The number of water molecules associated with copper sulphate molecule to form crystals is

1. Understanding Salts and Neutralization (basic)

To understand chemistry at its core, we must look at how substances interact to find balance. A salt is an ionic compound formed when an acid and a base react with each other. This specific type of chemical interaction is known as a neutralisation reaction. In its simplest form, the reaction can be summarized as:

Base + Acid → Salt + Water

During this process, the hydrogen ions (H⁺) from the acid and the hydroxide ions (OH⁻) from the base combine to form water (H₂O), often releasing energy in the form of heat Science-Class VII, NCERT (Revised ed 2025), Exploring Substances, p.18. For example, when Hydrochloric acid (HCl) reacts with Sodium hydroxide (NaOH), the result is Sodium chloride (NaCl)—the common salt we use in our kitchens—and water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21.

While we often think of salts as 'neutral,' their actual pH depends on the strength of the 'parents' (the acid and base) that created them. We can classify them based on this relationship:

Acid Strength	Base Strength	Nature of Salt	pH Value
Strong	Strong	Neutral	Around 7
Strong	Weak	Acidic	Less than 7
Weak	Strong	Basic	More than 7

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

Beyond their chemical identity, many salts have a unique physical characteristic: they contain water of crystallisation. This is a fixed number of water molecules chemically bonded within the salt's crystal structure. A famous example is Copper sulphate. In its natural crystalline state, it is a deep blue color because it contains five molecules of water for every one unit of salt (CuSO₄·5H₂O). If you heat these crystals, they lose their water and turn into a white anhydrous powder. Interestingly, this process is reversible—simply adding water restores the blue color Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32.

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.21; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32

2. Chemical Formulas and Symbols (basic)

In chemistry, Chemical Symbols and Formulas serve as a universal shorthand, transforming long-winded descriptions into precise, manageable language. Think of symbols as the 'alphabet' of chemistry and formulas as the 'words.' For instance, instead of writing 'magnesium' and 'oxygen,' we use Mg and O. This brevity allows scientists to represent complex reactions concisely and clearly Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3.

A Chemical Formula does more than identify elements; it tells us the exact ratio of atoms within a compound. For example, in H₂O, the subscript '2' indicates two atoms of Hydrogen for every one atom of Oxygen. To make these formulas even more informative, we attach physical state notations. These tell us whether a substance is a solid, liquid, gas, or dissolved in water Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5.

Notation	Physical State	Meaning
(s)	Solid	The substance is in a firm, stable shape.
(l)	Liquid	The substance is a pure fluid (like melted wax).
(g)	Gaseous	The substance is a gas or vapor.
(aq)	Aqueous	The substance is dissolved in water.

Sometimes, formulas include a 'dot' followed by water molecules, such as CuSO₄·5H₂O (Copper sulphate). This represents Water of Crystallisation—fixed water molecules integrated into the solid crystal structure. These molecules are not 'wet' in the traditional sense but are essential for the crystal's shape and color. In the case of Copper sulphate, these five water molecules give the crystal its beautiful blue color; if you heat it and remove the water, the formula becomes simply CuSO₄, and the blue color vanishes into a white powder Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32

3. Everyday Chemicals: Bleaching Powder and Baking Soda (intermediate)

To understand the chemistry of our kitchens and laundry rooms, we must first look at Sodium Chloride (NaCl), or common salt. Beyond seasoning food, salt acts as a fundamental 'raw material' for synthesizing several industrial and domestic chemicals Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30. Two of the most prominent derivatives are Bleaching Powder and Baking Soda, each serving distinct roles based on their chemical reactivity. 1. Bleaching Powder (CaOCl₂): Officially known as Calcium oxychloride, it is produced by the action of chlorine gas on dry slaked lime [Ca(OH)₂]. The chlorine used here is typically a byproduct of the electrolysis of brine (aqueous NaCl). Its primary power lies in its ability to act as a strong oxidizing agent. In the textile industry, it removes color from cotton and linen; in paper factories, it bleaches wood pulp; and most critically for public health, it is used to disinfect drinking water by killing harmful microorganisms. 2. Baking Soda (NaHCO₃): Chemically termed Sodium hydrogencarbonate, this mild, non-corrosive basic salt is produced using salt, water, carbon dioxide, and ammonia. It is a staple in baking because of its reaction when heated: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The Carbon Dioxide (CO₂) gas produced during this heating gets trapped in dough, causing it to rise and become light and spongy. Because it is alkaline, it is also an effective Antacid, helping to neutralize excess acid in the stomach Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74.

