Ammonia (NH3) obtained from different sources always has same proportion of Nitrogen and Hydrogen. It proves the validity of law of:

1. Classification of Matter: Pure Substances vs. Mixtures (basic)

In our daily lives, we often see labels like "pure milk" or "pure honey," but in the world of science, these are actually complex mixtures. To a scientist, a pure substance is a form of matter that consists of only one type of particle. These constituent particles behave identically, meaning they have uniform chemical properties throughout Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130. Pure substances are further categorized into elements (the simplest building blocks, like Oxygen or Iron) and compounds (substances like H₂O, where elements are chemically bonded in a fixed ratio).

In contrast, a mixture is formed when two or more substances are physically combined without undergoing a chemical reaction. The most important distinction is that in a mixture, the individual components retain their original properties Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130. For example, in a salt-water mixture, the salt still tastes salty and the water can be evaporated away to recover it. However, once elements react to form a compound, they lose their individual identity and take on entirely new properties.

Feature	Pure Substance (Compound)	Mixture
Composition	Fixed ratio of elements by mass.	Variable ratio of components.
Separation	Cannot be separated by physical methods.	Can be separated by physical methods (filtration, evaporation).
Properties	Different from its constituent elements.	Components retain their original properties.

Understanding this classification helps us predict how matter will behave. While a pure substance like baking soda (NaHCO₃) will always have the same chemical makeup regardless of its brand, a mixture like soil will vary greatly depending on where you collect it Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.121.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130

2. Foundations of Atomic Theory (basic)

The journey to understanding matter began with a simple question: How far can we break something down? Long before modern laboratories, ancient thinkers like the Indian philosopher Acharya Kanad proposed that matter is composed of tiny, indivisible, and eternal particles called Parmanu Science, Class VIII, Particulate Nature of Matter, p.101. While these early ideas were philosophical, they laid the conceptual groundwork for what we now call the Atomic Theory—the idea that the universe is built from discrete building blocks rather than continuous substance.

As chemistry transitioned from philosophy to an experimental science, researchers discovered that chemical substances don't combine randomly; they follow strict mathematical rules known as the Laws of Chemical Combination. One of the most vital of these is the Law of Constant Proportions (or the Law of Definite Proportions), formulated by Joseph Proust in 1799. This law states that in a pure chemical compound, the constituent elements are always present in a fixed proportion by mass, regardless of the source or the method used to prepare it.

To visualize this, consider Ammonia (NH₃). Ammonia is formed when Nitrogen and Hydrogen atoms share electrons to reach a stable state Science, Class X, Carbon and its Compounds, p.59. Because Nitrogen has a specific atomic mass (approx. 14) and Hydrogen has a specific mass (approx. 1), they always bond in a way that results in a mass ratio of 14:3. Whether you extract ammonia from a decaying organic pile or synthesize it in a high-tech factory, this ratio never changes. This predictability is why we can write universal chemical formulas; if the proportions were random, the "recipe" for water or ammonia would change every time you moved to a different city!

Sources: Science, Class VIII (NCERT 2025), Particulate Nature of Matter, p.101; Science, Class X (NCERT 2025), Carbon and its Compounds, p.59

3. Atomic Mass and Molecular Mass (intermediate)

To understand the building blocks of matter, we must first look at how we measure them. Since atoms are unimaginably small, weighing them in grams is like trying to weigh a single grain of sand on a truck scale—it's just not practical. Instead, scientists use a relative scale. The standard for this scale is the Carbon-12 isotope. One atomic mass unit (u) is defined as exactly 1/12th the mass of a single carbon-12 atom. On this scale, Carbon is assigned a mass of 12 u, and Hydrogen is approximately 1 u Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66. While we often use the term 'weight' in daily life—like saying a bag of wheat weighs 10 kg—in chemistry, we specifically focus on mass, which represents the actual amount of matter in an object regardless of gravity Science, Class VIII, Exploring Forces, p.75.

Once we know the masses of individual atoms, calculating the Molecular Mass of a substance becomes a simple 'recipe' calculation. The molecular mass is the sum of the atomic masses of all the atoms present in a molecule. For example, if we look at Methane (CH₄), which contains one carbon atom and four hydrogen atoms, its molecular mass is (1 × 12 u) + (4 × 1 u) = 16 u Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. This principle applies to any compound, allowing us to compare different molecules in a homologous series, where each successive compound (like Methane to Ethane) typically differs by a fixed unit like –CH₂–, resulting in a consistent increase in molecular mass Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66.

