ACIDS, BASES AND SALTS

0

Introduction

A wide variety of materials consists essentially of elements and compounds having different characteristics exist around us. Some of them are sour, some are bitter, while some are salty in taste.
For Example – Sour and bitter tastes of food are due to acids and bases, respectively, present in them. Acids react with bases to produce salt whose properties are different from acid and base.

Acids

The term “acid” is derived from the latin word “acidus” meaning sour to taste.
Example – Sour taste of lemon, unripened grapes, Vinegar, tomatoes etc.

  • According to Arrhenius theory :
    “An acid is a substance which dissolved in water, it ionizes and releases hydrogen ions [H+(aq.)] in solution”.
    HCl(aq.)    →   H+ (aq.)      +    Cl(aq.)
    Hydrochloric acid   Hydrogen ion  Chloride ion
    or     HCl(g) + H2O (l)  →    H3O(aq.) + Cl(aq.)Note :- Hydrogen ion do not exist as H+ ions in solution, they attach themselves to the polar  water molecules to form hydronium ions or hydroxonium ions, (H3O or H+(aq.))
    H+       +        H2O   →   H3O+
    Hydrogen ion       Water       Hydronium ionH2SO4(aq.)   →      2H+ (aq.)      +    SO42–(aq.)Sulphuric acid          Hydrogen ion        Sulphate ion
    HNO3(aq.)   →          H+ (aq.)      +    NO3(aq.)
    Nitric acid             Hydrogen ion         Nitrate ion

 

Classification of acids

(I) On the basis of their source acids are of two type –
(i) Mineral acids (ii) Organic acids

(i) Mineral Acids (Inorganic acids) :-
The acids which are usually obtained from minerals are known as inorganic acids.

Name Chemical FormulaWhere found or used
Hydrochloric acidHClIn purification of common salt, in textile industryIn purification of common salt, in textile industry as bleaching agent, to make aqua regia.
Sulphuric acidH2SO4 Commonly used in car batteries, in the manufacture of fertilizers (Ammonium phosphate, Super phosphateCommonly used in car batteries, in the manufacture of fertilizers (Ammonium phosphate, Super phosphate detergents etc, in paints, plastics, drugs) manufacture of artificial silk, in petroleum refining.
Nitric acidHNO3 Used in the manufacture of explosives (TNT, Nitroglycerine) and fertilizers (Ammonium nitrate, Calcium nitrate, Purification of Au, Ag.
Carbonic acidH2CO3 In soft drinks. In stomach as gastric juice, used in tanning industry
Phosphoric acidH3PO4 Used in antirust paints and in fertilizers

 

(ii) Organic Acids :- The acids which are usually obtained from plants and animals are known as organic acids.

Name Where found or used
Formic acid (HCOOH)Found in the stings of ants and bees, used in tanning leather, in medicines for treating gout.
Acetic acid (CH3COOH)Found in vinegar, used as solvent in the manufacture of dyes and perfumes.
Lactic acidResponsible for souring of milk in curd
Benzoic acidUsed as a food preservative
Citric acidPresent in lemon, orange and citrus fruits
Tartaric acidPresent in tamarind.

 

(II) On the Basis of their Basicity:-
“The  basicity of an acid is the number of replaceable hydrogen atoms present in a molecule that can be produced by the complete ionisation of one molecule of that acid in aqueous solution.”

or

“Basicity of an acid is determined by number of hydronium ions (H3O+/H+(aq)) produced per molecule of an acid on ionisation.”

