Crystals of a substance changed their colour on heating in a closed test tube but regained it after some time when were allowed to cool down. name the substance and write it's formula.
a) explain the phenomenon.
b) how is sodium carbonate prepared? give two uses of the compound.

●The Substance Is Hydrated Salt. Like copper sulphate pentahydrate (CuSO4.5H2O). The hydrated form of this compound is blue in color but the anhydrous form is grey-white. 

● PHENOMENON :- the crystals of hydrated salts are heated, they lose their water of crystallisation which is responsible for its color. when it is left to cool down, it absorbs moisture from air and again gains its water of crystallisation and thus, also its color.

●Solvay Process:

The Solvay process is used to manufacture sodium carbonate Na2CO3.

The steps involved in this process are:

1. Purify saturated brine

2. React it with gaseous ammonia

3. The ammoniated brine is carbonated with CO2 to form NaHCO3

4. This is insoluble in brine solution, so can be filtered out.

5. On heating, it decomposes to Na2CO3

6. CaCO3 is also used to provide CO2

 

●Reactions:

• CO2 reacts with ammonia to form ammonium carbonate.

2NH3 + CO2 + H2O --> (NH4)2CO3

• Ammonium carbonate further reacts with CO2 to form ammonium bicarbonate.

(NH4)2CO3 + CO2 + H2O --> 2NH4HCO3 (aq.)

• Ammonium bicarbonate then react with NaCl to form sodium bicarbonate.

NH4HCO3 + NaCl --> NaHCO3 + NH4Cl

Due to exothermic nature of above reactions, solubility of NaHCO3 increases. To counter this effect , the solution is cooled, precipitate of NaHCO3 are separated by vacuum filtration and washed to remove ammonium salts.

• Dry sodium bicarbonate is then heated to give anhydrous sodium carbonate. Carbon dioxide is then again recirculated.

2NaHCO3 --> Na2CO3 + CO2 + H2O

Sodium carbonate (Na2CO3), also known as washing soda, or soda ash, is the monohydrate form as crystal carbonate. It is the water-soluble sodium salt of carbonic acid.

Sodium Carbonate is  prepared by simply heat baking soda or sodium bicarbonate in a 200°F oven for about an hour. Carbon dioxide and water will be given off, leaving dry sodium carbonate. The reactions for this process are as follows.

NaHCO3 ---Heat--->  Na2CO3 + CO2 + H2O

Used as a cleansing agent in industries and household. It is used in the process of softening of water.

Hope this helps! :)

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