Earlier, acids, bases, and salts were characterized by the experimental testing of their aqueous solutions. An acid is defined as a substance whose water solution tastes sour, turns blue litmus red and neutralizes bases.  A substance is called base if its aqueous solution tastes bitter, turns red litmus blue or neutralizes acids.


Salt is a neutral substance whose aqueous solution does not affect litmus. According to Faraday: acids, bases, and salts are termed as electrolytes. Further, Liebig proposed that acids are compounds which contain hydrogen that can be replaced by metals.


Acidity is a characteristic property of acids. Acidic substances are usually very sour. Apart from hydrochloric acid, there are many other types of acids around us. Citrus fruits like lemons and oranges contain citric and ascorbic acids while tamarind paste contains tartaric acid.

In fact, the word ‘acid’ and ‘acidity’ are derived from the Latin word ‘acidus’ which means sour. If you dip a blue litmus paper into an acid, it will turn red while a red litmus paper will not change colour. Acids also liberate dihydrogen when they react with some metals.


Bases turn red litmus paper blue while the blue litmus paper stays blue. They taste bitter and also feel soapy. Some other common examples of bases include sodium bicarbonate that is used in cooking and household bleach.


Image: Litmus paper test. [Source: Wikimedia Commons]


Apart from sodium chloride, other common salts are sodium nitrate, barium sulfate etc. Sodium chloride or common salt is a product of the reaction between the hydrochloric acid (acid) and sodium hydroxide (base). Solid sodium chloride is made of a cluster of positively charged sodium ions and negatively charged chloride ions held together by electrostatic forces.

Electrostatic forces between opposite charges are inversely proportional to the dielectric constant of the medium. In other words, we can say that a compound that has acidity in its nature and a compound that has basicity as its nature, may yield salts when combined together.

The universal solvent, water, has a dielectric constant of 80. Therefore, when sodium chloride is dissolved in water, the dielectric constant of water reduces the electrostatic force, allowing the ions to move freely in the solution. They are also well-separated due to hydration with water molecules.


Image: Dissolution of sodium chloride in water [Source: Wikimedia Commons]

These are some examples of acids and bases:

Example of acid
Molecular formula
Examples of base
Molecular formula
Examples of salts
Molecular formula
H2SO4 Sulphuric acid LiOH Lithium hydroxide NaCl Sodium chloride
HI Hydroiodic acid NaOH Sodium hydroxide NH4Cl Ammonium chloride
HBr Hydrobromic acid KOH  Potassium hydroxide CH3COONH4 Ammonium acetate
HNO3 Nitric acid RbOH Rubidium hydroxide CuSO4 Copper sulphate
HF Hydrofluoric acid Ba(OH)2 Barium hydroxide MgSO4 Magnesium sulphate
HCl Hydrochloric acid Ca(OH)2 Calcium hydroxide Na2SO4 Sodium sulphate
HClO4 Perchloric acid NH4OH  Ammonium hydroxide Na2CO3 Sodium carbonate
CH3COOH Acetic acid Sr(OH)2 Strontium hydroxide NaHCO3 Sodium bicarbonate
H2CO3 Carbonic acid  Ferrous Hydroxide or Iron (II) Hydroxide Fe(OH)2 CaCO3 Calcium carbonate
H2S  Hydrosulphuric acid Ferric Hydroxide or Iron (III) Hydroxide Fe(OH)3 K2CO3  Potassium carbonate
Sulphurous Acid H2SO3 Zinc Hydroxide Zn(OH)2 K2SO4 Potassium sulphate
Phosphoric Acid H3PO4 Caesium hydroxide CsOH CaCl2 Calcium chloride
 Phosphorous Acid H3PO3 KCl Potassium chloride
Oxalic Acid H2C2O4 ZnCl2 Zinc chloride
Boric Acid H3BO3 AgCl Silver chloride
Silicic Acid H2SiO3 AgBr Silver bromide

Ionization And Dissociation

Dissociation is the separation of ions from an ionic crystal when a solid ionic compound dissolves in water. On the other hand, ionization is the process where a neutral molecule breaks into charged ions when dissolved in a solution. The extent of ionization depends on the strength of the bonds between ions and the extent of solvation of ions.

The three most important modern concepts of acids and bases are:

Arrhenius Concept

According to Arrhenius concept, Substances which produce H+ ions when dissolved in water are called acids while those which ionize in water to produce OH ions are called bases.

