When an alkali-metal reacts with water, which one of the following gases is produced?

1. Organization of the Modern Periodic Table (basic)

To understand the Modern Periodic Table, we must first understand the word "periodic." In nature, a periodic event is something that repeats at regular intervals—like the phases of the moon or the changing of the seasons Science, Class VIII, Keeping Time with the Skies, p.178. In chemistry, the Modern Periodic Table is an organized arrangement of elements where physical and chemical properties repeat in a predictable pattern. Unlike early versions that relied on atomic weights, the modern version is based on the Modern Periodic Law: the properties of elements are a periodic function of their atomic number (the number of protons in the nucleus).

The table is structured into a grid of horizontal rows and vertical columns, each serving a specific purpose in categorizing the 118 known elements:

• Periods (Horizontal Rows): There are 7 periods. Elements in the same period have the same number of occupied electron shells. As you move from left to right across a period, the atomic number increases by one, and the chemical properties gradually change.
• Groups (Vertical Columns): There are 18 groups. This is the most critical feature for a UPSC aspirant to remember. Elements in the same group have the same number of valence electrons (electrons in the outermost shell), which means they behave very similarly in chemical reactions.

Feature	Groups	Periods
Orientation	Vertical Columns (1 to 18)	Horizontal Rows (1 to 7)
Chemical Similarity	Elements in a group are chemically similar.	Elements in a period show a trend in properties.
Electron Shells	Number of shells increases downward.	Number of shells remains the same across.

By organizing elements this way, we can predict how an element will react even if we have never seen it before. For instance, knowing that Potassium (K) is in Group 1 helps us understand its high reactivity compared to metals like Gold (Au) which sit elsewhere in the table Science, class X, Metals and Non-metals, p.45. The table effectively separates metals (on the left) from non-metals (on the right), with metalloids acting as a bridge between them.

Sources: Science, Class VIII, Keeping Time with the Skies, p.178; Science, class X, Metals and Non-metals, p.45

2. Physical Properties of Group 1: Alkali Metals (basic)

In the world of chemistry, we often think of metals as hard, dense, and difficult to break—like iron or copper. However, the Alkali Metals (Group 1 of the Periodic Table, including Lithium, Sodium, and Potassium) challenge these expectations with some very peculiar physical properties. These elements are remarkably soft; while most metals require heavy machinery to shape, sodium and potassium are so soft that they can be easily cut with a simple laboratory knife Science, Class X (NCERT 2025 ed.), Chapter 3, p.40. This softness is a result of their atomic structure: they have only one electron in their outermost shell, which leads to relatively weak metallic bonding between their atoms.

Beyond their softness, Alkali Metals are known for having low densities and low melting points compared to transition metals like iron or gold Science, Class X (NCERT 2025 ed.), Chapter 3, p.40. In fact, lithium, sodium, and potassium are so light that they can actually float on water (though they react vigorously with it!). As you move down the group, the melting points tend to decrease even further. A fascinating example is Cesium (Cs): along with Gallium, it has such a low melting point that it will literally melt into a liquid just from the warmth of your palm Science, Class X (NCERT 2025 ed.), Chapter 3, p.40.

While they are physically "weak" in terms of hardness, they still retain the classic metallic lustre (shine) when freshly cut and are excellent conductors of heat and electricity. However, because they are so reactive, they quickly lose their shine when exposed to air, forming an oxide layer. To prevent this and ensure safety, metals like sodium are kept immersed in kerosene oil Science, Class X (NCERT 2025 ed.), Chapter 3, p.46.

Property	Typical Metals (e.g., Iron)	Alkali Metals (e.g., Sodium)
Hardness	Hard and strong	Soft (can be cut with a knife)
Density	High density	Low density
Melting Point	High melting point	Low melting point

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46

3. The Reactivity Series of Metals (intermediate)

The Reactivity Series (or Activity Series) is a vital tool in chemistry that ranks metals in decreasing order of their chemical reactivity. Instead of being a theoretical construct, this list is derived from empirical observations—specifically, how different metals react with water, oxygen, and acids. At the very top, we find metals like Potassium (K) and Sodium (Na), which are so reactive they must be stored in kerosene to prevent them from reacting violently with the moisture and oxygen in the air Science-Class VII, The World of Metals and Non-metals, p.52. At the bottom, we find "noble" metals like Gold (Au) and Silver (Ag), which are highly stable and often found in their pure, free state in the earth's crust Science, Metals and Non-metals, p.49.

