Which of the following salts are insoluble in water?

1. Introduction to Ionic Compounds and Solutions (basic)

Welcome to the first step of our journey into chemical principles! To understand how substances behave in water, we must first understand Ionic Compounds. These are compounds formed by the transfer of electrons from a metal to a non-metal, resulting in the creation of ions—atoms that carry a positive or negative charge. Because opposite charges attract, these ions are held together by powerful electrostatic forces, which define their physical nature. As noted in Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49, these compounds are typically hard solids and brittle, meaning they break into pieces when pressure is applied rather than bending.

One of the most defining characteristics of ionic compounds is their high melting and boiling points. Because the inter-ionic attraction is so strong, it requires a massive amount of thermal energy to break these bonds and change the state of the substance from solid to liquid or gas. This is a stark contrast to carbon-based covalent compounds, which generally have much lower melting points because their intermolecular forces are weaker Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

When it comes to solubility and conductivity, ionic compounds (also called electrovalent compounds) show a unique preference: they are generally soluble in water but insoluble in organic solvents like kerosene or petrol. In their solid state, they cannot conduct electricity because the ions are locked in a rigid structure. however, once dissolved in water or melted into a molten state, the ions become free to move. This movement of charged particles is exactly what allows them to conduct electricity Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.58.

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

2. Classification of Salts: Acidic, Basic, and Neutral (basic)

When we think of salts, the first thing that often comes to mind is common table salt (NaCl). However, in chemistry, a salt is a much broader category of compound formed during a neutralisation reaction where an acid reacts with a base Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18. While the reaction itself is called neutralisation, the resulting salt is not always chemically neutral. Its nature—whether it acts as an acidic, basic, or neutral substance in water—depends entirely on the relative strengths of the "parent" acid and base from which it was derived Science, Class X, Acids, Bases and Salts, p.29.

Think of this like a chemical tug-of-war. If a strong acid reacts with a strong base, they cancel each other out perfectly, resulting in a neutral salt with a pH of 7. However, if a strong acid reacts with a weak base, the "stronger" parent dominates, and the resulting salt will be acidic (pH < 7). Conversely, a strong base reacting with a weak acid produces a basic salt (pH > 7). This classification is vital because it determines how these salts interact with our environment, our bodies, and industrial processes.

Type of Salt	Parent Acid	Parent Base	pH Level	Example
Neutral	Strong	Strong	≈ 7	Sodium Chloride (NaCl)
Acidic	Strong	Weak	< 7	Ammonium Chloride (NH₄Cl)
Basic	Weak	Strong	> 7	Sodium Carbonate (Na₂CO₃)

Identifying the "family" of a salt is a helpful way to trace its origin. For instance, all salts containing the chloride ion (Cl⁻) belong to the chloride family and are typically derived from Hydrochloric acid (HCl). Similarly, salts containing the sodium ion (Na⁺) belong to the sodium family Science, Class X, Acids, Bases and Salts, p.29. By identifying these components, you can predict whether a salt will turn red litmus blue (basic) or blue litmus red (acidic).

Sources: Science, Class X, Acids, Bases and Salts, p.29; Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18

3. Chemical Hardness of Water (intermediate)

In chemistry, hardness of water refers to a high concentration of dissolved multivalent metallic cations, most commonly Calcium (Ca²⁺) and Magnesium (Mg²⁺). While water might look clear, these ions are dissolved within the spaces between water particles. As we understand from the particulate nature of matter, these particles are in constant motion, and the speed of this motion increases with temperature Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.110. This becomes critical because the solubility of these minerals changes with temperature, often leading to the scaling we see in kettles or industrial boilers.

To understand why some water is "hard" and some is "soft," we must look at solubility rules. Most nitrates (NO₃⁻) and chlorides (Cl⁻) are highly soluble in water, meaning they stay dissolved and invisible. However, carbonates (CO₃²⁻) and phosphates (PO₄³⁻) are generally insoluble unless they are paired with alkali metals like Sodium (Na) or Potassium (K). This is why Calcium Carbonate (CaCO₃) is a primary component of marine shells and skeletons in the ocean—it is a solid structure that does not easily dissolve Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.264. When these insoluble salts form in your pipes or during laundry, they create "scale" or prevent soap from lathering.

Hardness is typically classified into two types based on the anions paired with the calcium or magnesium:

• Temporary Hardness: Caused by dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). This type can be removed simply by boiling the water, which converts the soluble bicarbonates into insoluble carbonates that precipitate out.
• Permanent Hardness: Caused by chloride or sulfate salts of calcium and magnesium. These do not precipitate upon boiling because these specific salts remain soluble even at high temperatures.

