ACID, BASE & SALT (Sci-X)

ACID  BASE   &  SALT
1. Acids: Acids are sour in taste, turn blue litmus red and give H3O+ ion (hydronium ions in solution. e.g. HCl, H2SO4, HNO3 etc

2. Bases: Bases are bitter in taste, have soapy touch, turns red litmus blue and give hydroxide ions (OH-) in solution. Example – NaOH, KOH etc

3. Salts: - A salt is a compound which is formed by neutralization reaction between an acid and base. For example, sodium chloride is formed by reaction between hydrochloric acid and sodium hydroxide.

Acid + base à Salt + water 
 ACID
CLASSIFICATION OF ACID
On the basis of origin, acids are classified as:

a. Organic acids: Acids derived from living organisms like plants and animals are called organic acids. They are weak acids and are not harmful for living organisms. For example: citric acid is present in fruits, acetic acid present in vinegar, oxalic acid present in tomato, tartaric acid present in tamarind, lactic acid present in sour milk and curd.
b. Mineral acids: They are also called inorganic acids. They are dangerous and corrosive. Special precautions have to be taken while handling them. For example: sulphuric acid (H2SO4),
hydrochloric acid (HCl) etc.

On the basis of their strength, acids are classified as:

a. Strong acids: Strong acids are those acids which completely dissociate into its ions in aqueous solutions. Example: nitric acid (HNO3) , sulphuric acid(H2SO4), hydrochloric acid(HCl)
b. Weak acids: Weak acids are those acids which do not completely dissociate into its ions in aqueous solutions. For example: carbonic acid (H2CO3), acetic acid (CH3COOH)

On the basis of their concentration, acids are classified as:

a. Dilute acids: Have a low concentration of acids in aqueous solutions.
b. Concentrated acids: Have a high concentration of acids in aqueous solutions.

Chemical properties of acids

(i) Acids react with active metals to give hydrogen gas.

Zn + H2SO4à ZnSO4 + H2 

 

(ii) Acids react with metal carbonate and metals hydrogen carbonate to give carbon dioxide.

NaHCO3+HCl à NaCl + H2O+CO2

 

(iii) Acids react with bases to give salt and water. This reaction is called neutralization reaction.

NaOH +HCl à NaCl + H2O+CO2 

 

(iv) Acids react with metals oxides to give salt and water.

CuO + H2SO4 à CuSO4 + H2O

BASE

On the basis of their strength, bases are classified as:
a. Strong bases: Strong bases are those bases which completely dissociate into its ions in aqueous solutions. Example: sodium hydroxide (NaOH), potassium hydroxide (KOH)
b. Weak bases: Weak bases are those bases which do not completely dissociate into its ions in aqueous solutions. For example: ammonium hydroxide (NH4OH)

Chemical properties of Bases-

1. Reaction with Metals – Certain metals such as Zinc, Aluminum, Tin react with Alkali solutions on heating and hydrogen gas is evolved     2NaOH + Zn → Na2ZnO2 + H2 

2. Reaction with acids – Bases react with acids to form salt and water.

KOH + HCl → KCl + H2O 

 

3. Reaction with Non – metallic oxides –Non - metallic oxides are generally acidic in nature. They react with bases to form salt and water.   2NaOH + CO2 → Na2CO3 + H2O

INDICATORS
Indicators are those chemical substances which behave differently in acidic and basic medium and help in determining the chemical nature of the substance. Acid base indicators indicate the presence of an acid
or a base by a change in their colour or smell. Indicators can be natural or synthetic.

Synthetic/Chemical Acid Base Indicators

Phenolphthalein: Phenolphthalein remains colourless in acids but turn pink in bases.
Methyl orange: Methyl orange turns pink in acids and becomes yellow in bases.

Natural Indicators 

Litmus: Litmus is a natural indicator. Litmus solution is a purple dyewhich is extracted from lichen. Acids turn blue litmus red. Bases turnred litmus blue. Water is essential for acids and bases to change thecolour of litmus paper. Remember that litmus paper will act as anindicator only if either the litmus paper is moist or the acid or base is in the form of aqueous solution. This is because acids and bases release H+ and OH- ions respectively in aqueous solutions.

Olfactory Indicators

.Onion: Smell of onion diminishes in a base and remains as it is in an acid.. 

Vanilla essence: The odour of vanilla essence disappears when it is added to a base. The odour of vanilla essence persists when it is added to an acid.. 

Turmeric: In acids, yellow colour of turmeric remains yellow. In bases, yellow colour of turmeric turns red.

