Lesson Explainer: Acidity and Basicity Chemistry

In this explainer, we will learn how to define acids and bases, understand their characteristic properties, and identify acidic and basic species in chemical reactions.

Acids and bases are substances that we can find all around us. The table below lists some common acids and bases that we might be familiar with.

Chemical NameChemical FormulaFound In
AcidsAcetic acidCHCOOH3Vinegar
Carbonic acidHCO23Carbonated drinks
Citric acidCHO687Citrus fruits and sour candies
Hydrochloric acidHClThe stomach
Lactic acidCHO363Milk
BasesSodium hydroxideNaOHCleaning agents and plasticizers in cement mix.
Sodium bicarbonateNaHCO3Baking soda
Calcium carbonateCaCO3Antacids
Magnesium hydroxideMg(OH)2Milk of magnesia

We may recognize that lemons are acidic and cleaners like ammonia and bleach are basic, but what is it about a substance that designates it as an acid or a base?

The terms acid and alkaline (base) have been in existence for centuries. It was well known by the 1800s that acids and bases exhibited certain properties, but they still lacked a formal definition by which they could be classified.

Properties of AcidsProperties of Bases
Sour taste
Changes the color of litmus to red
Bitter taste
Changes the color of litmus to blue
Slippery feel

In 1887, Swedish scientist Svante Arrhenius proposed the first widely accepted definition of an acid and a base. Arrhenius defined acids as substances that ionize when dissolved in water to produce hydrogen ions (H+) or increase the concentration of hydrogen ions in solution.

Definition: Arrhenius Acid

An Arrhenius acid is a substance that produces hydrogen ions (H+) or increases the hydrogen ion concentration when dissolved in water.

Hydrochloric acid (HCl) is an example of an Arrhenius acid. When HCl is dissolved in water, it ionizes into H+ and Cl ions: HCl()H()+Cl()gaqaqHO()2l+

The H+ ions do not remain free in solution and are gained by the water molecules, forming hydronium ions, HO3+. The chemical equation above is often used to abbreviate the reaction. However, the chemical equation below is a more accurate representation: HCl()+HO()HO()+Cl()aqlaqaq23+

Under the Arrhenius definition, bases are classified as substances that ionize in water to produce hydroxide ions (OH) or increase the concentration of hydroxide ions in solution.

Definition: Arrhenius Base

An Arrhenius base is a substance that produces hydroxide ions (OH) or increases the hydroxide ion concentration when dissolved in water.

Sodium hydroxide (NaOH) is an example of an Arrhenius base. When NaOH is dissolved in water, it ionizes into Na+ and OH. The complete reaction is shown below: NaOH()Na()+OH()saqaqHO()2l+

Example 1: Identifying the Definition of an Arrhenius Acid

According to Arrhenius theory, which of the following can an acid be defined as?

  1. A substance that changes the color of an aqueous solution
  2. A substance that ionizes in an aqueous solution to produce OH ions
  3. A substance that fizzes when placed into an aqueous solution
  4. A substance that dissolves any substance placed into a solution of it
  5. A substance that ionizes in an aqueous solution to produce H+ ions

Answer

Svante Arrhenius defined an acid as a substance that produces hydrogen ions (H+) or increases the hydrogen ion concentration when dissolved in water. The hydrogen ions are produced when an acid ionizes in solution. The correct answer is E.

While the Arrhenius definitions of an acid and base are useful, they are limited. The definitions only apply to substances that have been dissolved in water. However, acid–base reactions can occur when the species are dissolved in other solvents and even when they are in the gaseous phase.

In 1932, two scientists, Johannes Brønsted and Thomas Lowry, independently proposed new definitions of an acid and a base to include more substances and reaction conditions. They believed that acid–base reactions involved the transfer of a proton (H+) from an acid to a base. Thus, under the Brønsted–Lowry definition, an acid is a proton donator and a base is a proton acceptor.

Definition: Brønsted–Lowry Acid

A Brønsted–Lowry acid is a substance that can lose or donate protons (H+) in a reaction.

Definition: Brønsted–Lowry Base

A Brønsted–Lowry base is a substance that can gain or accept protons (H+) in a reaction.

Substances that were classified as Arrhenius acids are still considered acids under the Brønsted–Lowry definition, and the same applies to Arrhenius bases. An Arrhenius base may sometimes be referred to as an alkali. An alkali is a base that is soluble in water. All alkalis are bases, but not all substances that we can classify as bases under the Brønsted–Lowry definition are alkalis.