Interestingly, different metals react with water at different intensities. While Calcium reacts with cold water to form bubbles of hydrogen, Magnesium requires hot water to react significantly Science, Class X NCERT (2025 ed.), Metals and Non-metals, p.43. These chemical properties dictate how these minerals interact with our water systems and why they are so prevalent in groundwater as it seeps through mineral-rich soil and rocks.

Sources: Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.110; Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.264; Science, Class X NCERT (2025 ed.), Metals and Non-metals, p.43

4. Heavy Metals and Environmental Chemistry (intermediate)

To understand environmental pollution, we must first look at the chemistry of solubility. In the context of heavy metals—elements with high atomic weights and densities—their impact on our health is largely determined by whether they dissolve in water or remain as solid sediments. As a general rule in inorganic chemistry, all nitrates (NO₃⁻) are soluble in water, meaning metals like lead or mercury become highly mobile and dangerous when they form nitrate salts. Conversely, carbonates (CO₃²⁻) and phosphates (PO₄³⁻) are generally insoluble unless paired with alkali metals like Sodium (Na) or Potassium (K). This explains why Lead (II) carbonate (PbCO₃) or Copper (II) carbonate (CuCO₃) often settle at the bottom of water bodies rather than dissolving completely, though they can still enter the food chain through aquatic organisms Environment, Shankar IAS Academy, Environmental Pollution, p.76.

The danger of heavy metals is compounded by their ability to bioaccumulate. Unlike organic waste, these metals do not degrade; instead, they get concentrated in higher trophic levels of the food chain Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.36. For instance, Mercury (Hg) can be converted into methylmercury, which partitions into the water column more easily under acidic conditions. This led to the famous Minamata disease in Japan, characterized by neurological damage after consuming contaminated fish. Similarly, Cadmium (Cd) contamination, often from mining or galvanized pipes, causes Itai-Itai disease (literally 'ouch-ouch'), which leads to painful bone softening and kidney failure Environment, Shankar IAS Academy, Environmental Pollution, p.105.

Lead (Pb) is particularly insidious because it mimics calcium in the body, interfering with brain development in children and causing anaemia or a characteristic bluish line around the gums. Aluminium (Al), while common in the earth's crust, becomes a threat when water acidifies (pH drops), leaching into the supply and potentially causing dialysis dementia, a central nervous system disorder Environment, Shankar IAS Academy, Environmental Pollution, p.105. Understanding these chemical properties—solubility and pH-dependency—is vital for managing water quality and public health.

Metal	Primary Health Impact	Key Chemical Property
Mercury (Hg)	Minamata Disease (Neurological)	Biomagnifies in fish; solubility increases with methylmercury partitioning.
Cadmium (Cd)	Itai-Itai (Bone/Joint pain)	Enters via corrosion; solubility increases fivefold as pH drops from 6.5 to 4.5.
Lead (Pb)	Mental deficiency; Anaemia	Carbonates are generally insoluble, but nitrates and some chlorides are soluble.
Aluminium (Al)	Dialysis Dementia	Leached from watersheds into acidified waters.

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.76, 105; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.36

5. Industrial Uses of Carbonates and Phosphates (intermediate)

In our study of basic chemistry, Carbonates (CO₃²⁻) and Phosphates (PO₄³⁻) stand out because of their unique solubility and reactivity, which makes them indispensable to modern industry. A fundamental rule to remember is that most carbonates and phosphates are insoluble in water. However, those paired with alkali metals (like Sodium or Potassium) are soluble, allowing us to use them in aqueous solutions for everything from cleaning clothes to growing crops. Sodium Carbonate (Na₂CO₃), commonly known as washing soda, is a powerhouse in the manufacturing sector. It is used extensively in the production of glass, soap, and paper, and serves as a precursor for other chemical compounds like borax Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. One of its most critical applications for a UPSC aspirant to know is its role in removing the permanent hardness of water. It reacts with dissolved Calcium and Magnesium salts, which cause hardness, to form insoluble precipitates, thereby "softening" the water for industrial and domestic use. Similarly, Sodium Bicarbonate (NaHCO₃) or baking soda is a staple in the food industry and fire safety, with the interesting property that its solubility increases significantly as the temperature of the water rises Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.138. On the other hand, Phosphates are the bedrock of global food security. Most industrial phosphates are derived from phosphate rocks through weathering or mining and are processed into fertilizers to replenish phosphorus in agricultural soil Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.27. However, this industrial utility comes with an environmental cost. Because these salts can be easily leached from the soil by rainwater, they often end up in nearby water bodies. This nutrient enrichment leads to Eutrophication—a process where excessive nutrients trigger algal blooms that deplete oxygen and harm aquatic life Environment, Shankar IAS Academy, Aquatic Ecosystem, p.37.

