Separation and purification
The Principle of Separation
Separation techniques rely on the physical properties of substances, such as solubility, boiling point, and state of matter. To separate a mixture, you must choose a method that exploits a difference in these properties between the components.
Building on previous concepts: Recall that a mixture contains two or more substances physically combined but not chemically bonded. This means the individual substances retain their own chemical properties, allowing them to be separated by physical means.
1. Filtration
Filtration is used to separate an insoluble solid from a liquid (solution or suspension).
Residue
The insoluble solid that remains on the filter paper.
Filtrate
The liquid that passes through the filter paper and is collected in the beaker below.
Separating Sand from Water
If you mix sand and water, the sand does not dissolve. Pouring this mixture through filter paper in a funnel will leave the sand (residue) on the paper and allow the water (filtrate) to pass through.
⚠︎ Diagram Labelling Errors
The Error: Students often forget to draw the filter paper inside the funnel or fail to label the filtrate and residue correctly.
The Fix: Always ensure your diagram shows the filter paper lining the funnel. Label the liquid collecting in the beaker as 'filtrate' and the solid on the paper as 'residue'. Examiners look for these specific terms.
Describing Filtration Apparatus
When to use: When asked to describe or draw the apparatus for filtration.
Why it matters: Examiners award marks for specific visual and verbal details. You must mention that the filter paper is placed inside a funnel, which is positioned over a beaker or conical flask to collect the filtrate.
Example: 'Place filter paper in a funnel. Position the funnel over a clean beaker. Pour the mixture into the funnel. The insoluble solid remains as residue on the paper, while the liquid filtrate collects in the beaker.'
Identifying Filtration
Q:
Which process is used to separate an insoluble solid from a liquid?
A:
Filtration
Q:
In a filtration experiment, what is the name of the solid left on the filter paper?
A:
Residue
2. Crystallisation
Crystallisation is used to obtain a pure soluble solid from its solution. It is the preferred method when you want to keep the solid, rather than evaporating all the solvent.
Saturated Solution
A solution in which no more solute can dissolve at that temperature. This is the point of crystallisation.
Obtaining Copper(II) Sulfate Crystals
- Heat the solution to evaporate some water.
- Stop heating when the solution is saturated (point of crystallisation). You can test this by dipping a cold glass rod into the solution; if crystals form on the rod, it is saturated.
- Allow to cool. As the temperature drops, the solubility decreases, and pure crystals form.
- Filter off the crystals.
- Dry the crystals between sheets of filter paper (do not heat them, or they may lose water of crystallisation).
⚠︎ Drying Crystals Incorrectly
The Error: Students often say 'dry in an oven' or 'heat strongly'.
The Fix: Many salts contain water of crystallisation. Heating them can drive this water out, changing the chemical structure. Always state: 'Dry between sheets of filter paper' or 'Leave in a warm place to dry slowly.'
Describing Crystallisation Steps
When to use: When asked 'How do you obtain pure crystals from a solution?'
Why it matters: You must mention cooling. If you only say 'evaporate', you might get powder or decomposed salt. The key is that solubility decreases with temperature.
Example: 'Heat the solution to the point of crystallisation (saturation). Allow the solution to cool slowly. Pure crystals will form as the solubility decreases. Filter off the crystals and dry them with filter paper.'
Crystallisation Logic
Q:
Why do crystals form when a hot saturated solution is cooled?
A:
Because the solubility of the solid decreases as the temperature decreases.
Q:
What is the purpose of filtering off the crystals after cooling?
A:
To separate the pure solid crystals from the remaining liquid (mother liquor).
3. Simple Distillation
Simple distillation is used to separate a solvent from a solution (to obtain the pure liquid) OR to separate a liquid from a non-volatile solid.
Distillate
The pure liquid collected after condensation.
Obtaining Pure Water from Sea Water
- Heat the mixture in a flask.
- The water (solvent) evaporates, but the salt (solute) does not.
- The water vapour travels into the condenser.
- The condenser is cooled by cold water flowing around it. This causes the vapour to condense back into liquid water.
- The pure water (distillate) drips into the collection flask.
⚠︎ Confusing Distillation with Filtration
The Error: Using filtration to separate salt from water.
The Fix: Salt is soluble in water. Filtration only works for insoluble solids. You must use distillation (or crystallisation) for soluble substances.
Explaining the Process
When to use: When asked 'Explain how pure water is obtained from aqueous copper(II) sulfate.'
Why it matters: You must mention both evaporation and condensation. Just saying 'boil the water' is incomplete.
Example: 'The mixture is heated. The water evaporates (leaving the solid behind). The vapour passes into the condenser where it cools and condenses back into liquid water, which is collected as the distillate.'
Simple Distillation Application
Q:
Which technique is used to obtain pure water from sea water?
A:
Distillation
Q:
What is the name of the liquid collected at the end of distillation?
A:
Distillate
4. Fractional Distillation
Fractional distillation is used to separate a mixture of liquids with different boiling points. It is more effective than simple distillation for liquids with close boiling points.
Fractionating Column
A glass column packed with glass beads or plates placed between the flask and the condenser. It allows for repeated evaporation and condensation cycles, improving separation.
Separating Ethanol and Water
- Heat the mixture.
- The liquid with the lower boiling point (ethanol, 78^\text{C}) evaporates first.
- The vapour rises up the fractionating column. Less volatile liquids (water, BP 100^\text{C}) condense and fall back down.
