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Common Ion Effect Statement Explanation and Examples

The solubility equilibrium is affected by the common ion effect, which occurs when the addition of an anion reduces the ability of a compound to dissolve. This phenomenon is due to the ionization process and the solubility product.In this blog post, we will study about Common Ion Effect Statement, Explanation and Examples.

This happens because of Le Chatelier’s principle, which says that a system at equilibrium will shift to balance out any changes in reactants and equilibria equations.

In this case, the change happens when there is more ionization of zinc ions already in the solution. The solubility equilibrium of salts is affected by this increase.

This makes compounds with ionization less able to dissolve salts, so they form solid particles or complex ions instead. The presence of a Lewis acid can also cause precipitation.

The Importance of the Common Ion Effect in Chemistry

The common ion effect is a critical concept in chemistry that helps us understand and predict various chemical reactions involving ionization equilibria. It allows us to determine the concentrations of ions in a solution when a molecule with a common ion is added.

Zinc and calcium play a crucial role in controlling these equilibria reactions, particularly equilibrium. Let’s take a closer look at why the common ion effect is so important in maintaining equilibria and how it helps with ionization reactions by affecting the concentrations of ions in solutions.

Predicting and Understanding Rain and Dissolving

The common ion effect helps us understand the formation and dissolution of complex ions, as well as the ionization equilibria that occur. It also allows us to observe how substances dissolve or form solid precipitates.

When two substances with a common ion are mixed, the solubility of the compound decreases due to the precipitate formation caused by the reaction and ionization at certain concentrations.

This can cause a solid substance to form. By examining the concentrations of the common ion in a reaction, we can determine if a substance will dissolve or if it will form a solid precipitate at equilibrium.

For example:

  • When we add sodium chloride (NaCl) to a solution containing silver nitrate (AgNO3), both compounds undergo a reaction resulting in the formation of chloride ions (Cl) at equilibrium.

  • The concentrations of NH3 and the reactants play a crucial role in establishing this equilibrium.

  • The increased concentrations of chloride ions due to NaCl reduces the solubility of silver chloride (AgCl), causing a reaction to occur and leading to the precipitation of AgCl as a white solid. This reaction reaches equilibrium with the NH3 present in the solution.

Maintaining Equilibrium in Solutions

Equilibrium is important in chemistry, especially with solutions. The common ion effect helps maintain equilibrium by altering the balance between reactants and products. “When the concentrations of NH3 and NH4+ions change, this effect occurs.”, resulting in a reaction that adjusts over time.

The presence of this common ion can affect the concentrations of one reactant and the equilibrium reaction when a solution contains an ion that is also a reactant. This makes more product form to restore equilibrium.

For example:

In an aqueous solution, the concentrations of acetate ions (CH3COO) are increased by adding sodium acetate (CH3COONa). This occurs due to the equilibrium reaction involving acetic acid (CH3COOH) and acetate ions.

This increase in concentrations shifts the equilibrium towards more production of undissociated acetic acid molecules in the reaction, maintaining the balance between reactants and products in the aq solution.

Examples of the Common Ion Effect in Chemical Reactions

Adding Chloride Ions to a Solution Containing Silver Ions

When you add chloride ions to a solution with existing silver ions, an interesting reaction that reaches equilibrium is influenced by the concentration of chloride ions.

The solubility of silver chloride decreases. This means that the concentration of silver chloride (AgCl) in the solution is lower, affecting the reaction and the equilibrium.

Adding Chloride Ions to a Solution Containing Silver Ions
Adding Chloride Ions to a Solution Containing Silver Ions

Carbonate Ions and Calcium Carbonate Solubility

The common ion effect also comes into play when there is a change in concentration or reaction involving carbonate ions. This effect can impact the equilibrium of the reaction, especially when NH3 is present.

When aq carbonate ions are present in water, they decrease the solubility of calcium carbonate by affecting the concentration and equilibrium of Cl.

As a result of the higher chloride ion concentrations in the aqueous solution, less calcium carbonate can dissolve in the water, leading to an equilibrium.

Sodium Acetate and Acetic Acid Dissociation

Let’s take a look at another example involving sodium acetate and acetic acid, where we will examine the NH3 and ion concentrations, particularly the chloride ion and calcium ion.

When sodium acetate is added to acetic acid, it affects the equilibrium concentration of NH3 in the aqueous solution. The presence of acetate ions from sodium acetate reduces the dissociation of acetic acid, affecting the equilibrium concentration of NH3 in the aqueous solution.

In summary, various chemical reactions involving concentration and equilibrium observe the common ion effect.

  • Adding chloride ions to a solution containing silver ions reduces the solubility of silver chloride. This is because the concentration of chloride ions affects the equilibrium of the complex formed by silver and chloride ions.

  • The presence of carbonate ions, a result of the common ion effect, reduces the solubility of calcium carbonate in water. This effect occurs when the concentration of a complex ion, such as carbonate (CO32-), reaches equilibrium with its constituent ions, in this case calcium (CO2+) and carbonate (CO32-).

  • Adding sodium acetate to acetic acid decreases the dissociation of acetic acid by reducing the concentration of acetate ions in the complex.

These examples demonstrate how the concentration of certain ions, such as NH3 and Cl, can influence the behavior and solubility of different substances in chemical reactions. By understanding the effect of ion concentrations, scientists can predict and control chemical equilibria involving NH3, calcium ions, and complex ions more effectively.

Relationship between Common Ions and pH

The concentration of NH3, a common ion, can greatly affect the pH of a solution. It can shift the equilibrium towards acidic or basic conditions, altering the overall acidity or alkalinity depending on the concentration of NH3. Let’s explore how common ions influence pH.

