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CIE iGCSE Co-ordinated Sciences-C7.3 Preparation of salts- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-C7.3 Preparation of salts – Study Notes

CIE iGCSE Co-ordinated Sciences-C7.3 Preparation of salts – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Preparation, Separation, and Purification of Soluble Salts

Soluble salts can be prepared by reacting an acid with an alkali, a metal, an insoluble base, or an insoluble carbonate. After preparation, the salt solution is separated from any solids and purified by crystallisation.

(a) Reaction of an Acid with an Alkali by Titration

  • A soluble salt can be prepared by carefully neutralising a known volume of acid with an alkali using an indicator (like methyl orange) to detect the endpoint.
  • The mixture is then evaporated to crystallise the salt.

Example 

Describe the preparation of sodium chloride by titration.

▶️Answer/Explanation
  1. Measure a known volume of hydrochloric acid into a conical flask and add methyl orange indicator.
  2. Titrate with sodium hydroxide solution until the endpoint is reached (colour changes from orange to pink).
  3. Remove the indicator and evaporate the solution to crystallise sodium chloride.
  4. Filter the crystals and allow them to dry.

(b) Reaction of an Acid with Excess Metal

  • Add a metal (e.g., zinc) in excess to an acid (e.g., sulfuric acid) until no more reaction occurs.
  • Filter the mixture to remove excess metal and evaporate the filtrate to crystallise the salt.

Example 

Describe the preparation of zinc sulfate.

▶️Answer/Explanation
  1. Add excess zinc to dilute sulfuric acid.
  2. Continue until bubbling stops (hydrogen gas ceases).
  3. Filter the solution to remove unreacted zinc.
  4. Evaporate the filtrate to obtain pure zinc sulfate crystals.

(c) Reaction of an Acid with Excess Insoluble Base

  • React an acid with an insoluble base (e.g., copper(II) oxide) in excess to ensure complete reaction.
  • Filter the mixture and evaporate the filtrate to crystallise the salt.

Example 

Describe the preparation of copper(II) sulfate.

▶️Answer/Explanation
  1. Add excess copper(II) oxide to dilute sulfuric acid.
  2. Stir until no more solid dissolves.
  3. Filter to remove unreacted copper(II) oxide.
  4. Evaporate the filtrate to obtain copper(II) sulfate crystals.

(d) Reaction of an Acid with Excess Insoluble Carbonate

  • React an acid with an insoluble carbonate (e.g., calcium carbonate) in excess.
  • Filter the mixture to remove excess carbonate and evaporate the solution to crystallise the salt.

Example 

Describe the preparation of calcium chloride.

▶️Answer/Explanation
  1. Add excess calcium carbonate to hydrochloric acid.
  2. Effervescence of carbon dioxide indicates reaction.
  3. Filter to remove unreacted calcium carbonate.
  4. Evaporate the filtrate to obtain pure calcium chloride crystals.

Hydrated and Anhydrous Substances

Hydrated Substance:

A hydrated substance is a substance that contains water molecules chemically combined within its structure. This water is called water of crystallisation.

Example: \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \) (copper(II) sulfate pentahydrate) contains 5 molecules of water chemically bound in each formula unit.

Anhydrous Substance:

An anhydrous substance is a substance that contains no water molecules in its structure. It is the dry form of a salt.

Example: \( \text{CuSO}_4 \) (anhydrous copper(II) sulfate) contains no water.

Example 

What is the difference between hydrated and anhydrous copper(II) sulfate?

▶️Answer/Explanation

Hydrated copper(II) sulfate: \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \), contains water of crystallisation and is blue in colour.
Anhydrous copper(II) sulfate: \( \text{CuSO}_4 \), contains no water and is white or greyish in colour.

Example 

How can you convert hydrated copper(II) sulfate to anhydrous copper(II) sulfate?

▶️Answer/Explanation

By heating hydrated copper(II) sulfate (\( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \)) gently, the water of crystallisation is removed, leaving anhydrous copper(II) sulfate (\( \text{CuSO}_4 \)).

Preparation of Insoluble Salts by Precipitation

Insoluble salts can be prepared by mixing two solutions containing ions that react to form an insoluble product. The insoluble salt forms as a precipitate, which can be separated by filtration and purified by washing and drying.

General Steps:

  • Mix two aqueous solutions containing the appropriate ions.
  • An insoluble salt precipitate forms immediately.
  • Filter the mixture to separate the solid precipitate from the solution.
  • Wash the precipitate with distilled water to remove impurities.
  • Dry the precipitate to obtain the pure insoluble salt.

Example 

Describe the preparation of barium sulfate (\( \text{BaSO}_4 \)).

▶️Answer/Explanation
  1. Mix aqueous solutions of barium chloride (\( \text{BaCl}_2 \)) and sulfuric acid (\( \text{H}_2\text{SO}_4 \)) or sodium sulfate (\( \text{Na}_2\text{SO}_4 \)).
  2. Barium sulfate forms as a white precipitate: \( \text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_4 \).
  3. Filter the mixture to separate the solid \( \text{BaSO}_4 \).
  4. Wash the precipitate with distilled water to remove soluble impurities.
  5. Dry the barium sulfate to obtain the pure insoluble salt.

Example 

Describe the preparation of lead(II) iodide (\( \text{PbI}_2 \)).

▶️Answer/Explanation
  1. Mix aqueous solutions of lead(II) nitrate (\( \text{Pb(NO}_3)_2 \)) and potassium iodide (\( \text{KI} \)).
  2. Lead(II) iodide forms as a bright yellow precipitate: \( \text{Pb}^{2+} + 2\text{I}^- \rightarrow \text{PbI}_2 \).
  3. Filter to separate the solid \( \text{PbI}_2 \).
  4. Wash with distilled water to remove soluble impurities.
  5. Dry the precipitate to obtain pure lead(II) iodide.
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