Understanding these masses is the foundation of the Law of Definite Proportions. Because every atom of an element has a specific average mass, a chemical compound will always contain its constituent elements in a fixed ratio by mass. Whether you obtain water (H₂O) from a river or create it in a lab, the ratio of the mass of hydrogen to the mass of oxygen will always be 1:8. This consistency is what allows chemists to predict exactly how much of a product will be formed in a reaction.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64, 66, 67; Science, Class VIII (NCERT 2025 ed.), Exploring Forces, p.75

4. Isotopes and Atomic Variations (intermediate)

At the heart of chemistry is the identity of an atom, which is strictly determined by the number of protons in its nucleus, known as the Atomic Number (Z). For example, every atom of Iron (Fe) — the primary constituent of the Earth's core — contains exactly 26 protons Physical Geography by PMF IAS, The Solar System, p.19. However, while the number of protons is a fixed 'ID card' for an element, the number of neutrons in the nucleus can vary. These variations of the same element that differ only in their neutron count are called Isotopes. Because neutrons carry mass but no charge, isotopes of an element have the same chemical properties but different Mass Numbers (A).

Consider the element Hydrogen, which was one of the first atoms to form after the Big Bang Physical Geography by PMF IAS, The Universe, p.2. In its most common form, Hydrogen has just one proton and no neutrons. However, it has two significant variations: Deuterium (one proton, one neutron) and Tritium (one proton, two neutrons). Tritium is particularly noteworthy because it is radioactive and is monitored as a significant air pollutant near nuclear power plants Environment, Shankar IAS Academy, p.437. Despite these differences in mass, all three are still 'Hydrogen' because they all possess exactly one proton.

Understanding isotopes is crucial because the Atomic Mass seen on the Periodic Table is actually a weighted average of all naturally occurring isotopes of that element. While isotopes behave almost identically in chemical reactions, their physical stability varies. For instance, while most carbon is stable, Carbon-14 is radioactive and used for dating ancient organic materials. This concept of 'atomic variation' adds a layer of complexity to the basic laws of chemistry, reminding us that even within a 'pure' element, there is a hidden diversity at the subatomic level.

Sources: Physical Geography by PMF IAS, The Solar System, p.19; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.437

5. Physical vs. Chemical Changes (basic)

In our study of chemistry, we categorize the transformations of matter into two primary types: physical and chemical. This distinction is fundamental to understanding how substances behave and interact. A physical change is one in which a substance undergoes a change in its physical properties—such as shape, size, or state—but its chemical identity remains the same Science-Class VII, Changes Around Us: Physical and Chemical, p.59. In these changes, no new substance is formed. For instance, when water (H₂O) freezes into ice or evaporates into steam, the molecules remain H₂O; only the physical state has shifted. Many physical changes are reversible, though some, like tearing paper into tiny bits, are not easily reversed even if the identity of the material remains unchanged.

A chemical change, on the other hand, is a process where one or more new substances are formed Science-Class VII, Changes Around Us: Physical and Chemical, p.60. This involves a chemical reaction where the original substances (reactants) transform into different substances (products) with entirely new properties. Common indicators of a chemical change include the production of heat or light, the evolution of a gas (like CO₂), or a permanent change in color Science-Class VII, Changes Around Us: Physical and Chemical, p.63. Examples include the rusting of iron, cooking, and combustion (burning) Science-Class VII, Changes Around Us: Physical and Chemical, p.68.

To master this concept, you must recognize that some complex events involve both changes simultaneously. Consider a burning candle: the melting of the wax is a physical change (change of state), but the burning of the wax vapor is a chemical change because it reacts with oxygen to produce new substances like carbon dioxide and water vapor Science-Class VII, Changes Around Us: Physical and Chemical, p.65.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Properties Affected	Physical properties (size, shape, state).	Chemical composition and properties.
Examples	Melting ice, breaking glass, dissolving sugar.	Burning wood, rusting of iron, digestion of food.

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.60; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.63; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.65; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68

6. Law of Conservation of Mass (basic)

The Law of Conservation of Mass is a cornerstone of chemistry, establishing that matter is neither created nor destroyed during a chemical reaction. In simpler terms, if you were to weigh all your starting materials (reactants) and then weigh everything produced after the reaction (products), the total mass would remain exactly the same. This principle, first formulated by Antoine Lavoisier in 1789, tells us that while substances might change their appearance or chemical identity, the total quantity of matter involved is constant Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3.