The addition of Hydroxide Ions Increases the pH

When the concentration of hydroxide ions (OH-) is increased in a solution, they act as a common ion, affecting the overall NH3 concentration. These hydroxide ions react with excess hydrogen ions (H+) present in the solution, neutralizing them and increasing the pH concentration. This leads to a shift towards more basic conditions.

For example:

  • If we add sodium hydroxide (NaOH) to an acidic solution containing excess H+ ions, the NH3 ions from NaOH will combine with H+ ions to form water (H2O). This reduces the concentration of H+ ions, resulting in an increase in pH. NH3 plays a role in this process.

Increase in Acidity through Hydronium Ions

On the other hand, when hydronium ions (H3O) and ammonia (NH3) introduce into a solution, they increase acidity by adding more H+ ions as a common ion.

For example:

  • Adding hydrochloric acid (HCl) to water increases the concentration of H3O+ ions, also known as NH3. The increased concentration of H+ ions and NH3 leads to a decrease in pH and makes the solution more acidic.

Understanding this relationship between common ions, such as NH3, and pH is crucial for predicting and controlling acidity or alkalinity levels in various chemical reactions and solutions.

How Common Ions Influence Solubility and Buffering

Le Chatelier’s principle states that a system at equilibrium when stressed by changes in ion concentrations or the introduction of NH3, will adjust to counteract the stress and restore balance. This principle also applies to the formation of complex ions. In solubility equilibrium, common ions like nh3 can reduce the solubility of a salt.

Adding a common ion like NH3 to a solution with a soluble salt reduces the number of free ions in the solution.

This makes the salt less likely to dissolve. For instance, if we add sodium chloride (NaCl) to a solution with silver chloride (AgCl), the extra chloride ions from NaCl will stop AgCl from dissolving more, as Le Chatelier’s principle says.

This is because the presence of NH3 can affect the solubility of AgCl in a similar way.

Buffering systems utilize a common ion to maintain a stable pH. A buffer solution consists of a weak acid and its conjugate base. “When you add an acid or base, it reacts with the weak acid or conjugate base to prevent changes in ion concentrations and pH.”

Buffering systems use a common ion to keep pH stable. A buffer solution has a weak acid and its conjugate base. When you add an acid or base, it reacts with the weak acid or conjugate base to prevent pH changes..

Separation of Hydrogen Sulfide and Barium Iodate

When hydrogen sulfide (H2S) dissolves in water, it separates into sulfate ions (SO42-) and hydroxide ions (OH). Barium iodate (Ba(IO3)2) separates into barium ions (Ba2+) and iodate ions (IO3).

Dissociation of Hydrogen Sulfide

When dissolved in water, H2S undergoes dissociation. The H2S molecules break apart to form sulfate ions (SO42-) and hydroxide ions (OH). This process occurs due to the reaction between HCl and sodium chloride solution, which results in the formation of sodium acetate and zinc hydroxide.

Dissociation of Barium Iodate

Barium iodate breaks apart in water into barium ions and iodate ions because of a strong acid like hydrochloric acid.

Both processes separate compounds into ions. These ions can react with other substances in the solution. For instance, sulfate ions can make insoluble salts like calcium sulfate or magnesium sulfate when they react with calcium or magnesium cations.


In chemistry, it’s important to know about the common ion effect. It affects chemical reactions by changing the balance of the reaction. This can cause changes in solubility and pH. It’s especially important when dealing with salts.

To understand the common ion effect, we need to look at examples where it happens. When we add a soluble salt with a common ion to a solution that already has that ion, it causes precipitation because the balance changes. Knowing how common ions affect pH levels helps us understand how they affect acid-base reactions and buffering systems.


What is meant by solubility?

Solubility refers to the ability of a substance (solute) to dissolve in another substance (solvent) to form a homogeneous mixture called a solution.

Typically, people express the ion concentration as grams of solute per volume (usually liters) of solvent at a given temperature This measurement helps determine the amount of complex ions present in a solution.

How does the common ion effect affect solubility?

The common ion effect reduces the solubility of an ionic compound by shifting its equilibrium towards precipitation. When we add an ionic compound with one or more ions in common with an existing solution, according to Le Chatelier’s principle, this addition causes some dissolved ions from that compound to combine with those already present and form an insoluble precipitate.

Can you give an example where pH is affected by common ions?

Certainly! Consider adding sodium acetate (NaCH3COO) to a solution of acetic acid (CH3COOH). Acetic acid is a weak acid that partially ionizes in water, producing hydrogen ions (H+) and acetate ions (CH3COO). When we add sodium acetate, it dissociates into sodium ions (Na+) and acetate ions. The presence of the common acetate ion shifts the equilibrium towards the formation of more undissociated acetic acid molecules, resulting in a higher pH.

How does the common ion effect impact buffering?

The common ion effect plays a crucial role in buffering systems. A buffer is a solution that contains a weak acid and its conjugate base or a weak base and its conjugate acid.

Buffers stabilize ion concentration and manipulate equilibrium by adding a common ion to a weak acid/base and its conjugate.

The system can resist pH changes when we add an acid or base by keeping the ion concentration stable and forming complex ions.

Is the common ion effect only observed in aqueous solutions?

Although the common ion effect is primarily observed in aqueous solutions due to its prevalence in chemistry, it can also occur in other solvents.

The principles behind this phenomenon remain applicable regardless of whether we can water or another solvent as long as there is an ionic compound present with one or more ions already present in the solution.