(i) Monobasic Acids:-
The acid on complete ionisation produce one hydronium ion in aqueous solution.
Example :
Hydrochloric acid (HCl)
Hydrobromic acid (HBr)
Hydrofluoric acid (HF)
Hydroiodic acid (HI)
Nitric acid (HNO3)
Acetic acid (CH3COOH)
Formic acid (HCOOH)

 

HCl          +           H2O              →           H3O                   +                Cl (Chloride ion)
Hydronium ion

CH3COOH          +      H2O          →              H3O         +             CH3COO
Hydronium ion             Acetate ion

HNO3                       +     H2O         →                 H3O⊕         +           NO3
Hydronium ion               Nitrate ion

 

(ii) Dibasic Acid:-
The acid on complete ionisation produces two hydronium ions in aqueous solution.
Example: Sulphuric acid (H2SO4)
Carbonic acid (H2CO3)
Oxalic acid (COOH)2

H2SO4           +            H2O          →            H3O⊕      +         HSO4–           (Step – I)
Bisulphate ion

HSO4–         +          H2O            ¾¾®      H3O          +       SO42–             (Step – II)
Sulphate ion

 

(iii) Tribasic Acid:-
The acid on complete ionisation produces three hydronium ions in aqueous solution.
Example :
H3PO4    +    3H2O       →      3H3O⊕      +               PO4–3
Hydronium ion               Phosphate ion

 


(III) Classification on the basis of their strength:-

(i) Strong Acid:-
The acid which undergoes complete ionisation in aqueous solution are known as strong acids.

Example :
HCl            + Water    →      H+(aq)  +  Cl(aq)
H2SO4    + Water    →     2H+(aq) + SO42–(aq)               Complete ionised
HNO    + Water     →     H+(aq)    + NO3 (aq)

(ii) Weak Acid:-
The acid which undergoes partial or incomplete ionisation in aqueous solution are known as weak acids. CH3COOH     +    Water           →       CH3COO (aq) + H+ (aq)
Acetic acid                                                 Acetate ion
Example: Formic acid (HCOOH), Oxalic acid (COOH)2
Carbonic acid (H2CO3), phosphoric acid (H3PO4)

 

 

 

(IV) Classification on the basis of concentration of the Acid:-

(i) Concentrated Acid:-  The acids which contains very small amount of water is called a concentrated acid.
(ii) Dilute Acid:- The acids which contains more amount of water is called a dilute acid. “Strength of an acid is not depend upon the concentration of an acid”
Strength of an Acid ∞ Concentration of hydronium ion.

 

Bases

Substances with bitter taste and give a soapy touch are known as bases but many bases have corrosive nature. So bases are defined as”

  • According to Arrhenius:
    those substances which give hydroxide or hydroxyl ion (OH–) in their aqueous solution” are called bases. 
    NaOH(aq.) ®  Na+(aq) + OH(aq)
    KOH(aq.) ®  K+(aq) + OH(aq)Example – Sodium hydroxide (NaOH), Zinc oxide (ZnO), Copper oxide (CuO), Calcium hydroxide [Ca(OH)2], Aluminium hydroxide [Al(OH)3].
  • The compounds which are either metallic oxides or metallic hydroxides. Which combines with acids to form salts and water only.
    CuO         +                 2HCl           →                    CuCl              +                  H2O
    Base                         Acid                                    Salt                                     Water
    NaOH     +              HCl                 →                    NaCl                  +                 H2O
    Base                         Acid                                      Salt                                       Water
    Mg(OH)2    +        H2SO4         →                    MgSO4             +               2H2O
    Base                        Acid                                       Salt                                      Water

 

♦ Alkalis 

Bases which completely dissolves in water are called alkalis.
Examples – KOH, NaOH, Ca(OH)2

 

All the alkalis are bases but all bases are not alkalis.
Examples – [Fe(OH)3] ferric hydroxide and cupric hydroxide [Cu(OH)2] are base, but not an alkali.

