Base Meaning

Base: A base is a chemical substance that can neutralize the effect of an acid. A basic substance is soapy or slippery in nature. It has the ability to turn red litmus paper to blue. Bases can react with an acid to form salt and water and the reaction is called neutralization reaction. A base has a pH always greater than 7. A few examples of bases are sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2), ammonia (NH3), soaps, toothpaste, etc.

There are three major theories and definitions of a base, and these are:

1. Arrhenius Base
2. Lewis Base
3. Bronsted-Lowry Base

1. Arrhenius's theory of bases: In 1887, the famous Swedish scientist Arrhenius described bases as the chemical substances which release OH-(hydroxide) ions in an aqueous solution. Sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂), ammonium hydroxide (NH4OH), etc., are a few examples of Arrhenius bases.

• NaOH (aqueous) → Na+ (aqueous) + OH- (aqueous)
  When sodium hydroxide is dissolved in water, the ions dissociate to form Na+ and OH- ions. Similarly, some other reactions are as follows:
• KOH (aqueous) → K+ (aqueous) + OH- (aqueous)
• Ca(OH)₂ (aqueous) → Ca₂+ (aqueous) + 2OH- (aqueous)
• NH₄OH (aqueous) → NH₄ (aqueous) + OH- (aqueous)

Neutralization Reaction: When a reaction between Arrhenius acid and Arrhenius base occurs, salt and water are formed as products. Such reactions are known as neutralization reactions. Here the bases neutralize the effect of acids.

• NaOH + HCl → NaCl (salt) + H₂O (water)
• CH₃COOH + NaOH → CH₃COONa(salt) + H₂O (water)
• KOH + HCl → KCl (salt) + H₂O (water)
• NH₄OH + HCl → NH₄Cl (salt) + H₂O (water)

2. Lewis's theory of bases: Lewis's theory of bases tells about the behavior of the bases in a non-aqueous solution. According to Lewis's theory of bases, a base is a molecule that can donate electrons. Thus, electron-rich speciesare Lewis bases.

• Anions such as F-, Cl-, Br-, and I- are a few examples of Lewis bases because they have an extra electron to donate.
• The molecules having lone pairs, such as ammonia (NH₃), methanide ion (CH3-), etc., are also Lewis bases.
• Benzene, ethyne, ethene, etc., are also rich in electrons. Thus, they are also considered Lewis bases.
• Compounds having Group-16 elements in them (Oxygen, Sulphur, Selenium, Tellurium) are also Lewis bases.

The reaction between Lewis Acid and Lewis Base

When a Lewis acid reacts with a Lewis base, a covalent bond formation takes place between the Lewis acid and the Lewis base. As an electron donor, the Lewis base shares the electron with the Lewis acid, thus, forming a covalent bond with the acid.

• NH₃ (ammonia) + H₂O (water) → NH₄+ (ammonium ion) + OH- (hydroxide ion)

Here, ammonia, having one lone pair, acts as a Lewis base and shares its pair of electrons with a water molecule, which acts as a Lewis acid (electron pair acceptor); a covalent bond is formed, resulting in the formation of ammonium ion and hydroxide ion.

• H+ (hydrogen ion) + H₂O (water) → H₃O+ (hydronium ion)

The hydrogen ion, being electron deficient, acts as a Lewis acid, and the water molecule has two lone pairs, acting as a Lewis base. Water molecule shares their electron with hydrogen ion, thus forming a covalent bond which results in the formation of hydronium ion.

3. Bronsted-Lowry theory of bases: In 1923, a Danish scientist, Johannes Nicolaus Bronsted, and a British scientist, Thomas Martin Lowry, discovered that a Bronsted-Lowry base is a chemical substance that can accept a proton. Thus, a Bronsted-Lowry base is a Hydrogen ion (Proton) acceptor. Thus, this theory was able to explain the behavior of acids or bases in both aqueous as well as non-aqueous solutions.

