Acids, Bases, Salt -IV (Class X)

Salt -IV

Salts are the ionic compounds which are produced after the neutralization reaction between acid and base.

Family of Salt -

Salts having common acidic or basic radicals are said to belong to same family.

Example:

• Sodium chloride (NaCl) and Calcium chloride (CaCl₂) belong to chloride family.
• Sodium sulphate (Na₂SO₄), Potassium sulphate (K₂SO₄) and Aluminium sulphate (Al₂(SO4)₃) belong to sulphate family.
• Calcium chloride (CaCl₂) and calcium sulphate (CaSO₄) belong to calcium family.
• Zinc chloride (ZnCl₂) and Zinc sulphate (ZnSO₄) belong to zinc family.

Conduction of Electricity -

As we saw that acid and base solution can conduct electricity due to formation of mobile ions.

Similarly when salt is dissolved in water, it ionizes to form ions. And these ions are responsible to conduct electricity same as in the case of acids and bases.

For example, if sodium chloride (NaCl) is dissolved in water, it ionizes to form Na⁺ and Cl^- ions.

NaCl (aq) Na⁺ + Cl^-

Hence salts can also conduct electricity.

pH Value of Acidic, Basic and Neutral Salt -

1.      Neutral Salt -

Salts produced because of reaction between strong acid and strong base are neutral in nature. The pH value of such salts is equal to 7, i.e. neutral.

Examples of such type of salts are sodium chloride, sodium sulphate, potassium chloride, etc.

• Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).  

2.      Basic Salt –

Salts which are formed after the reaction between weak acid and strong base are called basic salt. The pH value of acidic salt is greater than 7.

3.      Acidic salt -

Salts which are formed after the reaction between a strong acid and weak base are called acidic salt. The pH value of acidic salt is lower than 7.

Cause of formation of acidic, basic and neutral salt -

• When equally strong acid and base react they fully neutralize each other. Due to this a neutral salt is formed in this case.
• When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this an acidic salt is formed in this case.
• When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this a basic salt is formed in this case.

Characteristics of salt:

• Most of the salts are crystalline solid.
• Salts may be transparent or opaque.
• Most of the salts are soluble in water.
• Solution of salts conducts electricity. Salts conduct electricity in their molten state also.
• The salt may be salty, sour, sweet, bitter and savory.
• Neutral salts are odourless.
• Salts can be colourless or of coloured.

Water of crystallization

Many salts contain water molecule and are known as hydrated salts. The water molecules present in salt is known as water of crystallization. For example; copper sulphate pentahydrate (CuSO₄.5H₂O), ferrous Sulphate heptahydrate (FeSO₄.7H₂O), Plaster of Paris etc.

1.      Copper sulphate pentahydrate (CuSO₄.5H₂O):

• Blue colour of copper sulphate pentahydrate is due to presence of 5 molecules of water.
• When copper sulphate pentahydrate is heated, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate.

• After adding water; anhydrous copper sulphate becomes blue again.

2.      Plaster of Paris:

Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.

After addition of water, Plaster of Paris is again converted into gypsum.

Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.

     1)    Common Salt (Sodium Chloride) -

Sodium chloride (NaCl) is also known as common salt or table salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid.

NaOH + HCl → NaCl  +  H₂O

• It is a neutral salt.
• The pH value of sodium chloride is about 7.
• Sodium chloride is used in cooking as well as to prepare many other important chemicals such as chemicals used in manufacturing industries.

Methods of obtaining Sodium Chloride -

        1.      From sea water by evaporation: Spread sea water in open. In summer season, water of seawater evaporates in atmosphere and we get sodium chloride.

 2.   From rock salts found in mines: Sodium chloride is mined from deposits which form underground. Deposits of rock salt are usually the remains of inland seas which evaporated thousands or millions of years ago.

Making Other Chemical from Common Salt -

Sodium Hydroxide -

• Sodium hydroxide is a strong base. It is also known as caustic soda.
• It is obtained by the electrolytic decomposition of solution of sodium chloride (brine).
• In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide.
• In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process.

Use of products after the electrolysis of brine:

Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc.

Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid.

Sodium hydroxide is used in de-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.

Bleaching Powder (CaOCl₂) -

Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.

When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed.

Water solution of bleaching powder is basic in nature.

The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect.

Use of bleaching powder:

• Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc.
• Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry.
• Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc.

Baking Soda (NaHCO₃) -

Baking soda is another important product which can be obtained using by products of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO₃) or sodium bicarbonate.

Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.

In this process, calcium carbonate is used as the source of CO₂ and the resultant calcium oxide is used to recover ammonia from ammonium chloride.

Properties of sodium bicarbonate:

• Sodium bicarbonate is white crystalline solid, but it appears as fine powder
• Sodium hydrogen carbonate is amphoteric in nature
• Sodium hydrogen carbonate is sparingly soluble in water
• Thermal decomposition of sodium hydrogen carbonate (baking soda)
• When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water

Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating.

This reaction is known as dehydration reaction.

Use of Baking Soda:

• Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.
• Baking soda is used as antacid.
• Baking soda is used in toothpaste which makes the teeth white and plaque free.
• Baking soda is used in cleansing of ornaments made of sliver.
• Since, sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher.

Baking powder -

Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter.

Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder.

When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty.

Washing Soda (Sodium carbonate) -

Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process. The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate.

Since there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate.

Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water.

Use of sodium carbonate:

• It is used in cleaning of cloths; especially in rural areas.
• In making of detergent cake and powder.
• In removing permanent hardness of water.
• It is used in glass and paper industries.

Plaster of Paris:

Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.

After addition of water, Plaster of Paris is again converted into gypsum.

Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.

Base & Alkali - Alkalis are water soluble bases.

Bases in which complete dissociation of hydroxide ion takes place are called strong base. In alkali; complete dissociation of hydroxide ions takes place so they are considered as strong base. But it is not perfectly well defined that which substance is consider in category of Alkali and which is not.