 

♦ Classification of Bases

(I) Classification on the basis of their strength:-
(i) Strong alkalis or bases:- The alkalis or bases which undergo almost complete ionisation in aqueous solution are known as strong alkalis or bases.
Examples –
NaOH(aq.)   →   Na+(aq) + OH(aq)
Sodium hydroxide
KOH(aq.)    →   K+(aq) + OH(aq)              }Complete ionisation
Potassium hydroxide
Ba(OH)2(aq.)        →   Ba+(aq) + 2OH(aq)
Barrium hydroxide

 

(ii) Weak alkalis or bases:-
The alkalis or bases which undergo only partial ionisation in aqueous solution are known as weak alkalis or Bases.
Example –

Ca(OH)2(aq.)           →        Ca2+ (aq) + 2OH (aq.)
Calcium hydroxide                                                                             } Partial Ionisation
Mg(OH)2(aq.)           →     Mg2+ (aq) + 2OH (aq.)
Magnesium hydroxid

 

(II) Classification on the basis of their concentration –
(i) Concentrated Base or Alkali –
The bases or alkalis which contain very small amount of water is called a concentrated bases or alkalis.
(ii) Dilute Base – 
The bases or alkali which contain more amount of water is called a dilute bases or alkalis.

(III) Classification on the basis of their acidity – Acidity of a base is determined by the number of hydroxyl (OH–) ions produced by per molecule of a Base or Alkali on complete dissociation in water “or”  The “number of hydrogen ions of an acid with which a molecule of that alkali or base react to produce salt and water is known as acidity of an alkali or Base”.

(i) Mono acidic Bases or Alkali – The base or alkali on complete ionisation produce one hydroxyl (OH) ion in aqueous solution.
Example –
NaOH(aq.)       →     Na+(aq) + OH(aq)
Hydroxyl ion
KOH(aq.)         →    K+(aq) + OH(aq)
Hydroxyl ion

(ii) Diacidic Bases (or alkalis)
The base or alkali on complete ionisation produce two hydroxyl ion (OH–) in aqueous solution

Example – 

(A) Diacidic Bases of  –
Ca(OH)2(aq.)             →    Ca2+(aq.) + 2OH(aq.)
Mg(OH)2(aq.)            →     Mg2+(aq.) + 2OH(aq.)

(B) Diacidic Bases –
Ferrous hydroxide [Fe(OH)2] and copper hydroxide [Cu(OH)2]
Fe(OH)2(aq.)          →           Fe2+ + 2OH(aq.)
Fe+2(OH)2– + 2H+Cl(aq.)     →    FeCl2 + 2H2O

(iii) Tri Acidic Bases –
The base or alkali on complete ionisation produce three hydroxyl ion (OH)– in aqueous solution.
Example – Aluminium hydroxide [Al(OH)3], Ferric hydroxide [Fe(OH)3]
Al(OH)3(aq.)      →    Al3+(aq.) + 3OH(aq.)
Al3+(OH)3 + 3HCl(aq.)  →    AlCl3 + 3H2O

 

Properties of Acid

(I) Physical properties of Acid – 

The important properties of acids are given below
(i) Acids have a sour taste.
(ii) Acids turn blue litmus to red.
(iii) Acid solution conduct electricity (They are electrolytes).
(iv) Effect on Skin – All strong mineral acids have a corrosive action on skin and cause painful burns.

Example – Concentrated sulphuric acid stains the skin black.
Concentrated nitric acid & hydrochloric acid stains the skin yellow.

(v) Electrical Conductivity – All mineral acids are good conductors of electricity and conduct electricity in their aqueous solution. On electrolysis, they decompose liberating hydrogen at cathode.

 

(II) Chemical Properties of Acids
(i) Acids react with metal  to form hydrogen gas 
Most of the acids react with mtals to form salts and evolve hydrogen gas.
Zn(s) + H2SO4(aq)     →      ZnSO4(aq) + H2(g)
Most of the acid react with metal to form solid and evolve hydrogen gas.

