Which one among the following is a double salt?

1. Classification of Matter: Elements, Compounds, and Mixtures (basic)

To understand the universe, we must first understand matter—anything that has mass and occupies space. At a fundamental level, scientists classify matter based on its chemical composition into two broad categories: pure substances and mixtures. A pure substance is unique because it consists of the same type of particles throughout and cannot be separated into other kinds of matter by physical means, such as filtration or evaporation Science, Class VIII. NCERT (Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.121. These pure substances are further divided into elements (the simplest building blocks, like Iron or Oxygen, that cannot be broken down further) and compounds.

A compound is a substance formed when two or more elements combine chemically in a fixed, definite ratio. The most fascinating aspect of a compound is that its properties are entirely different from the elements that make it up. For example, while Hydrogen and Oxygen are both gases, they chemically react to form Water (H₂O), which is a liquid. In contrast, a mixture occurs when substances are simply blended together without any chemical reaction. In a mixture, the components retain their individual identities and can usually be separated by physical methods Science, Class VIII. NCERT (Revised ed 2025), Chapter 8, p.130.

Feature	Compound	Mixture
Composition	Fixed ratio (e.g., H₂O is always 2:1)	Variable ratio (e.g., more or less salt in water)
Properties	Entirely new properties	Constituents retain original properties
Separation	Only by chemical or electrochemical means	Physical methods (filtration, distillation)

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.117, 121, 130

2. Chemical Bonding: Ionic and Covalent Foundations (basic)

In nature, most atoms are like restless travelers looking for a home; they are unstable on their own and seek stability by completing their outermost electron shell (the octet rule). This journey toward stability leads to Chemical Bonding. The two most fundamental ways atoms achieve this are by either "giving and taking" electrons (Ionic) or "sharing" them (Covalent).

Ionic Bonding (also called electrovalent bonding) occurs when a metal atom transfers one or more electrons to a non-metal atom. This creates ions: the metal becomes a positively charged cation, and the non-metal becomes a negatively charged anion. Because opposite charges attract, a powerful electrostatic force holds them together in a rigid lattice structure Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.48. For example, in Magnesium Chloride (MgCl₂), Magnesium gives up electrons to Chlorine, resulting in a compound with a very high melting point because a huge amount of energy is needed to break those strong ionic attractions Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49.

Covalent Bonding, on the other hand, is the art of sharing. Instead of a total transfer, atoms (usually non-metals) share pairs of electrons to fill their shells. This is common in carbon-based compounds. Because these atoms are held together as individual molecules rather than a giant charged lattice, the intermolecular forces (attraction between molecules) are relatively weak. As a result, covalent compounds like methane or alcohol typically have much lower melting and boiling points compared to ionic salts Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

Feature	Ionic Compounds	Covalent Compounds
Mechanism	Transfer of electrons (Metal to Non-metal)	Sharing of electrons (Non-metal to Non-metal)
Melting/Boiling Point	Very High	Generally Low
Electrical Conductivity	Conducts in molten state or solution Science, class X, p.58	Generally poor conductors (no ions)
Physical State	Hard and brittle solids	Often liquids or gases at room temperature

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.48-49; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58-59

3. Acids, Bases, and the Formation of Salts (intermediate)

To understand the chemistry of salts, we must first look at how they are born. A salt is the product of a neutralization reaction between an acid and a base. This chemical marriage also produces water and is typically an exothermic process, meaning it releases heat Science-Class VII, Exploring Substances, p.18. For instance, when you mix Hydrochloric acid (HCl) and Sodium hydroxide (NaOH), they neutralize each other to form Sodium chloride (NaCl) — our common table salt — and water.

Salts are not just simple crystals; they come in various sophisticated forms. One common variation is hydrated salts, which contain a specific number of water molecules trapped within their crystal lattice, known as the water of crystallization Science, Class X, Acids, Bases and Salts, p.34. A well-known example is Copper sulphate pentahydrate (CuSO₄·5H₂O), which owes its vibrant blue color to these five water molecules. If heated, this water is lost, and the salt turns into a white powder.

Beyond simple and hydrated salts, we encounter Double Salts. These are unique substances formed by combining two different salts that crystallize together in a fixed ratio. A classic example used in water purification is Potash Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O). The distinguishing feature of a double salt is its behavior in water: it completely dissociates into its individual constituent ions Science, Class VIII, Nature of Matter, p.124. This is different from complex salts (like potassium ferrocyanide), which hold onto their internal structure even when dissolved.

Finally, the "personality" (pH) of a salt depends on the strength of its parents. If both the acid and base are strong, the resulting salt is neutral (pH 7). However, if a strong acid reacts with a weak base, the salt will be acidic; conversely, a strong base and a weak acid produce a basic salt Science, Class X, Acids, Bases and Salts, p.29.

