Which one of the following elements exists in liquid state at room temperature?

1. Physical Properties of Metals and Non-metals (basic)

In our journey to understand the building blocks of the universe, we first classify elements into two broad categories: Metals and Non-metals. This classification isn't just academic; it dictates how we use these materials in our daily lives—from the copper wires in our walls to the oxygen we breathe. Traditionally, metals are identified by their physical properties: they are typically hard, lustrous (shiny), and excellent conductors of heat and electricity. As noted in Science, Class X (NCERT 2025 ed.), Chapter 3, p.37, these properties are the very reason we choose specific materials for specific tasks, such as using metals for cooking vessels because of their high melting points and thermal conductivity.

Two of the most remarkable properties of metals are malleability (the ability to be beaten into thin sheets) and ductility (the ability to be drawn into thin wires). For instance, gold is so incredibly ductile that a single gram can be drawn into a wire approximately 2 kilometers long Science, Class X (NCERT 2025 ed.), Chapter 3, p.38. Conversely, non-metals are generally the mirror image: they are often brittle in solid form, lack a metallic luster, and act as poor conductors of heat and electricity. However, nature is rarely uniform, and exceptions are the favorite playground of examiners. For example, while most non-metals are poor conductors, graphite (a form of carbon) is an excellent conductor of electricity Science, Class X (NCERT 2025 ed.), Chapter 3, p.55.

The most critical exception to remember involves the physical state of elements at room temperature. While we think of metals as solid and sturdy, Mercury is the only metal that exists as a liquid at standard room temperature Science, Class X (NCERT 2025 ed.), Chapter 3, p.39. In the non-metal category, while many are gases (like Oxygen) or solids (like Sulfur), Bromine stands out as a liquid. Understanding these deviations is vital because they prove that physical properties alone are not always enough to categorize an element definitively.

Property	Metals	Non-Metals
Physical State	Mostly solids (Exception: Mercury is liquid)	Solids, liquids, or gases (Bromine is liquid)
Malleability	High (can be made into sheets)	Low (brittle; they break)
Conductivity	High (Silver and Copper are best)	Low (Exception: Graphite)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.37; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.38; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.39; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.55

2. Chemical Reactivity and the Activity Series (intermediate)

In the world of chemistry, not all metals are created equal. Some are hyper-reactive, bursting into flames at the mere touch of water, while others are so stable they remain unchanged for centuries at the bottom of the ocean. This "chemical personality" is captured in the Activity Series — a vertical arrangement of metals in the order of their decreasing reactivity Science, Class X (NCERT 2025 ed.), Chapter 3, p. 45.

At the root, reactivity is about a metal's desire to lose electrons and form positive ions. Metals at the top, like Potassium (K) and Sodium (Na), are so eager to react that they react violently even with cold water, releasing hydrogen gas that immediately catches fire due to the intense heat Science, Class X (NCERT 2025 ed.), Chapter 3, p. 43. As we move down the series, this enthusiasm wanes. Magnesium (Mg) needs hot water, Iron (Fe) and Zinc (Zn) only react with steam, and Gold (Au) or Silver (Ag) do not react with water at all.

We determine this hierarchy through Displacement Reactions. Think of it as a "tug-of-war" for a chemical partner. A more reactive metal will displace a less reactive metal from its salt solution. For example, if you put an Iron nail in a Copper(II) sulphate solution, the Iron (being higher in the series) will push the Copper out to form Iron sulphate Science, Class X (NCERT 2025 ed.), Chapter 3, p. 46. This principle is vital for the extraction of metals; metals at the bottom are so unreactive they are often found in their "free state" in nature, whereas those at the top must be extracted using heavy energy like electrolysis Science, Class X (NCERT 2025 ed.), Chapter 3, p. 49.

Reactivity Level	Metals	Behavior with Water
Most Reactive	K, Na, Ca	Reacts with cold water (violent)
Medium Reactivity	Mg, Al, Zn, Fe	Reacts with hot water or steam
Least Reactive	Cu, Ag, Au	No reaction with water

