Salts and Other Important Compounds

Salts

Salts are chemical compounds that result from the reaction between an acid and a base. They are formed when the hydrogen ions (H⁺) of an acid are replaced by metal ions or other positively charged ions. Salts play essential roles in chemistry, biology, and daily life. Here are some key characteristics and explanations of salts:

1. Ionic Compound: Salts are ionic compounds, meaning they are composed of positively charged ions (cations) and negatively charged ions (anions) held together by electrostatic forces. Typically, the cation comes from a metal or ammonium ion (NH₄⁺), and the anion comes from the acid.
2. Neutralisation Reaction: The formation of salt involves a neutralisation reaction, where the acidic properties of the hydrogen ions in the acid are neutralised by the basic properties of the hydroxide ions (OH^-) or other anions from the base. This results in the formation of water (H₂O) and salt.
3. Types of Salts: Salts can vary widely based on the specific acid and base involved in their formation. Some common types of salts include chlorides (from hydrochloric acid), sulphates (from sulfuric acid), carbonates (from carbonic acid), nitrates (from nitric acid), and acetates (from acetic acid). For example, sodium chloride (NaCl) is a chloride salt formed by the reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH).
4. Solid at Room Temperature: Most salts are solid at room temperature and have high melting and boiling points. They form crystalline structures due to the arrangement of ions in a repeating pattern.
5. Water Solubility: Many salts are highly soluble in water, which allows them to dissociate into their constituent ions when dissolved. This property makes them important in various chemical and biological processes.
6. Electrical Conductivity: Aqueous solutions of salts are good conductors of electricity because they contain mobile ions that can carry electric charges. This property is essential in electrolysis, battery chemistry, and biological processes like nerve signalling.
7. pH of Salt Solutions: While salts themselves are generally neutral in pH, some salt solutions can be acidic, basic, or neutral, depending on the nature of the cation and anion. Salts formed from strong acids and strong bases produce neutral solutions. Salts from strong acids and weak bases produce acidic solutions, while salts from weak acids and strong bases produce basic solutions.
8. Diverse Applications: Salts have numerous practical applications in various fields. They are used in food preservation, seasoning, water softening, agriculture (as fertilisers), chemical manufacturing, pharmaceuticals, cleaning products, and more.
9. Natural Occurrence: Many salts are naturally occurring minerals found in the Earth's crust, often in the form of mineral deposits or evaporated sea salts.
10. Biological Importance: Salts are essential for biological systems. Sodium chloride (table salt) and potassium chloride, for example, are vital for nerve function and muscle contractions in humans.

Neutral, Acidic and Basic Salts

Neutral, acidic, and basic salts are categories of salts based on their pH properties, which are determined by the nature of the acid and base used to form the salt.

Neutral Salts

1. Formation: Neutral salts are formed through the neutralisation of a strong acid with a strong base. In this type of reaction, the acidic properties of the hydrogen ions (H⁺) in the acid are neutralised by the basic properties of the hydroxide ions (OH^-) from the base. This results in the formation of water (H₂O) and salt.
2. Examples: Sodium chloride (NaCl), potassium sulphate (K₂SO₄), and calcium nitrate (Ca(NO₃)₂) are common examples of neutral salts.
3. Characteristics: Neutral salts do not contain replaceable hydrogen ions (H⁺). When dissolved in water, they produce a solution with a pH close to 7, indicating a neutral or near-neutral solution.
4. Reaction Example: NaOH (strong base) + HCl (strong acid) → NaCl (neutral salt) + H₂O (water)
   In this reaction, the strong base (Sodium hydroxide, NaOH) and the strong acid (Hydrochloric acid, HCl) neutralise each other, forming water and a neutral salt (Sodium chloride, NaCl).

