A compound that is a white solid which absorbs water vapour from the air is :

1. Ionic Compounds: Properties and Structures (basic)

To understand ionic compounds, we must first look at their atomic architecture. These compounds are formed when a metal transfers electrons to a non-metal, creating positively charged cations and negatively charged anions. These oppositely charged ions are held together by powerful electrostatic forces of attraction. This strong 'glue' results in the formation of a crystalline lattice, which explains why most ionic compounds are hard, brittle solids at room temperature Science, Class X, Metals and Non-metals, p.49. For example, common table salt (NaCl) or calcium chloride (CaCl₂) exist as rigid crystals that shatter when pressure is applied rather than deforming like metals. Because these inter-ionic bonds are so strong, it takes a massive amount of thermal energy to break them apart. This gives ionic compounds characteristically high melting and boiling points. As seen in Science, Class X, Metals and Non-metals, p.48, compounds like Calcium Oxide (CaO) have melting points as high as 2850 °C. Beyond heat, their behavior with water is also unique. Most are soluble in water (a polar solvent) because water molecules can surround and stabilize the individual ions, pulling them away from the crystal lattice. However, they generally remain insoluble in organic solvents like kerosene or petrol Science, Class X, Metals and Non-metals, p.49. One of the most critical functional properties of ionic compounds is their electrical conductivity. In a solid state, ions are locked in fixed positions and cannot move, meaning the compound does not conduct electricity. However, when molten (melted) or dissolved in water, the crystal structure breaks down, allowing the ions to move freely toward electrodes. This movement of charged particles is what allows an electric current to flow Science, Class X, Carbon and its Compounds, p.58. Additionally, certain ionic salts like Calcium Chloride (CaCl₂) have a unique affinity for moisture; they are hygroscopic, meaning they can actively attract and hold water molecules from the surrounding air, sometimes even dissolving in the moisture they absorb.

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

2. Water of Crystallization in Salts (basic)

At a first glance, many salt crystals appear completely dry. However, at a molecular level, they often contain a fixed number of water molecules chemically bonded within their crystal structure. This is known as the Water of Crystallisation. It is important to understand that this water is not 'wetness' in the sense of a damp cloth; rather, it is an integral part of the salt's geometric shape and often its color. For instance, Copper Sulphate (CuSO₄·5H₂O) contains five molecules of water for every one formula unit of the salt Science, Class X, Chapter 2: Acids, Bases and Salts, p.32.

The presence of this water can be demonstrated through a simple heating experiment. When you heat blue copper sulphate crystals in a boiling tube, the heat breaks the chemical bonds holding the water molecules. As the water evaporates (often appearing as droplets on the cooler sides of the tube), the salt loses its characteristic blue color and turns into a white powder. Interestingly, this process is reversible; if you add a few drops of water back to the white anhydrous salt, the vibrant blue color is restored, signifying that the crystal structure has reformed Science, Class X, Chapter 2: Acids, Bases and Salts, p.32.

We represent these salts by writing the number of water molecules after a dot in the chemical formula. Here are some common examples you should be familiar with:

Common Name	Chemical Formula	Water Molecules
Copper Sulphate	CuSO₄·5H₂O	5
Washing Soda	Na₂CO₃·10H₂O	10
Gypsum	CaSO₄·2H₂O	2
Plaster of Paris	CaSO₄·½H₂O	0.5 (Shared)

In the case of Plaster of Paris, the formula shows 'half' a water molecule. This doesn't mean a molecule is split; instead, it indicates that two formula units of Calcium Sulphate (CaSO₄) share a single molecule of water between them Science, Class X, Chapter 2: Acids, Bases and Salts, p.33.

Sources: Science, Class X, Chapter 2: Acids, Bases and Salts, p.32; Science, Class X, Chapter 2: Acids, Bases and Salts, p.33

3. Efflorescence: Losing Water to the Atmosphere (intermediate)

In our journey through chemistry, we often think of salts as dry, crystalline solids. However, many salts naturally contain a fixed number of water molecules within their crystal structure, known as water of crystallization. Efflorescence is the fascinating phenomenon where a hydrated salt loses some or all of this water of crystallization when exposed to the atmosphere. This happens because the vapor pressure exerted by the water molecules within the crystal is higher than the partial pressure of water vapor in the surrounding air, causing the salt to "shed" its moisture and often crumble into a powdery form.

