Which of the following is the best example of the law of conservation of mass?

1. Matter and Chemical Transformations (basic)

In our study of the physical world, we observe that matter is constantly undergoing transformations. These changes are broadly classified into two categories: physical changes and chemical changes. A physical change occurs when a substance alters its physical properties—such as shape, size, or state (solid, liquid, gas)—without forming a new substance. For instance, when you chop vegetables or when water freezes into ice, the internal identity of the molecules remains the same Science-Class VII, Changes Around Us: Physical and Chemical, p.59. Many physical changes are reversible, but the defining feature is that no new chemical species are created Science-Class VII, Changes Around Us: Physical and Chemical, p.68.

A chemical change, or a chemical reaction, is more profound. Here, the original substances (reactants) react to form entirely new substances (products) with different chemical properties. Common examples include the rusting of iron, the curdling of milk, or the combustion of fuel Science-Class VII, Changes Around Us: Physical and Chemical, p.68, 70. During these transformations, the atoms of the reactants are rearranged to create new combinations. This process is often accompanied by observable signs such as a change in color, the evolution of a gas, a change in temperature, or the formation of a precipitate Science, class X, Chemical Reactions and Equations, p.12.

Crucially, even though a substance might look entirely different after a chemical reaction, it must follow the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products Science, class X, Chemical Reactions and Equations, p.3. For example, if 12g of Carbon (C) reacts completely with 32g of Oxygen (O₂), they will produce exactly 44g of Carbon Dioxide (CO₂). The atoms have simply rearranged, but the total quantity of matter remains constant.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Nature	Only physical properties (size, shape, state) change.	Chemical properties and composition change.
Examples	Melting ice, tearing paper, dissolving salt.	Burning wood, cooking an egg, digestion.

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.59, 68, 70; Science, class X, Chemical Reactions and Equations, p.3, 12

2. Atoms and Dalton's Atomic Theory (basic)

Imagine you keep cutting a piece of gold into smaller and smaller bits. Eventually, you would reach a particle so tiny it cannot be divided further while still remaining 'gold.' This is the atom. While ancient philosophers speculated about these tiny units, it was John Dalton in the early 19th century who transformed these ideas into a scientific framework. Dalton proposed that all matter is composed of indivisible particles, which serve as the fundamental 'building blocks' of everything we see around us. Dalton’s Atomic Theory is built on several key pillars that help us understand how chemistry works at a microscopic level. He stated that atoms of a given element are identical in mass and chemical properties, whereas atoms of different elements differ in these traits. For instance, in the study of carbon compounds, we see that carbon atoms consistently have an atomic mass of 12 u Science, Class X (NCERT 2025 ed.), Chapter 4, p. 66. Furthermore, Dalton explained that compounds are formed when atoms of different elements combine in simple, fixed, whole-number ratios — which is why we see formulas like CO₂ or H₂O rather than fractional combinations. Crucially, Dalton’s theory provides the logical foundation for the Law of Conservation of Mass. He argued that chemical reactions do not create or destroy atoms; they merely rearrange them. If you start a reaction with a certain number of carbon and oxygen atoms, you must end with that same number, even if they have bonded differently Science, Class X (NCERT 2025 ed.), Chapter 1, p. 3. This is why the total mass of reactants must always equal the total mass of the products in a balanced chemical equation.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3

3. Laws of Chemical Combination (intermediate)

When we look at chemical reactions, it might seem like matter is appearing or disappearing—think of a log burning into a tiny pile of ash. However, chemistry is governed by the Laws of Chemical Combination, which ensure that nature keeps a perfect set of books. The most fundamental of these is the Law of Conservation of Mass. This principle states that mass can neither be created nor destroyed in a chemical reaction. In practical terms, this means the total mass of the reactants (the starting substances) must be exactly equal to the total mass of the products (the resulting substances) Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3.

To understand why this happens, we have to look at the atomic level. During a chemical transformation, atoms are not being wiped out of existence or summoned from nothing; they are simply rearranged into new groupings. This is why we must always work with balanced chemical equations. If you start with four hydrogen atoms on the left side of an equation, you must end with four on the right, regardless of what new molecules they have formed Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14. For example, consider the combustion of carbon:

Reactants	→	Products
Carbon (12g) + Oxygen (32g)	→	Carbon Dioxide (44g)
Total: 44g	=	Total: 44g

While the Law of Conservation of Mass focuses on quantity, other laws like the Law of Definite Proportions tell us that a chemical compound always contains the same elements in the same fixed ratio by mass. Together, these laws transformed chemistry from a qualitative "guessing game" into a precise quantitative science, allowing chemists to predict exactly how much product a specific amount of raw material will yield.

