A gas is evolved when a piece of zinc metal is placed in dilute sulphuric acid (H2S04). What is the gas?

1. Types of Chemical Reactions (basic)

At its core, a chemical reaction is the process of breaking and making bonds between atoms to produce new substances. It is important to remember that atoms of one element do not simply disappear or turn into atoms of another element; they merely rearrange themselves into new configurations. We classify these rearrangements into specific types based on how the reactants interact and what products they form. Understanding these patterns allows us to predict the outcome of chemical changes in both the lab and the natural world Science, Class X (NCERT 2025 ed.), Chapter 1, p.2.

The four primary types of chemical reactions are categorized by the movement of atoms or ions:

• Combination Reaction: Two or more substances (elements or compounds) combine to form a single product. For example, when Hydrogen gas and Chlorine gas meet, they react to form Hydrogen Chloride (H₂ + Cl₂ → 2HCl) Science, Class X (NCERT 2025 ed.), Chapter 1, p.15.
• Decomposition Reaction: This is the exact opposite of a combination reaction. Here, a single compound breaks down into two or more simpler substances. Because this involves breaking bonds, it usually requires energy in the form of heat, light, or electricity Science, Class X (NCERT 2025 ed.), Chapter 1, p.14.
• Displacement Reaction: A more reactive element "kicks out" or displaces a less reactive element from its compound. A classic example is placing Zinc metal in dilute Sulphuric acid. Since Zinc is more reactive than Hydrogen, it displaces the Hydrogen to form Zinc Sulphate and Hydrogen gas (Zn + H₂SO₄ → ZnSO₄ + H₂) Science, Class X (NCERT 2025 ed.), Chapter 2, p.20.
• Double Displacement Reaction: This involves an exchange of ions between two compounds. Often, one of the products formed is an insoluble solid called a precipitate that settles out of the solution Science, Class X (NCERT 2025 ed.), Chapter 1, p.12.

Beyond the movement of atoms, we also classify reactions by their energy profile. If heat is released during the reaction, making the surroundings warmer (like respiration), it is exothermic. If the reaction absorbs energy from the surroundings to proceed, it is endothermic Science, Class X (NCERT 2025 ed.), Chapter 1, p.15.

Reaction Type	General Equation	Key Feature
Combination	A + B → AB	Single product formed
Decomposition	AB → A + B	Single reactant breaks down
Displacement	A + BC → AC + B	More reactive element displaces less reactive one
Double Displacement	AB + CD → AD + CB	Exchange of ions; often forms a precipitate

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.2, 12, 14, 15; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.20

2. General Properties of Acids and Bases (basic)

To understand the chemistry of the world around us, we must first look at the general properties of acids and bases. At a fundamental level, an acid is a substance that produces hydrogen ions (H⁺) in an aqueous solution, while a base is one that produces hydroxide ions (OH⁻). This difference in ion production is what dictates their chemical behavior and strength Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 26.

One of the most defining chemical properties of acids is their reaction with metals. When a metal reacts with a dilute acid, it typically displaces hydrogen from the acid, resulting in the formation of a salt and the evolution of hydrogen gas (H₂). For example, when zinc reacts with sulphuric acid (H₂SO₄), it forms zinc sulphate (ZnSO₄) and releases H₂ gas. We can confirm the presence of hydrogen gas by performing the 'pop' test: if you bring a burning matchstick near the gas bubbles, they burst with a distinct popping sound Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 20.

It is important to distinguish between the strength of these substances. A strong acid, like hydrochloric acid (HCl), ionizes completely in water to release a high concentration of H⁺ ions. In contrast, a weak acid, such as acetic acid (ethanoic acid) found in vinegar, only partially ionizes, releasing fewer H⁺ ions Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p. 73. This concept applies to bases as well; strong bases produce more OH⁻ ions than weak bases.

Property	Acids	Bases
Ion Produced	Hydrogen ions (H⁺)	Hydroxide ions (OH⁻)
Reaction with Metal	Produces Salt + Hydrogen gas	Generally less reactive (some form salts/H₂)
Taste/Feel	Sour taste	Bitter taste; soapy feel

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.20, 22, 26; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.73

3. Nature and Formation of Salts (basic)

In the world of chemistry, a salt is not just the white crystals on your dining table; it is a broad class of ionic compounds formed through the interaction of acids and bases. At its most fundamental level, a salt is created when the hydrogen ions of an acid are replaced by metal ions or ammonium ions. This most commonly occurs through a process called a neutralisation reaction, where an acid and a base react to produce a salt and water Science, Class X (NCERT 2025 ed.), Chapter 2, p. 21. For instance, when Hydrochloric acid (HCl) meets Sodium hydroxide (NaOH), they produce Sodium chloride (NaCl) and Water (H₂O). This reaction effectively 'cancels out' the extreme properties of both the acid and the base, resulting in a solution that can be neutral Science-Class VII, NCERT (Revised ed. 2025), p. 18.

