7.3 Preparation of Salts Revision Notes

1. Overview

The preparation of salts is a fundamental laboratory process in chemistry. It involves selecting the correct method based on the solubility of the reactants and the desired salt product. Understanding these techniques is crucial for controlling chemical reactions and purifying substances for industrial and medicinal use.

Key Definitions

• Salt: A compound formed when the hydrogen ions ($H^+$) of an acid are replaced by metal ions or ammonium ($NH_4^+$) ions.
• Hydrated substance: A substance that is chemically combined with water molecules within its crystal structure.
• Anhydrous substance: A substance containing no water, often formed by heating a hydrated salt to drive off the water of crystallisation.
• Precipitate: An insoluble solid that emerges from a liquid solution during a chemical reaction.
• Filtrate: The liquid that passes through a filter.
• Residue: The solid left behind on the filter paper after filtration.

Core Content

Solubility Rules for Salts

To choose the correct preparation method, you must know if the salt is soluble:

• Sodium, potassium, and ammonium salts: All are soluble.
• Nitrates: All are soluble.
• Chlorides: All are soluble except lead(II) chloride ($PbCl_2$) and silver chloride ($AgCl$).
• Sulfates: All are soluble except barium sulfate ($BaSO_4$), calcium sulfate ($CaSO_4$), and lead(II) sulfate ($PbSO_4$).
• Carbonates: All are insoluble except sodium, potassium, and ammonium carbonates.
• Hydroxides: All are insoluble except sodium, potassium, ammonium, and calcium hydroxides (calcium hydroxide is partially soluble).

Preparation of Soluble Salts

There are two main methods for making soluble salts:

Method A: Reaction of an acid with an insoluble reactant (Excess Metal, Base, or Carbonate) Used when the starting material (metal, base, or carbonate) does not dissolve in water.

1. Add excess insoluble solid to the acid to ensure all acid is neutralised.
2. Heat gently to speed up the reaction (for bases and metals).
3. Filter the mixture to remove the unreacted excess solid.
4. Heat the resulting filtrate in an evaporating dish to the "point of crystallisation" (saturated solution).
5. Allow to cool so crystals form; then filter, wash with a little distilled water, and dry.

• Example (Excess Metal): $Magnesium + Sulfuric\ acid \rightarrow Magnesium\ sulfate + Hydrogen$ $Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)$
• Example (Excess Insoluble Base): $Copper(II)\ oxide + Hydrochloric\ acid \rightarrow Copper(II)\ chloride + Water$ $CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)$
• Example (Excess Insoluble Carbonate): $Calcium\ carbonate + Nitric\ acid \rightarrow Calcium\ nitrate + Water + Carbon\ dioxide$ $CaCO_3(s) + 2HNO_3(aq) \rightarrow Ca(NO_3)_2(aq) + H_2O(l) + CO_2(g)$

Method B: Reaction of an acid with an alkali (Titration) Used when both reactants are soluble (e.g., sodium hydroxide and hydrochloric acid).

1. Use a pipette to add a fixed volume of alkali to a conical flask.
2. Add an indicator (e.g., methyl orange or phenolphthalein).
3. Add acid from a burette until the indicator changes colour (the end-point).
4. Note the volume of acid used, then repeat the process without indicator using the same volumes.
5. Evaporate and crystallise the resulting solution as described in Method A.

• Example (Titration): $Sodium\ hydroxide + Hydrochloric\ acid \rightarrow Sodium\ chloride + Water$ $NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$

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Extended Content (Extended Only)

Preparation of Insoluble Salts (Precipitation)

Insoluble salts are prepared by mixing two solutions of soluble salts.

1. Mix the two soluble salt solutions.
2. A precipitate (the insoluble salt) forms immediately.
3. Filter the mixture to collect the precipitate (residue).
4. Wash the residue with distilled water to remove traces of soluble impurities.
5. Dry the precipitate in a warm oven or by patting with filter paper.

• Example (Precipitation): To make silver chloride ($AgCl$): $Silver\ nitrate(aq) + Sodium\ chloride(aq) \rightarrow Silver\ chloride(s) + Sodium\ nitrate(aq)$ $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$

Water of Crystallisation

Many salts form crystals that contain water molecules as part of their fixed chemical structure. This is called water of crystallisation.

• Definition: The water molecules present in hydrated crystals.
• Hydrated Copper(II) sulfate: $CuSO_4 \cdot 5H_2O$ (Blue crystals).
• Hydrated Cobalt(II) chloride: $CoCl_2 \cdot 6H_2O$ (Pink crystals).

When these are heated, the water is lost, leaving the anhydrous salt (Anhydrous $CuSO_4$ is white; anhydrous $CoCl_2$ is blue).

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Key Equations

Reaction Type	General Equation
Acid + Metal	$Acid + Metal \rightarrow Salt + Hydrogen$
Acid + Base	$Acid + Base \rightarrow Salt + Water$
Acid + Carbonate	$Acid + Carbonate \rightarrow Salt + Water + Carbon\ Dioxide$
Precipitation	$Soluble\ salt\ A + Soluble\ salt\ B \rightarrow Insoluble\ salt + Soluble\ salt\ C$

Specific Balanced Symbols Equations to Know:

1. $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$
2. $NaOH(aq) + HNO_3(aq) \rightarrow NaNO_3(aq) + H_2O(l)$
3. $BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$

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Common Mistakes to Avoid

• ❌ Wrong: Forgetting to use "excess" solid when making a soluble salt from an insoluble base.
• ✓ Right: Always state "add excess solid" to ensure all the acid is used up, so the salt isn't contaminated with acid.
• ❌ Wrong: Washing a soluble salt with lots of water after filtration.
• ✓ Right: Only wash insoluble precipitates. If you wash a soluble salt crystal with too much water, it will dissolve and be lost.
• ❌ Wrong: Heating a solution to dryness to get crystals.
• ✓ Right: Heat to the "point of crystallisation" (saturation) and let it cool slowly to get large, well-defined crystals.

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Exam Tips

• Command Word "Describe": When asked to describe a preparation, list the practical steps in order (Add, Stir, Filter, Evaporate, Dry).
• Identifying Methods: If the salt is insoluble, use Precipitation. If it is soluble and contains Na, K, or $NH_4$, use Titration. For all other soluble salts, use the "Excess Insoluble Base/Metal" method.
• State Symbols: In precipitation questions, the salt you are making must have the state symbol (s), while the reactants are (aq).
• Real-world Context: Barium sulfate is often used as a "Barium meal" in X-rays because it is insoluble and won't be absorbed into the blood, making it safe despite barium's toxicity.