A classic example of an efflorescent substance is Washing Soda, or sodium carbonate decahydrate (Na₂CO₃·10H₂O). When left in open air, it loses nine of its ten water molecules to become a monohydrate (Na₂CO₃·H₂O), turning from transparent crystals into a white opaque powder. This property is quite useful for identifying substances; for instance, while sodium carbonate is typically efflorescent, other compounds like calcium chloride act in the exact opposite way by greedily absorbing moisture from the air Science, Class X, Chapter 2: Acids, Bases and Salts, p.32.

Property	Efflorescence	Deliquescence
Action	Loses water to the atmosphere.	Absorbs water from the atmosphere.
Result	Crystalline solid often becomes powdery.	Solid may eventually dissolve into a solution.
Condition	Occurs when salt vapor pressure > air vapor pressure.	Occurs when salt vapor pressure < air vapor pressure.

Understanding efflorescence is vital for the proper storage of chemicals and in industries like construction. For example, you might have seen white, powdery deposits on old brick walls; this is often due to salts within the masonry losing water and crystallizing on the surface. While we study calcium carbonate in the context of lime water turning milky Science, Class VIII, Chapter 8: Nature of Matter, p.119, it is important to distinguish these chemical reactions from the physical-chemical process of moisture exchange that defines efflorescence.

Sources: Science, Class X, Acids, Bases and Salts, p.32; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.119

4. Desiccants and Dehydrating Agents (intermediate)

In chemistry, managing moisture is crucial, and we distinguish between substances based on how they interact with water. A desiccant is a substance used to sustain a state of dryness (desiccation) in its local vicinity. These materials work primarily through adsorption or absorption of water vapor from the air. A classic example is silica gel, which is frequently placed in containers to keep air dry and protect items like iron nails from rusting Science-Class VII (NCERT 2025 ed.), The World of Metals and Non-metals, p.49. On the other hand, a dehydrating agent is more aggressive; it doesn't just pull moisture from the air, but chemically removes the elements of water (hydrogen and oxygen) from other compounds. For instance, concentrated sulphuric acid (H₂SO₄) is a powerful dehydrating agent used in organic chemistry to remove water molecules from substances like ethanol Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.72. We also encounter terms like hygroscopy and deliquescence. A hygroscopic substance, such as Calcium Chloride (CaCl₂), has a high affinity for water. If a substance is so hygroscopic that it absorbs enough water from the atmosphere to eventually dissolve completely and form a liquid solution, it is called deliquescent. This is the opposite of efflorescence, where a hydrated crystal (like sodium carbonate) loses its water of crystallization to the dry air.

Term	Mechanism	Common Example
Desiccant	Physical removal of water vapor from the environment.	Silica Gel
Dehydrating Agent	Chemical removal of H and O elements from a compound.	Conc. Sulphuric Acid (H₂SO₄)
Deliquescent	Absorbs so much moisture that it turns into a solution.	Calcium Chloride (CaCl₂)

The effectiveness of these agents often depends on the relative humidity of the environment. In weather terms, relative humidity tells us how 'full' the air is with water vapor; air with low relative humidity (20%–40%) has more capacity to absorb moisture, which speeds up evaporation and drying Exploring Society: India and Beyond, Social Science-Class VII, Understanding the Weather, p.38.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.72; Exploring Society: India and Beyond, Social Science-Class VII, Understanding the Weather, p.38; Science-Class VII (NCERT 2025 ed.), The World of Metals and Non-metals, p.49

5. Salts in Daily Life and Industry (intermediate)

In our daily lives and industrial processes, salts are far more than just the seasoning on our table. Chemically, a salt is an ionic compound formed by the neutralization of an acid and a base. One of the most fascinating characteristics of certain salts is their interaction with atmospheric moisture. We categorize these interactions into three main phenomena: hygroscopy, deliquescence, and efflorescence. A salt like Calcium chloride (CaCl₂) is a powerhouse in this regard; it is hygroscopic, meaning it actively attracts and holds water molecules from the surrounding environment. When the humidity is high enough, it becomes deliquescent—it absorbs so much water that it eventually dissolves in it to form a concentrated liquid solution. This makes CaCl₂ an ideal industrial desiccant used to keep goods dry during shipping. Science, Acids, Bases and Salts, p.28

Conversely, some salts exhibit the opposite behavior. Sodium carbonate (washing soda) is typically efflorescent; when exposed to dry air, it loses its water of crystallization and turns into a powdery substance. Meanwhile, other calcium-based compounds like Calcium sulphate (found as Gypsum or Plaster of Paris) are prized for their structural stability rather than moisture absorption. Calcium oxide (CaO), also known as quicklime, is another vital industrial salt-like oxide produced through the thermal decomposition of limestone. It is a critical ingredient in the manufacture of cement, highlighting how these chemical properties dictate the material's utility in infrastructure. Science, Chemical Reactions and Equations, p.8