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.43, 45, 46, 49

3. Alkali Metals and Alkaline Earth Metals (intermediate)

In the study of the Periodic Table, the first two columns—Group 1 (Alkali Metals) and Group 2 (Alkaline Earth Metals)—contain some of the most fascinating and chemically active elements. Alkali metals, such as Sodium (Na) and Potassium (K), are so reactive that they are never found free in nature. They have a single valence electron which they are ‘eager’ to lose to achieve stability. This high reactivity is why sodium and potassium must be stored immersed in kerosene oil; otherwise, they would react violently with the moisture and oxygen in the air Science, Chapter 3, p.46. Physically, these metals are surprisingly soft and can be cut easily with a knife. Moving to Group 2, we find the Alkaline Earth Metals, including Magnesium (Mg) and Calcium (Ca). These elements have two valence electrons. While still very reactive, they are slightly less so than their Group 1 neighbors. A unique phenomenon occurs when these metals interact with water: Calcium reacts with cold water to form bubbles of hydrogen gas (H₂). Interestingly, the metal starts floating because these hydrogen bubbles stick to its surface Science, Chapter 3, p.43. Magnesium is slightly more ‘stubborn’—it does not react with cold water at all, requiring hot water to form Magnesium hydroxide and hydrogen gas, at which point it also begins to float due to bubble adhesion Science, Chapter 3, p.43. Understanding the Reactivity Series is crucial for UPSC aspirants, as it explains why certain metals displace others in chemical reactions. In terms of vigor, the order generally follows K > Na > Ca > Mg Science, Chapter 3, p.45. While Group 1 and 2 metals are typically solid at room temperature, it is important to remember that they represent the ‘active’ side of the metallic spectrum, contrasting sharply with ‘noble’ metals like Gold or Silver which do not react with water or dilute acids at all Science, Chapter 3, p.43.

Feature	Alkali Metals (Group 1)	Alkaline Earth Metals (Group 2)
Examples	Li, Na, K	Be, Mg, Ca
Valence Electrons	1	2
Reaction with Water	Violent (Cold Water)	Moderate (Ca: Cold; Mg: Hot)

Sources: Science, Chapter 3: Metals and Non-metals, p.43; Science, Chapter 3: Metals and Non-metals, p.45; Science, Chapter 3: Metals and Non-metals, p.46

4. Non-metals: Diverse Physical States (basic)

In our journey through the periodic table, we often think of elements as rigid solids. However, nature is far more diverse. While most of the 118 known elements exist in a solid state, non-metals are the true 'shape-shifters' of the chemical world, existing across all three states of matter—solid, liquid, and gas—at standard room temperature. Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.123. This diversity is a defining characteristic that sets them apart from metals, which are almost uniformly solid. To understand this clearly, let's look at the breakdown. Most non-metals are either **solids** (like Carbon, Sulphur, and Phosphorus) or **gases** (like Oxygen, Nitrogen, and the Noble Gases). In fact, all 11 elements that exist as gases at room temperature are non-metals. Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.123. However, the most unique case is **Bromine (Br)**. It is the only non-metal that exists as a liquid at room temperature. Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.39. It is helpful to compare this with metals to see the full picture. While metals like Sodium and Calcium are solid, **Mercury (Hg)** is the lone metallic exception that stays liquid at room temperature—often seen in traditional thermometers. Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.39. There are also 'borderline' cases like **Gallium (Ga)** and **Caesium (Cs)**; these are solids at 25°C but have such low melting points that they turn into liquid just from the heat of your palm (around 30°C). Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.123.

State at Room Temp	Metals	Non-metals
Solid	Most metals (e.g., Fe, Au, Na)	Several (e.g., Carbon, Sulphur, Iodine)
Liquid	Only Mercury (Hg)	Only Bromine (Br)
Gas	None	Many (e.g., Oxygen, Nitrogen, Helium)

Sources: Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.39

5. Trends in Melting and Boiling Points (intermediate)

At its heart, the Melting Point (MP) and Boiling Point (BP) of a substance are reflections of its internal 'grip'—the strength of the forces holding its atoms or molecules together. When we heat a substance, we are providing thermal energy to overcome these interparticle attractions. In the periodic table, metals generally exhibit high melting points because they are held together by strong metallic bonds. For instance, Iron (Fe) melts at a staggering 1538 °C Science, Class VIII, Particulate Nature of Matter, p.103. However, this is not a universal rule for all metals. Some, like the alkali metals (Lithium, Sodium, and Potassium), are so soft they can be cut with a knife and possess relatively low melting points and densities Science, Class X, Metals and Non-metals, p.40.

The periodic table also presents fascinating anomalies that defy the typical 'solid metal' stereotype. Mercury (Hg) is the most famous exception, being the only metal that remains liquid at standard room temperature. While most metals require extreme heat to melt, Gallium (Ga) and Caesium (Cs) have such low melting points that they will literally melt if placed on your palm, responding to your body heat Science, Class X, Metals and Non-metals, p.40. Among non-metals, the trend is usually toward lower MP and BP, yet Diamond (an allotrope of carbon) shatters this trend by being the hardest natural substance with an exceptionally high melting point due to its rigid 3D covalent structure.

