SCIJHS 1 • Term 1Topic 5Free Trial Lesson
Mixtures & Methods of Separating Mixtures
Differentiate mixtures from compounds, and master separation techniques: filtration, evaporation, distillation, magnetic separation, and chromatography.
Curated Video Lesson
Visual explanation and practical step-by-step walk-through
Comprehensive Study Notes
Aligned with Ghana NaCCA & WAEC BECE syllabus standards
Topic Introduction & Real-World Context:
In Ghana's economy, separating mixtures is vital: galamsey and commercial gold miners pan for gold by separating dense gold dust from river sand; salt makers in Ada and Songor Lagoon use solar evaporation to crystallize sea salt; palm oil processors clarify oil from water and chaff.
What You Will Master in This Lesson (NaCCA Objectives):
Define a mixture and classify mixtures into homogeneous and heterogeneous mixtures.
Compare the properties of mixtures and chemical compounds.
Describe practical separation techniques: filtration, evaporation, simple distillation, magnetic separation, sublimation, and chromatography.
Select and sequence appropriate separation techniques to separate complex multi-component mixtures.
1. What is a Mixture? Homogeneous vs Heterogeneous
A mixture consists of two or more substances that are physically combined in any proportion without undergoing a chemical reaction.
• Characteristics of Mixtures:
- The components retain their individual physical and chemical identities.
- Components can be separated by physical separation methods.
- There is no chemical bond formation and no fixed proportion by mass.
- Heat is generally neither released nor absorbed during mixing.
• Classification of Mixtures:
1. Homogeneous Mixtures (Solutions):
- The composition is uniform throughout; particles are distributed at the molecular level with no visible boundaries.
- Examples: Common salt dissolved in water, sugar solution, clean filtered air, brass (alloy of copper and zinc).
2. Heterogeneous Mixtures (Suspensions and Emulsions):
- The composition is non-uniform; different phases and constituent particles are visible to the naked eye or microscope.
- Examples: Muddy water, sand and water, oil and water, iron filings mixed with sulfur powder.
Key Takeaway: In mixtures, components keep their original properties and can be separated physically without chemical reactions.
Real-World Application: Gari fortor or shito is a heterogeneous mixture where distinct ingredients (oil, pepper, fish, gari) remain physically mixed.
2. Differences Between Mixtures and Compounds
A foundational BECE examination topic is contrasting mixtures and chemical compounds:
| Property | Mixture | Chemical Compound |
| :--- | :--- | :--- |
| **Method of Formation** | Physical mixing of substances | Chemical reaction with bond formation |
| **Proportion of Constituents**| Variable proportion (any ratio) | Fixed, definite proportion by mass |
| **Properties** | Retains properties of constituents | Completely new properties formed |
| **Separation Method** | Physical methods (filtering, boiling) | Chemical methods only (electrolysis) |
| **Energy Change** | Usually no heat/light released | Heat or light released/absorbed |
| **Melting/Boiling Point** | Melts/boils over a range of temps | Sharp, fixed melting and boiling points |
• Classic Demonstration: Iron Filings + Sulfur Powder
- When mixed at room temperature: It is a MIXTURE. A magnet attracts the iron away from the sulfur; dissolving in carbon disulfide dissolves only the sulfur.
- When heated strongly in a test tube: A chemical reaction occurs. Red glow spreads; a black solid, Iron(II) sulfide (FeS), is formed. The magnet no longer attracts the iron! It is now a COMPOUND.
Key Takeaway: Mixtures have variable ratios and can be separated physically; compounds have fixed ratios and completely new chemical properties.
Real-World Application: Air is a mixture of nitrogen, oxygen, and argon; water is a compound of hydrogen and oxygen.
3. Laboratory Separation Techniques and Principles
The choice of separation technique depends on the differences in the physical properties of the components:
1. Filtration:
- Principle: Difference in particle size and solubility.
- Used to separate an insoluble solid from a liquid (e.g. chalk dust or sand from water).
- Solid remaining on the filter paper is the RESIDUE; the clear liquid passing through is the FILTRATE.
2. Evaporation:
- Principle: High boiling point solute vs volatile liquid solvent.
- Used to recover a dissolved solid solute from a solution by heating to dryness (e.g. recovering salt crystals from brine). The solvent water is lost to the atmosphere.