HA →   H+ + A (Acid)

BOH → B+ + OH (Base)

Arrhenius proposed that acid-base reactions are characterized by acids if they dissociate in aqueous solution to form hydrogen ions (H+) and bases if they form hydroxide (OH) ions in aqueous solution.

Limitations of Arrhenius Concept

  • The presence of water is absolutely necessary for acids and bases. Dry HCl can’t act as an acid. HCl acts as an acid in water only and not any other solvent.
  • The concept does not explain the acidic and basic character of substances in non-aqueous solvents.
  • The neutralization process is only possible for reactions which can occur in aqueous solutions, although reactions involving salt formation can occur in the absence of a solvent.
  • The acidic character of some salts such as AlCl3 in aqueous solution can’t be explained.
  • An extended as well as artificial explanation is needed to define the basic nature of NH3.

Bronsted-Lowry Concept

Bronsted and Lowry in 1923 independently proposed a more general definition of acids and bases. According to them, an acid is defined as any hydrogen-containing material (molecule, anion or cation) which can donate a proton to other substance and a Base is any substance(molecule, cation or anion) that can accept a proton from any other substance. Therefore, acids are proton donor whereas bases are proton acceptor.


Conjugate Acid-Base Pairs

Consider a reaction

Acid1  +  Base2  →  Acid2  +  Base1

H2O + HCl ⇔ H3O+  + Cl

In this reaction, HCl donates a proton to H2O and is, therefore an acid. Water, on the other hand, accepts a proton from HCl, and is, therefore, a base. In the reverse reaction which at equilibrium proceeds at the same rate as the forward reaction, the H3O+ ions donate a proton to Cl– ion, hence H3O+, an ion is an acid. Cl ion, because it accepts a proton fromH3O+ ion, is a base.

Acid-base pairs in which the members of reaction can be formed from each other by the gain or loss of protons are called conjugate acid-base pairs.

Limitations of Bronsted Lowry Concept

  • Bronsted Lowry could not explain the reaction occurring in the non-protonic solvent like COCl3, SO2, N2O4, etc.
  • It cannot explain the reactions between acidic oxides like  etc and the basic oxides like  etc which can easily take place in the absence of solvent as well e.g. (No proton transfer)
  • Substances like BF3, AlCl3 etc, do not contain hydrogen which means they can’t donate a proton, still they behave as acids.

Lewis Concept

According to Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons.  Thus, it can be said that a Lewis acid is electron-pair acceptor.

The advantage of the Lewis theory is that complements the model of oxidation-reduction reactions. Oxidation-reduction reactions take place on a transfer of electrons from one atom to another, with a net change in the oxidation number of one or more atoms.

The Lewis theory further suggested that acids react with bases and share a pair of electrons but there is no change in the oxidation numbers of any atoms. Either an electron is transferred from one atom to another, or the atoms come together to share a pair of electrons.

Al(OH)+ 3H+ → Al3+ + 3H2O (Aluminium hydroxide is acting as a base)

Al(OH)3 + OH → Al(OH)4- (Aluminium hydroxide is acting as an acid)

These reactions are showing clearly: When Aluminium hydroxide accepts protons, it acts as a base. When it accepts electrons, it acts as an acid. This Lewis acid-base theory also explains why non-metal oxides such as carbon dioxide dissolve in H2O to form acids, such as carbonic acid H2CO3.

CO2(g) + H2O(l) → H2CO3(aq)

Limitations of Lewis Concept

  • Lewis concept gave a generalized idea including all coordination reactions and compounds. This is not true always.
  • An idea about the relative strength of acids and bases is not provided by Lewis concept.
  • Lewis concept is not in line with the acid-base reaction concept.
  • Lewis concept has not discussed the behaviour of protonic acids like HCl.


  • We taste food sour and bitter, it is only because of presence of acids and bases respectively.

Litmus Solution

  • Litmus, which is extracted from lichen, has purple color (see the image given below), but the condition is when it is neither acidic nor basic, i.e. neutral.

Litmus Solution

  • Litmus basically is a plant belongs to Thallophyta, and in chemical experiment, it is commonly used as an indicator.
  • The substances, which odor changes in acidic or basic media, are known as olfactory indicators.

