28 August 2015

Chapter 2 Acids Bases and Salts

CBSE NCERT Class X (10th) | Science

Chapter 2 Acids Bases and Salts


The word "acid" comes from the Latin word "acidus" which means sour. 
According to Arrhenius theory an acid is a substance which ionizes and gives hydrogen ions when dissolved in water

Example: Hydrochloric acid in water releases H+ ions. 
HCl + H2O → H(aq) + Cl(aq)

A hydrogen ion cannot exist on its own, so it combines with a water molecule to form a hydronium ion.  
Example: Hydrochloric acid when dissolved in water liberates a hydrogen ion and a chloride ion. The hydrogen ion combines with water to form a hydronium ion.
HCl + H2O → H3O(aq) + Cl(aq)

Classification of acids:
Classification of acids based on source:
Based on the source the acids were classified into two types. They are organic acids and in-organic acids.

Organic acids:
Acids obtained from food like curd, lemons, grapes, raw mango, citrus fruits and gooseberry are called organic acids. 

In-organic acids:
Acids which are synthesised in the laboratory are called as in-organic acids or mineral acids
Following table is the list of some acids which are used in the laboratory.
Name of the acidChemical formula
Sulphuric acidH2SO4
Nitric acidHNO3
Hydrochloric acidHCl
Acetic acidCH3COOH

Classification of acids based on concentration:
The word concentration indicates the quantity of acid in relative to the quinatity of water in the aqueous solution of that acid.
Highly concentrated acid contains high percentage of acid in comparision with water in that solution.
Low concentrated acid contains low percentage of acid in comparision with water in that solution.

Dilution of acid:
Mixing an acid with water reduces the concentration of hydronium ions of the acid per unit volume. This is called dilution of acid. The action of acids with water is exothermic as heat is generated on dilution.

Classification of acids based on strength:
Based on ionisation, the acids were classified into strong acids and weak acids.
Strong acids: Acids which ionises complely into its ions are called strong acids.
Example: HCl, H2SO4, HNO3...etc
Weak acids: Acids which ionises partially into its ions are called weak acids.
Example: CH3COOH, H2CO3...etc

Classification of acids based on basicity of acids:
Based on basicity acids were classified into different types. They are
Mono-basic acids
Di-basic acids
Tri-basic acids

Mono-basic acids:
Acids which on ionisation produces on hydronium ion in water are termed as mono-basic acids.
Example: HCl

Di-basic acids:
Acids which on ionisation produces two hydronium ions are called as di-basic acids.
Example: H2SO4, H2CO3..etc

Tri-basic acids:
Acids which on ionisation produces three hydronium ions are called astri-basic acids.
Example: H3PO4, H3PO3..etc

Properties of acids:
Acids have corrosive action on skin.
Acids are good conductors of electricity.
Acids neutralizes bases to form salt and water.

Chemical properties of acids:
Reaction of acids with active metals:
Acids reacts with metals to form metal salts. In this reaction, hydrogen gas is liberated.
Example: In the reaction of hydrochloric acid reacts with zinc produces hydrogen gas and zinc chloride.
 2HCl + Zn → ZnCl2 + H2

Reaction of acids with metal carbonates:
Acids reacts with metal carbonates to form corresponding salts, carbon dioxide and water.
Example: Hydrochloric acid on reaction with sodium carbonate forms sodium chloride, carbon dioxide and water.

 2HCl + Na2CO3 → 2NaCl +  CO2 + H2O

Reaction of acids with metal hydrogen carbonates:

Acids reacts with metal hydrogen carbonates and form corresponding salts, carbon dioxide and water.
Example: Hydrochloric acid on reaction with sodium bicarbonate forms sodium chloride, carbon dioxide and water.
 HCl + NaHCO3 → NaCl + CO2 + H2O

Reaction of acids with metal oxides:
Acids reacts with metal oxide to form salt and water.
Example: Sulphuric acid on reaction with cupric oxide forms copper sulphate and water.
 CuO + H2SO4 → CuSO4 + H2O

An acid base indicator is a substance which exhibits different colour in acids and bases. 
Red cabbage is a visual indicator used to detect acids. 
Onions are called olfactory indicators. They change their odour with change in the nature of solution.
Litmus is a natural indicator and is extracted from lichens. 
Apart from natural indicators there are a few synthetic indicators, such as methyl orange and phenolphthalein. 

