Saturday, September 18, 2021

Acids, Bases and Salts | Class 10 CBSE | Web Notes | Part 4 | More about Salts

ACIDS, BASES AND SALTS

MORE ABOUT SALTS


Family of Salts

Salts having same positive or negative radicals belong to a family. E.g. NaCl & Na2SO4 belong to family of sodium salts. NaCl & KCl belong to the family of chloride salts.


Salts & their Chemical formulae

Formed from which Acids & bases?

Potassium sulphate (K2SO4)

H2SO4 & KOH

Sodium sulphate (Na2SO4)

H2SO4 & NaOH

Calcium sulphate (CaSO4)

H2SO4 & CaCO3

Magnesium sulphate (MgSO4)

H2SO4 & Mg(OH)2

Copper sulphate (CuSO4)

H2SO4 & Cu(OH)2

Sodium chloride (NaCl)

HCl & NaOH

Sodium nitrate (NaNO3)

HNO3 & NaOH

Sodium carbonate (Na2CO3)

H2CO3 & NaOH

Ammonium chloride (NH4Cl)

HCl + NH4OH

 

pH of Salts

Salts of a strong acid & a strong base are neutral (pH = 7).

Salts of a strong acid and weak base are acidic (pH < 7).

Salts of a strong base and weak acid are basic (pH > 7).

pH of some salt samples (soluble in distilled water) and the acid & base used to form the salts are given below:

Salt

pH

Acid used

Base used

Sodium chloride

7

HCl

NaOH

Potassium nitrate

7

HNO3

KOH

Aluminium chloride

7

HCl

Al(OH)3

Zinc sulphate

7

H2SO4

Zn(OH)2

Copper sulphate

< 7

H2SO4

Cu(OH)2

Sodium acetate

> 7

CH3COOH

NaOH

Sodium carbonate

> 7

H2CO3

NaOH

Sodium hydrogen carbonate

> 7

H2CO3

NaOH


Chemicals from Common Salt

Common salt (sodium chloride, NaCl) is a neutral salt formed by the reaction of HCl & NaOH solution.

NaCl is separated from seawater containing many salts.

Deposits of large crystals of solid salt are also found in several parts. These are often brown due to impurities. This is called rock salt.

There are beds of rock salt formed when seas of bygone ages dried up. Rock salt is mined like coal.

Common salt is used in food. It is also a raw material to make sodium hydroxide, baking soda, washing soda, bleaching powder etc.


1. Sodium hydroxide (NaOH)


When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide. It is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.

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

Chlorine gas is given off at the anode, and hydrogen gas at the cathode. NaOH solution is formed near the cathode.

All three products are useful.

Uses of NaOH: De-greasing metals, soaps & detergents, paper making, artificial fibres etc.

Uses of Cl2: For water treatment, in swimming pools, PVC, disinfectants, CFCs, pesticides etc.
Uses of H2: Fuels, margarine, ammonia for fertilisers.


Important products from the chlor-alkali process


2. Bleaching powder (CaOCl2)


It is manufactured by using chlorine produced during the electrolysis of brine.

Chlorine acts on dry slaked lime to give bleaching powder.


Ca(OH)2 + Cl2 CaOCl2 + H2O

Actual composition of CaOCl2 is quite complex.

Uses of Bleaching powder:

  • For bleaching cotton & linen in the textile industry.
  • For bleaching wood pulp in paper factories.
  • For bleaching washed clothes in laundry.
  • As an oxidising agent in many chemical industries.
  • To make drinking water free from germs.


3. Baking soda


Chemical name: Sodium hydrogencarbonate (NaHCO3).

It is produced using sodium chloride.


It is a mild non-corrosive basic salt.

When it is heated for cooking, the following reaction occurs.


Uses of Baking soda:

  • To make tasty crispy pakoras, etc.
  • It is added for faster cooking.
  • To make baking powder (baking soda + mild edible acid such as tartaric acid). When baking powder is heated or mixed in water, the following reaction occurs:

NaHCO3 + H+         CO2 + H2O + Sodium salt of acid

                                     (From any acid)

CO2 causes bread or cake to rise making soft and spongy.

  • It is an ingredient in antacids. Being alkaline, it neutralises excess acid (acidity) in stomach.
  • Used in soda-acid fire extinguishers.


4. Washing soda (Na2CO3.10H2O)


A basic salt obtained from NaCl.

NaCl → NaHCO3 → Na2CO3 (sodium carbonate).

Recrystallisation of sodium carbonate → washing soda.

Na2CO3     +     10H2O     →    Na2CO3.10H2O

Uses of washing soda:

  • Used in glass, soap and paper industries.
  • To manufacture sodium compounds such as borax.
  • Used as a cleaning agent for domestic purposes.
  • For removing permanent hardness of water.

Are the Crystals of Salts really Dry?

Heat few copper sulphate crystals in a dry boiling tube.

Copper sulphate becomes white and some water droplets appear in the boiling tube.

When 2-3 drops of water are added, copper sulphate crystals restore its blue colour.

Copper sulphate crystals contain water of crystallisation. When heated the crystals, this water is removed and the salt turns white.


Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.

Five water molecules are present in one formula unit of hydrated copper sulphate (Cu SO4. 5H2O).

Similarly, the molecule of Na2CO3.10H2O is wet.

Gypsum is also a salt possessing water of crystallisation (CaSO4.2H2O).


Plaster of Paris


On heating gypsum at 373 K, it loses water molecules and becomes a white powder. This is called Plaster of Paris or Calcium sulphate hemihydrate (CaSO4. ½ H2O).

Plaster of Paris on mixing with water, it changes to gypsum again giving a hard solid mass.

CaSO4. ½ H2O +1½ H2O CaSO4.2H2O

(Plaster of Paris)                  (Gypsum)

Only half a water molecule is shown to be attached as water of crystallisation. It is written in this form because two formula units of CaSO4 share one molecule of water.

Uses of Plaster of Paris:

  • Used as plaster to support fractured bones.
  • To make toys and materials for decoration.
  • To make surfaces smooth.

Plaster of Paris gets its name from large gypsum deposits in Montmartre in Paris.


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