Compound Type	Primary Industrial Uses	Key Property/Impact
Carbonates (e.g., Na₂CO₃)	Glass, Soap, Paper, Water Softening	Effective at removing mineral "hardness" from water.
Phosphates (e.g., PO₄³⁻ salts)	Agricultural Fertilizers, Detergents	Essential for plant growth; major cause of Eutrophication.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.138; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.27; Environment, Shankar IAS Academy (ed 10th), Aquatic Ecosystem, p.37

6. The Solubility Rules for Inorganic Salts (exam-level)

To understand inorganic chemistry, we must understand how salts behave in water. Solubility refers to the ability of a salt (an ionic compound) to dissolve in a solvent like water. In our environment and in the lab, these rules help us predict whether a reaction will form a solid precipitate or stay in solution. For instance, when we test various salts like sodium chloride or potassium nitrate in a lab setting, we observe that they disappear into the water, forming clear solutions Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29. However, not all salts are so cooperative. Most Nitrates (NO₃⁻) and Chlorides (Cl⁻) are highly soluble. This has significant ecological implications; because nitrates dissolve so easily, they are often washed away from soil by surface runoff or groundwater, requiring bacteria to constantly fix more nitrogen for plants Environment, Shankar IAS Academy (10th ed.), Functions of an Ecosystem, p.20. While chlorides are generally soluble (like sodium chloride or the chlorides of iron and manganese), there are critical exceptions to remember: Silver chloride (AgCl) and Lead (II) chloride (PbCl₂) are famously insoluble in cold water. On the other hand, Carbonates (CO₃²⁻) and Phosphates (PO₄³⁻) are generally insoluble. They prefer to stay as solids. A classic example is the "milky" appearance of lime water when CO₂ is bubbled through it; this happens because Calcium carbonate (CaCO₃) forms as tiny insoluble white particles Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.119. The only common exceptions to this rule are when these ions are paired with Alkali metals (like Sodium or Potassium) or Ammonium (NH₄⁺). Therefore, while Sodium carbonate is soluble, salts like Lead (II) carbonate (PbCO₃) and Copper (II) carbonate (CuCO₃) will always remain insoluble precipitates in water.

Solubility Status	Common Ions	Key Exceptions
Always Soluble	Nitrates (NO₃⁻), Alkali Metals (Na⁺, K⁺), Ammonium (NH₄⁺)	Virtually none
Generally Soluble	Chlorides (Cl⁻)	Silver (AgCl), Lead (PbCl₂)
Generally Insoluble	Carbonates (CO₃²⁻), Phosphates (PO₄³⁻)	Soluble only with Na⁺, K⁺, or NH₄⁺

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Environment, Shankar IAS Academy (10th ed.), Functions of an Ecosystem, p.20; Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.119

7. Solving the Original PYQ (exam-level)

In your previous modules, you mastered the fundamental solubility rules and the behavior of ionic compounds in aqueous solutions. This question is a classic application of those building blocks, testing your ability to distinguish between universal solubility and specific exceptions—a core theme in NCERT Class 11 Chemistry. To solve this, you must apply the hierarchy of rules: the chemical identity of the anion usually dictates the general behavior, while the cation determines if an exception applies. This synthesis of rules is exactly what you practiced when studying lattice energy versus hydration energy.

Let’s walk through the reasoning as if we were in the exam hall. First, recall the most reliable rule: all nitrates are soluble. This immediately eliminates Option (B), regardless of the cations involved. Next, look at the chlorides; you know most are soluble except for the 'Big Three' (Silver, Lead, and Mercury). Since Iron (Fe) and Manganese (Mn) are transition metals not on that exception list, Option (A) is also soluble. Moving to the phosphates in Option (D), you might remember they are generally insoluble, but there is a crucial exception for Alkali metals (Na) and Ammonium (NH4). This leaves us with Option (C): Carbonates of Pb and Cu. Since carbonates are typically insoluble and neither Lead nor Copper falls under the Alkali metal exception, these salts will not dissolve, making them the correct choice.

UPSC often uses "familiarity traps" to trip up students. In Option (B), they pair Silver and Lead—cations you frequently associate with precipitates (like AgCl)—with Nitrates to see if you will ignore the universal solubility of nitrates. Similarly, Option (D) is a trap designed to test your knowledge of the exceptions to the exceptions. The examiners want to see if you can look past the "insoluble phosphate" label to recognize the solubilizing effect of Sodium and Ammonium. To succeed, always verify the anion rule first and then double-check for cation exceptions before committing to an answer.