NCERT SCIENCE CLASS 10- ACIDS BASES AND SALTS- NOTES-part-5- CHEMICALS FROM COMMON SALT
 |
| NCERT SCIENCE CLASS 10- ACIDS BASES AND SALTS- NOTES-part-5- CHEMICALS FROM COMMON SALT |
In the previous parts of chapter acids, bases and salts, you have learnt the chemical properties of acids and bases. In this part you will be learning some of the important compounds formed from sodium chloride and its properties.
Sodium chloride (NaCl) is formed by the combination of sodium hydroxide(NaOH) and hydrochloric acid(HCl). Common salt is commonly used in the kitchen for cooking purposes. It is mainly found in sea water. You have studied it earlier as a neutral salt because it is the salt of a strong base and a strong acid. The seawater contains many salts dissolved in it and NaCl is one of them.
Common salt can be extracted just by storing the seawater in salt fields so that water evaporates leaving behind the salts. Deposits of solid salt are also found in various places of earth.The salt obtained like this is called rock salt which is brown in colour due to the presence of impurities in it. It can be purified to get common salt.
 |
| ROCK SALT |
COMPOUNDS FROM COMMON SALT
Common salt is a raw material for making many chemical compounds. Some of them are sodium hydroxide, baking soda, washing soda and bleaching powder.
SODIUM HYDROXIDE
When common salt is dissolved in water, its solution is called brine and when it is electrolyzed by passing the electric current, it dissociates into sodium ions and chlorine ions. Sodium ions are deposited at cathode and chloride ions are deposited at anode where it forms chlorine gas. So chlorine gas is liberated at anode and sodium ions that are liberated at cathode combine with water to form sodium hydroxide and hydrogen gas.
2 NaCl + 2 H2O → 2NaOH + Cl2 + H2
So three substances are formed by the electrolysis of aqueous solution of NaCl. They are hydrogen gas, chlorine gas and NaOH.
Uses of H2, Cl2 and NaOH
All of them are very important raw materials for making so many useful things.
- Hydrogen gas is used as a fuel and it gives a large amount of energy on burning.
- It is also used for making margarine for dehydrogenation of vegetable oils.
- Chlorine gas as you know it is used to disinfect water and also in the swimming pools for disinfecting water.
- For making poly vinyl chloride .
- To make chlorofluorocarbons used in refrigerators and also used to make pesticides.
- Hydrogen and chlorine will combine together and form HCl gas. It is used for cleaning steel, for making cosmetics, medicines and ammonium chloride.
- NaOH is used for degreasing metals, for making soaps and detergent.
- It is also used for making paper and artificial fibers.
- NaOH and Cl2 are used to make bleach and is used to make household bleach and for bleaching fabric.
BLEACHING POWDER
Another important compound is bleaching powder. Its formula is CaOCl2. It is also called calcium oxychloride. It is formed by passing chlorine gas over Ca(OH)2.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
- Bleaching powder is widely used for bleaching cotton and linen in the textile industry.
- It is used in the paper industry also for bleaching wood pulp.
- In the laundry, it is used for bleaching washed clothes to remove any stain on them.
- It is also used as an oxidizing agent in many chemical industries.
- It is used to disinfect drinking water to make it germ-free.
BAKING SODA
The next compound which
can be prepared from common salt is baking soda. Baking soda is commonly used
in the kitchen for baking bread, baking cakes and for cooking. Baking soda is a
general name and chemically it is called sodium hydrogen carbonate and its formula
is NaHCO3.
It is prepared by a reaction
between NaCl, carbon dioxide, ammonia and water.
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3
Sodium hydrogen carbonate and ammonium chloride are formed. On heating sodium hydrogen carbonate, it forms sodium carbonate and water.
2 NaHCO3 → Na2CO3 + H2O + CO2
You have already studied baking soda and its basic nature. How it is used to treat acidity in the stomach? It is also used for baking bread and cakes.
BAKING POWDER
 |
| BAKING BREAD |
Baking powder is a
mixture of sodium hydrogen carbonate and tartaric acid. When baking soda is mixed with water, it liberates carbon
dioxide gas which makes the dough rise and makes the bread and the cake spongy
and fluffy. So it is widely used in baking.
NaHCO3 + H⁺ → CO2 + H2O + Sodium salt of acid
It is also used as an antacid to remove acidity in the stomach and it is also used for fire fighting in soda
acid fire extinguishers. It liberates carbon dioxide gas by reacting with an acid
which extinguishes the fire.
WASHING SODA
The next compound which can be prepared from sodium chloride(NaCl) is washing soda(Na2CO3.10H2O).
As we have
discussed, sodium hydrogen carbonate(NaHCO3) is formed from NaCl, carbon dioxide and
ammonia. On heating NaHCO3, it forms sodium carbonate(Na2CO3). This sodium
carbonate is dissolved in water and is recrystallized. On recrystallization,
it forms washing soda which is Na2CO3 .10 H2O. In the formula, there are 10 molecules
of water present. These molecules of water are called water of
crystallisation and they form a part of the crystal structure.
Na2CO3 + 10 H2O → Na2CO3.10H2O
- Washing soda is used in various places like in the glass, soap and paper industries.
- It is used in manufacturing sodium compounds like borax.
- It is used as a cleansing agent in homes.
- It removes the permanent hardness of the water. It makes the permanent hard water to soft water.
WATER OF CRYSTALLIZATION
We will discuss some more compounds which have water of crystallization.
Are the crystals of salts really dry? They are dry, but some crystals of salts of
some compounds have water molecules that become a part of the crystal
structure.
The water of crystallization is the number of water molecules attached to one formula unit of salt.
ACTIVITY
Let us, first of all, perform an activity. Take a dry test tube and take copper sulphate(CuSO4.5H2O) crystals in it which are blue in colour. Hold it with the help of a holder and heat it over the flame of the bunsen burner. After some time it is found that the blue colour changes to white. It forms anhydrous copper sulphate(CuSO4).
Anhydrous means the salt from which water molecules have been removed.
The salt in which there is the water
of crystallization in the crystal is called hydrated salt. Now on this white
salt, if water drops of water are sprinkled, it again changes back to blue.
PLASTER OF PARIS
We have already studied Na2CO3. 10 H2O. There is another example- gypsum CaSO4 . 2 H2O. It has 2 molecules of water in the crystal. When gypsum is heated to about 373 Kelvin, it forms Plaster of Paris(POP). The plaster of Paris is calcium sulphate hemihydrate. So on heating gypsum, we get calcium sulphate hemihydrate.
CaSO4.2H2O → CaSO4.1/2H2O + 11/2 H2O
Now you will be surprised why it is half H2O. Actually, one water molecule is shared by two calcium sulphate
molecules. We can also write it like this.
2CaSO4.H2O
Here 2 molecules of
calcium sulphate share one molecule of water.
As you all know, the Plaster of Paris is used by doctors to make the plaster over the fractured bones to cure them. Actually what happens when plaster of Paris is mixed with water. It forms gypsum again which is a hard mass. It is called setting the Plaster of Paris.
CaSO4.1/2H2O + 1 1/2H2O → CaSO4.2H2O
 |
| PLASTER |
It is also used to make toys and statues which are used for decorating the
houses and you must have heard about POP walls and the ceilings. So it is also used
for decorating the walls and the ceilings of the houses.
 |
| POP STATUE |
 |
| POP CEILING |
In chapter acids, bases and salts, there are many activities involved. There were many application-level questions from these topics in the previous years' board exams. The formula of all important salts to be studied. More emphasis is to be given to the method of preparation of each salt and its uses.
No comments:
Post a Comment