Understanding these properties is crucial for both chemistry and geography (specifically industrial location). For instance, the pharmaceutical industry—represented in India by bodies like the Indian Drugs and Pharmaceuticals Limited (IDPL)—relies heavily on the purity and moisture-control of various salts to ensure medicine stability. Geography of India, Industries, p.60 Whether it is using salts to treat icy roads or as preservatives, their ability to manage water is their most 'industrial' trait.

Term	Behavior	Common Example
Deliquescent	Absorbs moisture until it dissolves	Calcium chloride (CaCl₂)
Efflorescent	Loses water of crystallization to air	Sodium carbonate (Na₂CO₃.10H₂O)
Hygroscopic	Attracts water but may not dissolve	Silica Gel / Concentrated H₂SO₄

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.8; Geography of India, Majid Husain, Industries, p.60

6. Hygroscopy and Deliquescence (exam-level)

At its core, the interaction between solid substances and atmospheric moisture is a fascinating study of chemical affinity. Hygroscopy is the physical property of a substance that allows it to attract and hold water molecules from the surrounding environment. This happens either through absorption (taking water into the body of the material) or adsorption (sticking water to the surface). Common examples include silica gel packets found in shoe boxes or honey, which pulls moisture from the air. These materials are often used as desiccants or drying agents because of their ability to 'scavenge' water vapor.

When this property is taken to the extreme, we encounter deliquescence. A deliquescent substance is so 'thirsty' for water that it continues to absorb atmospheric moisture until it literally dissolves in the water it has collected, turning into a liquid solution. This occurs when the vapor pressure of the saturated solution formed is lower than the partial pressure of water vapor in the air. A classic example of such a compound is Calcium chloride (CaCl₂). As noted in practical chemistry contexts, when CaCl₂ is exposed to high humidity, it transforms from a white solid into a liquid pool. Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p. 74.

It is important to distinguish these from efflorescence, which is the opposite process. Efflorescent substances, like certain forms of sodium carbonate, actually lose their water of crystallization to the atmosphere, often turning from a moist-looking crystal into a dry powder. Understanding these transitions is vital for storing chemicals and understanding how soils or pollutants interact with the environment. For instance, while some soils might simply swell and crack due to moisture changes Geography of India, Majid Husain (9th ed.), Soils, p. 11, deliquescent chemicals undergo a complete phase change from solid to liquid.

Term	Process	Final State
Hygroscopy	Attracts and holds water molecules.	Remains solid (usually damp).
Deliquescence	Absorbs enough water to dissolve completely.	Becomes a liquid solution.
Efflorescence	Loses internal water to the dry air.	Becomes a dry powder.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74; Geography of India, Majid Husain (9th ed.), Soils, p.11

7. Solving the Original PYQ (exam-level)

In this question, we apply the foundational concepts of hygroscopy and deliquescence—the ability of certain substances to attract and hold water molecules from their surroundings. You have recently studied how different salts interact with atmospheric moisture, and this PYQ tests your ability to identify which specific compound behaves as a potent desiccant. The key takeaway here is the transition from a solid state to a liquid solution upon exposure to humid air, a characteristic trait of Calcium chloride.

To arrive at the correct answer, think like a scientist: which of these compounds is famously used in laboratory drying tubes? Calcium chloride is highly deliquescent, meaning it doesn't just attract a little moisture; it absorbs enough water vapor to eventually dissolve and form a liquid solution. As noted in Science, class X (NCERT 2025 ed.), this property makes it indispensable for removing water vapor from gases or protecting moisture-sensitive materials. Therefore, (B) Calcium chloride is the most logically sound choice for a white solid that actively clears moisture from the air.

UPSC often includes "traps" by listing compounds that look similar or have inverse properties. For instance, Sodium carbonate is typically efflorescent, meaning it tends to lose its water of crystallization to the atmosphere rather than gain it. Calcium sulphate, which you know as Gypsum or Plaster of Paris, interacts with water to set into a hard mass rather than absorbing vapor to form a liquid. Finally, while Sodium nitrate is soluble, it does not match the rapid, practical application of Calcium chloride as a drying agent. Understanding these opposing behaviors—efflorescence versus deliquescence—is crucial for navigating such chemistry-based questions.