 

(III) Acids react with bases (or alkalis) to form salt and water:
Acid               +            Base       →       Salt       +     Water
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
NaOH(aq)  +  HCl(aq)         →      NaCl(aq)  +  H2O(l)
Sodium         Hydrochloric             Sodium      Water
hydroxide          acid                          chloride

 

(IV) Acids react with metal oxides to form salt and
water: 
Acids react with metal oxides to form salt and water:
Metal oxide  + Acid       →       Salt       +      Water
Copper(II) oxide is a metal oxide. Dilute hydrochloric acid reacts with copper(II) oxide to form copper(II) chloride and water:
CuO(s)          +        2HCl(aq)                →          CuCl2(aq)      +      H2O(l)
Copper                Hydrochloric                            Copper                  Water

(II) oxide                      acid                                          (II) chloride
(Blue Green)

 

(V) Acids have corrosive nature:
This mineral acids causes severs burns on the skins and attack and eat up materials like clothes,wood,metal structures and stonework, so they are said to be corrosive. Acids are never stored in metal containers because they gradually corrode and eat up the metal container. The strong bases (or alkalies) such as sodium hydroxide are also very corrosive, and attack and destroy our skin.

 

♦ What is common in all acids?
An acid is a substance which dissociates (on ionises) on dissolving in water to produce hydrogen ion
[H+ (aq) ions].
HCl(aq)                       →               H+(aq)          +           Cl(aq)
Hydrochloric acid                   Hydrogen ions       Chloride ions

A common thing in all the acids is  that they produce hydrogen ions [H+ (aq) ions] when dissolved in water. H2SO4(aq)          →       2H+(aq) + SO42–(aq)
CH3COOH(aq)      →       CH3COO(aq) + H+(aq)

 

♦ Uses of Mineral Acids in Industry :
1. Sulphuric acid is used in the manufacture of fertilizers (like ammonium sulphate), paints, dyes, chemicals, plastics, synthesis fibres, detergents, explosives and car batteries.
2. Nitric acid is used for making fertilisers (like ammonium nitrate), explosivs (like TNT : Tri-Nitro Toluene), dyes and plastics.
3. Hydrochloric acid is used for removing oxide film from steel objects (before they are galvanised) and for removing ‘scale’ deposits from inside the boilers.

 

♦ Uses of Mineral Acids in Industry:

1. Sulphuric acid is used in the manufacture of fertilizers (like ammonium sulphate), paints, dyes, chemicals, plastics, synthesis fibres, detergents, explosives and car batteries.
2. Nitric acid is used for making fertilisers (like ammonium nitrate), explosivs (like TNT : Tri-Nitro Toluene), dyes and plastics.
3. Hydrochloric acid is used for removing oxide film from steel objects (before they are galvanised) and for removing ‘scale’ deposits from inside the boilers.

 

Properties of Bases

♦ PHYSICAL Properties of Bases
The important properties of water soluble bases (or alkali) are given below:
1. Bases have bitter taste.
2. Bases feel soapy to touch.
3. Bases turn red litmus to blue.
4. Bases conduct electricity in solution (They are electrolytes).

 

♦ Chemical Properties of Bases

1. Bases react with some metals to form hydrogen gas.

2NaOH(aq)               +        Zn(s)          \buildrel {heat} \over\longrightarrow          Na2ZnO2(aq)     +               H2(g)
sodium                                    Zinc            sodium            hydrogen
hydroxide                                                   zincate
(Base)                                                            (Salt)
All the metals do not react with bases to form salts and hydrogen.

 

Observations:

(a) A gas (hydrogen) is evolved, which on passing through soap solution form soap bubbles filled with H2 gas. (b) When we burn carefully the soap bubbles filled with H2 gas with a candle, a ‘pop’ sound is boserved.
(c) In the above reaction, hydrogen is displaced from the bases and is evolved as H2 gas.
(d) The metals combine with the remainder part of the bases to form, a compound, called sat. Thus:
Base          +          Metals             →       Salt    +   H2 (g)
e.g      2NaOH(aq) +    Zn(s)                  →        Na2 ZnO2 (aq) + H2(g)
sodium Zincate

Note: Such reactions are not possible with all metals and this reaction is different from reaction of metals with acids. In this reaction OH ion gives ZnO22– (zincate ion) and hydrogen gas.