Type of Salt	Key Characteristic	Example
Simple Salt	Formed from direct neutralization.	Sodium chloride (NaCl)
Double Salt	Two salts in one crystal; dissociates fully in water.	Potash Alum
Complex Salt	Retains a complex ion structure even in solution.	Potassium ferrocyanide

Sources: Science-Class VII . NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.18; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29, 34; Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.124

4. Hardness of Water and Mineral Salts (intermediate)

When we speak of the hardness of water, we aren't referring to its physical state, but rather its chemical composition—specifically the concentration of dissolved multivalent metallic cations. The primary culprits are Calcium (Ca²⁺) and Magnesium (Mg²⁺) ions. These minerals are picked up as water travels through deposits of limestone, chalk, or gypsum. Interestingly, these same minerals are vital for marine life; for instance, carbonate ions are essential for the calcification process that allows corals and molluscs to build their protective shells Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.264.

Hardness is generally classified into two categories based on the specific salts present:

Type of Hardness	Causative Salts	Removal Method
Temporary Hardness	Bicarbonates of Calcium and Magnesium	Physical methods like boiling.
Permanent Hardness	Chlorides and Sulfates of Calcium and Magnesium	Requires chemical treatment (e.g., Ion exchange).

The behavior of these metals in water is quite specific. For instance, while Calcium reacts with cold water, Magnesium requires hot water to react and form magnesium hydroxide Science class X (NCERT 2025 ed.), Metals and Non-metals, p.43. To treat hard water or clarify it, we often use Double Salts such as Potash Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O). A double salt is a substance formed by two different salts that crystallize together but lose their individual identity and dissociate into their constituent ions when dissolved in water Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter, p.124. Alums are particularly useful in purification because they help "clump" together fine particles through coagulation.

Sources: Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.264; Science class X (NCERT 2025 ed.), Metals and Non-metals, p.43; Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.124

5. Common Chemicals in Daily Life and Industry (exam-level)

To understand the chemistry around us, we must distinguish between different types of salts based on their structure and behavior. A simple salt, like Sodium Chloride (NaCl), consists of a single cation and anion pair. However, many salts exist as hydrated salts, where a fixed number of water molecules are chemically combined within the crystal structure, known as water of crystallisation. For instance, Copper Sulphate Pentahydrate (CuSO₄·5H₂O) appears blue due to these five water molecules; if heated, it loses this water and turns white. Science, Class X, Chapter 2, p.32 Beyond simple salts, we encounter double salts. These are formed by the combination of two different salts that crystallize together in a fixed ratio, such as Potash Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O). While they appear as a single crystalline substance, they completely dissociate into their individual constituent ions when dissolved in water. Science, Class VIII, Chapter 8, p.124 This differs from complex salts (like potassium ferrocyanide), which maintain their complex structure even in solution. In industry, specific chemical reactions yield essential materials. Bleaching Powder (CaOCl₂) is manufactured by the action of chlorine gas on dry slaked lime [Ca(OH)₂]. Science, Class X, Chapter 2, p.30 Similarly, Plaster of Paris (CaSO₄·½H₂O) is produced by heating Gypsum (CaSO₄·2H₂O) to exactly 373 K. At this temperature, it loses three-fourths of its water of crystallisation. If mixed with water again, it undergoes a reverse reaction to form a hard solid mass of gypsum, which is why doctors use it to support fractured bones. Science, Class X, Chapter 2, p.33 Lastly, the nature of the elements involved determines the acidity of their products. Generally, metallic oxides (like Magnesium oxide, MgO) are basic in nature, meaning they turn red litmus blue. In contrast, non-metallic oxides (like Sulphur dioxide, SO₂) are acidic, turning blue litmus red when dissolved in water. Science, Class X, Chapter 3, p.40

Common Name	Chemical Formula	Key Industrial Use
Bleaching Powder	CaOCl₂	Disinfecting drinking water; bleaching textiles
Plaster of Paris	CaSO₄·½H₂O	Castings for fractures; making toys/statues
Washing Soda	Na₂CO₃·10H₂O	Softening hard water; glass manufacture
Baking Soda	NaHCO₃	Antacid; ingredient in baking powder

Sources: Science, Class X (NCERT 2025), Chapter 2: Acids, Bases and Salts, p.30, 32-33; Science, Class X (NCERT 2025), Chapter 3: Metals and Non-metals, p.40; Science, Class VIII (NCERT 2025), Chapter 8: Nature of Matter, p.124

6. Classification of Salts: Simple, Double, and Complex (exam-level)

In our journey through chemistry, understanding how salts are structured is vital for both laboratory analysis and environmental science. While we often think of salt simply as the sodium chloride (NaCl) we use at the table, chemists classify salts into three distinct categories based on how they are formed and, more importantly, how they behave when dissolved in water.

