boiling points | Highly efficient |
| Example | Salt from seawater | Alcohol from water |
Example :
Why can simple distillation be used to separate salt water, but not alcohol and water?
▶️ Answer / Explanation
Step 1: In salt water, salt has a very high boiling point and does not evaporate, while water boils at 100°C.
Step 2: Alcohol and water both boil below 100°C with only a small temperature difference (22°C apart).
Step 3: Simple distillation cannot separate liquids with close boiling points — their vapors mix.
Final Answer: Simple distillation works for salt water but not for alcohol-water mixtures, which require fractional distillation.
Example :
Explain why the first liquid collected in fractional distillation is the one with the lowest boiling point.
▶️ Answer / Explanation
Step 1: Liquids with lower boiling points vaporize at lower temperatures.
Step 2: These vapors rise higher in the fractionating column and condense first.
Step 3: Liquids with higher boiling points condense lower down and are collected later.
Final Answer: The first distillate is the component with the lowest boiling point because it vaporizes earliest and reaches the condenser first.
Example :
How is fractional distillation applied in the petroleum industry?
▶️ Answer / Explanation
Step 1: Crude oil is a complex mixture of hydrocarbons with different boiling points.
Step 2: It is heated in a large fractionating tower — vapors rise and condense at different levels based on their boiling points.
Step 3: Lighter fractions (e.g., gasoline, kerosene) collect at the top; heavier fractions (e.g., diesel, bitumen) condense lower down.
Final Answer: Fractional distillation in petroleum refineries separates crude oil into useful products such as petrol, diesel, kerosene, and lubricating oil.