Acid or Base in a Water Solution

  • The hydrogen ions in HCl are produced because of the presence of water. Secondly, the separation of H+ ion from the HCl molecules cannot be done in the absence of water. The chemical formula is illustrated belowHCl + H2O → H3O+ + Cl
  • Furthermore, hydrogen ions cannot exist alone, but they can exist in presence of water molecules. Therefore, hydrogen ions are shown as H+(aq) or hydronium ion (H3O+). The chemical formula is −H+ + H2O → H3O+
  • The bases which are soluble in water are known as alkalis. But all bases are not soluble in water.
  • If water is added to a concentrated acid, then the heat is generated.
  • Mixing an acid or base with the water results into decrease in the concentration of ions (i.e. H3O+/OH–) per unit volume and the process is known as dilution.

pH Scale

  • A scale, used in measuring the hydrogen ion concentration in a solution, is known as pH scale.
  • The ‘p’ in pH stands for ‘potenz’, it is a German term, which means ‘power’.
  • pH value is taken simply as a number, which indicates the acidic or basic nature of a solution. So, if the concentration of hydronium ion is higher, then the value of pH would be lower.
  • The value of pH scale ranges between ‘0’ and ’14;’ so, if pH value is measured ‘0,’ it means – it is very acidic and if it is 14, then it means – it very alkaline.
  • The neutral value of pH scale is ‘7’.
  • On a pH scale, values less than 7 represent an acidic solution and values greater than 7 represent a basic solution.
  • Usually, paper impregnated with the common indicator is used for measuring the pH (see the image given below)−

pH Scale

  • Likewise, the strength of acids and bases substance mainly depends on the number of H+ ions and OH ions produced, respectively.
  • The following image roughly illustrates (variations in color) the pH value of some of the common substances −

pH Value

Importance of pH in Everyday Life

  • The pH value of a human body ranges between 7.0 and 7.8.
  • The stomach of a human body produces hydrochloric acid that helps in the digestion of food; surprisingly, it does not harm the stomach anyway.
  • However, when the stomach produces too much acid (known as indigestion), it causes pain and irritation. To get relief from this pain, doctors suggest the use of bases known as antacids.
  • These antacids neutralize and control the increased amount of acid.
  • The teeth, which are made up of calcium phosphate, is the hardest substance in the body. However, when the pH in the mouth decreases (below 5.5), it corrodes the teeth.
  • The salt, normally, is formed by the combination of hydrochloric acid and sodium hydroxide solution; and, the combination is known as sodium chloride.
  • When the pH value of rain water is measured as less than 5.6, it is known as acid rain.
  • When acid rain flows into the rivers, then it also lowers the pH of the river water
  • The acidic river water is threat for the survival of aquatic life.

Bleaching Powder

  • Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2] and it is represented as CaOCl2.
  • Bleaching powder is normally used in textile industry, paper factory, chemical industry, and disinfecting the drinking water.

Baking Soda

  • The baking soda is commonly used in the kitchen in order to cook tasty crispy food items. It also cooks some food items faster.
  • The chemical name of baking soda is sodium hydrogencarbonate and formula is NaHCO3.

Washing Soda

  • Recrystallization of sodium carbonate results into washing soda.
  • The chemical formula of washing soda is Na2CO3.10H2O.
  • Washing soda is commonly used in glass, soap, and paper industries.

Plaster of Paris

  1. Plaster of Paris is a white powder that doctors use as plaster for supporting fractured bones.
  2. The chemical name of plaster of paris is calcium sulphate hemihydrate and chemical formula is 2CaSO4.H2O.

All acids generate hydrogen gas in reaction with metals. Hydrogen is common to all acids. In fact,