Beyond neutralisation, salts can also form when an acid reacts with a metal. In these instances, the metal 'displaces' hydrogen from the acid, resulting in the evolution of hydrogen gas and the formation of a salt. A classic laboratory example is the reaction between Zinc (Zn) and dilute Sulphuric acid (H₂SO₄), which yields Zinc sulphate (ZnSO₄) and Hydrogen gas (H₂). To help chemists organise the vast number of possible salts, we group them into families based on their shared positive or negative ions (radicals) Science, Class X (NCERT 2025 ed.), Chapter 2, p. 29. Salts like NaCl and KCl belong to the 'Chloride family' because they share the same negative ion, while NaCl and Na₂SO₄ belong to the 'Sodium family' because they share the same positive ion.

Understanding these families is crucial for predicting how a substance will behave in more complex chemical processes. Many common salts serve as the 'building blocks' for industrial chemicals; for example, common salt (NaCl) is the primary raw material used to manufacture bleaching powder, baking soda, and washing soda Science, Class X (NCERT 2025 ed.), Chapter 2, p. 30.

Feature	Neutralisation Reaction	Metal-Acid Reaction
Reactants	Acid + Base	Acid + Reactive Metal
Primary Product	Salt	Salt
By-product	Water (H₂O)	Hydrogen Gas (H₂)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.21; Science-Class VII, NCERT (Revised ed. 2025), Exploring Substances: Acidic, Basic, and Neutral, p.18; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.29; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.30

4. The Reactivity Series of Metals (intermediate)

In the world of chemistry, not all metals are created equal. Some are incredibly "eager" to react and lose electrons, while others are noble and indifferent to their surroundings. The Reactivity Series (or Activity Series) is a vertical arrangement of metals in order of their decreasing chemical reactivity Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45. At the very top, we find metals like Potassium (K) and Sodium (Na), which are so reactive they must be stored under oil to prevent them from reacting with air. At the bottom, we find "noble" metals like Gold (Au) and Platinum, which remain unchanged for centuries, often found in nature in their pure, free state Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49.

The core logic governing this series is the Displacement Principle. A metal higher in the series is more "powerful" (more electropositive) and can displace any metal below it from its salt solution. For example, if you place an iron nail in a blue copper sulphate solution, the iron (being higher in the series) will displace the copper, turning the solution green and depositing reddish-brown copper on the nail. This ability to lose electrons easily is what defines a metal's position. Interestingly, Hydrogen—though a non-metal—is included in the series as a benchmark. Metals placed above Hydrogen can displace it from dilute acids to evolve Hydrogen gas (H₂), whereas those below it, like Copper or Silver, cannot Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55.

Understanding this hierarchy is vital for Metallurgy (the extraction of metals). Metals at the top are so reactive they require electrolysis to be separated from their ores, while those in the middle (like Zinc and Iron) can be reduced using Carbon. Those at the bottom are so unreactive they often don't need much processing at all Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49.

Reactivity Category	Metals	Key Characteristic
High Reactivity	K, Na, Ca, Mg, Al	Displace Hydrogen from water; extracted by electrolysis.
Medium Reactivity	Zn, Fe, Pb	Displace Hydrogen from dilute acids; found as oxides/sulphides.
Low Reactivity	Cu, Hg, Ag, Au	Very stable; often found in free state in nature.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55

5. Corrosion and Galvanization (intermediate)

Corrosion is a natural process that gradually destroys metals through a chemical reaction with their environment. When a metal surface is attacked by substances like moisture, oxygen, or acids, it undergoes oxidation, leading to the formation of metal oxides, sulfides, or carbonates. This isn't just an aesthetic issue; it causes significant structural damage to bridges, ships, and vehicles Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13. While we often focus on iron, other metals corrode in distinct ways:

• Silver: It turns black because it reacts with sulfur in the air to form a layer of silver sulfide (Ag₂S).
• Copper: It reacts with moist carbon dioxide to lose its brown shine and gain a green coat of basic copper carbonate [CuCO₃·Cu(OH)₂].
• Iron: When exposed to moist air, it develops a brown, flaky substance called rust, which is essentially hydrated ferric oxide (Fe₂O₃·xH₂O) Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.53.