When we look at compounds rather than pure elements, the trends continue to follow the logic of bonding strength. Ionic compounds, formed by the transfer of electrons, have high melting points because of the powerful electrostatic forces between the positive and negative ions Science, Class X, Metals and Non-metals, p.49. Conversely, in organic carbon compounds, we often see a gradation: as the molecular mass increases within a homologous series (like methane to ethane to propane), the melting and boiling points also increase because the cumulative attractive forces between larger molecules become stronger Science, Class X, Carbon and its Compounds, p.67.

Sources: Science, Class VIII (NCERT), Particulate Nature of Matter, p.103; Science, Class X (NCERT), Metals and Non-metals, p.40, 49; Science, Class X (NCERT), Carbon and its Compounds, p.67

6. Exceptions to General Elemental Rules (exam-level)

In chemistry, we often learn "general rules" to simplify the vast complexity of the periodic table. For instance, we say metals are hard, high-melting solids, and non-metals are dull, non-conductive gases or brittle solids. However, nature is rarely so rigid. For the UPSC, the real analytical gold lies in the exceptions to these rules, as they reveal deeper truths about atomic bonding and electron configuration.

The most famous exception involves the physical state of elements at room temperature (roughly 25°C). While nearly all metals are solid due to their strong metallic bonding, mercury (Hg) is the only metal that exists as a liquid at this temperature—a property that historically made it indispensable for thermometers Science, Class X, Chapter 3, p. 39. Interestingly, only two elements in the entire periodic table are liquid at standard conditions: mercury and the non-metal bromine (Br) Science, Class VIII, Nature of Matter, p. 123.

Another striking deviation concerns hardness and melting points. We typically associate metals like iron (which melts at a staggering 1538°C) with extreme durability Science, Class VIII, Particulate Nature of Matter, p. 103. However, alkali metals such as lithium, sodium, and potassium are so soft that they can be easily cut with a simple kitchen knife Science, Class VII, The World of Metals and Non-metals, p. 43. Furthermore, elements like gallium (Ga) and caesium (Cs) have melting points so close to human body temperature (around 30°C) that they will literally turn into liquid if you hold them in your palm Science, Class X, Chapter 3, p. 40.

Finally, non-metals provide their own set of surprises. While most non-metals are non-lustrous and poor conductors, iodine is a non-metal that shines with a metallic-like luster. Carbon, through its different forms or allotropes, breaks even more rules: diamond is the hardest natural substance known, while graphite is a rare non-metal that conducts electricity efficiently Science, Class X, Chapter 3, p. 40.

General Rule	The "Rule Breakers" (Exceptions)
Metals are Solids	Mercury (Liquid)
Metals are Hard	Sodium, Potassium, Lithium (Soft as wax)
Non-metals are Dull	Iodine (Lustrous/Shiny)
Non-metals are Insulators	Graphite (Conducts electricity)

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.39-40; Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.123; Science, Class VII (NCERT 2025 ed.), The World of Metals and Non-metals, p.43

7. Solving the Original PYQ (exam-level)

This question tests your ability to move from the general characteristics of metals—which you have learned are typically hard solids with high melting points—to the critical exceptions that the UPSC frequently targets. While the metallic bond generally creates a rigid lattice structure, the building blocks you studied regarding atomic structure and periodicity explain why certain elements behave differently. As highlighted in Science, class X (NCERT 2025 ed.), most metals are solid at room temperature, but Mercury stands out as the unique metallic exception that remains in a liquid state under standard conditions.

To arrive at the correct answer, you must apply a process of elimination based on specific physical properties. When you encounter a question about states of matter at room temperature, remember the "Rule of Two": only two elements on the entire periodic table are liquid at 25°C—Mercury (a metal) and Bromine (a non-metal). Since the options provided are all metals, Mercury is the only logical choice. You might recall that elements like Gallium or Caesium have very low melting points and can melt in your palm, but at the standard room temperature defined in your modules, they remain solid, making (A) Mercury the definitive answer.

UPSC often uses distractors like Sodium or Lead to test the depth of your conceptual clarity. A common trap is confusing "softness" with "state of matter"; while Sodium is an alkali metal so soft it can be cut with a knife, it is still a solid. Similarly, Lead and Calcium possess the high density and strong metallic bonding typical of solids. By distinguishing between mechanical softness and thermal melting points, you can avoid these traps and correctly identify the unique liquid nature of Mercury, a property that makes it essential for tools like thermometers.