Preparation of salts
Solubility Rules for Salts
Before preparing salts, you must know which salts dissolve in water. This determines the method used.
| Details |
|---|
| All nitrates are soluble. |
| All salts containing these ions are soluble. |
| Most chlorides are soluble. Exceptions: Silver chloride (AgCl), Lead(II) chloride (PbCl_2). |
| Most sulfates are soluble. Exceptions: Barium sulfate (BaSO_4), Lead(II) sulfate (PbSO_4), Calcium sulfate (CaSO_4). |
Why this matters: If a salt is soluble, we use titration or excess reactant methods. If it is insoluble, we must use precipitation.
Preparation of Soluble Salts
Soluble salts are prepared using different methods depending on the reactants. The goal is always to obtain a pure salt solution free from unreacted starting materials.
| Method Description |
|---|
| Titration is used. You must determine the exact volume of acid needed to neutralize the alkali using an indicator. Repeat without indicator to get pure solution. |
| Add excess metal (e.g., Zinc, Magnesium) to acid. The reaction stops when acid is used up. Filter off the unreacted metal. Evaporate filtrate. |
| Add excess base (e.g., Copper(II) oxide) to acid. Filter off the unreacted solid base. Evaporate filtrate. |
| Add excess carbonate (e.g., Calcium carbonate) to acid. Effervescence (bubbling of CO_2) stops when reaction is complete. Filter off unreacted solid. Evaporate filtrate. |
Key Principle for Excess Reactants: We add the solid in excess to ensure all the acid is neutralized. This prevents contamination of the final salt with leftover acid. The excess solid is removed by filtration because it is insoluble.
Preparation of Insoluble Salts (Precipitation)
Insoluble salts cannot be made by evaporation because they do not dissolve. They are made by precipitation.
The Process:
- Mix two soluble salt solutions that contain the ions needed for the insoluble salt.
- A solid precipitate forms immediately.
- Filter the mixture to collect the solid residue (the salt).
- Wash the residue with distilled water to remove soluble impurities.
- Dry the residue between filter papers or in a warm place.
Example: To make Lead(II) sulfate (PbSO_4):
Reactants: Lead(II) nitrate solution + Sodium sulfate solution.
Equation: Pb^{2+}(aq) + SO_4^{2-}(aq) \rightarrow PbSO_4(s)
Reactants: Lead(II) nitrate solution + Sodium sulfate solution.
Equation: Pb^{2+}(aq) + SO_4^{2-}(aq) \rightarrow PbSO_4(s)
Crystallization and Drying
To obtain crystals from a soluble salt solution:
| Action |
|---|
| Heat the solution in an evaporating basin until it reaches the point of crystallisation (saturation). This is when crystals just start to form on the surface or a 'skin' forms. |
| Allow the saturated solution to cool slowly. Crystals grow as the solubility decreases with temperature. |
| Filter off the crystals and dry them with filter paper or in a warm place. Do not heat strongly, or you may decompose the salt or lose water of crystallisation. |
Hydrated Substance
A hydrated substance is a salt that is chemically combined with water. The water molecules are part of the crystal structure.
Example: Copper(II) sulfate crystals are hydrated. Formula: CuSO_4 \cdot 5H_2O. This means 1 mole of CuSO_4 is combined with 5 moles of water.
Anhydrous Substance
An anhydrous substance is a salt containing no water. It is the dry form of the salt.
Example: Anhydrous copper(II) sulfate is white powder (CuSO_4). When water is added, it turns blue (hydrated).
Water of Crystallisation
Water of crystallisation refers to the specific number of water molecules present in the crystal lattice of a hydrated salt.
Examples:
- Cobalt(II) chloride hexahydrate: CoCl_2 \cdot 6H_2O (6 water molecules).
- Copper(II) sulfate pentahydrate: CuSO_4 \cdot 5H_2O (5 water molecules).
