An acid is a chemical substance having a strong hydrogen ion concentration. An acid can turn the blue litmus into the red and has the ability to neutralize alkali. It also has the capacity to dissolve some metals. Acids can be found in many of our  day-to-day usable items in their mild form, for example, lemon contains citric acid. Acids can be dilute or concentrated  with each acid having its own concentration and act differently on a variety of materials.


Classification of Acids

There are various criteria to classify acids. They are classified based on their source, the presence of oxygen, acid strength or concentration, etc. The table below explains the different ways in which we can classify the acids.


Based on source

Organic acids are - extracted from

Mineral Acids are  obtained from minerals.

Organic materials such as plants and animals. For e.g. Citric acid (Citrus fruits), Acetic acid (Vinegar). Mineral Acids are also known as inorganic acids as they don’t have carbon in them. For e.g.  H2SO4, HCl. HNO3, etc.

Classification based on the presence of Oxygen




They consist of oxygen in their composition, e.g. H2SO4, HNO3, etc. Hydracids have hydrogen combined with other elements. They do not contain oxygen in their composition e.g. HCl, HI, HBr, etc.

Classification based on the Strength of the acid


 Strong Acids

Weak Acids

Strong acids are those which can be dissociated completely for e.g.  Sulfuric acid, nitric acid, hydrochloric acid, etc. Weak acids does not dissociate completely or dissociates negligibly in water. Example, citric acid, acetic acid, etc.

Classification based on the concentration

Concentrated Acids


Diluted acid


These are aqueous solutions with a relatively high percentage of acid dissolved in it. E.g. concentrated hydrochloric acid, concentrated Sulfuric acid, concentrated nitric acid, etc., It is an aqueous solution that has a relatively lesser percentage of acid dissolved in it. E.g. dilute hydrochloric acid, dilute nitric acid, etc.

Classification based on the basicity of the acid


Monobasic Acid

It has only one hydrogen ion. Therefore, these acids combine with one hydroxyl group of the base to form salt and water.  For e.g. HCl, HCOOH, HBr, etc.

Dibasic Acids share two hydroxyl groups and these are dibasic acids.

H2SO4(aq) <—> H+(aq) + HSO-4(aq)

2NaOH(aq) + H2SO4(aq) <—-> Na2SO4(aq) + 2H2O(l)


Tribasic acids can combine with three hydroxyl groups, and they produce 3 types of salts. For e.g. H3PO4

NaOH(aq) + H3PO4(aq) <—> NaH2PO4(aq) + H2O(l)

2NaOH(aq) + H3PO4(aq) <—> Na2HPO4(aq) + 2H2O(l)



Chemical Properties of Acids

Chemicals act differently when combined with different organic and inorganic materials, metals, salts, carbonates, etc. 3 main reactions of acids are as follows.

1. Acid+metal→salt+hydrogen gas

2. Acid+base→salt+water

3. Acid+carbonate→salt+water+carbon dioxide gas.



Common acid-base tests

Testing acids and Bases Reaction with Metals


Acids react with metals to form salt and hydrogen gas. Metals  are more active than acids, and they can undergo  single displacement reaction. For example, when zinc metal combined with hydrochloric acid they produce  zinc chloride and hydrogen gas. i,e  Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g) Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)


Bases reacts with certain metals like zinc or aluminium, for example,sodium hydroxide reacts with zinc and water to form sodium zincate and hydrogen gas as follows;

Zn(s)+2NaOH(aq)+2H2O(l)→Na2Zn(OH)4(aq)+H2(g). i,e 

Base + Metal →Salt + Hydrogen .i, e NaOH + Zn+ Na2ZnO2 (Sodium Zincate) + H2



Testing of acids, and bases (Neutralization Reactions)

When HCl is mixed with a base such as NaOH, the resulting products are salt (sodium chloride) and water (H2O). It can be written as


In this case, salts are ionic compounds containing a positive ion other than H+ and a negative ion other than the hydroxide ion, OH-. Double displacement reactions of this type are called neutralization reactions. When a strong acid and a strong base are combined in the proper amounts - i,e, when [H+] [H+] equals [OH− [OH−]\) - a neutral solution results in which pH = 7. The acid and bases will get neutralized with each other, and the acidic and basic properties are no longer present. Salt solutions do not always have a pH of 7, however. Through a process known as hydrolysis, the ions produced when an acid and base combine may react with the water molecules to produce a solution that is slightly acidic or basic.