 

2. Bases react with acids to form salts and water 
2NaOH(aq)        +         H2SO4(aq)                             Na2SO4(aq)             +             2H2O(l)
sodium                                sulphuric                                   sodium                                       Water
hydroxide                             acid                                            sulphate
(Base)                                     (acid)                                              (salt)

when an acid and base combine then the real neutralisation reaction occurs due to the combination to hydrogen ions present in acid and hydroxide ions present in base to form water.

H+(aq) + OH(aq)            \mathrel{\mathop{\kern0pt\longrightarrow}\limits_{reaction}^{Neutralisation}}          H2O(I)
hydrogen      hydroxide                                          Water
ions                       ions
(From acid)                                                              (From base)

 

3. Bases react with non-metal oxides to form salt and water:

Non – metal oxide + Base      →      Salt   +   Water
Ca(OH)2 (aq) + CO2(g)            →       CaCO3(s) + H2O(l)
calcium                carbon                          calcium          water
hydroxide             dioxide                   carbonat
(Base)       (Non-metal oxide)                 (salt)
The reactions of non-metal oxides with bases to form salt and water show that non-metal oxides are acidic in nature.

 

♦ Uses of Bases :

(i) Sodium hydroxide is used in the manufacture of  soap, paper and a synthesis fibre called ‘rayon’.
(ii) Calcium hydroxide ( slaked lime) is used in the manufacture of bleaching powder.
(iii) Magnesium  hydroxide is used as an antacid to neutralise excess acid in the stomach and cure indigestion.

 

DIFFERENCE BETWEEN ACIDS & BASES

 

What happens when water is mixed with an acid or a base?
Mixing of acid or base in water is called dilution and the acid or the base is said to be diluted. During dilution, concentration of ions (H3O+ OH) per unit volume decreases. This process is generally exothermic in nature, which produces heat. Heat produced may be harmful because it may cause the mixture to splash out and cause burns. Moreover the glass container may also break due to excessive local heating. So precautions are recommended during dilution of an acid or bases.

Strength of acid and base solutions: PH scale

The basic solutions have excess of hydroxide ions. Acidic solutions have excess of hydrogen ions. The basic solutions have excess of hydroxide ions. In 1909 Sorenson developed a scale (known as pH scale) on which the strength of acid solutions as well as basic solutions could be represented by making use of the hydrogen ion concentration in them.  The pH of a solution is inversely proportional to the concentration of hydrogen ions in it.
The strength of an acid or base in measured on a scale of number called the pH scale. The pH scale has values from 0 to 14.

1. Neutral  substances have a pH of exactly 7.  The pH of pure water is 7.
A substance having pH 7 will have no effect on litmus or any other common indicator such as methyl orange or phenolphthalein.
2. All the solutions having pH less than 7 are acidic in nature and hence they turn blue litmus to red. They also turn methyl orange indicator red.
3. Bases (or basic solutions) have a pH or more than 7. The more basic a solution is, the higher will be its pH. The higher thepH, the stronger the base (or alkali). All the substances having pH more than 7 are basic in nature (or alkaline in nature) and hence they turn red litmus to blue. They also turn phenolphthalin indicator pink.


Indicators

Indicator as the name suggests, indicates the nature of particular solution whether acidic, basic or neutral. Apart from this, indicator also represents the change in nature of the solution from acidic to basic & vice-versa, Depending upon the property of the indicator, we have the following two types of acid-base indicators:
(1) Indicators showing different colours in acidic & basic medium.  Examples, Litmus, phenolphthalein and methyl orange.
(2) Indicators giving different odour in acidic and basic medium/olfactory indicators.  Examples, Onion extract, vanilla and clove oil.

Discussion: The changes observed are as follows:

Conclusion: 
(i) Acids turn blue litmus red but have no effect on red litmus.
(ii) Bases turns red litmus blue but have no effect on blue litmus.
(iii) Phenolphthalein is colourless in acidic medium and turns pink in basic medium.
(iv) Methyl orange is yellow in basic medium and red in acidic medium.

 

♦ Universal Indicators:

Universal indicator is a mixture of many different indicators (or dyes) which gives different colours  at different pH values of the entire pH scale. When an acid or base solution is added to the universal indicator, the indicator  produces a new colour. The colour produced by universal indicator is used to find the pH value of the acid or base solution by matching the colour with the colours on pH colour  chart.