Feature	Bleaching Powder	Baking Soda
Chemical Name	Calcium oxychloride (CaOCl₂)	Sodium hydrogencarbonate (NaHCO₃)
Primary Property	Strong Oxidizing Agent	Mild Non-corrosive Base
Key Use	Disinfecting water/Whitening textiles	Making bread spongy/Antacid

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74

4. Plaster of Paris and Gypsum (intermediate)

At the heart of many construction and medical applications lies the relationship between two forms of calcium sulphate: Gypsum and Plaster of Paris (PoP). Gypsum is a naturally occurring mineral found in sedimentary rock beds like limestone and sandstone Geography of India, Resources, p.28. Chemically, it is known as Calcium Sulphate Dihydrate (CaSO₄·2H₂O), meaning each unit of calcium sulphate is associated with two molecules of 'water of crystallisation'. In India, Rajasthan is the titan of production, accounting for about 99% of the country's gypsum Geography of India, Resources, p.28.
The transformation from Gypsum to Plaster of Paris is a delicate chemical process involving heat. When Gypsum is heated to exactly 373 K (100°C), it loses three-fourths of its water content to become Calcium Sulphate Hemihydrate (CaSO₄·½H₂O) Science, Acids, Bases and Salts, p.32. This 'hemihydrate' form is what we call Plaster of Paris. It is a white powder that possesses a unique property: when mixed with water, it re-hydrates and sets into a hard, solid mass of Gypsum again Science, Acids, Bases and Salts, p.33.
You might find the formula CaSO₄·½H₂O curious—how can there be 'half' a water molecule? In reality, the crystal structure is arranged such that two formula units of CaSO₄ share one single molecule of water Science, Acids, Bases and Salts, p.33. This substance is vital for supporting fractured bones in the correct position, manufacturing cement, and creating decorative ceramic casts Geography of India, Resources, p.28.

Feature	Gypsum	Plaster of Paris (PoP)
Chemical Name	Calcium Sulphate Dihydrate	Calcium Sulphate Hemihydrate
Formula	CaSO₄·2H₂O	CaSO₄·½H₂O
Physical State	Hard, solid mineral	Fine white powder

Sources: Science, Acids, Bases and Salts, p.32; Science, Acids, Bases and Salts, p.33; Geography of India, Resources, p.28

5. The Concept of Water of Crystallisation (exam-level)

When we look at crystals like blue copper sulphate, they appear perfectly dry to the touch. However, at a molecular level, they contain a fixed number of water molecules chemically combined within their crystal structure. This is known as the Water of Crystallisation. It is not "free water" that makes the substance wet; rather, it is part of the salt's formula unit and often dictates the crystal's shape and color Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 32.

Take Copper Sulphate (CuSO₄·5H₂O) as a classic example. Each formula unit of copper sulphate is associated with five molecules of water. In this state, it is a beautiful blue crystalline solid. If you heat these crystals in a boiling tube, the heat energy breaks the bonds holding the water molecules. As the water evaporates (often visible as droplets on the cooler walls of the tube), the crystal collapses into a white, powdery form called anhydrous copper sulphate. Interestingly, this process is reversible: adding a few drops of water to the white powder restores its blue color and crystalline structure Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 32.

