Name: how acids and bases form ions
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North Carolina K-12 Science Standard Course of Study

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Physical Science

Matter and its Interactions

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Acids, Bases and Neutralization Reactions

The human stomach is a muscular organ containing gastric juices, including hydrochloric acid, which help digest food. Certain kinds of foods raise the acidity levels of our stomach, causing acid reflux. 

Consuming alkaline food, or a base like an antacid, neutralizes the excess acid, relieving the burning sensation. This is an example of an acid-base reaction.

The simplest acids are substances that contain dissociable hydrogen atoms. In aqueous solutions, they release positively charged hydrogen ions, or protons, along with the corresponding anions. The protons react with water molecules to form hydronium ions. 

For example, an aqueous solution of nitric acid contains hydronium ions and nitrate ions. Other commonly known acids are hydrochloric acid, sulfuric acid, and phosphoric acid. 

Acids are monoprotic or polyprotic depending on the number of dissociable hydrogen atoms per molecule. 

Hydrochloric acid and nitric acid, with one dissociable hydrogen each, are both examples of monoprotic acids, while sulfuric acid and phosphoric acid are polyprotic. Sulfuric acid is diprotic and releases its two protons sequentially, whereas phosphoric acid is triprotic and dissociates sequentially in three steps.

The simplest bases are compounds that yield negatively charged hydroxide ions in aqueous solutions. Easily identifiable bases contain hydroxyl groups; for example, sodium hydroxide, potassium hydroxide, and calcium hydroxide. 

Such bases, when added to water, dissociate into hydroxide ions and the corresponding metal cations.

Non-hydroxide bases like ammonia react with water by accepting protons from water molecules to produce ammonium ions and hydroxide ions. 

In general, an acid and a base react to form water and a salt. Acid&#8211;base reactions are also called neutralization reactions when the moles of protons supplied by the acid and the moles of hydroxide ions supplied by the base are equal.

For example, when equal amounts of aqueous nitric acid and aqueous potassium hydroxide are mixed, the amounts of hydronium and hydroxide ions supplied to the solution are also equal.

The hydronium and hydroxide ions, therefore, completely neutralize each other by combining to form water, leaving no hydronium or hydroxide ions behind. Additionally, the nitrate anions and potassium cations pair up to form a salt &#8212; potassium nitrate. 

Certain neutralization reactions are gas-evolution reactions. For example, hydrochloric acid and potassium sulfite react to form potassium chloride, water, and sulfur dioxide gas through the sulfite&#8211;bisulfite equilibrium system.

acids, bases and neutralization reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to numerous natural and technological processes, ranging from the chemical transformations within cells or lakes and oceans to the industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to the society.

There are several ways of defining an acid. In the context of aqueous solutions, an acid is a substance that dissolves to produce hydrogen ions.

<img alt="HCl dissociation equation, HCl(aq) &#8594; H&#8314;(aq) + Cl&#8315;(aq), chemical reaction illustration." src="/files/core/CDNSource/lessons/1029/1029_Equations-01.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="HCl dissociation equation, chemical reaction, produces H&#8314; and Cl&#8315; ions in aqueous solution." />

This is the Arrhenius definition of an acid, named after Swedish chemist Svante Arrhenius (1859&#8211;1927). A hydrogen ion, represented by the symbol H+, is called a proton. In solution, the protons chemically combine with water molecules, through the lone pairs on oxygen, to form hydronium ions, H3O+.

<img alt="Acid-base reaction equation H&#8314;(aq) + H&#8322;O(l) &#8594; H&#8323;O&#8314;(aq), chemical equation diagram." src="/files/core/CDNSource/lessons/1029/1029_Equations-02.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Acid-base reaction equation H&#8314;(aq) + H&#8322;O(l) &#8594; H&#8323;O&#8314;(aq) illustrating proton transfer process." />

The chemical equation for the ionization of an acid is written as

<img alt="Acid dissociation equation: HA(aq) + H&#8322;O(l) &#8594; H&#8323;O&#8314;(aq) + A&#8315;(aq) for equilibrium analysis." src="/files/core/CDNSource/lessons/1029/1029_Equations-03.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Acid-base reaction equation; HA(aq) + H2O(l) &#8594; H3O+(aq) + A&#8722;(aq)." />

Acids that react completely with water in this manner are called strong acids. HCl, HNO3, and HBr are a few examples.