Just like litmus, universal indicator can be used either in the form of a solution or in the form of universal indicator paper. Water will produce a green colour with universal indicator.


♦ Olfactory indicators:
  Those substances whose smell (or odour) changes in acidic or basic solutions are called olfactory indicators. Onion and vanilla extract are olfactory indicators.

 

Importance of pH in everyday life

1. pH in our Digestive System: 
Our stomach produces hydrochloric acid helps in digesting our food.  The excess acid in the stomach causes indigestion which produces pain and irritation. Antacids are a group of mild bases which have no toxic effects on the body, react with excess acid in the stomach and neutralise it. The two common antacids used for curing indigestion due to acidity are: Magnesium hydroxide (Milk of Magnesia) and Sodium hydrogen carbonate (Baking soda).

2.  pH change as the cause of Tooth decay:  The bacteria present in our mouth break down the sugar to form acids. This acid lowers the pH in the mouth (making decay)  is to clean the mouth thoroughly after eating food. Tooth enamel is made up of calcium phosphate, the hardest substance in our body and it does not dissolve in water. Tooth starts decaying, when pH of the mouth is less than 5.5, due to corrosion of calcium phosphate. Bacteria present in the mouth produce acid by the degradation of carbohydrates (like sugar) and food particles still remaining in the mouth after eating. The best method of preventing tooth decay is to clean the mouth after eating and avoid eating sugar foods (such as sweets, toffees etc.). Most common tooth pastes are basic in nature so by using these for cleaning the teeth, the excess acid gets neutralized thereby pH becomes greater than 5.5 and tooth decay is prevented.

3. Plants and Animals are Sensitive to pH Changes:  Soil pH and Plant Growth : Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic (having low pH), then it is treated with materials like quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate). If the soil is too alkaline then its alkalinity can be reduced by adding decaying  organic matter (manure or compost) which contains acidic materials. In plants and animals: Living organisms can survive only in a narrow range of pH values. eg.g, our body can work within 7.0 to 7.5 pH range. When pH of rain water is below 5.6 (due to absorption of acidic gases like CO2, SO2, NO2 etc. present n the atmosphere), it is known as acid rain, which on flowing into the rivers lowers the pH of the river water, thereby survival of aquatic life in such rivers becomes difficult.

4. Self Defence by Animals and Plants Through Chemical Warfare:  Many animals and plants protect themselves from their enemies by injecting painful and irritating acids and bases  into their skin. For example when a honey bee stings a person, it injects an acidic liquid into the skin which causes immense pain and irritation. An ant’s sting injects methanoic acid (HCOOH) into the skin of a person causing burning pain. When a person happens to touch the leaves of a nettle plant accidently, the stinging hair of nettle leaves inject methanoic acid into the skin of the person causing burning pain.

 

pH Value of Some Common Substances

 

Salts

Salts are the ionic compounds consisting of two parts, one part carrying a positive charge called positive ion or cation and the other part carrying a negative charge called a negative ion or anion.
Some common salts
1. Sodium chloride (NaCl)
2. Sodium sulphate (Na2SO4)
3. Potassium nitrate (KNO3)
4. Copper sulphate (CuSO4)
5. Zinc sulphate (ZnSO4)
6. Calcium carbonate (CaCO3)
7. Calcium chloride (CaCl2)
8. Aluminium sulphate [Al2(SO4)3]
Salts are formed when acids reacts with bases
Types of Salts
(i) The salts of ‘hydrochloric acid’ are called ‘chlorides’.
(ii) The salts of ‘sulphuric acid’ are called ‘sulphates’,
(iii) The salts of ‘nitric acid’ are called ‘nitrates’.
(iv) The salts of ‘carbonic acid‘ are called ‘carbonates’.
(v) The salts of ‘acetic acid’ are called ‘acetates’ and so on.