Example 2: Completing the Definition of a Brønsted–Lowry Base

Fill in the blank: In Brønsted–Lowry theory, a base is defined as being .

  1. an H+ acceptor
  2. an OH acceptor
  3. an H+ donor
  4. an OH donor
  5. an HO2 donor

Answer

In Brønsted–Lowry theory, acid–base reactions occur when an acid transfers a proton to a base. The H+ ion and proton are often used interchangeably, particularly when referring to acids and bases, as the H+ ion contains one proton and no electrons or neutrons. If the H+ ion is transferred to the base, we can refer to the base as an H+ acceptor. Thus, we should fill in the blank with the words “an H+ acceptor.”

Arrhenius stated that the products of an acid–base reaction are a salt and water. This reaction definition relies on the base containing hydroxide. However, there are many Brønsted–Lowry bases that do not contain hydroxide. Therefore, when reacting an acid with a base that does not contain hydroxide, it is more useful to consider the proton transfer.

The chemical equation below shows the reaction between a Brønsted–Lowry acid and base:

A Brønsted–Lowry acid must contain a hydrogen atom that can be lost as an H+ ion and a Brønsted–Lowry base must be able to accept an H+ ion. The species produced when an acid loses a proton is called the conjugate base, and the species produced when a base gains a proton is called the conjugate acid:

Example 3: Identifying the Chemical Equation for the Ionization of Nitric Acid

In a solution, nitric acid (HNO3) ionizes completely to form an acidic solution. Which of the following equations shows the ionization of HNO3?

  1. HNO()+H()HNO()3+23aqaqaq
  2. HNO()H()+NO()3+3aqaqaq
  3. HNO()NO()+OH()32aqaqaq
  4. HNO()HNO()+O()32aqaqaq
  5. 2HNO()2H()+N()+3O()3+22aqaqgg

Answer

An acid, as defined by Brønsted and Lowry, is a substance that can lose or donate a proton (H+) in a reaction. The nitric acid can donate a proton to become a nitrate ion (NO3). As the acid is in solution, the water molecules can accept the proton to become hydronium ions (HO3+):

None of the answers include the water molecules as the second reactant. We can abbreviate the chemical equation by removing HO2 from both sides of the equation and adding the state symbol (aq), indicating that all of the species are dissolved in water: HNO()H()+NO()3+3aqaqaq

The equation that shows the ionization of nitric acid is the one in answer B.

Conjugate acids and conjugate bases have the same behavior as acids and bases. The conjugate acid could donate a proton to the conjugate base to reform the original acid and base:

Thus, acid–base reactions have the potential to proceed in both the forward and backward directions. The overall direction of an acid–base reaction is determined by the strength of the acid and base.

The strength of an acid or base is dependent on the ability of the substance to ionize. Acids and bases that can completely ionize when dissolved in water are deemed strong.

Definition: Strong Acids or Bases

Strong acids or bases are acids or bases that can completely ionize when dissolved in water.

Hydrochloric acid is an example of a strong acid. When dissolved in water, all of the hydrochloric acid molecules ionize into H+ and Cl ions. The H+ ions are picked up by water molecules to form HO3+: HCl()+HO()HO()+Cl()aqlaqaq23+

The conjugate base of a strong acid and the conjugate acid of a strong base are not able to ionize to reform the original reactants in solution:

As such, reactions involving strong acids and bases react to completion. The chemical equation for these reactions uses a forward reaction arrow ().

Acids and bases that do not completely ionize when dissolved in water are considered weak. The conjugate base of a weak acid and the conjugate acid of a weak base are capable of ionizing to reform the original reactants.

Definition: Weak Acids or Bases

Weak acids or bases are acids or bases that only partially ionize when dissolved in water.

Ammonia (NH3) is a weak base. When dissolved in water some of the ammonia molecules ionize into ammonium ion (NH4+). At the same time, some of the newly formed ammonium ions can lose a proton to become ammonia: NH()+HO()NH()+OH()32+4aqlaqaq

Reactions involving a weak acid and base exist in equilibrium. The chemical equation for these reactions uses an equilibrium arrow ().

The figures below represent the difference in behavior of a strong acid (HI) and a weak acid (HF) in water.

Notice how in the solution of HI, a strong acid, there is complete ionization into I and HO3+ ions, while in the solution of HF, a weak acid, there are still some HF molecules present in the solution that have not ionized.

It is important to recognize the difference between acid/base strength and concentration. Strength indicates the substance’s ability to ionize. Acids and bases are strong when all of the molecules completely ionize and are weak when only some of the molecules ionize.

