In chemistry, a precipitation reaction is a chemical reaction between two dissolved substances that forms one or more solid products. The solid is the precipitate. The remaining solution is the supernate or supernatant.
There are two common ways to indicate precipitation in a chemical reaction.
- State of matter symbol: Including the symbol (s) following a chemical formula means the product is a solid.
- Down arrow: Otherwise, a down arrow (↓) after the name or formula indicates a precipitate.
How Precipitation Reactions Work
Precipitation results from the concentration of a chemical exceeding its solubility. There are a few ways this happens:
- Double replacement reaction: Often, precipitation results from a double replacement reaction between two aqueous solutions. The dissolved salts react and one or more of the products is insoluble (or at least partially insoluble).
- Crystallization: Even in a pure solution, concentration often exceeds solubility. Particles aggregate during the nucleation stage and the substance falls out of solution until equilibrium is reached. Controlling temperature and pressure causes chemicals to precipitate out of solution.
- Changing the solution: Introducing a new solvent in which a chemical is insoluble often causes precipitation. Adding ions is another option, driving a compound toward solidification.
A double replacement reaction that forms a precipitate is a precipitation reaction. The other ways of forming precipitates are more processes than reactions.
In all cases, precipitation starts with nucleation. During nucleation, tiny particles adhere to each other and to surface imperfections on the container. Other nucleation sites include solid impurities in the solution and gas bubbles. Initially, nucleation may lead to the formation of a suspension of tiny solid particles in liquid. When the particles become large enough, they precipitate or fall out of solution.
Examples of Precipitation Reactions
Here are common examples of precipitation reactions. Note the way precipitation reactions appear as molecular equations and net ionic equations. Compare the different ways to write the reactions.
- Reaction between potassium iodide and lead nitrate in water, forming lead iodide as a precipitate and aqueous potassium nitrate:
2KI(aq) + Pb(NO3)2(aq) ⟶ PbI2(s) + 2KNO3(aq) (molecular equation)
Pb2+(aq)+2I−(aq)⟶PbI2(s) (net ionic equation)
- Reaction between sodium fluoride and silver nitrate in water, forming solid silver fluoride and aqueous sodium nitrate:
NaF(aq) + AgNO3(aq) ⟶ AgF(s) + NaNO3(aq) (molecular)
Ag+(aq) + F−(aq) ⟶ AgF(s) (net ionic)
- Reacting copper sulfate and sodium hydroxide forms sodium sulfate and copper hydroxide.
CuSO4 + 2NaOH ⟶ Na2SO4 + Cu(OH)2↓
- The reaction between sodium sulfate and strontium chloride forms sodium chloride and strontium sulfate, which is a precpitate.
Na2SO4 + SrCl2 ⟶ 2NaCl + SrSO4↓
- The reaction between cadmium sulfate and potassium sulfide in water forms potassium sulfate and cadmium sulfide.
CdSO4(aq) + K2S(aq) ⟶ K2SO4(aq) + CdS(s)
Common Precipitate Colors
The color of a precipitate is one clue to its identity. Here are some common transition metal precipitate colors. Note, these colors also occur from other compounds, plus these compounds may appear very different if the oxidation state of the ions changes.
|Chromium||blue, green, orange, yellow, or brown|
|Cobalt||pink (when hydrated)|
|Iron(III)||rusty reddish brown|
How to Predict a Precipitation Reaction
In a chemical reaction, predict whether or not a precipitate forms using solubility rules. Identify the products and determine whether or not they remain as ions in aqueous solution or form compounds.
For a pure substance, consult a solubility chart. Usually, temperature is an important controllable factor that determines where a solution becomes saturated and supersaturated. At a constant temperature, precipitate formation depends on concentration.
Precipitate vs Precipitant
While the words supernate and supernatant mean the same thing, the words precipitate and precipitant do not. A chemical added to a reaction to cause precipitation is called a precipitant. The solid that forms is the precipitate. The liquid portion of the solution is the supernate. The recovered solid from a precipitation reaction is the flower.
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- Zumdahl, Steven S.; DeCoste, Donald J. (2012). Chemical Principles. Cengage Learning. ISBN 978-1-133-71013-4.
- Zumdahl, Steven S.; DeCoste, Donald J. (2018). Introductory Chemistry: A Foundation. Cengage Learning. ISBN 978-1-337-67132-3.