 

 

CHEMICALS FROM COMMON SALT (NaCl)

1. Sodium Chloride (Common salt/table salt) We know that

hydrochloric acid and sodium hydroxide combine with each other to form sodium chloride (NaCl) which in common language is also known as common salt. This is the salt which you sprinkle on your salads and use in your kitchens. Common salt is an ionic compound of sodium and chlorine
(Na+Cl)n.
The main source of common salt (sodium chloride) is the sea water. Sea water contains about 3.5% of soluble salts, the most common of which is sodium chloride (2.7 to 2.9%). Saline water of inland lakes, such as Sambhar lake in Rajasthan is also a good source of common salt. Sodium chloride is also found as rock salt. Beds of rock salt were formed when lakes/seas dried up in past. Common salt act as raw material for making various materials of daily use. Let us discuss some of them.

2. Sodium hydroxide (NaOH) (chlor alkali processes): When electricity is passed through an acidulated aqueous solution of brine (or sodium chloride) the salt undergoes decomposition to produce sodium metal at cathode and chlorine gas at anode. The sodium metal then reacts with water to form sodium hydroxide and evolves hydrogen gas at cathode.
NaCl (aq)      \buildrel {Electicity} \over\longrightarrow      Na+ (aq) + Cl (aq)
At cathode:
Na+ (aq) + e       →    Na(s)
2 Na(s) + 2 H2O    →   2 NaOH(aq) + H2(g)
At anode:
Cl (aq) ¾® Cl + e
2 Cl  → Cl2 (g)

 

3. Bleaching powder (CaOCl2): The chlorine gas (obtained during the electrolysis of brine) is passed through dry slaked lime [Ca(OH)2], then bleaching powers (CaOC2) is obtained.
Ca(OH)2(s) + Cl2 (g)        →      CaOCl2(s)        +       H2O(l)
Dry slaked lime                        Calcium oxychloride
(Bleaching powder)

Uses:  (i) For disinfecting (or sterlizing) water (making water free of germs). (ii) As a germicide and deodorant in medicine. (iii) For rendering wool unshrinkable. (iv) For laboratory preparation of chlorine and oxygen. (v) In the manufacture of chloroform. (vi) For bleaching, cotton and linen in the textile industry and wood pulp in paper factories and for bleaching wahed clothes in laundary.

4. Baking soda (or sodium hydrogen carbonate, NaHCO3): Is prepared by passing carbon dioxide gas through a cold saturated solution of sodium carbonate.
Na2CO3 + H2O + CO2     →      2 NaHCO3
sodium carbonate                      Sodium hydrogen carbonate

Due to its low solubility in cold water, it separates out as white crystals. It is also the primary product of the Solvay process.

Uses:
(i) As an ingredient in antacid medicines, since it is alkaline in nature and neutralizes excess acid in the stomach.
(ii) In cooking food in the form of baking powers, which contains sodium hydrogen carbonate and an acid like tartaric acid (or salt of tartaric acid such as potassium hydrogen tartarate). When baking powder is either dissolved in water or heated, its constituent sodium hydrogen carbonate undrgoes decomposition to liberate carbon dioxide, which causes the bread and cake to rise. Tartaric acid present in the baking powder neutralizes sodium carbonate.
2 NaHCO3   \buildrel {Heat} \over\longrightarrow    Na2CO3 + H2O + CO2
Na2CO3 + Tartaric acid   →   Neutralization
If tartaric acid (or its acidic salt) is not present in the baking powder, then the cake/bread will taste bitter, due to the presence of sidum carbonate.

(iii) In soda-acid fire extinguisher, which contains a solution of sodium hydrogen carbonate surrounding a glass bottle containing sulphuric acid. In case of fire, the knob (provided at the top of extinguisher) is pressed, thereby bottle breaks and the two solutions come in contract, thereby liberating carbon dioxide gas.
2 NaHCO3 + H2SO4     →    Na2SO4 + 2 H2O + 2 CO2
The liberated carbon dioxide forces a stream of effervescing liquid on the fire. In this way, carbon dioxide surrounds the combustible substances and cut-off the supply of air, thereby it assits to put out the fire.