Preparing Pure, Dry Copper(II) Sulfate Crystals
Reactants: Dilute sulfuric acid (H_2SO_4) and excess Copper(II) oxide (CuO, an insoluble base).
| Explanation |
|---|
| Add excess CuO to warm dilute H_2SO_4. Stir until no more CuO dissolves (excess acid is neutralized). |
| Filter the mixture. The filtrate contains pure aqueous copper(II) sulfate. The residue is unreacted CuO. |
| Heat the filtrate to the point of crystallisation. Allow to cool. Crystals form. |
| Filter off crystals and dry with filter paper. Result: Blue hydrated crystals (CuSO_4 \cdot 5H_2O). |
Preparing Pure, Dry Lead(II) Chloride (Insoluble Salt)
Reactants: Lead(II) nitrate solution and Sodium chloride solution.
| Explanation |
|---|
| Mix the two solutions. White precipitate of PbCl_2 forms. |
| Filter to collect the solid residue (PbCl_2). |
| Wash the residue with distilled water to remove soluble impurities (like sodium nitrate). |
| Dry between filter papers. Result: White solid PbCl_2. |
⚠︎ Confusing Soluble vs Insolulent Salt Methods
Error: Using titration to prepare Lead(II) sulfate.
Correction: Titration is only for soluble salts. Lead(II) sulfate is insoluble, so it must be made by precipitation (mixing two soluble solutions).
Correction: Titration is only for soluble salts. Lead(II) sulfate is insoluble, so it must be made by precipitation (mixing two soluble solutions).
Error: Evaporating the filtrate when making an insoluble salt.
Correction: For insoluble salts, you filter to get the solid. You do not evaporate the solution to get the salt; the salt is already the solid residue.
Correction: For insoluble salts, you filter to get the solid. You do not evaporate the solution to get the salt; the salt is already the solid residue.
⚠︎ Definition of Hydrated vs Anhydrous
Error: Saying a hydrated salt is 'wet' or 'damp'.
Correction: A hydrated salt is chemically combined with water. The water is part of the crystal structure, not just surface moisture.
Correction: A hydrated salt is chemically combined with water. The water is part of the crystal structure, not just surface moisture.
Error: Saying anhydrous means 'no hydrogen'.
Correction: Anhydrous means no water. It can still contain hydrogen (e.g., in other ions), but no H_2O molecules are attached.
Correction: Anhydrous means no water. It can still contain hydrogen (e.g., in other ions), but no H_2O molecules are attached.
Describing Crystallization Steps
Context: When asked to describe how to obtain crystals from a solution.
Why accepted: Examiners look for the specific term point of crystallisation or saturation. Simply saying 'evaporate until dry' is incorrect because it may decompose the salt or remove water of crystallisation.
Why accepted: Examiners look for the specific term point of crystallisation or saturation. Simply saying 'evaporate until dry' is incorrect because it may decompose the salt or remove water of crystallisation.
Correct Phrase: "Heat the solution to the point of crystallisation (or until a saturated solution forms) and allow to cool."
Defining Hydrated Substances
Context: When asked to define 'hydrated'.
Why accepted: The key phrase is chemically combined. Vague answers like 'contains water' are often rejected because aqueous solutions also contain water but are not hydrated salts.
Why accepted: The key phrase is chemically combined. Vague answers like 'contains water' are often rejected because aqueous solutions also contain water but are not hydrated salts.
Correct Phrase: "A substance that is chemically combined with water."
Washing Insoluble Salts
Context: When describing the preparation of an insoluble salt.
Why accepted: Students often forget to remove soluble impurities. Washing is essential for purity.
Why accepted: Students often forget to remove soluble impurities. Washing is essential for purity.
Correct Phrase: "Wash the residue with distilled water to remove soluble impurities."
Past Paper Style Questions
Q:
Describe how to prepare pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
A:
- Add excess copper(II) oxide to dilute sulfuric acid.
- Filter the mixture to remove unreacted copper(II oxide (residue).
- Heat the filtrate to the point of crystallisation.
- Allow to cool and filter off crystals.
- Dry crystals with filter paper.
Q:
Name two soluble salts that can be used to prepare lead(II) sulfate.
A:
Lead(II) nitrate and Sodium sulfate (or Potassium sulfate).
Q:
Define the term 'water of crystallisation'.
A:
The water molecules present in hydrated crystals.
Q:
Why is excess zinc used when preparing zinc sulfate from dilute sulfuric acid?
A:
To ensure all the acid is neutralized/reacted, so no acid remains in the final salt.