Which one of the following is the correct sequence of the given substances in the decreasing order of their densities?

1. Matter and its Fundamental Properties (basic)

Welcome to your first step in mastering Applied Everyday Chemistry. To understand the complex reactions of the world, we must first define the building blocks. Matter is scientifically defined as anything that possesses mass and occupies space (volume). Whether it is the air you breathe, the water you drink, or the gold in a ring, they all share these two fundamental characteristics. It is equally important to distinguish what is not matter: phenomena such as light, heat, electricity, or even human emotions and thoughts do not occupy space or have mass, and thus fall outside this definition Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130.

While mass and volume tell us how much of something we have, Density tells us how "compact" that substance is. Mathematically, Density = Mass / Volume. This property is what explains why a small piece of lead feels much heavier than a large piece of cork. An essential rule to remember is that the density of a pure substance is independent of its shape or size; a drop of pure gold has the same density as a massive gold bar. However, density can change based on external factors like temperature and pressure, though pressure mostly affects gases while having a negligible effect on solids and liquids Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.140.

Category	Examples	Key Characteristics
Matter	Iron, Water, Oxygen, Wood	Has Mass; Occupies Space (Volume)
Non-Matter	Light, Heat, Sound, Love	Lacks physical mass; Does not occupy space

In the scientific world, we measure density using the SI unit of kilogram per cubic metre (kg/m³). However, in laboratory settings or everyday chemistry, you will frequently encounter smaller units like grams per cubic centimetre (g/cm³) or grams per millilitre (g/mL) Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141. Understanding these units and the concept of density is the secret to predicting whether an object will sink or float, or why certain metals feel "richer" or "heavier" than others.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.140; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141

2. States of Matter and Molecular Packing (basic)

To understand the world around us, we must look at the microscopic level. Every object is composed of extremely small particles held together by interparticle forces of attraction. Think of these forces as 'invisible glue.' The strength of this glue determines how particles are packed, which in turn defines whether a substance behaves as a solid, liquid, or gas Science, Class VIII (NCERT), Particulate Nature of Matter, p.113.

In solids, the 'glue' is at its strongest. Particles are so tightly packed that there is minimum interparticle space, leaving no room for them to move freely. This is why solids like iron or gold maintain a fixed shape and volume. Because they are so densely packed, it often takes a massive amount of energy (heat) to break these forces—for instance, iron requires a staggering 1538 °C just to melt Science, Class VIII (NCERT), Particulate Nature of Matter, p.103. Liquids represent a middle ground; the forces are slightly weaker, allowing particles to slide past one another. This provides the 'flow' we associate with water or mercury, giving them a definite volume but no fixed shape.

Finally, in gases, these attractions are almost negligible. Particles fly around freely with maximum interparticle space. This concept of 'packing' is the fundamental reason behind Density—which is simply mass per unit volume. The more massive the individual particles (molecular mass) and the more tightly they are packed, the higher the density of the material Science, Class X (NCERT), Carbon and its Compounds, p.67.

Feature	Solids	Liquids	Gases
Interparticle Force	Strongest	Moderate	Negligible (Weakest)
Interparticle Space	Minimum	Small	Maximum
Shape/Volume	Fixed Shape & Volume	Fixed Volume only	Neither fixed

Sources: Science, Class VIII (NCERT), Particulate Nature of Matter, p.113; Science, Class VIII (NCERT), Particulate Nature of Matter, p.103; Science, Class X (NCERT), Carbon and its Compounds, p.67

3. Archimedes' Principle and Buoyancy (intermediate)

At the heart of why a massive steel ship floats while a tiny pebble sinks is a concept called Buoyancy. When any object is immersed in a fluid—whether a liquid or a gas—it experiences an upward force known as upthrust or the buoyant force. This force acts in direct opposition to gravity. According to Archimedes' Principle, this upward force is exactly equal to the weight of the fluid that the object displaces Science Class VIII, Exploring Forces, p.76. Essentially, the object is "pushing" the water out of the way, and the water pushes back with a force proportional to how much of it was moved.

Whether an object sinks or floats depends on the tug-of-war between its own weight and this buoyant force. If the weight of the object is greater than the weight of the liquid it can displace, it sinks. However, if the object is shaped such that it displaces a volume of water heavier than itself, it floats. This is why we use Relative Density—the ratio of a substance's density to that of water—to predict behavior Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.141. If the relative density is less than 1, the object floats; if greater than 1, it sinks.

Scenario	Force Comparison	Outcome
Weight > Buoyant Force	Object is denser than the fluid.	Sinks to the bottom.
Weight = Buoyant Force	Object's density matches the fluid.	Floats (fully submerged/neutral).
Weight < Buoyant Force	Object is less dense than the fluid.	Floats on the surface.