Following table gives
alein Colourless Pink
 Blue litmus paper Red colour No Change
 Red litmus paper No change Blue colour

Universal indicator is a mixture of different number of indicators which shows different colours in different solutions.

Uses of acids:
        • Sulphuric acid is used in the manufacture of fertilisers, paints, dyes, chemicals , plastics  and synthetic fibres.
        • Sulphuric acid is also used in car batteries.
        • Nitric acid is used in the manufacture of fertilizers, explosives like TNT, dyes and drugs.
        • Hydrochloric acid is used before galvanizing, to remove oxide film from steel and also as a descaling agent for boilers. It is also used in the textile, leather and food industry.


According Arrhenius theory any substance that can produce hydroxide ions when dissolved in water is called as a base.
        Substance + Water → Metal ion + OH
        NaOH (aq)  → Na+ (aq) + OH– (aq)

A base is said to be an alkali if it is soluble in water. In general hydroxides of alkali metals and alkaline earthmetals are considered as alkalies.
        KOH  (aq) → K+(aq) + OH (aq) 
        Ca(OH)2(aq) → Ca+2 (aq) + OH (aq)

It is not a necessary that a base should contain hydroxide ion.
There are some bases even they does not contain hydroxide ion, can be considered as bases.

Example: Ammonia (NH3)
Ammonia when dissoled in water forms ammonium hydroxide which is a weak base.
NH3 + H2O → NH4OH (aq)

Oxides of alkali metals and alkaline earthmetals are also considered as basic in nature.
Example: CaO, MgO, Na2O, K2O...etc

Classification of bases:
Classification based on the strength:
Based on the extent of ionisation bases are classified into strong bases and weak bases.

Strong bases:
The bases which undergoes complete ionisation in aquesous solution are called as strong bases.
Example: NaOH, KOH...etc

Weak bases:
The bases which undergoes partial ionisation in aqueous solution are called weak bases.
Example: NH4OH, NH3...etc

Classification of based on acidity:
Based on acidity bases can be classified into different types. They are:
Mono acidic base
Di acidic base
Tri acidic base

Mono acidic bases:
Bases which produces only one hydroxide (OH-) ion in aqueous solutions are called mono acidic bases.
Example: NaOH, KOH...etc

Di acidic bases:

Bases which produces two hydroxide ions in aqueous solutions are called di acidic bases.
Example: Ca(OH)2, Mg(OH)2...etc

Tri acidic bases:
Bases which produces three hydroxide ions in aqueous solutions are called tri acidic bases.
Example: Al(OH)3, Fe(OH)3...etc

Physical properties of bases:
        • Bases are bitter to taste, soapy to touch.
        • Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.
        • Bases liberates heat on dilution.

Indicators in pressence of bases:
Bases turns red litmus to blue.
Phenolphthalein turns pink in pressence of bases.
Methyl orange turns to yellow in pressence of bases.

Chemical properties:
Reaction with active metals:
Bases react with metals to liberate hydrogen gas
Example: Sodium hydroxide react with zinc and liberate hydrogen and sodium zincate.
           NaOH + Zn → Na2ZnO2 + H2
Reaction with non-metal oxides:
Bases react with non-metallic oxides to form salt and water. This is similar to a neutralization reaction between an acid and a base.            
Example: Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water
              Ca(OH)2 + CO2 → CaCO3 + H2O
From this reaction, it can be concluded that non-metallic oxides are acidic in nature.