The core logic used to build this series is the Displacement Reaction. A more reactive metal has the power to "displace" or push out a less reactive metal from its salt solution. For example, if you place a piece of Iron in a Copper(II) sulphate solution, the Iron will displace the Copper because it sits higher on the reactivity series. By systematically testing which metals can displace others, scientists have mapped out the relative strength of these elements Science, Metals and Non-metals, p.46. Interestingly, Hydrogen, though a non-metal, is often included in the series as a reference point to show which metals can displace hydrogen from acids or water.

Reactivity Level	Metals	Key Characteristics
High	K, Na, Ca, Mg	React vigorously with cold water; never found free in nature.
Medium	Al, Zn, Fe, Pb	React with steam or dilute acids; found as oxides or sulphides.
Low	Cu, Hg, Ag, Au	Very unreactive; often found in the "native" (pure) state.

Science, Metals and Non-metals, p.45

Sources: Science, Metals and Non-metals, p.45; Science, Metals and Non-metals, p.46; Science, Metals and Non-metals, p.49; Science-Class VII, The World of Metals and Non-metals, p.52

4. Nature of Metal Oxides and Hydroxides (intermediate)

To understand the chemistry of the periodic table, we must look at how metals interact with oxygen and water. At their core, metals are electropositive—they prefer to lose electrons. This fundamental trait dictates the nature of the compounds they form. Most metals react with oxygen to form metal oxides. Because these oxides react with acids to produce salt and water, we classify them as basic oxides. For example, when copper is heated, it forms a black coating of copper(II) oxide (2Cu + O₂ → 2CuO), which shows basic properties when it reacts with hydrochloric acid Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41.

However, the periodic table offers fascinating exceptions. Not all metal oxides are purely basic. Elements like aluminium and zinc produce amphoteric oxides. These are unique because they exhibit both acidic and basic behavior, reacting with both acids and strong bases to form salts and water Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.55. For instance, zinc oxide (ZnO) can react with sodium hydroxide to form sodium zincate (Na₂ZnO₂), a reaction that also releases hydrogen gas Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20.

When we introduce water into the mix, the reaction proceeds in stages. A metal first reacts with water to produce a metal oxide and hydrogen gas. If that metal oxide is soluble in water (like those of alkali metals), it dissolves further to form a metal hydroxide (Metal oxide + H₂O → Metal hydroxide) Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.43. The reactivity varies wildly across the table: while sodium and potassium react violently with cold water, catching fire instantly, calcium reacts more calmly, and some metals don't react with water at all.

Nature of Oxide	Characteristics	Examples
Basic Oxide	Reacts with acids to form salt and water; turns red litmus blue.	Na₂O, MgO, CuO
Amphoteric Oxide	Shows both acidic and basic properties; reacts with both acids and bases.	Al₂O₃, ZnO

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.43; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.55; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20

5. Adjacent Concept: Reaction of Metals with Dilute Acids (intermediate)

When we look at the interaction between metals and dilute acids, we are essentially observing a displacement reaction. In this process, a reactive metal acts as a more dominant partner, pushing hydrogen atoms out of the acid to take their place. This chemical 'hand-off' results in two distinct products: a metal salt and hydrogen gas (H₂). The general chemical equation for this interaction is:

Metal + Dilute Acid → Salt + Hydrogen gas

This behavior is a defining characteristic of acids; because all acids generate hydrogen ions in solution, they all tend to produce hydrogen gas when reacting with metals Science, Class X (NCERT), Acids, Bases and Salts, p.22. For instance, when iron reacts with dilute sulfuric acid (H₂SO₄), it forms iron(II) sulfate and releases hydrogen gas: Fe + H₂SO₄ → FeSO₄ + H₂ Science, Class X (NCERT), Metals and Non-metals, p.46. You can confirm the presence of this hydrogen gas by performing the 'squeaky pop' test—holding a burning splint near the mouth of the test tube, which causes the gas to ignite with a characteristic sound.