At the atomic level, this law explains why chemical reactions occur the way they do. A reaction is essentially a high-speed shuffle; atoms break their old bonds and form new ones, but the atoms themselves do not disappear into thin air, nor do new ones spontaneously appear. Therefore, the number of atoms of each element must be identical on both the reactant and product sides of a chemical equation Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3. This is the fundamental reason why we must balance chemical equations—it is not just a mathematical exercise, but a physical necessity to satisfy the Law of Conservation of Mass.

When we write these equations, we often use the hit-and-trial method to ensure the number of atoms matches on both sides, using the smallest whole-number coefficients Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5. To make these descriptions even more precise, chemists include the physical states of the substances—solid (s), liquid (l), gas (g), or aqueous (aq)—to show how matter is transforming, even though its total mass remains unchanged Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14

7. Laws of Constant and Multiple Proportions (exam-level)

In our journey through the building blocks of chemistry, we encounter two fundamental laws that govern how elements decide to bond with one another. These laws, established in the late 18th and early 19th centuries, moved chemistry from mere alchemy to a precise science. Let's break them down from first principles.

1. The Law of Constant (or Definite) Proportions
Proposed by the French chemist Joseph Proust in 1799, this law states that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or how it was prepared. For example, whether you extract Ammonia (NH₃) from a synthetic lab process or from the decay of organic matter, it will always consist of Nitrogen and Hydrogen in a mass ratio of approximately 14:3. This tells us that nature is consistent; a molecule of water (H₂O) in a glacier is identical in composition to a molecule of water in a tropical rainstorm.

2. The Law of Multiple Proportions
Formulated by John Dalton in 1803, this law addresses what happens when elements are a bit more flexible and form more than one compound. It states that if two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers. Think of Carbon and Oxygen: they can form Carbon Monoxide (CO) or Carbon Dioxide (CO₂). If we fix the mass of Carbon at 12g, it combines with 16g of Oxygen in CO and 32g in CO₂. The ratio of Oxygen masses (16:32) simplifies to a clean 1:2.

It is interesting to note how the concept of "proportions" changes depending on the field of study. While in chemistry, these ratios are often fixed and definite, in fields like economics, we encounter the law of variable proportions, where the ratio of inputs can change, leading to different levels of marginal product Microeconomics (NCERT class XII 2025 ed.), Production and Costs, p.41. In chemistry, however, these "fixed" proportions are the very signature of a pure substance.

Feature	Law of Constant Proportions	Law of Multiple Proportions
Core Idea	One compound = One fixed ratio.	Two or more compounds = Simple ratios between them.
Key Scientist	Joseph Proust	John Dalton
Example	H₂O always has 1:8 mass ratio of H:O.	Ratio of Oxygen in CO vs CO₂ is 1:2.

Sources: Microeconomics (NCERT class XII 2025 ed.), Production and Costs, p.41

8. Solving the Original PYQ (exam-level)

Having just explored the fundamental laws of chemical combination, this question serves as a perfect bridge from theory to application. You have learned that matter follows strict mathematical rules when forming compounds. This specific question tests your ability to identify the "identity" of a chemical substance. By focusing on the fact that Ammonia ($NH_3$) maintains a fixed composition regardless of its origin, you are applying the building blocks of stoichiometry and molecular composition that we recently covered in our study of NCERT Class 9 Science.

To arrive at the correct answer, look for the "trigger phrase" in the stem: "different sources always has same proportion." This is the textbook definition of the Law of Constant Proportion (also known as the Law of Definite Proportions). As a coach, I suggest you think of this as the "fixed recipe" rule—no matter who cooks the dish or where the ingredients come from, the ratio of Nitrogen to Hydrogen by mass will always be approximately 14:3. Therefore, the observation directly validates (B) Constant proportion.

UPSC often uses the other laws as "distractors" to test if you can distinguish between similar-sounding principles. The Law of Multiple Proportions is a common trap; it applies only when the same two elements form multiple different compounds (like $CO$ and $CO_2$). Reciprocal Proportion is even more complex, involving the weight ratios of three different elements. Since this question remains focused on a single compound ($NH_3$) across various sources, you can confidently eliminate those options and avoid the trap of over-complicating the relationship between the elements.