3. Simple Distillation:
- Principle: Significant difference in boiling points between liquid and solid, or two liquids.
- Used to obtain the pure liquid solvent while retaining the solute (e.g. obtaining pure distilled water from seawater).
- Involves two successive processes: Boiling (Vaporization) followed by Condensation in a water-cooled Liebig condenser.
4. Magnetic Separation:
- Principle: Difference in magnetic properties.
- Used to separate magnetic materials (iron, steel, nickel, cobalt) from non-magnetic substances (e.g. iron filings from sulfur or sand).
5. Sublimation:
- Principle: One component sublimes (solid → gas) upon heating while the other does not.
- Used to separate ammonium chloride, camphor, or iodine from common salt or sand.
6. Paper Chromatography:
- Principle: Differences in solubility in a mobile solvent and rate of migration through porous filter paper.
- Used to separate mixtures of colored dyes, pigments in black ink, or plant leaf extracts.
Key Takeaway: Filtration leaves residue on filter paper; distillation recovers the liquid solvent using a Liebig condenser; chromatography separates colored dyes.
Real-World Application: In Ada, Ghana, salt miners use solar evaporation to crystallize sea salt, while the local brewing of akpeteshie uses distillation.
Common Mistakes Students Make in BECE Examinations:
⚠️Confusing the terms Residue (substance left on filter paper) and Filtrate (liquid collected in the beaker).
⚠️Choosing evaporation when asked how to obtain pure WATER from sea water—evaporation loses the water; distillation must be used to collect the water!
⚠️Using an open flame to boil alcohol/ethanol mixtures instead of a water bath.
⚠️Claiming that iron filings and sulfur heated together can still be separated by a magnet.
Teacher's BECE Exam Pro-Tips:
⭐In a 4-step separation problem (e.g. mixture of iron, sand, and salt): State the steps in logical order: (1) Use magnet to remove iron, (2) Add water to dissolve salt, (3) Filter to collect sand as residue, (4) Evaporate filtrate to obtain dry salt crystals.
⭐Remember the Liebig condenser in distillation: Cold water always enters through the BOTTOM inlet and exits through the TOP outlet to ensure the condenser jacket remains completely filled.
Quick Revision Summary Checklist:
Can clearly contrast a mixture with a compound.
Define residue and filtrate in filtration.
Understand the setup and function of the Liebig condenser in distillation.
Know the principles of magnetic separation, sublimation, and paper chromatography.
Step-by-Step Worked Examples (2)
Real BECE exam-standard problems with complete solution steps
Example 1: Separating a Mixture of Sand, Salt, and Iron Filings
Problem StatementDescribe a step-by-step laboratory procedure to separate a dry mixture of iron filings, common salt (sodium chloride), and fine sand.
Step-by-Step Solution:
Step 1 (Magnetic Separation): Pass a bar magnet over the dry mixture to attract and remove all the iron filings.
Step 2 (Dissolution): Add distilled water to the remaining sand and salt mixture and stir thoroughly. The salt dissolves completely while the sand remains insoluble.
Step 3 (Filtration): Pour the mixture through a filter funnel lined with filter paper. Sand is collected as the residue on the filter paper; wash and dry the sand.
Step 4 (Evaporation): Pour the salt solution filtrate into an evaporating dish and heat gently over a Bunsen burner to evaporate the water, leaving dry pure salt crystals.
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Key Takeaway / Exam Rule: Always choose separation methods based on contrasting physical properties: magnetism -> solubility -> filtration -> evaporation.
Example 2: Separating Common Salt from Ammonium Chloride
Problem StatementWhich method is most suitable for separating a mixture of ammonium chloride and sodium chloride, and why?
Step-by-Step Solution:
Step 1 (Method): Sublimation.
Step 2 (Procedure): Place the mixture in an evaporating dish covered with an inverted glass funnel plugged with cotton wool at the top, and apply gentle heat.
Step 3 (Principle): Ammonium chloride sublimes directly into vapor upon heating and recrystallizes on the cool inner walls of the funnel as a sublimate, leaving sodium chloride behind in the dish.
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Key Takeaway / Exam Rule: Sublimation works whenever one substance changes directly from solid to vapor upon heating while the other does not.
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