However, not all metals react with the same intensity. The 'vigor' of the reaction depends on the metal's position in the reactivity series. In a laboratory setting, if you were to drop different metal samples into dilute hydrochloric acid (HCl), you would notice a clear hierarchy in the rate of bubble formation and heat released (exothermic nature):

Metal	Observation	Reactivity Level
Magnesium (Mg)	Fastest bubble formation; highly exothermic.	Highest
Aluminium (Al)	Vigorous reaction.	High
Zinc (Zn)	Steady bubbling.	Moderate
Iron (Fe)	Slow bubbling.	Low
Copper (Cu)	No bubbles; no temperature change.	Non-reactive with dilute HCl

As noted in Science, Class X (NCERT), Metals and Non-metals, p.44, the reactivity decreases in the order Mg > Al > Zn > Fe. Metals like copper, gold, and silver are so unreactive that they do not displace hydrogen from dilute hydrochloric acid at all.

Sources: Science, Class X (NCERT), Metals and Non-metals, p.44, 46; Science, Class X (NCERT), Acids, Bases and Salts, p.20, 22

6. Chemical Reaction: Alkali Metals and Water (exam-level)

When we look at the Alkali Metals (Group 1 elements like Lithium, Sodium, and Potassium), we are looking at the most "eager" elements in the periodic table. Because they have a single electron in their outermost shell, they are incredibly keen to lose it to achieve stability. This makes their reaction with water one of the most famous and vigorous demonstrations in chemistry. When an alkali metal meets water, it undergoes a highly exothermic redox reaction, where the metal displaces hydrogen from the water molecule.

The process actually occurs in two rapid stages. First, the metal reacts with water to form a metal oxide and hydrogen gas. Since alkali metal oxides are highly soluble, they immediately dissolve in the water to form a metal hydroxide Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.43. The general chemical equation for this reaction is:

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g) + Heat Energy

As you move down the group from Lithium to Potassium, the reaction becomes increasingly violent. For Sodium and Potassium, the heat released is so intense that the evolved hydrogen gas catches fire almost instantly Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.43. Sodium melts into a silvery ball and skims across the water surface, while Potassium burns with a characteristic lilac flame. Because these metals react so vigorously even with the moisture in the air, they must be stored immersed in kerosene oil to prevent accidental combustion Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

Metal	Reaction Intensity	Key Observation
Lithium (Li)	Steady	Bubbles of hydrogen gas; metal floats and gradually disappears.
Sodium (Na)	Violent	Melts into a ball; darts on surface; hydrogen may ignite with an orange flame.
Potassium (K)	Extremely Violent	Catches fire immediately; burns with a distinct lilac flame.

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.43; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

7. Solving the Original PYQ (exam-level)

In your previous modules, you explored the periodic properties of elements and the nature of chemical bonding. This question brings those building blocks together by testing your understanding of Group 1 elements, also known as alkali metals. Because these metals have a single valence electron, they are highly electropositive and "eager" to react. When they encounter water, a single displacement reaction occurs where the metal displaces hydrogen from the water molecule to achieve a stable electronic configuration.

To arrive at the correct answer, visualize the reaction: the metal (M) reacts with water (H2O) to form a basic metal hydroxide (MOH) and release gas. The chemical equation 2M + 2H2O → 2MOH + H2 confirms that the evolved gas is Hydrogen. As noted in Science, Class X (NCERT), this reaction is so exothermic for elements like Sodium and Potassium that the evolved hydrogen often catches fire instantly, producing a 'squeaky pop' or a flame. This is a classic example of a redox reaction where the metal is oxidized and the hydrogen in water is reduced.

UPSC often includes distractors to test the precision of your conceptual clarity. Options like Oxygen and Ozone are common traps; while they are associated with atmospheric chemistry or the electrolysis of water, they are not products of simple metal-water displacement. Similarly, Hydrogen peroxide is a liquid compound often used as a reagent, but it is not a gaseous byproduct of this specific reaction. By focusing on the displacement principle you learned, you can confidently eliminate these and identify Hydrogen as the only logical gaseous product.