Precipitation Reactions
Precipitation reactions
Sometimes, ions in solution may react with each other to form a new substance that is insoluble. This is called a precipitate. The reaction is called a precipitation reaction.
Definition: Precipitate
A precipitate is the solid that forms in a solution during a chemical reaction.
Optional Experiment: The reaction of ions in solution
Aim
To investigate the reactions of ions in solutions.
Apparatus
4 test tubes; copper(II) chloride solution; sodium carbonate solution; sodium sulphate solution
Method
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Prepare 2 test tubes with approximately \(\text{5}\) \(\text{mL}\) of dilute copper(II) chloride solution in each
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Prepare 1 test tube with \(\text{5}\) \(\text{mL}\) sodium carbonate solution
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Prepare 1 test tube with \(\text{5}\) \(\text{mL}\) sodium sulphate solution
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Carefully pour the sodium carbonate solution into one of the test tubes containing copper(II) chloride and observe what happens
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Carefully pour the sodium sulphate solution into the second test tube containing copper(II) chloride and observe what happens
Results
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A light blue precipitate forms when sodium carbonate reacts with copper(II) chloride.
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No precipitate forms when sodium sulphate reacts with copper(II) chloride. The solution is light blue.
It is important to understand what happened in the previous demonstration. We will look at what happens in each reaction, step by step.
For reaction 1 you have the following ions in your solution: \(\text{Cu}^{2+}\), \(\text{Cl}^{-}\), \(\text{Na}^{+}\) and \(\text{CO}_{3}^{2-}\). A precipitate will form if any combination of cations and anions can become a solid. The following table summarises which combination will form solids (precipitates) in solution.
|
Salt |
Solubility |
|
Nitrates |
All are soluble |
|
Potassium, sodium and ammonium salts |
All are soluble |
|
Chlorides, bromides and iodides |
All are soluble except silver, lead(II) and mercury(II) salts (e.g. silver chloride) |
|
Sulphates |
All are soluble except lead(II) sulphate, barium sulphate and calcium sulphate |
|
Carbonates |
All are insoluble except those of potassium, sodium and ammonium |
|
Compounds with fluorine |
Almost all are soluble except those of magnesium, calcium, strontium (II), barium (II) and lead (II) |
|
Perchlorates and acetates |
All are soluble |
|
Chlorates |
All are soluble except potassium chlorate |
|
Metal hydroxides and oxides |
Most are insoluble |
Table: General rules for the solubility of salts
Tip:
Salts of carbonates, phosphates, oxalates, chromates and sulphides are generally insoluble.
If you look under carbonates in the table it states that all carbonates are insoluble except potassium sodium and ammonium. This means that \(\text{Na}_{2}\text{CO}_{3}\) will dissolve in water or remain in solution, but \(\text{CuCO}_{3}\) will form a precipitate. The precipitate that was observed in the reaction must therefore be \(\text{CuCO}_{3}\). The balanced chemical equation is:
\[2\text{Na}^{+}\text{(aq)} + \text{CO}_{3}^{2-}\text{(aq)} + \text{Cu}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)} \rightarrow \text{CuCO}_{3}\text{(s)} + 2\text{Na}^{+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)}\]Note that sodium chloride does not precipitate and we write it as ions in the equation. For reaction 2 we have \(\text{Cu}^{2+}\), \(\text{Cl}^{-}\), \(\text{Na}^{+}\) and \(\text{SO}_{4}^{2-}\) in solution. Most chlorides and sulphates are soluble according to the table. The balanced chemical equation is:
\[2\text{Na}^{+}\text{(aq)} + \text{SO}_{4}^{2-}\text{(aq)} + \text{Cu}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)} \rightarrow 2\text{Na}^{+}\text{(aq)} + \text{SO}_{4}^{2-}\text{(aq)} + \text{Cu}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)}\]Both of these reactions are ion exchange reactions.
This lesson is part of:
Reactions in Aqueous Solution