What Is a Precipitate? Step-by-Step Guide to Predicting Precipitation

Imagine mixing two aqueous solutions and obtaining an insoluble product (a solid) as a result. This process, which appears to be a magic trick, is called precipitation, and the solid formed is known as a precipitate.

In this article, we present some interesting facts and curiosities about precipitates, along with additional information. However, if you are looking for different types of ionic reactions, you need to access our excellent net ionic equation calculator.

Keep reading with us to know the following subjects deeply:

• What is a precipitate in chemistry​? Precipitate formula and definition.
• Example of a precipitation chemical reaction​.
• Precipitation reaction​ and the solubility rules.
• How does temperature affect precipitation in chemistry​?
• Does tap water affect precipitate in chemistry​?
• What causes precipitation in chemistry​?
• And much more.

So, let’s mix your cations and anions to create your own precipitate.

We know that you want an answer to the question, “What is a precipitate?” We’re here to help you. Essentially, a precipitate in chemistry is an insoluble solid formed from a chemical reaction involving two aqueous solutions, each comprising cations and anions. To better understand the concepts behind ions, cations, and anions, check out our articles titled “What are cations and anions?” and “What are spectator ions?”

Besides the precipitate, the remaining aqueous solution at the end of the reaction is referred to as the supernate.

The generic form of a precipitate reaction is:

where:

• $\mathrm{AB}$ and $\mathrm{CD}$ — Reagents;
• $\mathrm{AD}$ and $\mathrm{CB}$ — Products;
• $\rm (aq)$ — Means the substance is dissolved in water;
• $\rm (s)$ — Indicates the substance is solid (i.e., a precipitate).

The final result of a precipitation reaction may depend on various factors, such as temperature, pH, the solutions, and their concentrations. Moreover, this reaction is applicable in both inorganic and organic chemistry, and can be applied in various contexts.

Some applications include water and wastewater treatment, mining and metallurgy, the removal of impurities from a desired compound, and food production.

Let us show you how the precipitation reaction works with some examples. Consider a solution with aqueous potassium chromate $\mathrm{K_2\,Cr\,O_4\,(aq)}$ (which is yellow), mixed together with barium nitrate $\mathrm{Ba\,(NO_3)_2}\,\rm (aq)$ (which is colorless). The result of this reaction will be a yellow solid precipitate. However, how can we predict such a result?

To answer this question, we need to follow the steps below:

1. First, write the left-hand side of your chemical reaction equation:

   $\mathrm{K_2\,Cr\,O_4\,(aq)} + \mathrm{Ba\,(NO_3)_2\,(aq)} \rightarrow \mathrm{products}$

2. Include the ion charges for a more accurate representation:

   $\mathrm{2\,K^{+}\,(aq)}+\mathrm{Cr\,O_{4}^{\,2-}\,(aq)} + \mathrm{Ba^{\,2+}\,(aq)}+\mathrm{2NO_3^{-}\,(aq)} \rightarrow \mathrm{products}$

3. Combine the ions to produce a zero net charge. The possible products from this reaction are:

   $\mathrm{KNO_3}$ and $\rm BaCrO_4$

4. Investigate the features of the precipitate. The $\mathrm{KNO_3}$ is a colorless product while the ion $\mathrm{Cr\,O_{4}^{\,2-}}$ is yellow. Therefore, the yellow solid must be $\mathrm{BaCrO_4}$.

5. You can also write the final equation for the chemical reaction:

   $\mathrm{K_2\,Cr\,O_4\,(aq)} + \mathrm{Ba\,(NO_3)_2\,(aq)} \rightarrow \mathrm{BaCrO_4(s)} + \mathrm{2\,KNO_3\,(aq)}$

As another example, let’s consider aqueous silver nitrate $\mathrm{AgNO_3\,(aq)}$ together with potassium chloride $\mathrm{KCl\,(aq)}$. In this case, the possible products are: $\mathrm{AgCl}$ and $\mathrm{KNO_3}$. The second product, $\mathrm{KNO_3}$, is easily soluble in water, so it cannot form a solid. Therefore, the final precipitate is $\mathrm{AgCl}$, and the precipitate formula is:

$\mathrm{AgNO_3\,(aq)} + \mathrm{KCl\,(aq)} \rightarrow \mathrm{AgCl\,(s)} + \mathrm{KNO_3\,(aq)}$

As you see, the prediction of the final result requires not only quantitative knowledge of the properties of the substances, but also experimental observations of the products.

We can summarize the general rules for a precipitation reaction as follows:

1. Most nitrate salts, such as $\mathrm{NO_3^{-}}$ are soluble.
2. Most salts containing alkali metal ions $\left(\mathrm{Li^{+}}, \mathrm{Na^{+}}, \mathrm{K^{+}}, \mathrm{Cs^{+}}, \mathrm{Rb^{+}}\right)$, and ammonium ion $\left(\mathrm{NH_4^{+}}\right)$ are soluble.
3. Chloride, bromide, and iodide are mostly soluble, with some well-known exceptions as $\mathrm{Ag^{+}}$, $\mathrm{Pb^{\,2+}}$, and $\mathrm{Hg_2^{\,2+}}$.
4. Most sulfate salts are soluble. Some exceptions are $\mathrm{BaSO_4}$, $\mathrm{PbSO_4}$, $\mathrm{Hg_2SO_4}$, and $\mathrm{CaSO_4}$.
5. Most hydroxide salts are slightly soluble. The most important ones to memorize are $\mathrm{NaOH}$ and $\mathrm{KOH}$.
6. Most sulfide $\left(\mathrm{S^{\,2-}}\right)$, carbonate $\left(\mathrm{CO_3^{\,2-}}\right)$, chromate $\left(\mathrm{CrO_4^{\,2-}}\right)$, and phosphate $\left(\mathrm{PO_4^{\,3-}}\right)$ are only slightly soluble.

If you combine these rules with your precipitate formula, you can find the proper products of your reaction. To complement your knowledge about solubility rules, check out our dedicated articles: “Ionic vs. Covalent Compounds” and “Solubility Rules: Which Compounds Dissolve in Water?”