To prevent this decay, we use various barrier methods like painting, oiling, or greasing. However, one of the most effective industrial techniques is Galvanization. This involves coating iron or steel with a thin layer of Zinc. What makes galvanization remarkable is that the protection persists even if the zinc coating is scratched or broken. This happens because zinc is more reactive than iron; it effectively "sacrifices" itself by reacting with the environment first, thereby preventing the underlying iron from oxidizing. This principle is often called sacrificial protection Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54.

Another sophisticated way to prevent corrosion is by Alloying. Pure iron, for instance, is very soft and stretches easily when hot. By mixing it with small amounts of carbon, it becomes hard and strong. If we mix iron with nickel and chromium, we get stainless steel, which is hard and does not rust at all. Thus, alloying doesn't just prevent corrosion; it allows us to engineer the metal to have the exact properties we need Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.53; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54

6. Reaction of Metals with Dilute Acids (intermediate)

When we talk about the chemistry of metals, one of the most fundamental interactions to understand is their reaction with dilute acids. At its heart, this is a single displacement reaction. In this process, a reactive metal "displaces" or kicks out the hydrogen atoms from the acid, releasing them as hydrogen gas (H₂) while the metal combines with the remaining part of the acid to form a salt. The general principle can be written as:

Metal + Dilute Acid → Salt + Hydrogen gas

For example, when zinc granules are added to dilute sulphuric acid, the zinc displaces hydrogen to form zinc sulphate (the salt) and hydrogen gas Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20. The chemical equation for this is: Zn + H₂SO₄ → ZnSO₄ + H₂. You can visually confirm this reaction in a lab by observing the formation of bubbles around the metal surface. If you bring a burning matchstick near these bubbles, the gas burns with a characteristic 'pop' sound, which is the definitive test for hydrogen Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2.

However, not all metals react with the same intensity. The rate of reaction depends on the metal's position in the reactivity series. For instance, Magnesium reacts very vigorously and the reaction is highly exothermic (releases significant heat), whereas Iron reacts much more slowly Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44. Interestingly, some metals like Copper (Cu) do not react with dilute hydrochloric acid at all because they are less reactive than hydrogen and cannot displace it from the acid.

Metal	Reaction Observation	Relative Reactivity
Magnesium (Mg)	Rapid bubble formation; high heat release	Highest (in this group)
Aluminium (Al)	Fast reaction (after oxide layer dissolves)	High
Zinc (Zn)	Moderate bubble formation	Medium
Iron (Fe)	Slow bubble formation	Low
Copper (Cu)	No bubbles; no temperature change	No reaction

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44

7. Solving the Original PYQ (exam-level)

Now that you have mastered the Reactivity Series and the nature of Displacement Reactions, you can see these building blocks in action. This question tests your ability to apply the fundamental principle that more reactive metals displace hydrogen from dilute acids. In this scenario, Zinc acts as the displacing agent because it sits higher than hydrogen on the activity scale, a core concept found in Science, class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations. By recognizing the reactants, you are essentially identifying a standard Metal-Acid interaction.

To arrive at the correct answer, follow the chemical logic: when Zinc (Zn) reacts with dilute Sulphuric Acid (H2SO4), the zinc atoms transfer electrons to the hydrogen ions. This redox process results in the formation of Zinc Sulphate and the liberation of (A) Hydrogen gas. Remember the 'pop' sound test mentioned in Science, class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts; it is the definitive laboratory signature for this specific gas. The equation Zn + H2SO4 → ZnSO4 + H2 represents a clean displacement where the acid's hydrogen is released in molecular form.

UPSC often includes distractors like Sulphur dioxide to trap students who overcomplicate the reaction or confuse dilute acids with concentrated, highly oxidizing acids. Oxygen and Water vapour are classic decoys used to test if you can distinguish between oxidation-reduction in acids versus simple evaporation or decomposition. By sticking to the Activity Series, you can safely ignore these traps, knowing that a reactive metal like Zinc will always prioritize the displacement of hydrogen in a dilute acidic medium.