<section>
<h2 class="title">Precipitation reactions</h2>
<p>Sometimes, ions in solution may react with each other to form a new substance that is <em>insoluble</em>. This is called a <strong>precipitate</strong>. The reaction is called a precipitation reaction.</p>
<div class="ns-school-emp">
<h3>Definition: Precipitate</h3>
<div class="ns-school-defn">
<p>A precipitate is the <a data-lesson="melting-and-freezing" data-subject="chemistry" data-tutorial="liquids-and-solids" href="https://app.ulearngo.com/chemistry/liquids-and-solids/melting-and-freezing">solid</a> that forms in a solution during a chemical reaction.</p>
</div>
</div>
<div class="ns-school-emp">
<h2>Optional Experiment: The reaction of ions in solution</h2>
<div class="ns-school-defn">
<section class="activity">
<h3>Aim</h3>
<p>To investigate the reactions of ions in solutions.</p>
</section>
<section class="activity">
<h3>Apparatus</h3>
<p>4 <a data-lesson="reaction-rates-in-analysis-test-strips-for-urinalysis" data-subject="chemistry" data-tutorial="chemical-kinetics" href="https://app.ulearngo.com/chemistry/chemical-kinetics/reaction-rates-in-analysis-test-strips-for-urinalysis">test</a> tubes; copper(II) chloride solution; sodium <a data-lesson="occurrence-preparation-and-properties-of-carbonates" data-subject="chemistry" data-tutorial="metals-metalloids-nonmetals" href="https://app.ulearngo.com/chemistry/metals-metalloids-nonmetals/occurrence-preparation-and-properties-of-carbonates">carbonate</a> solution; sodium sulphate solution</p>
<img alt="12f2c7c127c4c114f3d46aaa361b3412.png" src="https://data.ulearngo.com/assets/3cfe6460-fe95-4cf8-b4b3-416656038868"/></section>
<section class="activity">
<h3>Method</h3>
<ol>
<li>
<p>Prepare 2 test tubes with approximately \(\text{5}\) \(\text{mL}\) of dilute copper(II) chloride solution in each</p>
</li>
<li>
<p>Prepare 1 <a data-lesson="investigating-elements-and-compounds" data-subject="chemistry" data-tutorial="chemistry-essential-ideas" href="https://app.ulearngo.com/chemistry/chemistry-essential-ideas/investigating-elements-and-compounds">test tube</a> with \(\text{5}\) \(\text{mL}\) sodium carbonate solution</p>
</li>
<li>
<p>Prepare 1 test tube with \(\text{5}\) \(\text{mL}\) sodium sulphate solution</p>
</li>
<li>
<p>Carefully pour the sodium carbonate solution into one of the test tubes containing copper(II) chloride and observe what happens</p>
</li>
<li>
<p>Carefully pour the sodium sulphate solution into the second test tube containing copper(II) chloride and observe what happens</p>
</li>
</ol>
</section>
<section class="activity">
<h3>Results</h3>
<ol>
<li>
<p>A light blue precipitate forms when sodium carbonate reacts with copper(II) chloride.</p>
</li>
<li>
<p>No precipitate forms when sodium sulphate reacts with copper(II) chloride. The solution is light blue.</p>
</li>
</ol>
</section>
</div>
</div>
<p>It is important to understand what happened in the previous demonstration. We will look at what happens in each reaction, step by step.</p>
<p>For <strong>reaction 1</strong> 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 <a data-lesson="electronic-structures-of-anions" data-subject="chemistry" data-tutorial="chemical-bonding" href="https://app.ulearngo.com/chemistry/chemical-bonding/electronic-structures-of-anions">anions</a> can become a solid. The following table summarises which combination will form solids (precipitates) in solution.</p>
<div>
<table summary="            ">
<tbody>
<tr>
<td>
<p><strong>Salt</strong></p>
</td>
<td>
<p><strong>Solubility</strong></p>
</td>
</tr>
<tr>
<td>
<p>Nitrates</p>
</td>
<td>
<p>All are <strong>soluble</strong></p>
</td>
</tr>
<tr>
<td>
<p>Potassium, sodium and ammonium salts</p>
</td>
<td>
<p>All are <strong>soluble</strong></p>
</td>
</tr>
<tr>
<td>
<p>Chlorides, bromides and iodides</p>
</td>
<td>
<p>All are <strong>soluble</strong> except silver, lead(II) and mercury(II) salts (e.g. silver chloride)</p>
</td>
</tr>
<tr>
<td>
<p>Sulphates</p>
</td>
<td>
<p>All are <strong>soluble</strong> except lead(II) sulphate, barium sulphate and calcium sulphate</p>
</td>
</tr>
<tr>
<td>
<p>Carbonates</p>
</td>
<td>
<p>All are <strong>insoluble</strong> except those of potassium, sodium and ammonium</p>
</td>
</tr>
<tr>
<td>
<p>Compounds with fluorine</p>
</td>
<td>
<p>Almost all are <strong>soluble</strong> except those of magnesium, calcium, strontium (II), barium (II) and lead (II)</p>
</td>
</tr>
<tr>
<td>
<p>Perchlorates and acetates</p>
</td>
<td>
<p>All are <strong>soluble</strong></p>
</td>
</tr>
<tr>
<td>
<p>Chlorates</p>
</td>
<td>
<p>All are <strong>soluble</strong> except potassium chlorate</p>
</td>
</tr>
<tr>
<td>
<p>Metal hydroxides and oxides</p>
</td>
<td>
<p>Most are <strong>insoluble</strong></p>
</td>
</tr>
</tbody>
</table>
<div>
<p><em>Table: General rules for the <a data-lesson="summarizing-solubility" data-subject="chemistry" data-tutorial="solutions-and-colloids" href="https://app.ulearngo.com/chemistry/solutions-and-colloids/summarizing-solubility">solubility</a> of salts</em></p>
</div>
</div>
<div class="ns-school-defn2">
<h4>Tip:</h4>
<p>Salts of carbonates, phosphates, oxalates, chromates and sulphides are generally insoluble.</p>
</div>
<p>If you look under carbonates in the table it states that all carbonates are <strong>insoluble</strong> 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:</p>

\[2\text{Na}^{+}\text{(aq)} + \text{CO}_{3}^{2-}\text{(aq)} + \text{Cu}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)} \rightarrow \text{CuCO}_{3}\text{(s)} + 2\text{Na}^{+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)}\]

<p>Note that sodium chloride does not precipitate and we write it as ions in the equation. For <strong>reaction 2</strong> 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:</p>

\[2\text{Na}^{+}\text{(aq)} + \text{SO}_{4}^{2-}\text{(aq)} + \text{Cu}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)} \rightarrow 2\text{Na}^{+}\text{(aq)} + \text{SO}_{4}^{2-}\text{(aq)} + \text{Cu}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)}\]

<p>Both of these reactions are ion exchange reactions.</p>
</section>
<section></section>

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Precipitation Reactions

Chemistry generally explores the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances. 

Chemistry

Explore how different substances dissolve and react with each other in water, including ionization and conductivity, precipitation reactions, tests for anions, and various types of reactions such as redox and ion exchange.


Precipitation Reactions

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