Another fascinating example is Plaster of Paris (CaSO₄·½H₂O). You might wonder how "half" a water molecule exists. In reality, two formula units of Calcium Sulphate share one single molecule of water between them Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 33. When Plaster of Paris is mixed with more water, it absorbs it to become Gypsum (CaSO₄·2H₂O), a hard solid mass often used to support fractured bones.

This principle isn't just confined to chemistry labs; it has significant geological impacts. In geography, this process is known as Hydration. When minerals in rocks take up water of crystallisation, they expand in volume. This expansion creates physical stress within the rock, leading to exfoliation and granular disintegration, which are key drivers of physical weathering Physical Geography by PMF IAS, Geomorphic Movements, p. 91.

Common Name	Chemical Formula	Water Molecules per Unit
Blue Vitriol	CuSO₄·5H₂O	5
Gypsum	CaSO₄·2H₂O	2
Plaster of Paris	CaSO₄·½H₂O	0.5 (1 water shared by 2 units)
Washing Soda	Na₂CO₃·10H₂O	10

Sources: Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.32; Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.33; Physical Geography by PMF IAS, Geomorphic Movements, p.91

6. Blue Vitriol: Structure and Properties (exam-level)

Blue Vitriol, scientifically known as Copper(II) sulphate pentahydrate (CuSO₄·5H₂O), is one of the most iconic salts in chemistry due to its striking deep blue color. This color and its distinct crystalline shape are not inherent to the copper sulphate itself, but are the result of water of crystallisation. These are water molecules that occupy fixed positions within the crystal lattice of the salt. While the crystals appear dry to the touch, they contain a specific hydration number—in this case, five molecules of water for every one formula unit of copper sulphate Science, Class X, Chapter 2, p.32.

The structural arrangement of these five water molecules is quite unique and a favorite topic for conceptual science questions. In the crystal, four water molecules are directly coordinated (attached) to the central Copper (Cu²⁺) ion, forming a square planar geometry around it. The fifth water molecule, however, is not attached to the metal ion; instead, it is held deeply within the crystal structure through hydrogen bonding between the sulphate (SO₄²⁻) group and the coordinated water molecules. This specific "4+1" arrangement is vital for the stability of the blue crystal lattice.

When you apply heat to these blue crystals, a thermal decomposition (specifically, dehydration) occurs. As the water of crystallisation is driven off as steam, the crystal lattice collapses, and the salt loses its blue color, transforming into a white anhydrous powder Science, Class X, Chapter 2, p.32. This process is a classic example of a reversible chemical change. If you add a few drops of water back to the white powder, the blue color is instantly restored as the water molecules re-enter their specific positions in the lattice.

Sources: Science, Class X, Acids, Bases and Salts, p.32

7. Solving the Original PYQ (exam-level)

This question bridges your theoretical understanding of water of crystallisation with its practical application in identifying common chemical compounds. You have learned that many salts exist as hydrated crystals, where a specific number of water molecules are chemically combined in a definite ratio. The case of copper sulphate, or blue vitriol, is the primary example used in Science, class X (NCERT 2025 ed.) to demonstrate how these molecules are responsible for both the crystalline shape and the distinct blue color of the salt.

To reach the correct answer, you must recall the full chemical name: copper(II) sulphate pentahydrate. In chemistry, the prefix "penta-" explicitly denotes the number 5, leading to the formula CuSO4·5H2O. As a coaching tip, always visualize the structural arrangement: while four water molecules are directly coordinated to the central copper ion, a fifth molecule is uniquely tucked into the crystal lattice via hydrogen bonding. Therefore, the total number of water molecules associated with the crystal is (C) 5. If these molecules are removed through heating, the substance becomes anhydrous and turns white.

UPSC often includes numbers that correspond to other famous salts to distract you. For example, 2 is a common trap because it refers to the water molecules found in gypsum (CaSO4·2H2O). The option 4 is particularly tricky because it represents the number of molecules directly bonded to the copper ion, but it ignores the fifth molecule that completes the crystalline structure. By sticking to the standard pentahydrate formula, you can avoid these common conceptual pitfalls.