Most acids encountered in daily life are weak acids. The citric acid in fruits, acetic acid in vinegar, and lactic acid in milk are a few examples. A familiar example of a weak acid is acetic acid, the main ingredient in food vinegar. When dissolved in water under typical conditions, only about 1% of acetic acid molecules are present in the ionized form, CH3CO2&#8722;. 

<img alt="Chemical equilibrium: acetic acid dissociation equation, CH3COOH(aq) &#8652; CH3COO-(aq) + H3O+(aq)." src="/files/core/CDNSource/lessons/1029/1029_Equations-04.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Acetic acid ionization equation, CH&#8323;COOH + H&#8322;O &#8652; CH&#8323;COO&#8315; + H&#8323;O&#8314;, chemical equilibrium process." />

The use of a double-arrow in the above equation denotes the partial reaction aspect of this process.

A base is a substance that will dissolve in water to yield hydroxide ions, OH&#8722;. The most common bases are ionic compounds composed of alkali or alkaline earth metal cations (groups 1 and 2) combined with the hydroxide ion&#8212;for example, NaOH and Ca(OH)2. Unlike acid-compounds discussed previously, these compounds do not react chemically with water; instead, they dissolve and dissociate, releasing hydroxide ions directly into the solution. For example, KOH and Ba(OH)2 dissolve in water and dissociate completely to produce cations (K+ and Ba2+, respectively) and hydroxide ions, OH&#8722;. These bases, along with other hydroxides that completely dissociate in water, are considered strong bases.

Consider the dissolution of sodium hydroxide in water as an example:

<img alt="NaOH dissociation equation; chemical formula; static equilibrium; educational diagram." src="/files/core/CDNSource/lessons/1029/1029_Equations-05.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Dissolution equation NaOH(s) &#8594; Na&#8314;(aq) + OH&#8315;(aq), chemical reaction process, formula." />

This equation confirms that sodium hydroxide is a base. When dissolved in water, NaOH dissociates to yield Na+ and OH&#8722; ions. This is also true for any other ionic compound containing hydroxide ions. Since the dissociation process is essentially complete when ionic compounds dissolve in water under typical conditions, NaOH and other ionic hydroxides are all classified as strong bases.

Unlike ionic hydroxides, some compounds produce hydroxide ions when dissolved by chemically reacting with water molecules. In all cases, these compounds react only partially and so are classified as weak bases. These types of compounds are also abundant in nature and are important commodities in various technologies. For example, global production of the weak base ammonia is typically well over 100 metric tons annually because it is widely used as an agricultural fertilizer, a raw material for chemical synthesis of other compounds, and an active ingredient in household cleaners. When dissolved in water, ammonia reacts partially to yield hydroxide ions, as shown here:

<img alt="Ammonia equilibrium reaction, NH3(aq) &#8652; NH4+(aq) + OH-(aq), chemical equation for acid-base balance." src="/files/core/CDNSource/lessons/1029/1029_Equations-06.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Ammonia equilibrium formula: NH3(aq) &#8652; NH4+(aq) + OH&#8722;(aq), chemical equilibrium concept." />

This is, by definition, an acid-base reaction involving the transfer of H+ ions from water molecules to ammonia molecules. Under typical conditions, only about 1% of the dissolved ammonia is present as NH4+ ions.

