CIE iGCSE Co-ordinated Sciences-C12.4 Separation and purification- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C12.4 Separation and purification – Study Notes
CIE iGCSE Co-ordinated Sciences-C12.4 Separation and purification – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe and explain methods of separation and purification using:
(a) a suitable solvent
(b) filtration
(c) crystallisation
(d) simple distillation
(e) fractional distillation - Suggest suitable separation and purification techniques, given information about the substances involved
- Identify substances and assess their purity from melting point and boiling point information
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Separation and Purification Methods
(a) Using a Suitable Solvent
- A solvent is used to selectively dissolve one substance while leaving another undissolved.
- This method separates mixtures of solids where one is soluble and the other is insoluble.
- Example: Salt and sand can be separated by dissolving salt in water (solvent), leaving sand behind.
(b) Filtration
- Filtration separates an insoluble solid from a liquid.
- The mixture is poured through filter paper in a funnel.
- The residue (insoluble solid) remains on the paper, while the filtrate (liquid/solution) passes through.
- Example: Separating sand (residue) from salt solution (filtrate).
(c) Crystallisation
- Crystallisation is used to obtain pure crystals of a soluble solid from a solution.
- The solution is heated until it becomes saturated, then cooled to allow crystals to form.
- Example: Obtaining pure salt crystals from seawater.
(d) Simple Distillation
- Simple distillation separates a solvent from a solution.
- The solution is heated; the solvent boils and evaporates, then condenses into a separate container.
- It works when the solvent has a much lower boiling point than the solute.
- Example: Obtaining pure water from salt water.
(e) Fractional Distillation
- Fractional distillation separates a mixture of liquids with different boiling points.
- The mixture is heated; the liquid with the lowest boiling point evaporates first.
- A fractionating column improves separation by allowing repeated condensation and evaporation.
- Example: Separating ethanol (bp 78°C) from water (bp 100°C), or separating crude oil into fractions.
Example :
Which method would you use to separate a mixture of ethanol and water, and why?
▶️ Answer/Explanation
Step 1: Ethanol and water are both liquids, so methods like filtration or crystallisation will not work.
Step 2: Their boiling points are different (ethanol 78°C, water 100°C).
Step 3: Fractional distillation can separate them based on boiling point differences.
Final Answer: Use fractional distillation because it separates liquids with different boiling points.
Choosing Suitable Separation and Purification Techniques
Different mixtures require different separation techniques. The method depends on whether the substances are solids, liquids, soluble, insoluble, or have different boiling points.
General Guide:
- Insoluble solid + liquid → Use filtration (e.g. sand + water).
- Soluble solid + liquid → Use crystallisation (to get the solid) or simple distillation (to get the liquid solvent).
- Two or more soluble coloured substances → Use paper chromatography (e.g. inks, dyes).
- Two or more liquids with different boiling points → Use fractional distillation (e.g. ethanol + water, crude oil).
- Gas → Collect in a gas syringe or over water if insoluble in water (e.g. oxygen, hydrogen).
- Purification of a solid → Dissolve in a suitable solvent, filter, then recrystallise.
Key Idea: The choice of technique depends on physical properties such as solubility and boiling point, not chemical reactions.
Example
How would you separate a mixture of salt and sand?
▶️ Answer/Explanation
Step 1: Add water → salt dissolves, sand does not.
Step 2: Filter → sand (residue) and salt solution (filtrate).
Step 3: Evaporate water from filtrate → pure salt crystals.
Final Answer: Use dissolving, filtration, and crystallisation.
Example
How would you separate ethanol from water?
▶️Answer/Explanation
Step 1: Ethanol (bp 78°C) boils before water (bp 100°C).
Step 2: Heat mixture → ethanol evaporates first.
Step 3: Vapour is condensed into a separate container.
Final Answer: Use fractional distillation.
Example
How would you check if black ink contains more than one dye?
▶️ Answer/Explanation
Step 1: Place a spot of ink on chromatography paper.
Step 2: Run solvent up the paper.
Step 3: If more than one spot appears → ink is impure.
Final Answer: Use paper chromatography.
Identifying Substances and Assessing Purity from Melting and Boiling Points
Every pure substance has a sharp, fixed melting point and boiling point. Impure substances show changes in these values.
1. Pure Substances
- A pure substance has a sharp melting point (melts at a single temperature).
- A pure liquid has a fixed boiling point (boils at one temperature).
- Example: Pure water boils at 100°C and melts at 0°C at 1 atm pressure.
2. Impure Substances (Mixtures)
- Impurities lower the melting point and cause melting over a range of temperatures.
- Impurities raise or broaden the boiling point range.
- Example: Salt water boils above 100°C and does not have a sharp boiling point.
3. Identifying Substances
- Measure the melting or boiling point of an unknown substance.
- Compare with known values in reference tables.
- If the experimental value matches closely, the substance can be identified as that pure compound.
Summary:
- Sharp melting/boiling point = Pure substance
- Lowered, raised, or broad range = Impure substance
Example :
A student measures the boiling point of a sample of ethanol and finds it boils between 76°C and 79°C. Pure ethanol has a boiling point of 78°C. What does this suggest about the sample?
▶️ Answer/Explanation
Step 1: Pure ethanol has a sharp boiling point of 78°C.
Step 2: The student’s sample boiled over a range (76–79°C).
Step 3: This suggests the ethanol is impure, containing another substance.
Final Answer: The ethanol sample is not pure because it boiled over a range of temperatures.