Reaction with acids:
Bases reacts with acids to form salts and water.
Potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water.
KOH + HCl → KCl + H2O

Uses of Bases:
        •  Mild bases neutralise the acidity in the stomach.
        • Sodium hydroxide is used in the manufacture of soaps, paper and synthetic fibres like rayon.
        • Calcium hydroxide is used in the manufacture of bleaching powder. Bleaching powder is used as a disinfectant.
        • Magnesium hydroxide is used as an antacid to neutralize the acid in the stomach.
        • Ammonium hydroxide is used in the preparation of fertilizers like ammonium phosphate and ammonium sulphate.

Strength of Acids and Bases

Neutralization is a chemical reaction in which an acid reacts with a base to form salt and water. In this process, a hydrogen ion of the acid combines with a hydroxide ion of the base to form a water molecule. The anion of the acid combines with the cation of the base to form a salt.

For example when hydrochloric acid reacts with sodium hydroxide the chlorine of hydrochloric acid combines with sodium of sodium hydroxide to form sodium chloride. The hydrogen of hydrochloric acid combines with the hydroxyl part of the sodium hydroxide and forms water.
 HCl + NaOH → NaCl + H2O

Strength of acids or bases:
Based on extent of ionization acids and bases are classified into strong acids, weak acids and strong bases, weak bases.

Strong acids or strong bases ionizes completly (100%) to form ions in the aqueous solution.
Hydrochloric acid ionizes completly to form ions.
HCl (aq) → H+(aq) + Cl(aq)
Sodium hydroxide ionizes completly to form ions.
NaOH (aq) → Na+ (aq) + OH (aq)

Weak acids or weak bases ionizes partially (<100%) to form ions in the aqueous solution.
Acetic acid ionizes partially in aqueous solution to form ions.
CH3COOH (aq) ⇔ H+(aq) + CH3COO– (aq)
Ammonium hydroxide ionizes partially in aqueous solution to form ions.
NH4OH ⇔ NH4+ + OH (aq)

An acid or base is considered as strong or weak depending on the concentration of hydrogen and hydroxide ions within it.
This concentration or the power of hydrogen differs from substance to substance and can be measured using a scale, called the pH scale.  
A solution that has a pH value of less than 7 is acidic and a solution with a pH value of more than 7 is basic. A neutral solution is indicated by a pH value of 7 on the scale. 
Strong acids will posses pH values between 0-2 and weak acids posses pH values more than 3.
Strong bases will posses pH values between 12-14 and weak bases posses pH values less than 12.

CBSE NCERT Class X (10th) | Science, Chapter 2 Acids Bases and Salts, NCERT CBSE Solved Question Answers, KEY NOTES, NCERT Revision Notes, Free NCERT Solutions Online

Applications of neutralization concept in daily life:
        • Antacids like Milk of Magnesia are mild bases that neutralize the acids in the stomach and aid digestion.
        • If the pH lowers, the acidity in the mouth increases and leads to tooth decay.  Toothpastes are basic in nature and they counteract the acid in the mouth.
        • Hydrangea produces pink flowers when the soil has a pH value of 6.8 or higher and blue flowers when the pH value is 6.0 or less.
        • If the soil is acidic, then the applied pesticides, herbicides and fungicides will not be absorbed by the soil. In order to neutralize the soil, suitable bases are used. Generally, salts of calcium or magnesium, which are basic are used to neutralize soil acidity.

        • When a bee stings, formic acid is released. That is what makes the skin burn. Baking soda, which is a base, neutralizes the formic acid and provides relief from the pain.

Salts and their Properties

The compounds formed by the reaction between an acids and a bases are known as a salts.
                                Acid + Base → Salt + Water

Salts are ionic compounds which contain positively charged cations and negatively charged anions. During salt formation cation is coming from base and anion is coming from acid.
Example: In Sodium chloride (NaCl) formation cation sodium is coming from sodium hydroxide and anion chlorine is coming from hydrochloric acid.