- The ethanol vapour reaches the condenser, cools, and is collected.
- If you continue heating, water will eventually boil over and be collected.
⚠︎ Confusing Simple and Fractional Distillation
The Error: Using simple distillation to separate two liquids.
The Fix: If the question involves two or more liquids, you must use fractional distillation. Simple distillation is only for separating a liquid from a solid (or a single liquid from a solution).
Explaining Fractional Distillation
When to use: When asked 'How are crotyl alcohol and water separated?'
Why it matters: You must reference boiling points. Examiners want to see that you understand the lower boiling point component vaporizes first.
Example: 'Water has a lower boiling point than crotyl alcohol. When heated, water boils off first and is collected as the distillate. Crotyl alcohol remains in the flask because its boiling point is higher.'
Fractional Distillation Logic
Q:
Which component is collected first during fractional distillation?
A:
The liquid with the lowest boiling point.
Q:
What apparatus distinguishes fractional distillation from simple distillation?
A:
A fractionating column.
5. Separation Using a Suitable Solvent
This refers to Chromatography (paper or thin-layer) or Solvent Extraction. Both rely on the principle that different substances have different solubilities in a specific solvent.
Suitable Solvent (for Chromatography)
A solvent that dissolves the components of the mixture to different extents. It must be able to move up the paper/plate via capillary action.
Suitable Solvent (for Extraction)
A solvent that is immiscible (does not mix) with the original solvent (usually water) and in which the target substance is more soluble than in the original solvent.
Chromatography Principle
In paper chromatography, a 'suitable solvent' moves up the paper. Substances that are more soluble in the solvent travel further (higher R_f value). Substances that are less soluble (or bind more to the paper) travel less.
⚠︎ Choosing the Wrong Solvent for Extraction
The Error: Suggesting water to extract a substance already in water.
The Fix: For extraction, the solvent must be immiscible with the original liquid. Common examples include dichloromethane or hexane. If asked to choose a solvent, ensure it forms a separate layer.
Describing Chromatography Solvent Choice
When to use: When asked 'Why is a suitable solvent used in chromatography?'
Why it matters: You must explain that the solvent allows separation based on differential solubility.
Example: 'A suitable solvent is one in which the components of the mixture have different solubilities. This causes them to travel at different speeds up the paper, separating the spots.'
Solvent Selection
Q:
What property must a solvent have to be used for extracting a substance from water?
A:
It must be immiscible with water.
Q:
In chromatography, what determines how far a spot travels?
A:
Its solubility in the solvent (and affinity for the stationary phase).
6. Purification and Purity Testing
Impurities affect the physical properties of a substance, specifically its melting point and boiling point.
| Property | Pure Substance | Impure Substance |
|---|---|---|
| Melting Point | Sharp, specific temperature (e.g., 0^\text{C} for pure ice) | Melts over a range, lower than pure MP |
| Boiling Point | Sharp, specific temperature (e.g., 100^\text{C} for pure water) | Boils over a range, higher than pure BP |
Pure Substance
A substance consisting of only one type of atom or molecule. It has a sharp melting/boiling point.
Impure Substance
A mixture containing other substances. It melts/boils over a range of temperatures.
Assessing Purity of Ethanol
Pure ethanol boils at 78^\text{C}. If a sample of ethanol contains glucose (an impurity), the boiling point will be higher than 78^\text{C} and it will boil over a range of temperatures.
⚠︎ Melting Point Trends
The Error: Thinking impurities raise the melting point.
The Fix: Impurities lower the melting point and broaden the range. They disrupt the crystal lattice, making it easier to break apart.
Identifying Purity from Data
When to use: When given experimental data (e.g., 'Sample X melts at 120-125^\text{C}, pure Y melts at 123^\text{C}').
Why it matters: You must compare the sharpness and the value.
Example: 'Sample X is impure because it melts over a range (120-125^\text{C}) rather than at a sharp temperature. Also, its melting point is lower than the pure substance.'
Purity Identification
Q:
How can you tell if a solid is pure using melting point data?
A:
It melts at a sharp, specific temperature.
Q:
What happens to the boiling point of water when salt is dissolved in it?
A:
It increases (boils at a higher temperature).
Choosing the Right Technique
To answer 'Suggest a suitable method' questions, follow this decision tree:
| Mixture Type | Recommended Technique |
|---|---|
| Insoluble solid + Liquid | Filtration |
| Soluble solid + Liquid (want solid) | Crystallisation |
| Soluble solid + Liquid (want liquid) | Simple Distillation |
| Mixture of liquids (different BPs) | Fractional Distillation |
| Separating components for identification | Chromatography |
⚠︎ Evaporation vs. Crystallisation
The Error: Saying 'evaporate to dryness' to obtain crystals.
The Fix: Evaporating to dryness often produces a powder or decomposes the salt. Use crystallisation (heat to saturation, then cool) to get pure crystals.
Justifying Your Choice
When to use: When asked 'Why is this method suitable?'
Why it matters: You must link the method to the property difference.
Example: 'Fractional distillation is suitable because ethanol and water have different boiling points.'
Technique Selection
Q:
Which method separates sand from aqueous sodium chloride?
A:
Filtration (to remove sand) then Crystallisation/Distillation (for salt).
Q:
How do you separate a mixture of several liquids with different boiling points?
A:
Fractional distillation.