Tag: Salts Definitions

Salts:

Salts are produced when an acid reacts with a base and such reactions are known as neutralization reactions.

Family of Salts:

Salts having the same positive or negative radicals are said to belong to the same family.
Examples:

1. NaCl and Na₂SO₄ belong to the family of sodium salts as they have the common positive radical, namely, sodium.
2. KCl and CaCl₂ belong to the family of chloride salts as they both have the common negative radical, namely, chloride.

pH of Salts:

Salts may be neutral, acidic or basic depending upon the strength of the acid and base from which it has been derived or formed.

Neutral Salts:

Salts of a strong acid and a strong base are neutral with pH value of 7.
Examples:

1. NaCl: Obtained from strong acid, HCl and strong base NaOH.
2. K₂SO₄: Obtained from strong acid, H₂SO₄ and strong base KOH.

Acidic Salts:

Salts of a strong acid and weak base are acidic with pH value less than 7.
Examples:

1. NH₄Cl: Obtained from strong acid, HCl and weak base NH₄OH.
2. (NH₄)₂SO₄: Obtained from a strong acid, H₂SO₄ and weak base NH₄OH.

Basic Salts:

Salts of a strong base and weak acid are basic in nature, with pH value of more than 7.
Examples:

1. CaCO₂: Obtained from a weak acid, H₂CO₃, and strong base, Ca(OH)₂^–
2. CH₃COONa: Obtained from a weak acid, CH₃COOH, and strong base, NaOH.

Common Salt:

The salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride and is the salt that is used in food.

Preparing Common Salt

1. Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts by the process of evaporation.
2. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.

Chemicals Obtained from Common Salt:

Common salt is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.

Sodium Hydroxide:

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process 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.
Sodium hydroxide solution is formed near the cathode.

Uses of sodium hydroxide:

• It is used for making soaps and detergents.
• It is used for making artificial textile fibres such as rayon.
• It is used in the manufacture of paper.
• It is used in purifying bauxite ore from which aluminium metal is extracted.
• It is used in degreasing metals, oil refining, and making dyes and bleaches.

Uses of chlorine:

• It is used to sterilize the drinking water supply and the water in swimming pools as it is a disinfectant.
• It is used in the production of bleaching powder.
• It is used in the production of hydrochloric acid.
• It is used to make plastics such as PVC (polyvinyl chloride), pesticides, CFCs, chloroform, paints and dyestuffs.
• It is used for making solvents for dry cleaning (such as trichloroethane).

Uses of hydrogen:

• It is used in the hydrogenation of oils to obtain solid fats called vegetable ghee.
• It is used in the production of hydrochloric acid.
• It is used to make ammonia for fertilizers.
• It is used to make methanol.
• Liquid hydrogen is used as a fuel for rockets.

Uses of hydrochloric acid:

• It is used for cleaning iron sheets before tin plating or galvanisation.
• It is used in the preparation of chlorides such as ammonium chloride which is used in dry cells.
• It is used in medicines and cosmetics.
• It is used in textile, dyeing and tanning industries.
• It is used in making plastics like PVC.

Bleaching Powder:

Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. Bleaching powder is represented as CaOCl₂, though the actual composition is quite complex.
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Properties of bleaching powder

1. It is a white powder that gives a strong smell of chlorine.
2. It is soluble in cold water.
3. It reacts with dilute acids to produce chlorine.

Uses of bleaching powder:

1. It is used for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in the laundry.
2. It is used as an oxidising agent in many chemical industries; and
3. It is used for disinfecting drinking water to make it free of germs.