Acidic Salts

1. Formation: Acidic salts are formed through the neutralisation of a strong acid with a weak base. In this case, the acid is still strong, but the base used is weaker, so not all of the hydrogen ions (H⁺) from the acid are completely neutralised by the weak base. This results in a solution that has an excess of H⁺ ions and is acidic.
2. Examples: Ammonium chloride (NH₄Cl) and sodium hydrogen sulphate (NaHSO₄) are examples of acidic salts.
3. Characteristics: Acidic salts contain replaceable hydrogen ions (H⁺), which can be released in an aqueous solution.
   When dissolved in water, they produce a solution with a pH less than 7, indicating an acidic solution.
4. Reaction Example:
   NH₄OH (weak base) + HCl (strong acid) → NH₄Cl (acidic salt) + H₂O (water)
   In this reaction, the weak base (Ammonium hydroxide, NH₄OH) does not completely neutralise the strong acid (Hydrochloric acid, HCl), resulting in the formation of an acidic salt (Ammonium chloride, NH₄Cl).

Basic Salts

1. Formation: Basic salts are formed through the neutralisation of a strong base with a weak acid. In this case, the base is strong, but the acid used is weaker, so not all of the hydroxide ions (OH^-) from the base are neutralised by the weak acid. This results in a solution that has an excess of OH^- ions and is basic (alkaline).
2. Examples: Sodium acetate (NaCH₃COO) and potassium carbonate (K₂CO₃) are common examples of basic salts.
3. Characteristics: Basic salts contain replaceable hydroxide ions (OH^-), which can ionise in an aqueous medium. When dissolved in water, they produce a solution with a pH greater than 7, indicating a basic (alkaline) solution.
4. Reaction Example:
   CH₃COOH (weak acid) + NaOH (strong base) → NaCH₃COO (basic salt) + H₂O (water)
   In this reaction, the weak acid (Acetic acid, CH₃COOH) is not fully neutralised by the strong base (Sodium hydroxide, NaOH), leading to the formation of a basic salt (Sodium acetate, NaCH₃COO).

Some Important Compounds

1. Sodium Chloride (NaCl)

NaCl, or sodium chloride, is commonly known as table salt or common salt. It is a chemical compound composed of two elements: sodium (Na) and chlorine (Cl).

1. Chemical Composition: Common salt is composed of two elements, sodium (Na) and chlorine (Cl), in a 1:1 ratio. It is a chemical compound formed by the chemical bonding of sodium and chlorine atoms.
2. Appearance: Common salt typically exists as small, white, crystalline grains or as a fine powder. It is colourless, odourless, and has a salty taste.
3. Natural Occurrence: Salt is one of the most abundant minerals on Earth and is found in various forms in nature. It occurs in large deposits underground, in seawater, in salt pans, and in some rocks and minerals. The salt obtained from seawater or salt mines is commonly used for various purposes.
4. Preparation: Common salt can be prepared through several methods, but one common method is the extraction of salt from seawater or salt mines. Seawater is collected and allowed to evaporate, leaving behind salt crystals, which are then harvested and purified. Salt can also be obtained through mining salt deposits in underground mines.

Properties and Uses

1. Seasoning and Flavouring: Common salt is primarily used as a seasoning and flavour enhancer in cooking. It adds a salty taste to food, which is a fundamental and widely appreciated flavour.
2. Preservation: Salt has been used for centuries as a natural preservative. It helps extend the shelf life of various food items, such as meats and fish, by inhibiting the growth of spoilage-causing microorganisms. The process of preserving food with salt is known as "salting" or "curing."
3. Chemical Industry: Sodium chloride is a crucial raw material in the chemical industry. It is used in the production of various chemicals, including sodium hydroxide (caustic soda), sodium carbonate (soda ash), and chlorine gas. These chemicals have numerous industrial applications.
4. Water Softening: Salt is used in water softeners to remove calcium and magnesium ions from hard water, improving the quality of water for domestic and industrial purposes.
5. De-Icing: Salt is commonly used for de-icing roads and walkways during winter. When spread on icy surfaces, it lowers the freezing point of water, causing the ice to melt and making travel safer.
6. Food Processing: In addition to seasoning and preservation, salt is used in various food processing applications, such as cheese production, baking, and the manufacture of processed foods like snacks and canned goods.
7. Health: Sodium chloride is an essential nutrient for the human body, providing the necessary sodium and chloride ions for bodily functions. However, excessive salt consumption is associated with health risks, including high blood pressure and cardiovascular diseases. Therefore, moderation in salt intake is recommended for maintaining good health.