(iv) In effervescent cold drinks and fruit salts (e.g. Eno).

5. Sodium carbonate or washing soda, Na2CO3. 10 H2O: Is manufactured by solvay process. The raw materials employed are sodium chloride (NaCl), limestone (CaCO3) and ammonia (NH3). Actually, most of the ammonia is recovered in the process itself. In this process:
Step I: A cold and concentrated solution of sodium chloride (called brine) is saturated with ammonia to get ammoniacal brine.
Step II: The ammoniacal brine is fed from the top of carbonating tower, which is provided with perforated plates. As the ammoniacal brine trickles down this tower, upcoming carbon dioxide reacts with ammoniacal brine to form insoluble precipitate of sodium hydrogen carbonate (or sodium bicarbonate, NaHCO3). Thus:

The sodium hydrogen carbonate is filtered; while ammonium chloride is recycled in step I to prepare ammonia.

 

Step III: Sodium hydrogen carbonate precipitate is heated to get soda ash (sodium carbonate).
2 NaHCO3      \buildrel {Heat} \over\longrightarrow      Na2CO3 + H2O + CO2
Soda ash
(Sodium carbonate)
The CO2 is recycled to step (II).

Step IV: The soda ash (Na2CO3) so-obtained is dissolved in water and crystallized to get washing soda. Na2CO3 + 10 H2O     \buildrel {Crystallization} \over\longrightarrow     Na2CO3 . 10 H2O
Soda ash                                     Washing soda

It dissolves in water to give alkaline solution. So its aqueous solution turns red litmus blue.

Uses:
(i) For washing purposes in the laundry.
(ii) For softening hard water.
(iii) As a laboratory regent.
(iv) For manufacturing fusion mixture (Na2CO3 + K2CO3), borax, sodium phosphate, glass, soap etc.

6. Plaster of Paris, CaSO4. ½H2O: Is prepared by heating gypsum (CaSO4 . 2H2O) at 373 K in a kiln.
2 [CaSO4. 2H2O]      \buildrel {373K} \over\longrightarrow        [CaSO4 . ½H2O] + 3 H2O
Gypsum                                           Plaster of Paris

It may be pointed that heating should be carried our carefully at controlled temperature of 373 K. If temperature of heating exceeds 373 K, there is every possibility of formation of anhydrous calcium sulphate (CaSO4), which possesses no property of Plaster of Paris.

Properties:
(i) A white powder, which abosrbs water with evolution of heat.
(ii) Plaster of Paris (CaSO4 . ½H2O) possesses the property of “setting up” on mixing with water. During this process, heat is evolved and the material quickly sets with a little expansion. The setting of Plaster of Paris is due to its hydration to gypsum.

2 [CaSO4 . ½H2O] + 3 H2O     →     2 [CaSO4 . 2 H2O]
Plaster of Paris                                                              Gypsum

 

Uses:
(i) For making casts for statues, metals etc.
(ii) In orthopedic to maintain bone joints in fixed position.
(iii) In making wall-boards.
(iv) As a fire-proofing material.
(v) For making smooth the surfaces of walls, ceilings, etc. This process is known as POP.
(vi) For making ornate designs on walls, ceilings etc.
(vii) For making toys and decorative materials.
(viii) In laboratories and industries for sealing the air gap in apparatus/equipment so as to make it air tight.

Water of Crystallisation
It is the fixed number of water molecules present in one formula unit of a crystalline salt, e.g..,
Blue vitriol CuSO4. 5H2O                                              Green vitriol FeSO4. 7H2O
Glauber’s salt Na2SO4. 10H2O                                 White vitriol ZnSO4. 7H2O
Gypsum CaSO4. 2H2O                                                    Epsom salt MgSO4. 7H2O

 

 

 

 

 


Leave A Reply

Your email address will not be published.

IBPS Clerk 2017 Video Lecturesx