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.
Precipitate
A precipitate is the solid that forms in a solution during a chemical reaction.
The reaction of ions in solution
Aim
To investigate the reactions of ions in solutions.
Prepare 2 test tubes with approximately \(\text{5}\) \(\text{mL}\) of dilute copper(II) chloride solution
in each
Prepare 1 test tube with \(\text{5}\) \(\text{mL}\) sodium carbonate solution
Prepare 1 test tube with \(\text{5}\) \(\text{mL}\) sodium sulphate solution
Carefully pour the sodium carbonate solution into one of the test tubes containing copper(II) chloride
and observe what happens
Carefully pour the sodium sulphate solution into the second test tube containing copper(II) chloride and
observe what happens
Results
A light blue precipitate forms when sodium carbonate reacts with copper(II) chloride.
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 18.1: General rules for the solubility of salts
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:
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:
Both of these reactions are ion exchange reactions.
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Tests for anions (ESAFS)
We often want to know which ions are present in solution. If we know which salts precipitate, we can derive
tests to identify ions in solution. Given below are a few such tests.
Test for a chloride
Prepare a solution of the unknown salt using distilled water and add a small amount of silver
nitrate solution. If a white precipitate forms, the salt is either a chloride or a carbonate.
(\(\text{Ag}_{2}\text{CO}_{3}\) is white precipitate)
The next step is to treat the precipitate with a small amount of concentrated nitric acid.
If the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed and the
precipitate disappears, the salt is a carbonate.
As was the case with the chlorides, the bromides and iodides also form precipitates when they are reacted
with silver nitrate. Silver chloride is a white precipitate, but the silver bromide and silver iodide
precipitates are both pale yellow. To determine whether the precipitate is a bromide or an iodide, we use
chlorine water and carbon tetrachloride (\(\text{CCl}_{4}\)).
Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a reddish brown.
Add a small amount of barium chloride solution to a solution of the test salt. If a white precipitate forms,
the salt is either a sulfate or a carbonate.
If the precipitate is treated with nitric acid, it is possible to distinguish whether the salt is a sulphate
or a carbonate (as in the test for a chloride).
(It is the insoluble \(\text{CaCO}_{3}\) precipitate that makes the limewater go milky)
Precipitation reactions and ions in solution
Textbook Exercise 18.2
Silver nitrate (\(\text{AgNO}_{3}\)) reacts with potassium chloride (\(\text{KCl}\)) and a white
precipitate is formed.
Write a balanced equation for the reaction that takes place. Include the state symbols.
What is the name of the insoluble salt that forms?
Which of the salts in this reaction are soluble?
Solution not yet available
Barium chloride reacts with sulphuric acid to produce barium sulphate and hydrochloric acid.
Write a balanced equation for the reaction that takes place. Include the state symbols.
Does a precipitate form during the reaction?
Describe a test that could be used to test for the presence of barium sulphate in the products.
Solution not yet available
A test tube contains a clear, colourless salt solution. A few drops of silver nitrate solution are
added to the solution and a pale yellow precipitate forms. Chlorine water and carbon tetrachloride
were added, which resulted in a purple solution. Which one of the following salts was dissolved in the
original solution? Write the balanced equation for the reaction that took place between the salt and
silver nitrate.