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Salts and their Properties

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Salts and their Properties - Lesson Summary

  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.



Classification of salts:
Based on nature the salts have been classified into different types. They are:
Normal salts
Acidic salts
Basic salts
Double salts
Complex salts


Normal salts:
These salts are formed by the complete replacement of hydrogen in acids by other metal cations from the bases.
NaCl is normal salt formed by the reaction of HCl with NaOH.
HCl + NaOH → NaCl + H2O

Acidic salts:
Salts which are formed by the partial replacement of hydrogens atoms of acids are called acidic salts.
Example:
NaHSO4 is formed when partial replacement of hydrogen atoms by the sodium atoms of base.
H2SO4 + NaOH → NaHSO4 + H2O
In general these salts formed when the reacted base is not sufficient for the neutralisation of acid.

Basic salts:
Salts which are formed by the partial replacement of hydroxyl group are called basic salts.
Example: 
Ca(OH)Cl is formed by the partial replacement of hydroxide group from Ca(OH)2 by chloride ions of acid.
Ca(OH)2 + HCl → Ca(OH)Cl + H2O
In general these salts formed when the reacted acid is not sufficient for the neutralisation of base.

Table below giving neutral, acidic and basic salts.

Type of Salt Type of Acid Type of Base Example          Neutral
        pH = 7   Strong Acids
  Examples:
  HCl
  H 2SO 4   Strong Bases
  Examples:
  NaOH
  KOH   NaCl
  K 2SO 4         Acidic
       pH < 7   Strong Acids
  Examples:
  HCl
  HNO 3   Strong Bases
  Examples:
  NH 4OH
  Mg(OH) 2   NH 4Cl
  Mg(NO 3) 2         Basic
        pH > 7   Weak Acids
  Examples:
  H 2CO 3
  CH 3COOH   Strong Bases
 Examples: 
  NaOH
  KOH   Na 2CO 3
  CH 3COOK



Double salts: 
Salts that are formed by mixing of two simple salts which are obtained crystallisation.
Example:
Potash alum - K2SO4 Al2 (SO4)3 .24H2O
Dolomite - CaCO3.MgCO3


Complex salts:
The salts which contains different types of metal atoms which on hydrolysis produces complex ions along with simple ions are called complex salts.
Example:
[Ag(NH3)2]Cl  ⇄  [Ag ( NH3 )2 ]+ + Cl-


Salts in our daily life:
Baking soda

Chemical name: Sodium hydrogen carbonate
Molecular formula: 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.

Washing soda:
Molecular formula:  Na2CO3.10H2
Chemical formula: Sodium carbonate.decahydrate
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 + 10H2O → Na2CO3.10H2O
In general sodium carbonate is prepared by passing CO2 gas through concentrated NaOH.
2NaOH + CO2 → Na2CO3 + H2O

Properties:
It is a white crystalline solid. It exists as decahydrate of sodium carbonate.
When exposed to dry air and heating it loses water molecules to change into anhydrous form.
Na2CO3.10H2O + Exposure to open dry air → Na2CO3.H2O + 9H2
Na2CO3.H2O + Heating → Na2CO3
It is soluble in water and during dilution heat will releases out.
On reaction with acids sodium carbonate releases carbon dioxide along with the formation of sodium salts and water.
Na2CO3 + HCl → 2NaCl + H2O + CO2

Sodium carbonate is used to manufacture of glass, cleansing agents, soap, glass and paper.


Bleaching powder (CaOCl2):
Bleaching powder chemically known as calcium oxy chloride.
It is prepared by the reaction between chlorine and slaked lime at about 40 0C.
Ca(OH)2 + Cl2 → Ca(OCl)Cl + H2O + Cl2
Ca(OH)2 + H2SO4 → CaSO4 + H2O + Cl2

It acts a strong oxidising agent to bleach substances.
CaOCl2 + KNO2 → CaCl2 + KNO3
CaOCl2 + H2S → CaCl2 + H2O + S
It is used to bleach cotton, linen textiles and wood pulp.
Coloured matter + Bleaching powder → Colourless product 
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.
Example:
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  Na 2CO 3.10 H 2O  Zinc Sulphate heptahydrate or White vitriol  ZnSO 4.7H 2O  Magnesium sulphate heptahydrate or Epsom salt  MgSO 4.7H 2O  Potash alum  K 2SO 4 Al 2 (SO 4) 3 .24H 2O  Copper (II) sulphate pentahydrate or Blue vitriol  CuSO 4.5H 2O  Calcium sulphate dihydrate or Gypsum  CaSO 4.2H 2O


Plaster of paris (CaSO4. 1 2 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    373 K →      CaSO4  1 2 H2O      +    1 1 2 H2O    
  Gypsum                         Plaster of Paris            Water         

Uses:
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.  

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