In the real world, density isn't a fixed constant for a substance; it changes with environment. For instance, in our oceans, cold water and highly saline (salty) water are denser than warm or fresh water. This causes denser water to sink and lighter water to rise, driving the massive conveyor belts of ocean currents that regulate our planet's climate Physical Geography PMF IAS, Ocean Movements, p.487.

Sources: Science Class VIII, Exploring Forces, p.76; Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.141; Physical Geography PMF IAS, Ocean Movements, p.487

4. The Chemistry of Transition and Noble Metals (intermediate)

In our journey through chemistry, we encounter a fascinating group of elements known as transition metals. These elements occupy the central block of the periodic table and are the backbone of human civilization—from the iron in our buildings to the gold in our technology. What sets them apart is their unique electronic structure, which allows them to form colorful compounds, act as catalysts, and possess remarkable physical strength. Some of these, like iron, are quite active, while others like gold and silver are famously resistant to corrosion. Elements that show very low chemical reactivity are often called Noble Metals because they do not easily react with oxygen or acids, maintaining their luster for centuries Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.123.

One of the most defining physical characteristics of transition metals is their density. Density is defined as mass per unit volume—essentially how much "stuff" is packed into a specific space. Metals like Gold (Au) are incredibly dense because their atoms are very heavy and packed extremely tightly together. Gold has a density of approximately 19.3 g·cm⁻³, which is significantly higher than Mercury (Hg), the only metal that remains liquid at room temperature (around 13.5 g·cm⁻³). In contrast, Steel—which is primarily composed of iron—is much lighter, with a density of about 7.8–8.0 g·cm⁻³. This is why a small gold bar feels surprisingly heavy compared to a steel bolt of the same size.

The chemical behavior of these metals is best understood through the Reactivity Series. This series ranks metals from the most reactive (like Potassium) to the least reactive. At the very bottom of this list, you will find the noble metals: Copper, Mercury, Silver, and Gold Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.45. Their lack of reactivity is due to their stable electronic configurations; they have little tendency to lose electrons to form compounds, which is why they are often found in nature in their pure, metallic form rather than as ores Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

Property	Noble Metals (e.g., Gold)	Structural Metals (e.g., Steel/Iron)
Reactivity	Extremely low; does not rust or tarnish easily.	Moderate to high; reacts with moisture/oxygen (rusts).
Density	Very High (Heavier for the same size).	Moderate (Lighter for the same size).
Occurrence	Often found in pure "native" state.	Usually found as oxides or sulfides (ores).

Sources: Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

5. Mercury: The Liquid Metal (intermediate)

Mercury (Hg), often called 'quicksilver', is a fascinating outlier in chemistry. While most metals are hard solids with high melting points, mercury is the only metal that remains in a liquid state at room temperature Science, Class VIII, Nature of Matter, p.123. This unique property allows it to flow easily and expand uniformly with heat, which is why it has been traditionally used in clinical thermometers and barometers Science, Class VII, The World of Metals and Non-metals, p.43. Despite being a liquid, it retains metallic properties like high electrical conductivity and a lustrous appearance.

One of the most counter-intuitive aspects of mercury is its extreme density. It is roughly 13.6 times heavier than an equal volume of water. To put this in perspective, common structural materials like steel have a density of about 7.8 g/cm³, meaning solid steel will easily float on liquid mercury. However, even mercury is surpassed by precious metals like gold, which has a much higher density of approximately 19.3 g/cm³.

Material	Typical State	Approx. Density (g/cm³)
Pure Gold	Solid	19.3
Mercury	Liquid	13.6
Steel (Carbon/Alloy)	Solid	7.8 – 8.0

Chemically, mercury is relatively unreactive and is found low in the reactivity series. It is primarily extracted from its sulfide ore, Cinnabar (HgS). The extraction process is unique because it doesn't require complex reducing agents; simply heating Cinnabar in air converts it to mercuric oxide (HgO), and further heating reduces it directly to liquid mercury Science, Class X, Metals and Non-metals, p.51. However, we must handle it with extreme care. Mercury is highly toxic, particularly in the form of methylmercury, which can accumulate in fish and cause severe neurological damage in humans Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.413.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, Class VII NCERT, The World of Metals and Non-metals, p.43; Science, Class X NCERT, Metals and Non-metals, p.51; Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.413

6. Alloys and Structural Materials (intermediate)

In the world of structural engineering and everyday chemistry, pure metals often fall short of our needs. For instance, pure iron is quite soft and stretches easily when hot. However, by creating an alloy—a homogeneous mixture of a metal with other metals or non-metals—we can radically transform its properties Science, Class X, Metals and Non-metals, p.54. This process of alloying allows us to customize materials for specific roles, such as increasing hardness, preventing rust, or improving heat resistance.

