Filtration and Crystallisation

This lesson covers two essential separation techniques — filtration and crystallisation — as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You need to know when to use each technique, the practical method for each, and the scientific principles behind them. These techniques link directly to Edexcel core practicals, so you must be able to describe the methods in detail.

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Filtration

Filtration is used to separate an insoluble solid from a liquid (or a solution). The mixture is poured through filter paper, which has tiny pores that allow the liquid to pass through but trap the solid particles.

Key Vocabulary

Term	Definition
Residue	The insoluble solid that remains on the filter paper
Filtrate	The liquid that passes through the filter paper
Insoluble	Does not dissolve in a given solvent
Soluble	Dissolves in a given solvent

Apparatus Needed

• Filter paper
• Filter funnel
• Conical flask (to collect the filtrate)
• Beaker (containing the mixture)
• Retort stand and clamp (to hold the funnel)

Method

1. Fold a piece of filter paper into a cone shape and place it inside the filter funnel.
2. Place the filter funnel in the neck of a conical flask.
3. Carefully pour the mixture into the filter funnel (use a glass rod to guide the liquid).
4. The liquid passes through the tiny pores in the filter paper and collects in the conical flask as the filtrate.
5. The insoluble solid is trapped on the filter paper as the residue.
6. Wash the residue with a small amount of distilled water to remove any soluble impurities.

When to Use Filtration

Use filtration when you need to separate:

• Sand from water
• An insoluble product from a reaction mixture (e.g. precipitate from a solution)
• Solid impurities from a liquid

Limitations

• Filtration only works for insoluble solids. You cannot use it to separate a dissolved solid from a solution.
• Very fine particles may pass through the filter paper if the pore size is too large.

Exam Tip: Make sure you know the terms "residue" and "filtrate." The residue is what stays behind on the filter paper (the solid). The filtrate is what passes through (the liquid). These terms appear frequently in exam questions and mark schemes.

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Evaporation

Evaporation is a simple technique used to obtain a dissolved solid from a solution by heating the solution until all the solvent has evaporated.

Method

1. Pour the solution into an evaporating basin.
2. Heat the evaporating basin gently using a Bunsen burner or a water bath.
3. Continue heating until all the water (or other solvent) has evaporated.
4. The dissolved solid is left behind in the evaporating basin.

Limitations of Simple Evaporation

• If heated too quickly, the crystals may spit out of the basin (a safety hazard).
• Rapid evaporation produces small, imperfect crystals.
• Heating too strongly may decompose some salts (e.g. copper sulfate loses its water of crystallisation and turns from blue to white, then may decompose further).
• Some substances are thermally unstable and cannot be recovered by simple evaporation.

Exam Tip: If a question asks you to obtain crystals of a soluble salt, do NOT say "evaporate all the water." This gives poor-quality, small crystals and may decompose the salt. Instead, use crystallisation (described below) for better results.

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Crystallisation

Crystallisation is a more controlled technique for obtaining a dissolved solid from a solution. It produces larger, more regular, purer crystals than simple evaporation.

Method for Crystallisation

1. Pour the solution into an evaporating basin.
2. Heat the solution gently (using a water bath or Bunsen burner) to evaporate some of the water — do NOT evaporate all the water.
3. To test whether the solution is concentrated enough, dip a clean glass rod into the solution and allow it to cool. If crystals form on the glass rod, the solution is saturated (sufficiently concentrated).
4. Remove the evaporating basin from the heat and leave it to cool slowly. As the solution cools, the solubility of the solute decreases and crystals begin to form.
5. Filter the crystals using filter paper and a funnel to separate them from the remaining solution (called the mother liquor).
6. Wash the crystals with a small volume of cold distilled water to remove impurities.
7. Dry the crystals by leaving them on filter paper in a warm place, or by placing them in a desiccator or a warm oven (at low temperature).

Why Slow Cooling Produces Better Crystals

When a solution cools slowly:

• Particles have time to arrange themselves in a regular pattern.
• This produces large, well-formed crystals with a regular shape.

When a solution cools quickly:

• Particles do not have time to arrange regularly.
• This produces small, irregular crystals.

Exam Tip: When describing crystallisation, the key steps examiners look for are: (1) heat to evaporate SOME water (not all), (2) test with a glass rod to check saturation, (3) leave to cool SLOWLY, (4) filter the crystals, (5) wash with cold distilled water, (6) dry. If you include all six steps, you will gain full marks on a describe-the-method question.

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Core Practical: Preparing Copper Sulfate Crystals

This links to the Edexcel Core Practical on preparing copper sulfate crystals by reacting copper oxide with sulfuric acid.

Method

1. Warm approximately 25 cm\u00b3 of dilute sulfuric acid in a beaker on a tripod and gauze.
2. Add copper oxide powder (an insoluble base) a spatula at a time, stirring after each addition.
3. Continue adding copper oxide until it is in excess — you will know it is in excess when unreacted black powder remains at the bottom of the beaker and does not dissolve.
4. Filter the mixture to remove the excess copper oxide (the residue).
5. The blue filtrate is a copper sulfate solution.
6. Pour the filtrate into an evaporating basin.
7. Heat gently to evaporate some of the water.
8. Test with a glass rod for saturation.
9. Leave to cool slowly to allow blue copper sulfate crystals to form.
10. Filter the crystals, wash with cold distilled water, and dry.

Why Add Excess Copper Oxide?

Adding excess copper oxide ensures that all the sulfuric acid has reacted. This means the final solution contains only copper sulfate (and water) — no unreacted acid remains. The excess copper oxide can be easily removed by filtration.