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Any substance that in water solution tastes sour and changes the colour of certain indicators (e.g., reddens blue litmus paper), reacts with some metals (e.g., iron) to liberate hydrogen, reacts with bases to form salts, and promotes certain chemical reactions (acid catalysis) is called Acid. Examples of acids include the inorganic substances known as the mineral acids—sulfuric, nitric, hydrochloric, and phosphoric acids—and the organic compounds belonging to the carboxylic acid, sulfonic acid, and phenol groups. Such substances contain one or more hydrogen atoms that, in solution, are released as positively charged hydrogen ions (see Arrhenius theory).

The word acid is derived from the Latin word acidus which means sour. We use acids and bases in our daily routine. We use orange and lemon and they both have sour taste due to the presence of citrus acid in them.

Broader definitions of an acid, to include substances that exhibit typical acidic behaviour as pure compounds or when dissolved in solvents other than water, are given by the Brønsted–Lowry theory and the Lewis theory. Examples of nonaqueous acids are sulfur trioxide, aluminum chloride, and boron trifluoride.

Types of acids based on proton donor property

There are two types of acids Monoprotic acid and polyprotic acids.

Monoprotic Acid

The acid which can donate one proton is called monoprotic acid.


HCl, HBr and HCN are monoprotic acids.

Polyprotic Acid

Those acids which can donate two or more than two protons in an aqueous solution is called polyprotic acid.


H2SO4 etc

Amphoteric substance

That substance which can both like acid and like the base is called Amphoteric substance. When we treat an amphoteric substance with an acid it starts behaving like base while if we treat an amphoteric substance with a base it behaves like an acid.


Water is amphoteric because it can act as an acid and base. Water is the most common example,

acting as a base when reacting with an acid such as hydrogen chloride:

H2O + HCl —> H3O+ + Cl

acting as an acid when reacting with a base such as ammonia:

H2O + NH3 —> NH4+ + OH


Types of acids based on dissociative property

Acids may be identified as either strong or weak acids based on how completely they dissociate into their ions in water.

Strong acid 

Acid which ionizes completely in water and gives the maximum number of H+ ion in aqueous the solution is known as strong acid.


HCl, HNO3 are strong acid

Ionization of HCl is 92%

Ionization of HNO3 is 84%

HCl —>  (H+) + (Cl)

It has been proved that in gaseous state HCl ionization is 100%.

Weak acid

Weak acid is that substance which ionizes partially in an aqueous solution and gives lower a number of H+ ion in an aqueous solution.


CH3COOH ionization is 13%

H3PO4 ionization is 28%



Properties of Acid

The following are properties of acids:

  1. Dilute acids have a sour taste.
  2. They turn moist blue litmus paper red.
  3. Some acids are corrosive, however, this is not a general property.
  4. Most acids are solids.
  5. More reactive metals, e.g. Zn, Mg, Al and Fe react with acids to liberate hydrogen. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
  6. Stronger and less volatile acids displace weaker and more volatile acids from their salts.
  7. Acids, except very weak acids like H2CO3, when reacted with a trioxocarbonate(IV) liberate CO2, which turns lime water milky.
    1. Example,  2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g)
    2. H2CO3 dissolves a trioxocarbonate(IV) to give a bicarbonate (i.e., hydrogen trioxocarbonate(IV)).
    3. H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq)
  1. Acids react with bases or alkalis to give salts and water.
  2. Solutions of acids can conduct electricity (i.e., they are electrolyte).
  3. Acids have pH values below 7.

Acid Examples

These are examples of types of acids and specific acids:

  • Arrhenius acid: By this definition, an acid is a substance which increases the concentration of hydronium ions (H3O+) when added to water.
  • Brønsted-Lowry Acid – By this definition, an acid is a material capable of acting as a proton donor. This is a less restrictive definition because solvents besides water are not excluded. Essentially, any compound that can be deprotonated is a Brønsted-Lowry acid, including typical acids, plus amines and alcohol.
  • Lewis acid: A Lewis acid is a compound which can accept an electron pair to form a covalent bond. By this definition, some compounds which don’t contain hydrogen qualify as acids, including aluminium trichloride and boron trifluoride.
  • Hydrochloric acid
  • Sulfuric acid
  • Hydrofluoric acid
  • Acetic acid
  • Stomach acid (which contains hydrochloric acid)
  • Vinegar (which contains acetic acid)
  • Citric acid (found in citrus fruits)

Summary of Acid and Base Properties

This table offers an overview of key properties of acids compared with bases:

Property Acid
pH less than 7
litmus paper blue to red
taste sour (e.g. vinegar)
odour burning sensation
texture sticky
reactivity reacts with metals to produce hydrogen gas

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