Concentration indicates the amount of substance dissolved in a given volume. If a large amount of a substance is dissolved, the solution is concentrated, and when a small amount of substance is dissolved, the solution can be considered dilute. It is possible to have dilute solutions of strong acids and concentrated solutions of weak bases.

In addition, we may describe solutions as being more or less acidic/basic than other solutions. Solutions become more acidic as the concentration of hydrogen ions in the solution increases and the concentration of hydroxide ions decreases. Solutions become more basic as the concentration of hydroxide ions in solution increases and the concentration of hydrogen ions decreases.

Example 4: Recognizing the Difference Between Strong and Weak Acids

Which of the following statements correctly describes the difference between a strong and a weak acid?

  1. In an aqueous solution, a strong acid ionizes partially, whilst a weak acid ionizes completely.
  2. A strong acid is more concentrated than a weak acid.
  3. In an aqueous solution, a strong acid will contain fewer ions than a weak acid.
  4. In an aqueous solution, a strong acid ionizes completely, whilst a weak acid only partially ionizes.
  5. A weak acid is more concentrated than a strong acid.

Answer

A strong acid is a substance that ionizes completely when in solution. A forward reaction arrow () is used to indicate that all of the acid molecules ionize. A weak acid is a substance that only partially ionizes when in solution. An equilibrium arrow () is used to indicate that only some of the acid molecules ionize.

The concentration of an acid has no relation to the acid strength. The statement that correctly describes the difference between a strong and weak acid is the one in answer D.

There are a limited number of strong acids and bases. The seven strong acids and eight strong bases are shown in the tables below:

Strong Acids
Acid Name (Formula)Ionization Reaction
Hydrochloric acid (HCl)HCl()+HO()HO()+Cl()aqlaqaq23+
Hydrobromic acid (HBr)HBr()+HO()HO()+Br()aqlaqaq23+
Hydroiodic acid (HI)HI()+HO()HO()+I()aqlaqaq23+
Nitric acid (HNO3)HNO()+HO()HO()+NO()323+3aqlaqaq
Chloric acid (HClO3)HClO()+HO()HO()+ClO()323+3aqlaqaq
Perchloric acid (HClO4)HClO()+HO()HO()+ClO()423+4aqlaqaq
Sulfuric acid (HSO24)HSO()+HO()HO()+HSO()2423+4aqlaqaq
Strong Bases
Base Name (Formula)Ionization Reaction
Lithium hydroxide (LiOH)LiOH()Li()+OH()saqaqHO()2l+
Sodium hydroxide (NaOH)NaOH()Na()+OH()saqaqHO()2l+
Potassium hydroxide (KOH)KOH()K()+OH()saqaqHO()2l+
Rubidium hydroxide (RbOH)RbOH()Rb()+OH()saqaqHO()2l+
Cesium hydroxide (CsOH)CsOH()Cs()+OH()saqaqHO()2l+
Calcium hydroxide (Ca(OH)2)Ca(OH)()Ca()+2OH()22+saqaqHO()2l
Strontium hydroxide (Sr(OH)2)Sr(OH)()Sr()+2OH()22+saqaqHO()2l
Barium hydroxide (Ba(OH)2)Ba(OH)()Ba()+2OH()22+saqaqHO()2l

The majority of acids and bases are considered weak. The table below lists some common weak acids and bases.

Weak Acids
Acid Name (Formula)Ionization Reaction
Hydrofluoric acid (HF)HF()+HO()HO()+F()aqlaqaq23+
Hydrogen cyanide (HCN)HCN()+HO()HO()+CN()aqlaqaq23+
Hydrogen sulfide (HS2)HS()+HO()HO()+HS()223+aqlaqaq
HS()+HO()HO()+S()23+2aqlaqaq
Carbonic acid (HCO23)HCO()+HO()HO()+HCO()2323+3aqlaqaq
HCO()+HO()HO()+CO()323+32aqlaqaq
Phosphoric acid (HPO34)HPO()+HO()HO()+HPO()3423+24aqlaqaq
HPO()+HO()HO()+HPO()2423+42aqlaqaq
HPO()+HO()HO()+PO()2423+43aqlaqaq
Acetic acid (CHCOOH3)CHCOOH()+HO()HO()+CHCOO()323+3aqlaqaq
Water (HO2)HO()+HO()HO()+OH()223+llaqaq
Weak Bases
Base Name (Formula)Ionization Reaction
Ammonia (NH3)NH()+HO()NH()+OH()324+aqlaqaq
Trimethylamine ((CH)N)33(CH)N()+HO()(CH)NH()+OH()33233+aqlaqaq
Water (HO2)HO()+HO()HO()+OH()223+llaqaq

Example 5: Identifying Which Solution is a Weak Base

Which of the following solutions is an example of a weak base?