  • Acids, bases and salts are three main categories of chemical compounds. They have certain definite properties which distinguish one class from the other.
  • The sour taste of many fruits and vegetables, lemon for instance, is due to various types of acids present in them.
  • The digestive fluids of most animals and humans also contain acids. The word ‘acid’ is derived from a Latin word, which means “sour”.
  • The acids we use in the laboratory are very strong acids like hydrochloric acid and sulfuric acid. Strong acids are corrosive and can burn your skin.
  • Bases, on the other hand, are the chemical opposite of acids. They are bitter in taste and soapy to touch.
  • Sea water and detergents are some examples of substances that are basic. Many bases are oxide or hydroxide compounds of metals.
  • Strong bases can also burn one’s skin. Tasting a substance is not the right way of finding out if it is an acid or a base! Acids and bases can be better distinguished with the help of indicators.
  • Indicators are substances that undergo a change of color with a change of acidic, neutral or basic medium.
  • Litmus, a purple dye extracted from the lichen plant, is commonly used as an indicator in laboratories.
  • Acids change the color of litmus solution to red, and bases change the color of litmus solution to blue. Turmeric is another common household indicator.
  • A stain of turmeric based food spill on a white cloth becomes reddish-brown when soap is scrubbed on it.
  • Soap is basic in nature and changes the color of the turmeric stain. It turns yellow again when the cloth is washed with plenty of water.


Properties of Acids Bases and Salts

Properties of Acids

  • Acid is a compound which yields hydrogen ion (H+), when dissolved in water.
  • Acid is sour to the taste and corrosive in nature. The pH value of acids is less than 7.
  • Generally, all acids readily react with metal to release hydrogen gas. For example, metal zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.
  • Acids react with limestone (CaCO3) to produce carbon dioxide. For example, hydrochloric acid reacts with limestone to produce carbonic acid and calcium chloride.
  • Acids can be classified into organic and inorganic acids. Acetic acid (CH3COOH) is the best example of organic acid, and acids produced from minerals are termed as inorganic acids like sulfuric acid (H2SO4), hydrochloric acid (HCl), etc.
  • Acid converts blue litmus paper to red in color.
  • Acids have a tendency to corrode metal surfaces quickly.
  • Phenolphthalein solution is colorless in an acidic solution and turns methyl orange solution to red. Red cabbage juice which is purple in color changes to red in an acidic medium. 
On the basis of number of hydrogen ion, acids can be classified as
  1. Monoprotic acid – Such type of acids produce one mole of H+ ions per mole of acid, e.g., HCl , HNO3, etc.
  2. Diprotic acid – They can produce two moles of H+ ions per mole of acid, e.g., H2SO4.
  3. Triprotic acid – They produce three moles of H+ ions per mole of acid, e.g., H3PO4.
  4. Polyprotic – They can produce more than three H+ ion per mole of acid.

On the basis of strengths or capacity to donate hydrogen ions, acids can be described as:

  • Strong acids: Which are completely (100%) ionized in aqueous solutions. Hence at equilibrium, the concentration of acid molecules is very less and concentration of hydrogen ion reaches to maximum, e.g., HCl, HNO3, HClO4.
  • Weak acids: They are only partially ionized in solution at equilibrium state. At equilibrium state, acid molecules are present and the concentration of hydrogen ion is less, e.g., HF, CH3COOH.

Properties of bases

  • Bases are compounds which yield hydroxide ion (OH), when dissolved in water.
  • Bases are bitter to taste and corrosive in nature. They feel slippery and soapy.
  • Bases are good conductor of electricity and show a pH value of more than 7.
  • Bases react with oils and grease to form soap molecules.
  • Bases convert red litmus paper to blue in color.
  • Bases also have the tendency to corrode metal surfaces.
  • A reaction between a base and a metal is similar as for acid to form salt and release hydrogen gas. But this reaction can only occur when a metal is strong enough to displace another metal from its parent constituent.

2NaOH + Zn 

Na2ZnO2 + H2

  • Phenolphthalein solution turns pink in color in a basic solution. Bases turn methyl orange to yellow. Red cabbage juice which is purple in color changes to yellow in a basic medium.

Strength of bases

  • Strong bases: They are completely ionized in water to produce hydroxide ions, e.g, sodium hydroxide: NaOH(s) 

     Na+(aq) + OH(aq)

  • Weak bases: Partially ionize and equilibrium lies mostly towards reactants side, e.g., ammonia in water: NH3(aq) + H2O(l) 

     NH4+(aq) + OH(aq)

On the basis of the number of hydroxide groups present in a base, they can be classified as monobasic (one OH), dibasic (two OH), tribasic (three OH) bases and so on.

Arrhenius Concept of Acid and bases

Arrhenius concept is the oldest concept to explain acids and bases. According to this concept.

  • An acid is a hydrogen-containing compound which can give hydrogen ion (H+) in aqueous solution. For example, Hydrochloric acid (HCl) gives H+ ion in its aqueous solution.