HCl (hydrochloric acid) + H₂O (water) → H₃O+ (hydronium ion) + Cl- (chloride ion)

Here, hydrochloric acid donates a Hydrogen ion (proton) to the water molecule thus, HCl acts as Bronsted-Lowry acid, and H₂O acts as Bronsted-Lowry base.

• CH₃COOH (l)(acetic acid) + H₂O (l) (water) → CH3COO- (aq) (acetate ion) + H3O+ (aq)(hydronium ion)

Here, CH₃COOH acts as Bronsted-Lowry acid because it donates its hydrogen ion (proton) to water, acting as Bronsted-Lowry base (proton acceptor).

Concept of Conjugate Acids and Bases:

Conjugate Base: When a Bronsted-Lowry acid releases a proton (hydrogen ion), the resulting species formed is known as its conjugate base.

Conjugate Acid: When a Bronsted-Lowry base accepts a proton (hydrogen ion), the resulting species formed is its conjugate acid.

In the below-mentioned reaction, CH₃COO- acts as the conjugate base of CH₃COOH, and H₃O⁺ acts as the conjugate acid of H₂O.

• CH₃COOH (l)(acetic acid) + H₂O (l) (water) → CH₃COO- (aq) (acetate ion) + H₃O+ (aq)(hydronium ion)

Conjugate Pairs: Acid and its conjugate base or base and its conjugate acid are termed conjugate pairs. In the above reaction, CH3COOH and CH3COO- a conjugate pair as well as H2O and H3O+ is a conjugate pair.

Properties of Bases

1. Bases are soapy or slippery substances.
2. Bases have the ability to react with acid to form salt and water, and the reaction is termed a neutralization reaction.
3. Bases are very good conductors of electricity.
4. They are bitter to taste.
5. PH of bases is always greater than 7 (between 8-14).
6. Some bases are used as electrolytes, such as sodium hydroxide and potassium hydroxide.
7. Bases turn red litmus paper to blue.
8. Strong bases have the tendency to react with active metals such as potassium, lithium, calcium, etc. to produce hydrogen gas.
9. Bases react with non-metallic (acidic oxides) to produce salt and water.
10. Bases that are soluble in water are termed alkalis.

NOTE: All alkalis are bases, but all bases are not alkalis.

Types of Bases

1. Based on the Strength of the Bases

• Strong Base
• Weak Base

i) Strong Base:A strong base is defined as a chemical substance that can remove a proton from a molecule of a weak acid when an acid-base reaction occurs. A strong base dissociates completely in an aqueous solution, giving one or more than one hydroxide ions per molecule.

Some examples of strong bases are:

• BOH + H₂O → B⁺(aq) + OH^-(aq)

Lithium Hydroxide (LiOH), Sodium Hydroxide (NaOH), Potassium Hydroxide (KOH), Magnesium Hydroxide Mg(OH)?, etc are examples of strong bases.

ii) Weak Base: A weak base is a chemical substance that dissociates partially into its constituent ions in an aqueous solution. It does not completely ionize in water, thus resulting in few hydroxide ions, basic radicals, and many undissociated molecules.

• B + H₂O → BH⁺ + OH^-

Some examples of weak bases are:

Ammonia (NH₃), Zinc Hydroxide (Zn(OH)₂), Pyridine (C₅H₅N), Aluminum hydroxide( Al(OH)₃), etc.

NOTE: Water (H₂O) is a weak base and a weak acid because it dissociates to give hydroxide and hydrogen ions.

H₂O (acid) + H₂O (base) → H3O⁺ + OH^-

2. Based on the Acidity of Bases

• Monoacidic Base
• Diacidic Base
• Triacidic Base

Acidity of a base: The acidity of a base is the total number of replaceable hydroxyl groups in a base that it can release when dissolved in water or an aqueous solution as ions. The number of OH^- ions formed determines the strength of the base.

For example, NaOH, when dissolved in water, dissociates into Na⁺ and OH^- thus, the acidity of NaOH is 1.