  1. A 2 M solution of NH3
  2. A 0.1 M solution of NaOH
  3. A 0.2 M solution of Ba(OH)2
  4. A 1 M solution of LiOH
  5. A 0.5 M solution of KOH

Answer

Metal hydroxides such as NaOH can ionize when dissolved in water into a metal cation and hydroxide. Substances that produce hydroxide ions in solution are considered bases according to the Arrhenius definition. Ammonia (NH3) has a lone pair of electrons that can accept an H+ ion. Substances that can act as H+ ion acceptors are considered bases according to the Brønsted–Lowry definition. Thus, all of the answers represent basic solutions.

The concentration of each given solution is different. Concentration is a measure of the amount of substance in a given volume, but it does not indicate the strength of a base. Base strength is dependent on the ability of a substance to ionize in solution. Substances that completely ionize in solution are considered strong while substances that only partially ionize in solution are considered weak.

There are eight strong bases, all of which are either alkali metal hydroxides or alkaline earth metal hydroxides. They are LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. These eight compounds will completely ionize when dissolved in water. All other bases only partially ionize when dissolved in water.

Looking at the answers, we can see that answers B through E all contain a strong base. The answer that is an example of a weak base is answer A.

Acids that ionize to produce only one H+ ion are called monoprotic because they produce one proton. Carbonic acid and phosphoric acid, shown in the weak acid table above, can produce more than one H+ ion in solution. Acids capable of producing two separate protons are called diprotic, while acids capable of producing more than two protons are called polyprotic.

Diprotic and polyprotic acids donate one proton at a time, with the first proton removal occurring more readily then the second or third. In the case of sulfuric acid, a strong acid, the first proton is readily and completely ionized from the sulfuric acid molecule: HSO()+HO()HO()+HSO()2423+4aqlaqaq

However, the second proton in the product HSO4 only ionizes to a small extent. Thus, HSO4 is a weak acid: HSO()+HO()HO()+SO()423+42aqlaqaq

There are several acids that contain more than one hydrogen atom, but not all of the hydrogen atoms can ionize. Acetic acid (CHCOOH3) contains four hydrogen atoms, but only the hydrogen atom bonded to the oxygen will ionize when dissolved in water:

+H2OH3O+++H2OH3O++CHHHCOOHCHCHHOOCHCHHOOHCHCOOHH

Water is listed as both a weak acid and a weak base. If we look at a water molecule, we can see that it can lose a proton and become the hydroxide ion (OH):

We can also see that the oxygen atom in the water molecule has lone pair electrons that can accept a proton. This would transform the water molecule into the hydronium ion (HO3+):

Molecules, like water, that can behave as either an acid or a base are called amphoteric.

Definition: Amphoteric

Amphoteric species are species that can behave as either an acid or a base.

If water can behave as an acid or a base, this means that one water molecule can undergo an acid–base reaction with another water molecule. In this acid–base reaction, the acidic water molecule will lose a proton and the basic water molecule will gain a proton:

The reaction shown above is known as the autoionization of water.

Reaction: Autoionization of Water

The chemical equation for the acid–base equilibrium reaction of two water molecules is HO()+HO()OH()+HO()223+llaqaq

This reaction occurs in every water sample, but since water is a weak acid and a weak base, only a fraction of the water molecules participate in the reaction. Despite the presence of this acid–base reaction in pure water, the solution is neutral. This is because an equal amount of hydronium ions and hydroxide ions are produced.

Let us summarize what we have learned in this explainer.

Key Points

  • Arrhenius defined acids as substances that produce H+ ions or increase the concentration of H+ ions when dissolved in water.
  • Arrhenius defined bases as substances that produce OH ions or increase the concentration of OH ions when dissolved in water.
  • In the Brønsted–Lowry theory, an acid is a proton (H+) donator and a base is a proton acceptor.
  • Strong acids and bases completely ionize in solution, while weak acids and bases only partially ionize.
  • Water molecules can behave as either an acid or a base.
  • Two water molecules can participate in an acid–base reaction, known as the auto-ionization of water: HO()+HO()HO()+OH()223+llaqaq

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