Salts formed from any hydroxide and hydrochloric acid are ‘chlorides ‘. 
Example: Potassium hydroxide reacts with hydrochloric acid to form a salt called potassium chloride.
                      KOH + HCl → KCl + H2O

Salts formed from any hydroxide and sulphuric acid are called ‘sulphates.‘ 
Example: Magnesium hydroxide reacts with sulphuric acid to form magnesium sulphate. 
                Mg(OH)2 + H2SO4 → MgSO4 + 2H2O

Salts formed from any hydroxide and nitric acids are known as ‘nitrates‘. 
Example: Ammonium hydroxide reacts with nitric acid to form the salt ammonium nitrate.
          NH4OH + HNO3 → NH4(NO3) + H2O

Salts formed from any hydroxide and carbonic acid are ‘carbonates ‘. 
Example: Sodium hydroxide reacts with carbonic acid to form the salt sodium carbonate.
           NaOH + H2CO3 → Na2CO3 + H2O

Based on the strength of the reacting acid and the base, salts can be classified into three types.
They are neutral salts, acidic salts and basic salts.
Type of SaltType of AcidType of BaseExample
        pH = 7
  Strong Acids
  Strong Bases
       pH < 7
  Strong Acids
  Strong Bases
        pH > 7
  Weak Acids
  Strong Bases

Salts in our daily life:
Sodium chloride (NaCl):

To preserve pickles, fish and meat.
To manufacture soap.
To melt ice formed on roads in cold countries and
As a raw material for the manufacture of other compounds.

Sodium hydrogen carbonate (NaHCO3):
Sodium hydrogen carbonate is commenly called as baking soda.
Sodium hydrogen carbonate is used in the baking industry.
It is used in preparation of soda acid.
It is also used in foam type fire extinguishers.

Sodium carbonate (Na2CO3):
Sodium carbonate is used to manufacture of glass, cleansing agents, soap, glass and paper, sodium compounds like borax.
Adding water to sodium carbonate and this allowing this mixture to cool to forms decahydrated sodium carbonate. This is commenly called as washing soda.
         Na2CO3 + 10 H2O → Na2CO3.10H2O

Bleaching powder (CaOCl2):
Bleaching powder chemically known as calcium oxy chloride.
It is used to bleach cotton, linen textiles and wood pulp.
It is also used to disinfect drinking water.

Hydrated salts:
The molecules of salts which contain fixed number of water molecules in them are called hydrated salts.
In general they exists as dry in pure form. 
These salts on heating loses water molecules in them and forms anhydrous salts.
Ferrous sulphate heptahydrate (FeSO4.7H2O) on heating loses water molecules in it.
          FeSO4.7H2O (on heating) → FeSO4 + 7H2O

Some of the hydrated salts along with their chemical formula.
                    Name of the salt      Chemical formula
 Sodium carbonate decahydrate Na2CO3.10 H2O
 Zinc Sulphate heptahydrate or White vitriol ZnSO4.7H2O
 Magnesium sulphate heptahydrate or Epsom salt MgSO4.7H2O
 Potash alum K2SO4 Al2 (SO4)3 .24H2O
 Copper (II) sulphate pentahydrate or Blue vitriol CuSO4.5H2O
 Calcium sulphate dihydrate or Gypsum CaSO4.2H2O

Plaster of paris (CaSO4.H2O):
Plaster of paris which is chemically called calcium sulphate hemihydrate.
Since it is brought to use from paris, called as "plaster of paris".
It is prepared by heating of gypsum at 373K.

CaSO4.2H2O         CaSO4  H2O      +    1 H2O    
  Gypsum                         Plaster of Paris            Water         

It is used as a bandage, proofing material, sealing agent.
It is used for making statues, toys and decorative articles.
It is also used for smoothening wall surfaces. colours of the indicators in presence of acids and bases.
           Indicator        Acid        Base
 Methyl orange Red Yellow