HCl(g) H2O

→H2O H+(aq) + Cl-1(aq)

Such types of acids are called Arrhenius acids, like nitric acid (HNO3), acetic acid (CH3COOH), etc.

  • Base is also a hydrogen containing compound, which can give hydroxide ion (OH) in an aqueous solution. For example, Sodium hydroxide in water form s sodium ion (Na) and hydroxide ion (OH). Such type of bases are known as Arrhenius bases. Other example of Arrhenius base are ammonium hydroxide (NH4OH), aluminium hydroxide (Al(OH)3) and magnesium hydroxide(Mg(OH)2).

NaOH(s) H2O

→H2O Na+(aq) + Cl-1(aq)

Properties of Salts

  • Salts form by the combination of acid and base through neutralization reaction.
  • The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction.
Acid Base Salt Example
 Strong  Strong  Neutral NaOH + HCl 

→ NaCl + H2O

Strong Weak Acidic HCl + NH4OH 

→ NH4Cl + H2O

Weak Strong Basic CH3COOH + NaOH 

→ CH3COONa + H2O

Weak Weak Neutral  CH3COOH + NH4OH 


The most common salt is sodium chloride or table salt which forms by the combination of sodium hydroxide (base) and hydrochloric acid. Other examples include Epsom salts(MgSO4) used in bath salts, ammonium nitrate (NH4NO3) used as fertilizer, and baking soda (NaHCO3) used in cooking.

The pH of a salt solution depends on the strength of acids and bases combined in the neutralization reaction.

Salt pH
 strong  strong pH=7
weak strong pH>7
strong weak pH<7
weak weak  depends on which is stronger

Is Salt An Acid or A Base?

Acids form hydrogen ions (H+) in aqueous solution. In place of hydrogen ions, it’s better to write hydronium ions (H3O+) which form due to reaction of H+ with water. For example, Hydrochloric acid (HCl) in water.

HCl + H2

 H3O+ + Cl

At the same time bases are the substances that produce hydroxide ion OH- in water. Acids and bases react with one another to yield two products: water, and an ionic compound known as salt. The reaction is called as neutralization reaction.

NaOH + HCl 

 H2O + Na+Cl

Neutralization reactions are always exothermic in nature, which means they liberate the same amount of heat. This amount is fixed for any acid and base that is 57.7kJ per mole of H+. For any acid and base combination the net reaction is

H+(aq) + OH (aq) 


Because salt produced in neutralization reaction is ionic in nature and exists in the form of ions just before the reaction. ­Hence salts are the main products of a neutralization reaction.

Formation of Ammonium

When ammonia is dissolved in water it forms ammonium ions (NH4+) and hydroxyl (OH) ions by the formation of a co-ordinate covalent bond.

This can be shown as follows. 
Bases and acids neutralize each other, therefore, another way to define a base is, “a compound which reacts with an acid to give salt and water only.” Like acids, alkalis can be strong or weak. The more hydroxide ions they produce, the stronger the alkali is.
The acidic property of an acid is due to the presence of hydrogen ions (H+) while that of a base or alkali is due to the presence of hydroxyl (OH) ions in them.When strong acids and bases (alkali) combine, the positively charged hydrogen ion of the acid combines with the negatively charged hydroxyl ion of the base to form a molecule of water. Hence, the water molecule formed does not have any charge because the positive and negative charges of the hydrogen ions and hydroxyl ions get neutralized.

H+(aq) + OH(aq) 


Neutralization can be viewed as a reaction in which an acid combines with a base, neutralizing the positively charged hydrogen ion and the negatively charged hydroxyl ion, to form a molecule of water and the respective salt.

Addition of Acids or Bases to Water

The process of dissolving an acid or a base in water is a highly exothermic one. As this reaction generates a lot of heat, care must be taken while mixing concentrated acids with water, specially nitric acid or sulfuric acid with water.

As a rule: The acid must be added slowly to the water with constant stirring. If one mixes the other way by adding water to a concentrated acid, the heat generated causes the mixture to splash out and cause burns.

The glass container may also break due to excessive local heating and cause damages! Mixing an acid or base with water results in dilution. It decreases the concentration of ions (H3O+/OH) per unit volume thereby dissipating the heat effect easily. The list of acids and bases are shown below. Naming of acids and bases is done on the acid base scale.



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