2. Sodium Hydroxide (NaOH)

Sodium hydroxide (NaOH), commonly known as caustic soda or lye, is a highly versatile and important chemical in various industries.

1. Production of Sodium Hydroxide: Sodium hydroxide is primarily produced through the electrolysis of a concentrated aqueous solution of sodium chloride (common salt), a process known as the chlor-alkali process. The reaction occurs as follows:
2. At the Anode (Positive Electrode): 2NaCl (aq) → Cl₂ (g) + 2Na⁺ (aq) + 2e^-
   Chlorine gas (Cl₂) is produced at the anode.
3. At the Cathode (Negative Electrode): 2H₂O (l) + 2e^- → H₂ (g) + 2OH^- (aq)
   Hydrogen gas (H₂) is produced at the cathode, and hydroxide ions (OH^-) are formed in the solution.
4. Overall Reaction: 2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)
   Sodium hydroxide (NaOH) is formed in the process, along with chlorine gas and hydrogen gas. These products are collected separately.

Uses of Sodium Hydroxide

Sodium hydroxide has a wide range of applications in various industries due to its strong alkaline properties and ability to react with various substances. Some important uses include:

1. Soap and Detergent Manufacturing: Sodium hydroxide is a crucial ingredient in soap production. It reacts with fats and oils through a process called saponification to form soap. It is also used in detergent production for cleaning purposes.
2. Textile Industry: Sodium hydroxide is used in the production of synthetic fibres like rayon and in mercerization, a process that improves the properties of cotton fibres.
3. Paper Manufacturing: It is used in the pulping and bleaching of wood pulp in papermaking.
4. Aluminium Extraction: Sodium hydroxide is employed in refining bauxite ore to extract aluminium metal in the aluminium industry.
5. Metal Degreasing: It is used for cleaning and degreasing metals, making them ready for further processing or coating.
6. Oil Refining: In the petroleum industry, sodium hydroxide is used in the refining of crude oil to remove impurities and sulphur compounds.
7. Chemical Industry: It serves as a strong base and reagent in various chemical processes, including the production of other chemicals such as sodium carbonate (soda ash) and sodium bicarbonate (baking soda).
8. Water Treatment: Sodium hydroxide is used in water treatment to adjust pH levels, neutralise acidic wastewater, and remove heavy metals from industrial effluents.
9. Food Preparation: In food processing, it is used for purposes like curing, peeling fruits and vegetables, and regulating acidity in some food products.
10. Cleaning Agents: Sodium hydroxide is an essential ingredient in various household and industrial cleaning products.

3. Sodium Hydrogen Carbonate (NaHCO₃)

Baking soda, also known as sodium hydrogen carbonate or sodium bicarbonate, is a chemical compound with the formula NaHCO₃.

Production of Sodium Hydrogen Carbonate (Baking Soda):

Sodium hydrogen carbonate is produced by reacting a cold and concentrated solution of sodium chloride (brine) with ammonia (NH₃) and carbon dioxide (CO₂):
NaCl + NH₃ + H₂O + CO₂ → NaHCO₃ + NH₄Cl
Sodium chloride (common salt) in brine reacts with ammonia and carbon dioxide to form sodium hydrogen carbonate (baking soda).

Properties of Sodium Hydrogen Carbonate

1. Sodium hydrogen carbonate exists as white crystals that are sparingly soluble in water.
2. It is a mild, non-corrosive base, and its aqueous solution is mildly alkaline.
3. When heated, sodium hydrogen carbonate decomposes, releasing carbon dioxide gas. This property makes it useful in baking, as the carbon dioxide released causes the dough to rise and become soft and spongy.
   2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Uses of Sodium Hydrogen Carbonate (Baking Soda)

1. Antacid: Sodium hydrogen carbonate is used as an antacid in medicine to relieve stomach acidity. It neutralises excess stomach acid and helps alleviate indigestion.
2. Baking: Baking soda is used in baking as a raising agent. When mixed with an acidic ingredient, such as yoghurt or vinegar, it reacts to release carbon dioxide gas, causing dough or batter to rise and become light and fluffy.
3. Fire Extinguisher: Baking soda is used in certain types of fire extinguishers, known as soda-acid fire extinguishers. These extinguishers contain sodium hydrogen carbonate and sulfuric acid, which react to produce carbon dioxide gas. The gas forms a blanket around the fire, cutting off its oxygen supply and extinguishing it.

