Salts of a strong acid and a strong base are neutral with a pH value of 7. On the other hand, salts of a strong acid and weak base are acidic with a pH value less than 7, and salts of a strong base and weak acid are basic in nature, with a pH value of more than 7.
What is a Salt?
Salt is a substance formed by the reaction of an acid with a base. Salt contains a positive ion or cation of a base and a negative ion or anion of an acid. Salts are ionic compounds. We use the term ‘salt’ for common table salt as well, which is also known as sodium chloride.
Chemicals from Common Salt
By now you have learned that the salt formed by the combination of hydrochloric acid and sodium hydroxide is called sodium chloride. This is the salt that you use in food. Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world.
These large crystals are often brown due to impurities and are called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal. You must have heard about Mahatma Gandhi’s Dandi March. Did you know that sodium chloride was such an important symbol in our struggle for freedom?
Common salt — A raw material for chemicals
The common salt thus obtained is an important raw material for various substances of daily use, such as:
- Sodium hydroxide
- Baking soda
- Washing soda
- Bleaching powder
Let us see how one substance is used for making all these different substances.
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide.
2NaCl + 2H2O → Cl2+ H2 + 2NaOH
The process is called the chlor-alkali process because of the products formed where chlor stands for chlorine and alkali stands for sodium hydroxide.
The reaction being Sodium Chloride reacts with water to form NaOH aqueous plus chlorine and hydrogen gas.
Chlorine gas is given off at the anode, the reaction being:
2Cl– → Cl2 + 2e–
and hydrogen gas at the cathode, the reaction being:
2H2O + 2e– → H2 + 2OH–
Sodium hydroxide solution is formed near the cathode. It is important to note that the three products produced in this process are all useful in our daily lives.
You already know that chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder.
Bleaching powder is produced by the action of chlorine on dry slaked lime that is Ca(OH)2. Bleaching powder is represented as CaOCl2 though the actual composition is quite complex.
The reaction being:
Ca(OH)2 + Cl2 → CaOCl2 + H2O
Let us now understand some uses of bleaching powder
Bleaching powder is used – for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories, and for bleaching washed clothes in laundry; It is used as an oxidizing agent in many chemical industries, and for disinfecting drinking water to make it free of germs.
The soda commonly used in our kitchen for making tasty crispy pakoras is baking soda. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogen carbonate (NaHCO3). It is produced using sodium chloride as one of the raw materials.
The reaction being:
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3 (sodium hydrogen carbonate )
It is important to understand that sodium hydrogen carbonate is a mild non-corrosive base.
The following reaction takes place when it is heated during cooking –
2NaHCO3 on heating → Na2CO3 (sodium carbonate) +H2O+CO2
Sodium hydrogen carbonate has got various uses in the household.
Let us now study some uses of sodium hydrogen carbonate:
It is used for making baking powder, which is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid.
When baking powder is heated or mixed in water, the following reaction takes place –
NaHCO3 + H+ (from any acid)→ CO2 + H2O + Sodium salt of acid
(i) Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.
(ii) Sodium hydrogen carbonate is also an ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
(iii) It is also used in soda-acid fire extinguishers.
Another chemical that can be obtained from sodium chloride is Na2CO3.10H2O (washing soda).
You have seen above that sodium carbonate can be obtained by heating baking soda; recrystallization of sodium carbonate gives washing soda.
It is also a basic salt.
The reaction being:
Na2CO3 + 10H2O → Na2CO3.10H2O
In this equation, can you tell what does 10H2O signifies?
Does it make Na2CO3 wet?
Sodium carbonate and sodium hydrogen carbonate are useful chemicals for many industrial processes as well.
Let us understand the uses of washing soda:
(i) Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
(ii) It is used in the manufacture of sodium compounds such as borax.
(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.
(iv) It is used for removing the permanent hardness of the water.