NaOH (aq) (sodium hydroxide) → Na⁺ + OH^-

i) Monoacidic Base

In a monoacidic base, one hydroxyl ion has the ability to combine with only one hydrogen ion. Examples: Sodium hydroxide (NaOH), Ammonium hydroxide (NH₄OH), etc.

ii) Diacidic Base

In a diacidic base, two hydroxyl ions have the ability to combine with two hydrogen ions. Examples: Calcium hydroxide (Ca(OH)₂), Magnesium hydroxide (Mg(OH)₂), etc.

iii) Triacidic Base

In a triacidic base, three hydroxyl ions have the ability to combine with three hydrogen ions. Examples: Aluminum hydroxide (Al(OH)₃), ferric hydroxide (Fe(OH)₃), etc.

Based on Concentration in Aqueous Solutions

• Concentrated Base
• Dilute Base

i) Concentrated Base:When the base concentration is highin a solution, it is known as a concentrated base. For example, concentrated NaOH, concentrated KOH, etc.

ii) Dilute Base: When the base concentration is relatively lessin the solution, it is known as a diluted base. For example, dilute KOH, dilute NaOH, etc.

Some Characteristic Reaction of Bases

1. When an alkali is reacted with active metals, hydrogen gas is releasedalong with the formation of a salt.

Metal + Base → Salt + Hydrogen gas↑

For example:

2NaOH (aq) +Zn(s) → Na₂ZnO₂ (aq) + H₂ (g) ↑

Note: Because of this property of a base, it cannot be stored in a metallic container.

2. Bases have the property of reacting with non-metallic oxides (acidic oxide) to produce salt and water. This reaction is proof that the non-metallic oxides are acidic.

Base + Non-Metallic Oxide → Salt + Water

2NaOH + CO₂ (g) → Na₂CO₃ (salt) + H₂O

3. Bases react with acid to form salt and water (neutralization reaction).

Acid + Base → Salt + Water

HCl + NaOH → NaCl (salt) + H₂O (water)

What is pH?

pH: pH, also known as the Potential of Hydrogen is the ability to measure the concentration of Hydrogen ions in a solution. It can also be defined as the negative logarithm of H+ ion concentration.

• pH = -log[H+]

The range of pH scale is from 0 to 14 and it is used to measure a solution's pH (Hydrogen ion concentration).

When the pH of a solution is less than 7, the solution is considered acidic, whereas when the pH of a solution is greater than 7, it is considered basic. When the pH equals 7, the solution is considered neutral.

Solutions having pH equal to 0: Strongly acidic (example: Sulphuric Acid)

Solutions having pH equal to 14: Strongly basic (example: Sodium Hydroxide (NaOH))

Day-to-Day Uses of Bases

1. Soaps and toothpaste: Soaps we use daily to clean our bodies and clothes contain a salt of a strong base and a weak acid, which is a salt of basic pH. Generally, Caustic Soda (NaOH) and Potassium Hydroxide (KOH) are used to make soaps. Bases are also present in mild amounts in our toothpaste to neutralize germs in our teeth.
2. Electrolytes: Bases are good electrolytes; thus, Potassium Hydroxide (KOH) and Sodium Hydroxide (NaOH) are used as electrolytes for industrial purposes.
3. Medicines: Magnesium Hydroxide (Mg(OH)2) is used in medicines as an antacid to neutralize the effect of acids produced in our stomach. Aluminum Hydroxide (Al(OH)₃) is used in medicines to treat peptic ulcers. Sodium Bicarbonate (NaHCO₃) is used for treating indigestion.
4. Household:Sodium Carbonate (Na₂CO₃), also known as washing soda, is used for household cleaning. Sodium Bicarbonate (NaHCO₃), also known as baking soda, is used for cooking/baking purposes. Dilute Ammonium Hydroxide (NH₄OH) is also used for household cleaning purposes. Calcium Hydroxide (Ca(OH)₂) is used for whitewashing.
5. Manufacturing: Ammonia (NH₃) is used in the manufacturing of fertilizers. Lithium Carbonate (LiCO₃) is used in the manufacturing of cement. Sodium Hydroxide (NaOH) is used to manufacture a fiber called rayon, used in the textile industry. Sodium Hydroxide (NaOH) is also used to manufacture paper in the paper industry.