4. Sodium Carbonate Decahydrate (Na₂CO₃·10H₂O)

Washing soda, chemically known as sodium carbonate decahydrate (Na₂CO₃·10H₂O), is a versatile chemical compound with various applications.

Production of Washing Soda:

Washing soda is derived from sodium chloride (common salt) through a series of chemical reactions:

1. Formation of Sodium Hydrogen Carbonate (Sodium Bicarbonate): A cold and concentrated solution of sodium chloride (brine) is reacted with ammonia (NH₃) and carbon dioxide (CO₂).
   Sodium hydrogen carbonate, which is only slightly soluble in water, precipitates out as a solid.
2. Conversion to Sodium Carbonate (Soda Ash): The sodium hydrogen carbonate obtained in the first step is separated by filtration, dried, and then heated.
   Heating sodium hydrogen carbonate causes it to decompose, forming sodium carbonate:
   2NaHCO₃ → Na₂CO₃ + CO₂ + H2O
   The anhydrous sodium carbonate obtained in this step is referred to as soda ash.
3. Crystallisation to Form Washing Soda: Anhydrous sodium carbonate (soda ash) is dissolved in water and then recrystallised to produce washing soda crystals containing ten molecules of water of crystallisation:
   Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O

Properties of Washing Soda

1. Washing soda is a transparent crystalline solid.
2. It is one of the few metal carbonates that are soluble in water.
3. The solution of washing soda in water is alkaline, turning red litmus paper blue.
4. Washing soda exhibits detergent properties, making it effective in removing dirt and grease from various surfaces, especially clothes. It attacks dirt and grease, forming water-soluble products that are easily rinsed away.

Uses of Sodium Carbonate (Washing Soda)

1. Laundry Detergent: Sodium carbonate is widely used as a cleansing agent in laundry detergents and is a component of many dry soap powders. It helps in the removal of stains, dirt, and grease from clothing.
2. Water Softening: Sodium carbonate is used to remove the permanent hardness of water by precipitating calcium and magnesium ions as their carbonates, which can then be filtered out.
3. Glass Manufacturing: It is a crucial ingredient in the manufacture of glass. Sodium carbonate lowers the melting point of silica, making it easier to melt and shape glass.
4. Soap Production: Sodium carbonate is used in soap production, where it acts as a pH regulator and a water softener.
5. Papermaking: In the paper industry, sodium carbonate is used as a pH regulator and helps in the pulping and bleaching processes.
6. Manufacture of Sodium Compounds: Sodium carbonate is used in the production of various sodium compounds, including borax (sodium borate).

5. Calcium Oxychloride (CaOCl₂)

Bleaching powder, also known as calcium oxychloride or chloride of lime, is a chemical compound with the formula CaOCl₂.

Production of Bleaching Powder:

Bleaching powder is produced by passing chlorine gas over dry slaked lime (calcium hydroxide):
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Chlorine gas reacts with calcium hydroxide (slaked lime) to form bleaching powder and water.

Properties of Bleaching Powder

1. Bleaching powder appears as a white powder with a strong smell of chlorine.
2. It is soluble in cold water, with the remaining insoluble part being lime.
3. Bleaching powder reacts with dilute acids to liberate chlorine gas, which acts as a bleaching agent.

Uses of Bleaching Powder

1. Textile Industry: Bleaching powder is used for bleaching cotton and linen in the textile industry, as it effectively removes colour from fabrics.
2. Disinfecting Drinking Water: It is used for disinfecting drinking water supplies, helping make water safe for consumption by eliminating germs.
3. Manufacturing Chloroform: Bleaching powder is used in the production of chloroform (CHCl₃).
4. Wool Processing: It is used to make wool unshrinkable during processing.
5. Oxidising Agent: Bleaching powder serves as an oxidising agent in various chemical industries.