1. Simple Salts: These are the most basic form, produced by the neutralization of an acid and a base. Examples include NaCl, K₂SO₄ (potassium sulphate), and MgSO₄ (magnesium sulphate) Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28. Even when they contain "water of crystallization," like CuSO₄·5H₂O (copper sulphate pentahydrate), they are still considered simple salts because they contain only one type of metal cation and one type of acid anion. Salts sharing the same positive or negative radicals are grouped into families; for instance, NaCl and Na₂SO₄ both belong to the sodium salt family Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29.

2. Double Salts: These occur when two different simple salts crystallize together in a fixed ratio. The hallmark of a double salt is that it loses its identity in water. When you dissolve a double salt, it breaks down completely into its individual constituent ions. A classic example is Potash Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O). In solid form, it is one substance, but in water, it exists simply as a mixture of K⁺, Al³⁺, and SO₄²⁻ ions.

3. Complex Salts: Unlike double salts, complex salts retain their identity in aqueous solution. They contain a "complex ion"—a central metal atom bonded to molecules or ions called ligands—that does not break apart in water. For example, in Potassium ferrocyanide [K₄Fe(CN)₆], the [Fe(CN)₆]⁴⁻ unit stays together as a single entity and does not further dissociate into Fe²⁺ and CN⁻ ions. This stability makes complex salts chemically distinct from the simple salts they were made from.

Feature	Double Salt	Complex Salt
Dissociation	Completely dissociates into simple ions.	Does not completely dissociate; complex ion remains intact.
Identity	Identity is lost in solution.	Retains identity even in solution.
Example	Potash Alum, Mohr's Salt.	Potassium Ferrocyanide, Tetraamminecopper(II) sulfate.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29

7. The Alum Family and Potash Alum Properties (exam-level)

In the study of chemistry, we classify salts based on their structure and behavior in water. A double salt, like those in the Alum family, is formed when two different simple salts crystallize together in a fixed ratio. The most iconic example is Potash Alum (Fitkari), with the formula K₂SO₄·Al₂(SO₄)₃·24H₂O. While these salts exist as a single crystalline unit in their solid state, they are unique because they lose their identity when dissolved in water, completely dissociating into their constituent ions (K⁺, Al³⁺, and SO₄²⁻). This distinguishes them from complex salts (like potassium ferrocyanide), which maintain their complex molecular structure even in aqueous solutions NCERT Class VIII, Nature of Matter, p.124.
The 'Alum' family refers to a specific group of isomorphous double sulfates. They typically follow the general chemical formula M₂SO₄·M'₂(SO₄)₃·24H₂O, where 'M' represents a monovalent cation (like Potassium or Ammonium) and 'M'' represents a trivalent cation (like Aluminum, Chromium, or Iron). While we often focus on their chemical structure, it is worth noting that for industrial and agricultural use, India's entire requirement for potash—a key component of these salts—is actually met through imports Indian Economy by Nitin Singhania, Agriculture, p.303. This highlights the strategic importance of understanding these chemical compounds beyond the laboratory.
To help you distinguish between the various types of salts you might encounter in your exams, such as sodium chloride or copper sulfate NCERT Class X, Acids, Bases and Salts, p.28, refer to the comparison below:

Salt Type	Example	Behavior in Water
Simple Salt	NaCl (Sodium Chloride)	Dissociates into simple ions.
Double Salt	Potash Alum	Dissociates completely; loses molecular identity.
Complex Salt	K₄[Fe(CN)₆]	Retains the complex ion structure; does not fully dissociate.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.124; Indian Economy by Nitin Singhania, Agriculture, p.303; Science, Class X NCERT, Acids, Bases and Salts, p.28

8. Solving the Original PYQ (exam-level)

You have just mastered the classification of matter, specifically how elements and compounds organize into different chemical structures. This question tests your ability to distinguish between compounds based on their behavior in water. A double salt is a substance formed by two different salts that crystallize together in a fixed ratio but entirely lose their identity when dissolved, dissociating into all their constituent ions. This is the bridge between simple ionic bonding and the more complex coordination chemistry you’ve studied.

To identify the correct answer, look for a formula that represents a combination of two distinct, stable salts. Option (B) K2SO4·Al2(SO4)3·24H2O, commonly known as potash alum, is the classic example. It consists of potassium sulphate and aluminium sulphate molecules bound with water of crystallization. As explained in Science, Class VIII NCERT (Revised ed 2025), when this substance is placed in water, it breaks down completely into K+, Al3+, and SO4(2-) ions, which is the defining characteristic of a double salt.

UPSC often uses distractors to test your precision. Option (A) K4[Fe(CN)6] is a complex salt; unlike a double salt, it maintains its complex ion [Fe(CN)6]4- in solution. Option (C) CuSO4·5H2O is a common trap—it is a hydrated simple salt, not a double salt, because it contains only one type of metal cation. Finally, Option (D) NaCl is a simple salt. The key to winning here is remembering that a double salt must contain two different types of metal ions (or ammonium) that exist as separate salts in the same crystal lattice.