Example 1.
Name the substance which on treatment with chlorine yields bleaching powder.
Answer:
When dry slaked lime or calcium hydroxide is treated with chlorine gas, we get bleaching powder.
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Baking Soda:

The chemical name of the compound is sodium hydrogen carbonate (NaHCO₃). It is produced using sodium chloride as one of the raw materials.
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

Properties of baking soda:

1. It consists of white crystals which are sparingly soluble in water.
2. It is a mild, non-corrosive base.
3. Its solution in water is mildly alkaline.
4. When heated, it decomposes to give sodium carbonate with the evolution of carbon dioxide gas.
5. The following reaction takes place when it is heated during cooking:
   2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Uses of sodium hydrogen carbonate:

1. 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:

NaFICO(aq) + FI + (from tartaric acid) → CO₂(g) + H₂O(l) + Sodium salt of acid
Carbon dioxide produced during the reaction causes bread or cake to rise to make them soft and spongy.

2. Sodium hydrogen carbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

3. It is also used in soda-acid fire extinguishers.

Example 2.
What will happen if a solution of sodium hydro carbonate is heated? Give the equation of the reaction involved.
Answer:
When a solution of sodium hydrogen carbonate is heated, carbon dioxide is liberated along with the formation of sodium carbonate and water. The equation of the reaction involved is given below:

Washing soda
Sodium carbonate is obtained by heating baking soda and recrystallisation of sodium carbonate gives washing soda (Na₂CO₃10H₂O).
Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O

Properties of Washing Soda:

1. It Is a transparent crystalline solid.
2. It is soluble in water.
3. The solution of washing soda in water is alkaline which turns red litmus blue.
4. It has cleansing properties due to which it is used in detergents.

Uses of washing soda:

1. Sodium carbonate (washing soda) is used in glass, soap and paper industries.
2. It is used in the manufacture of sodium compounds such as borax.
3. Sodium carbonate can be used as a cleaning agent for domestic purposes.
4. It is used for removing the permanent hardness of the water.

Example 3.
Name the sodium compound which is used for softening hard water.
Answer:
The sodium compound which is used for softening hard water is sodium carbonate (Na₂CO₃ 10H₂O).

Plaster of Paris:

On heating gypsum at 373 K, it loses water molecules and
becomes calcium sulphate hemihydrate (CaSO₄ \(\frac{1}{2}\) H₂O).

This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position.

Properties of plaster of Paris:

1. It is a white powder.
2. It has a property of setting into a hard mass on wetting with water which is due to its hydration to form crystals of gypsum which set to form a hard solid mass. Plaster of Paris should therefore be stored in moisture-proof containers.
   CaSO₄. 2 H₂O + 1 \(\frac{1}{2}\) H₂O → CaSO₄.2H₂O

Note: 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:

1. Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth.
2. It is used for setting fractured bones in the right position by doctors.
3. It is used as a fire-proofing material.
4. It is used in chemical laboratories for sealing air gaps in apparatus.

Example 4.
Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:
The plastic of Paris should be stored in moisture-proof containers because if it comes in contact with water, it sets into a hard solid mass, Gypsum.

The equation for the reaction taking place is:
CaSO₄ \(\frac{1}{2}\) H₂O + 1 \(\frac{1}{2}\) H₂O → CaSO₄.2H₂O

Water of Crystallization:

The water of crystallisation is the fixed number of water molecules present in one formula unit of salt. Five water molecules are present in one formula unit of copper sulphate. The chemical formula for hydrated copper sulphate is CuSO₄.5H₂0.

Copper sulphate crystals that seem to be dry contain water of crystallisation. When we heat the crystals, this water is removed and the salt turns white.
CUSO₄.5H₂O → CuSO₄ + 5H₂O

The dehydration of copper sulphate crystals is a reversible process. When water is added to anhydrous copper sulphate, it gets hydrated and turns blue:
CuSO₄ + 5H₂O → CuSO₄

Some examples of hydrated salts:

1. Copper sulphate – CuSO₄.5H₂O
2. Sodium carbonate (washing soda) – Na₂CO₃.10H₂O
3. Calcium sulphate (Gypsum) – CaSO₄.2H₂O
4. Iron sulphate – FeSO₄.H₂O

Class 10 Science Notes