6. Calcium Sulphate Hemihydrate (CaSO₄·1/2H₂O) or Plaster of Paris

Plaster of Paris, commonly known as P.O.P., is a chemical compound known as calcium sulphate hemihydrate (CaSO₄·1/2H₂O).

Production of Plaster of Paris:

Plaster of Paris is prepared from gypsum, which is calcium sulphate dihydrate (CaSO₄·2H₂O). The production involves heating gypsum to a temperature of 100^oC (373 K) in a kiln. During this process, gypsum loses three-fourths of its water of crystallisation and forms plaster of Paris:

CaSO₄·2H₂O (Gypsum) + Heat to 100^oC → CaSO₄·1/2H₂O (Plaster of Paris) + 3/2H₂O

The heating process must be controlled to avoid temperatures exceeding 100^oC, as this would result in the formation of anhydrous calcium sulphate (dead burnt plaster), which does not set like the plaster of Paris when mixed with water.

Properties of Plaster of Paris

1. Plaster of Paris is a white powder.
2. It exhibits a unique property of setting into a hard mass upon wetting with water. This setting occurs in about half an hour.
3. The setting process is due to the hydration of plaster of Paris to form gypsum crystals, resulting in the hardening of the material.

Uses of Plaster of Paris

1. Medical Application: Plaster of Paris is extensively used in the medical field for setting fractured bones in the correct position to ensure proper healing. It helps immobilise fractured bone joints. It is also used for making casts in dentistry.
2. Arts and Crafts: Plaster of Paris is used in various art and craft applications. It is used for making toys, decorative items, inexpensive ornaments, and cosmetic products. Additionally, it is used to make blackboard chalk and casts for statues.
3. Fireproofing: Due to its ability to harden into a solid mass and resist high temperatures, plaster of Paris is used as a fireproofing material.
4. Laboratory Use: In chemistry laboratories, plaster of Paris is employed to seal air gaps in apparatus where an airtight seal is required.
5. Construction and Decoration: Plaster of Paris is used in construction for smoothing surfaces, especially walls, before painting. It is also used for creating ornamental designs on the ceilings of buildings.

Water of Crystallisation and Hydrated Salts

Water of crystallisation is a term used to describe the water molecules that are an essential part of the crystal structure of certain salts. These salts are known as hydrated salts because they contain a fixed number of water molecules within their crystal structure.

Some of the hydrated salts and their corresponding water of crystallisation are:

1. Copper Sulphate Pentahydrate: Copper sulphate crystals contain 5 molecules of water of crystallisation in one formula unit and are written as CuSO₄.5H₂O.
2. Sodium Carbonate Decahydrate: Sodium carbonate crystals (washing soda) contain 10 molecules of water of crystallisation per formula unit and are written as Na₂CO₃.10H₂O.
3. Calcium Sulphate Dihydrate: Calcium sulphate crystals (gypsum) contain 2 molecules of water of crystallisation in one formula unit and are written as CaSO₄.2H₂O.
4. Iron Sulphate Heptahydrate: Iron sulphate crystals contain 7 molecules of water of crystallisation per formula unit and are written as FeSO₄.7H₂O.

Water of crystallisation is a part of the crystal structure of these salts and is not free water. Therefore, hydrated salts appear to be dry even though they contain water within their crystals.

Importance of Water of Crystallisation

Water of crystallisation plays a crucial role in the physical properties of hydrated salts. It gives crystals their characteristic shape and, in some cases, imparts colour to them. For example, the presence of water of crystallisation in copper sulphate crystals gives them a blue colour.

Action of Heat on Hydrated Salts

When hydrated salts are heated strongly, they lose their water of crystallisation, resulting in the formation of anhydrous salts. Anhydrous salts do not contain water of crystallisation and appear as colourless powders. This process is reversible, and anhydrous salts can regain their water of crystallisation when water is added to them.

For example, when blue copper sulphate crystals (CuSO₄.5H₂O) are heated strongly, they lose their water of crystallisation, turning white and forming anhydrous copper sulphate (CuSO₄). However, when water is added to anhydrous copper sulphate, it regains its water of crystallisation and turns blue again.

This property of hydrated salts losing and regaining water of crystallisation is used in various applications, including detecting the presence of moisture in a substance.