Preparation of insoluble salts

Insoluble salts are prepared by double decomposition or precipitation method. In this method, two soluble salts are mixed to form two new salts by exchange of radicals. One of the new salts formed is a soluble salt and one is an insoluble salt that appear as precipitates. The precipitate is filtered off and washed then dried.
Precipitation is the formation of solids when solutions are mixed.
A precipitate is the solid formed when two or more solutions are mixed.
Example
Preparation of barium sulphate (by reacting barium nitrate and sodium sulphate)
Procedure

  • Put a solution of barium nitrate in a beaker and add a solution of sodium sulphate to it. A white precipitate of barium sulphate immediately appears,
  • Filter off the precipitate and wash with distilled water.
  • Dry the precipitate (salt formed) under sun shine, in an oven or between filter papers.
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This method can be used to prepare salts such as lead sulphate, aluminium chloride, silver chloride, silver carbonate and barium sulphate.


Preparation of lead (II) sulphate (by reacting lead (II) nitrate and sulphuric acid)
Procedure

  • Add dilute nitric acid to lead (II) nitrate solution in a beaker and stir the mixture. White precipitates of lead (II) sulphate is formed.
  • Filter off the precipitates and wash with distilled water to remove traces of the acid.
  • Dry the precipitates in a steam oven or leave it to dry in air.

NB. If any of the compounds to be used in the preparation of the salt is insoluble in water, it must first be made to dissolve in a mineral acid. For example, in the preparation of lead (II) sulphate using lead (II) oxide, the lead (II) is first divvolved in nitric acid to form lead (II) nitrate.

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Preparation of salts by direct synthesis
Salts consisting of two elements (binary salts) can be prepared by direct synthesis/ direct combination.
Example:
In the preparation of sodium chloride from sodium and chlorine, burning sodium is lowered in a gas jar of chlorine. Sodium continues to burn in chlorine forming white fumes which settle into white solids (sodium chloride).
Equation

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Effects of heat on salts

  1. Carbonates
    Potassium and sodium carbonates are very stable and are not decomposed by heat. But if the salts are hydrated, they lose their water of crystallization. In such a process, salts lose their crystalline nature and become amorphous.
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All the other metallic carbonates decompose upon heating to give the oxide of the metal and carbondioxide gas.
Example
When white zinc carbonate is heated, it produces a colorless gas that turns lime water milky leaving a yellow residue when hot which turns white on cooling.

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When lead (II) carbonate is heated, a brown residue (when hot) which becomes yellow on cooling and a colorless gas that turns lime water milky are produced.

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Ammonium carbonate decomposes to give ammonia gas, carbondioxide and water vapor.

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Hydrogen carbonate of metals decompose to form carbonate of metals, carbondioxide gas and water vapor.
Example

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  1. Sulphates
    Sulphates of sodium and potassium donot decompose on heating. When hydrated sulphates of potassium or sodium is heated, it loses its water of crystallization and becomes amorphous.
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Sulphates of heavy metals decompose to give metal oxides and white fumes of sulphur trioxide gas. When heated more strongly, the sulphur trioxide gas decomposes to give sulphur dioxide and oxygen gas.
Examples
When hydrated copper (II) sulphate crystals are heated, they lose their water of crystallization and changes from blue crystals to white powder. The water condenses as a colorless liquid on the cooler parts of the test tube.

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When green solid of iron (II) sulphate is heated, it loses it water of crystallization and changes from green to dirty-yellow anhydrous solids.

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  1. Nitrates
    All nitrates decompose upon heating.
    Sodium and potassium nitrates melt into colorless liquids then decompose upon heating to give their corresponding nitrites that form yellow solids on cooling and oxygen gas.
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  1. Chlorides
    Metallic chlorides are not affected by heat because hey are very stable. However, if they are hydrated, the lose their water of crystallization.
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Ammonium chloride sublimes on slight heating and on further heating, it decomposes to give ammonia and hydrogen chloride gases.

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Effects of heat on hydroxides
Hydroxides of sodium and potassium are not decomposed by heat. If they are in solid forms, they absorb moisture and melt to form solutions.
However, the hydroxides of other metals decompose to give the corresponding oxides and water vapor.

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Sample questions on salts

  1. Briefly explain what is meant by the following and in each case give an example (formula and name): salt, acid salt, normal salt and basic salt.
  2. What is solubility of a salt? Describe with the aid of a labeled drawing how you can determine the solubility of sodium chloride at 40˚C
  3. (a) State one method for preparing: (i) lead (II) nitrate other than from lead (II) carbonate (ii)lead (II) sulphate. Describe how a pure dry sample of lead (II) nitrate can be prepared in the laboratory, starting from lead (II) carbonates.
  4. Describe what would be observed and write equations when each of the following salts are heated: (a) lead(II) carbonate (b) lead(II) nitrate (c) sodium carbonate crystals (d) zinc carbonate (e) ammonium carbonate (f) hydrated copper(II) sulphate (g) sodium nitrate (h) zinc nitrate crystals (i) silver nitrate and ammonium nitrate.
  5. Describe in detail how you would prepare pure dry samples of (a) zinc sulphate from zinc carbonate (b) lead(II) sulphate from lead(II) nitrate (c) sodium chloride by action of an acid and a base.
  6. Write down the formula of the precipitate which is produced when pairs of aqueous solutions are mixed as follows: (i) Ba(NO3)2 and Na2SO4; (ii) H2S and Pb(NO3)2; (iii) NH4Br and AgNO3; (iv) CaCl2 and K2CO3.
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