Testing  acids with carbonates

Acids react with carbonate or hydrogen carbonate to form  salt, water, and carbon dioxide gas as shown in the equation.

 E.g. Calcium carbonate (limestone), CaCO3 and dilute hydrochloric acid react to give off carbon dioxide gas.

B. CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g).

The carbon dioxide given off produces an effervescence. Carbon dioxide turns the lime water chalky.



What happens to acid and base in water?

Acids in water solution dissociates H+ ions. Let us consider the reaction between water and hydrochloric acid (HCl).  HCl in the presence of water produces H+ ion. This ion cannot exist alone  hence it combines with water molecules to form H3O+.

The reaction is written as  HCL+H2O =H3o+Cl. 

Base in water : Base, when dissolved in water produces OH- ion. Let us consider the reaction between water and sodium hydroxide NaOH. NaOH in the presence of water produces OH- ion. The reaction is NaOH(s) + H2O (l) => Na+ + OH- + H20 + HEAT.



The Pop Test

When a burning candle is held close to the test tube containing hydrogen gas, it burns with a ‘Pop’ sound ( see image below). This will demonstrate the presence of hydrogen gas. Equipment required for the test includes, test tube and rack, dilute acid (egg. hydrochloric, Sulphuric or nitric acid) and a piece of magnesium Wooden splint, Safety glasses




STEP1: Take about 2 cm depth of diluted acid in a test tube and add a small piece of magnesium to this and wait for a minute, bubbles of Hydrogen will come off the magnesium. Press and hold the mouth of the test tube firmly.

STEP2: Bring a lighted split lamp to the end of the test tube carefully while releasing thumb. Soon you will see that there is a distinctive 'squeaky pop' sound. If you do not hear the ' pop' then try the experiment again.

STEP3: Dispose of it carefully while separating the metal and liquid carefully. Pour the liquid down the sink and place the metal in the bin.



Lime water Test

Lime water is chemically composed of Calcium hydroxide Ca (OH) 2 and it is a colourless solution with alkaline structure.


1. Make lime water by adding a small amount of calcium hydroxide to a test tube containing water. Now filter out the clear solution and transfer it to the beaker.

2. Add about 2 cm depth lime water to a test tube and blow it through a drinking straw to bubble CO2 through the lime-water solution. What will you see? You can see a thick precipitate or cloudy substance in 20-30 seconds.

3. Turn the lime water clear again. Continue to bubble CO2 through the solution until the precipitate just disappears.

The equation for the test is written as,

=Ca(OH)2 (aq) + CO2(g) = CaCO3(s) + H2O(l)

=On passing excess CO2 it leads to CaCO3(s) + H2O(l) + CO2(g)  = Ca(HCO3)2 aq Soluble in water



Litmus test



The litmus test is a quick way to find whether a liquid or gaseous solution is acidic or basic (alkaline). The test is based on how the liquid can change the colour of a litmus paper. To perform this test, place a drop of liquid sample on a small strip of paper or dip a piece of litmus paper in a small specimen of the sample. The litmus test can produce a range of colours. A change in the colour will decide whether the solution is acidic, alkaline or neutral. It can be performed with a blue litmus paper where the blue paper changes to red, indicating acidity somewhere between the pH range of 4.5 to 8.3. (Note, however, that 8.3 is alkaline.) Red litmus paper can indicate alkalinity with a change to blue.


Check your understanding

  1. Give 2 examples of organic acids.

  2. Explain the litmus test.

  3. What is an acidic pH?

  4. Explain the reaction between the limestone and dilute hydrochloric acid

  5. Exclude the weak acid among the sulfuric acid, nitric acid, hydrochloric acid, and citric acid.

  6. What is a neutralization reaction?


More readings in Chemistry 


Acids, Bases and Salts              


pH in daily life


 S-Block elements                                           


Equilibrium in Chemistry





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