A neutralization reaction is a specific type of acid-base reaction in which the reactants are an acid and a base (but not water), and the products are often salt and water: 

<img alt="Neutralization reaction formula: Acid + Base &#8594; Salt + Water, chemical equation diagram." src="/files/core/CDNSource/lessons/1029/1029_Equations-07.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Neutralization reaction formula: Acid + Base &#8594; Salt + Water." />

To illustrate a neutralization reaction, consider what happens when a typical antacid such as milk of magnesia (an aqueous suspension of solid Mg(OH)2) is ingested to ease symptoms associated with excess stomach acid (HCl):

<img alt="Chemical reaction: Mg(OH)&#8322; + 2HCl &#8594; MgCl&#8322; + 2H&#8322;O; equation in chemical process diagram." src="/files/core/CDNSource/lessons/1029/1029_Equations-08.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction equation: Mg(OH)&#8322;(s) + 2HCl(aq) &#8594; MgCl&#8322;(aq) + 2H&#8322;O(l)." />

Note that, in addition to water, this reaction produces a salt, magnesium chloride. The H+ (aq) from the acid (strong or weak) combines with the OH&#8722; (aq) from the base (or produced by the reaction of a weak base with water) to form H2O (l). For example, the reaction between aqueous solutions of HCl (strong acid) and NaOH (strong base) is written as follows:

<img alt="Neutralization reaction: HCl(aq) + NaOH(aq) &#8594; NaCl(aq) + H2O(l) equation, chemistry diagram." src="/files/core/CDNSource/lessons/1029/1029_Equations-09.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Acid-base reaction equation HCl(aq) + NaOH(aq) &#8594; NaCl(aq) + H2O(l) in neutralization process." />

Both HCl and NaOH undergo 100% ionization. The complete ionic equation, therefore, may be written as:

<img alt="Neutralization reaction equation: H&#8314;(aq) + OH&#8315;(aq) &#8594; H&#8322;O(l) in chemical formula diagram." src="/files/core/CDNSource/lessons/1029/1029_Equations-10.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Acid-base neutralization equation: H&#8314;(aq) + OH&#8315;(aq) &#8594; H&#8322;O(l); chemical reaction diagram." />

Cl- and Na+ are called the spectator ions, which cancel out, giving the net ionic equation as: 

<img alt="Neutralization reaction equation: H&#60;sup&#62;+&#60;/sup&#62;(aq) + OH&#60;sup&#62;-&#60;/sup&#62;(aq) &#8594; H&#60;sub&#62;2&#60;/sub&#62;O(l)." src="/files/core/CDNSource/lessons/1029/1029_Equations-11.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Neutralization reaction equation: H&#60;sup&#62;+&#60;/sup&#62;(aq) + OH&#60;sup&#62;-&#60;/sup&#62;(aq) &#8594; H&#60;sub&#62;2&#60;/sub&#62;O(l), chemical process." />

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">OpenStax Chemistry 2e, Section: 4.2: Classifying Chemical Reactions.</a>

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

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Chemical Quantities and Aqueous Reactions

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how acids and bases form ions

Acids and bases form ions in water through acid-base reactions. These reactions move a hydrogen ion, H+, from one chemical species to another. They matter in many natural and technological processes, from chemistry inside cells and in lakes and oceans to the large-scale production of fertilizers, pharmaceuticals, and other useful substances.

In water, an acid is a substance that produces hydrogen ions. This is the Arrhenius definition of an acid. The H+ ion is also called a proton. In solution, these protons combine with water molecules to form hydronium ions, H3O+.

Acids that react completely with water are called strong acids. Common examples include HCl, HNO3, and HBr. Most acids people encounter in daily life are weak acids. Citric acid in fruits, acetic acid in vinegar, and lactic acid in milk are examples. Acetic acid is a familiar weak acid, and under typical conditions only about 1% of its molecules are in the ionized form, CH3CO2−. A double arrow shows that this reaction is only partial.