Stainless steel is perhaps the most famous structural alloy. While ordinary steel is mostly iron (Fe) with a tiny bit of carbon (C) for strength, stainless steel adds chromium (Cr) and nickel (Ni) to the mix Science, Class VIII, Nature of Matter, p.118. The chromium reacts with oxygen to form a thin, invisible layer of chromium oxide that protects the iron from rusting. This makes the material indispensable for everything from surgical tools to kitchen cutlery and high-end machine parts Certificate Physical and Human Geography, Manufacturing Industry, p.284.

Beyond chemical resistance, we must consider physical properties like density (mass per unit volume). While steel is strong and durable, it is relatively "light" compared to heavy metals. For perspective, typical steels have a density of around 7.8–8.0 g/cm³, whereas mercury (Hg) is nearly twice as dense (≈13.6 g/cm³), and gold (Au) is even denser (≈19.3 g/cm³). This is why a small gold bar feels surprisingly heavy for its size compared to a steel block of the same dimensions.

Alloy	Primary Components	Key Property/Use
Stainless Steel	Iron, Chromium, Nickel, Carbon	Corrosion resistance; tools & machinery
Brass	Copper and Zinc	Acoustic properties; decorative items
Bronze	Copper and Tin	Hardness; statues and bearings

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54; Science, Class VIII (NCERT Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.118; Certificate Physical and Human Geography (GC Leong 3rd ed.), Manufacturing Industry and The Iron and Steel Industry, p.284

7. Comparing Densities of Common Substances (exam-level)

To understand why some materials feel "heavier" than others even when they are the same size, we must look at density. Simply put, density is the amount of mass packed into a specific volume (Density = Mass / Volume). In the world of chemistry and materials science, density tells us how tightly the atoms of a substance are squeezed together. For competitive exams, it is crucial to recognize the relative densities of common materials like gold, mercury, and steel, as these frequently appear in questions about buoyancy and material properties.

Gold is one of the densest naturally occurring elements, with a density of approximately 19.3 g/cm³. This means a small bar of gold is nearly twice as heavy as a lead bar of the same size. While we often prize gold for its ductility—the ability to be drawn into thin wires (Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.38)—its high density is a primary reason why it sinks rapidly in almost any other liquid. Mercury, on the other hand, is a unique metal because it remains liquid at room temperature. Despite being a liquid, it is incredibly dense (about 13.6 g/cm³). It is so dense that solid iron or steel will actually float on top of it! When mercury is mixed with other metals, it forms what we call an amalgam (Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54).

Finally, we have Steel (an alloy primarily consisting of iron). Structural steels typically have a density of around 7.8 to 8.0 g/cm³. While steel feels very heavy to us in daily life, it is significantly less dense than both gold and mercury. This hierarchy is why an iron pillar or a steel bolt would sink in water but float on a pool of mercury.

Substance	Approx. Density (g/cm³)	Key Characteristic
Gold	19.3	Extremely dense; most ductile metal.
Mercury	13.6	Dense liquid; forms amalgams.
Steel / Iron	7.8	Common structural metal; floats on mercury.

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.38; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54

8. Solving the Original PYQ (exam-level)

You’ve just mastered the fundamental principle that density is the ratio of mass to volume, reflecting how closely atoms are packed within a substance. This question tests your ability to apply that conceptual hierarchy to real-world materials. While we often intuitively think of solids as "heavier" than liquids, this specific comparison highlights that atomic weight and lattice structures are the true drivers of density. As discussed in NCERT Class 9 Science (Matter in Our Surroundings), density is a characteristic property that allows us to distinguish between elements like Gold, Mercury, and the alloy Steel.

To arrive at the answer, reason through the material properties of each substance. Gold is a heavy transition metal with a very high atomic mass, resulting in a massive density of approximately 19.3 g/cm³. Next, you must recognize the unique nature of Mercury; despite being a liquid at room temperature, its atoms are so heavy and closely packed that it reaches a density of about 13.6 g/cm³. Finally, Steel—which is primarily Iron—is relatively lighter, with a density of roughly 7.8–8.0 g/cm³. By aligning these values from highest to lowest, the logical sequence emerges as Gold > Mercury > Steel, which corresponds to Option (B).

UPSC frequently uses the Mercury Trap to catch students off-guard. Many aspirants mistakenly assume that because Steel is a rigid solid and Mercury is a liquid, the solid must be denser. This leads them to incorrectly choose Options (C) or (D). The key takeaway here is that state of matter does not dictate density as much as the internal atomic mass does. Always remember: Mercury is so dense that a piece of solid Steel will actually float on it! Recognizing these outliers is essential for navigating the "General Science" section of the Prelims.