 Some Important Points :-
1- pH of our digestive system: Our stomach produces hydrochloric acid for digestion of food. But during indigestion, excess of acid is produced in the stomach and therefore, the pH decreases. This causes pain and
irritation. So, to neutralise this excess acid, a mild base is used. This mild base works as an antacid. An antacid is any substance, generallya base or basic salt, which counteracts stomach acidity.

2- Tooth decay: Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel is made up of calcium phosphate which is the hardest substance in the body. It does not dissolve in water, but is
corroded when the pH in the mouth is below 5.5. If food particles remain in the mouth after eating, bacteria present in our mouth produce acid by degradation of sugar. This decreases the pH of mouth and hence tooth decay occurs. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.

11. Some Important Chemical Compounds

(a) Common Salt (NaCl)

Sodium chloride is known as common salt .Its main source is sea water. It also exists in the form of rocks and sodium chloride obtained from rocks is called rock salt. Common salt is an important component of our food. It is also used for preparing sodium hydroxide, baking soda, washing soda etc.

(b) Sodium Hydroxide or Caustic Soda (NaOH)

Sodium hydroxide is prepared by passing electricity through an aqueous solution of sodium chloride (also known as brine).

2NaCl (aq) + 2H2O (l) 2NaOH (aq) + Cl2 (g) + H2 (g)

This process is known as chlor -alkali process due to the formation of chlorine and sodium hydroxide (an alkali) as the products.

Properties –

1. It is a white translucent solid.

2. It is readily soluble in water to give a strongly alkaline solution.

3. Crystals of sodium hydroxide are deliquescent.

(c) Bleaching Powder (CaOCl2)

Its chemical name is calcium oxychloride .It is prepared by passing chlorine gas through dry slaked lime.

Ca (OH) 2 + Cl2 CaOCl2 + H2O Slaked lime Bleaching powder

Uses-

1. For bleaching cotton and linen in textile industry.

2. For bleaching wood pulp in paper industry.

3. For disinfecting drinking water. 

 

(d) Baking soda (NaHCO3)

Its chemical name is sodium hydrogen carbonate .It is prepared by passing CO2 gas through brine solution saturated with ammonia.

NaCl + H2O + CO2 + NH3 ------   NH4Cl              +             NaHCO3

                                                Ammonium chloride                     Sodium Hydrogen carbonate

The precipitated sodium hydrogen carbonate is filtered off.

 Properties –

1. It is a white crystalline solid, sparingly soluble in water at room temperature.

2. Its aqueous solution is weakly alkaline due to hydrolysis.

3. On heating, it decomposes to give sodium carbonate and carbon dioxide.

2NaHCO3 Na2 CO3 + H2O + CO2 4.

It reacts with acids to give CO2 gas.

NaHCO3 + HCl NaCl + H2O + CO2

Uses-

1. It is used as a component of baking powder. In addition to sodium hydrogen carbonate baking soda contains tartaric acid.

2. It is used in soda- acid fire extinguisher.

3. It acts as mild antiseptic and antacid. 

 

(e) Washing soda (Na2 CO3 .10 H2O)

Its chemical name is sodium carbonate decahydrate. It is obtained by heating baking soda in turn is obtained by passing CO2 gas through sodium chloride solution saturated with ammonia .

NaCl + H2O + CO2 + NH3 ------à       NH4Cl + NaHCO3

2 NaHCO3 ----------D----->      Na2 CO3             +       H2O + CO2

Sodium hydrogen carbonate            Sodium Carbonate

Recrystallisation of sodium carbonates gives washing soda.

Na2 CO3 + 10 H2O Na2 CO3 .10 H2O

Uses-

1. It is used in glass, soap and paper industries. 2. It is used for removing permanent hardness of water. 3. It can be used as a cleaning agent for domestic purposes. 

 

(f)Plaster of Paris (CaSO4 ..1/2 H2O)

Its chemical name is calcium sulphate hemihydrate. It is obtained by heating Gypsum upto 373 K.

CaSO4 .2H2O ---D----à   CaSO4 ..1/2 H2O             + 11/2 H2O

     Gypsum                                                    Plaster of Paris

On treatment with water it is again converted into gypsum and sets as a hard mass.

 CaSO4 ..1/2 H2O  + 11/2 H2O -------à     CaSO4 .2H2O

Plaster of Paris                                                                                     Gypsum

Uses –

1. It is used by Doctors for setting fractured bones.

2. It is used for making statues, models and other decorative materials.