A base is a substance that dissolves in water to yield hydroxide ions, OH−. The most common bases are ionic compounds made of alkali or alkaline earth metal cations from groups 1 and 2 with hydroxide ions, such as NaOH and Ca(OH)2. These compounds do not react chemically with water. Instead, they dissolve and dissociate, releasing hydroxide ions directly into solution.

KOH and Ba(OH)2 are examples of strong bases because they dissociate completely in water. The same is true for sodium hydroxide. When NaOH dissolves, it gives Na+ and OH− ions. Any ionic compound that contains hydroxide ions behaves this way under typical conditions, so these hydroxides are classified as strong bases.

Some compounds make hydroxide ions by reacting with water molecules instead of simply dissociating. These compounds react only partially, so they are weak bases. Ammonia is a common example. It is produced on a very large scale because it is used as an agricultural fertilizer, a starting material for chemical synthesis, and an ingredient in household cleaners. When ammonia dissolves in water, it reacts partially to form OH− and NH4+ ions. This is an acid-base reaction because H+ ions move from water molecules to ammonia molecules. Under typical conditions, only about 1% of dissolved ammonia is present as NH4+.

A neutralization reaction is a special acid-base reaction in which an acid and a base react, but water is not a reactant. The products are often a salt and water. Milk of magnesia is a common antacid made of solid Mg(OH)2 in water. It helps relieve excess stomach acid, HCl, by neutralizing it. This reaction forms water and the salt magnesium chloride.

In neutralization, H+(aq) from the acid combines with OH−(aq) from the base to make H2O(l). For example, HCl and NaOH are both completely ionized in water. Their complete ionic equation includes Na+ and Cl− ions, but these are spectator ions because they cancel out. The net ionic equation shows the essential change: H+(aq) + OH−(aq) → H2O(l).

How Acids and Bases Form Ions

precipitation reactions

Predicting Precipitation in Aqueous Reactions

Precipitation reactions happen when two aqueous solutions of soluble salts are mixed and an insoluble ionic compound forms. That solid is called a precipitate. The ions in solution exchange partners, so these reactions are also called double displacement, double replacement, exchange, or metathesis reactions. These reactions are useful in chemistry for identifying metal ions in a compound. They also support gravimetric methods, which measure composition by the mass of a substance. Whether a...

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Acids and bases form ions in water through acid-base reactions. These reactions move a hydrogen ion, H+, from one chemical species to another. They matter in many natural and technological processes, from chemistry inside cells and in lakes and oceans to the large-scale production of fertilizers, pharmaceuticals, and other useful substances. In water, an acid is a substance that produces hydrogen ions. This is the Arrhenius definition of an acid. The H+ ion is also called a proton. In...

limiting reactant

Limiting Reactant in Water Formation

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synthesis and decomposition reactions

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chemical quantities and aqueous reactions

Learn how acids and bases form ions in water, including strong and weak acids, strong and weak bases, ammonia, and neutralization reactions.

How Acids and Bases Form Ions | JoVE K12

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Neutralization Reactions

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

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Molecules and Compounds

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Phase Transitions

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Phase Transitions: Vaporization and Condensation

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Vapor Pressure

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Clausius-Clapeyron Equation

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Phase Transitions: Melting and Freezing

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Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

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Phase Diagrams

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Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

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Molecular Comparison of Gases, Liquids, and Solids

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Intermolecular vs Intramolecular Forces

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Intermolecular Forces

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Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

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The Atomic Theory of Matter

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Subatomic Particles

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The Bohr Model

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Emission Spectra

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The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

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Use models to interpret the data presented in Bohr diagrams and electron dot diagrams for neutral atoms of elements 1 through 18.

PS.PSc.1.4

Ions and Ionic Charges

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PS.PSc.1.5

The Periodic Table

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Periodic Classification of the Elements

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Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

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Electron Affinity

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Alkali Metals

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Halogens

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Noble Gases


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Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

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Understand types, properties, and structure of matter.

How Acids and Bases Form Ions

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