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

4. Law of Definite Proportions (intermediate)

Once we understand that mass is conserved in a reaction, the next logical question is: in what specific quantities do elements combine? The Law of Definite Proportions (also known as the Law of Constant Proportions) answers this by stating that in a chemical substance, the elements are always present in definite proportions by mass. This principle, formulated by Joseph Proust, implies that a pure chemical compound will always contain the same elements combined in the exact same mass ratio, regardless of its source or the method used to prepare it. Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.124 To visualize this, consider Water (H₂O). Whether you collect water from a Himalayan glacier, a local tap, or synthesize it in a laboratory, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8. This occurs because two atoms of hydrogen (atomic mass ~1 each) always combine with one atom of oxygen (atomic mass ~16). As noted in academic texts, the ratio of the number of atoms in water is 2:1, but the ratio by mass remains constant at 1:8. Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.124 Similarly, in Ammonia (NH₃), nitrogen and hydrogen are always present in a mass ratio of 14:3.

Compound	Elements Involved	Ratio by Mass	Ratio by Number of Atoms
Water (H₂O)	H : O	1 : 8	2 : 1
Carbon Dioxide (CO₂)	C : O	3 : 8	1 : 2
Ammonia (NH₃)	N : H	14 : 3	1 : 3

This law is fundamental for UPSC aspirants because it distinguishes compounds from mixtures. While atmospheric gases can vary in concentration based on location or altitude Physical Geography by PMF IAS, Earths Atmosphere, p.271, the components of a chemical compound are locked in a rigid, unchanging mathematical relationship. This predictability allows chemists to calculate exactly how much of a reactant is needed to produce a specific amount of a product without any waste.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.124; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Earths Atmosphere, p.271

5. Balancing Chemical Equations (exam-level)

In chemistry, a chemical equation is more than just a recipe; it is a precise mathematical statement. The fundamental reason we balance equations is rooted in the Law of Conservation of Mass. As established by Antoine Lavoisier, mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the elements in the reactants (the starting substances) must exactly equal the total mass of the elements in the products (the substances formed) Science, Class X (NCERT 2025 ed.), Chapter 1, p.3.

When you first write a reaction, it is often a skeletal chemical equation—a simple representation that shows the formulas but doesn't yet account for the quantity of atoms Science, Class X (NCERT 2025 ed.), Chapter 1, p.3. To balance it, we use the Hit-and-Trial method. We adjust the coefficients (the numbers in front of the formulas) to ensure the number of atoms of each element is the same on both sides. It is crucial to remember: never change the subscripts in a chemical formula (like the '2' in H₂O) because doing so changes the identity of the substance itself Science, Class X (NCERT 2025 ed.), Chapter 1, p.5.

To make an equation truly informative for scientific study, we also include physical states. These notations tell us the form the matter takes during the reaction:

• (s) for solid
• (l) for liquid
• (g) for gas
• (aq) for aqueous (a substance dissolved in water)

For example, when iron reacts with steam to form iron oxide and hydrogen gas, we write it as: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) Science, Class X (NCERT 2025 ed.), Chapter 1, p.4-5.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.3; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.4; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.5

6. Law of Conservation of Mass (intermediate)

At the heart of every chemical change lies a fundamental rule of the universe: the Law of Conservation of Mass. This principle states that mass can neither be created nor destroyed in a chemical reaction. Think of a chemical reaction not as a magic trick where things disappear, but as a cosmic LEGO set—you can pull the bricks apart and rebuild them into a new shape, but the total number of bricks remains exactly the same. Consequently, the total mass of the reactants (the starting materials) must always equal the total mass of the products (the substances formed) Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3.

To understand this quantitatively, let us look at the combustion of carbon. If you take 12g of Carbon (C) and react it completely with 32g of Oxygen (O₂), you will produce exactly 44g of Carbon Dioxide (CO₂). The math is simple and absolute: 12 + 32 = 44. This isn't just a coincidence; it is because the number of atoms of each element remains identical before and after the reaction. Because each atom has a specific mass, if the count of atoms doesn't change, the total mass cannot change either Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3.

This law is the reason why scientists must balance chemical equations. If an equation shows more atoms on one side than the other, it violates this law, appearing to "create" matter out of thin air. By using the "hit-and-trial" method to add coefficients, we ensure the equation accurately reflects the physical reality that matter is conserved Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5. While we often use the term "weight" in daily life to describe the quantity of matter, in chemistry, we focus on mass—the actual quantity of matter present—because it remains constant regardless of the environment Science, Class VIII (NCERT 2025 ed.), Exploring Forces, p.75.

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 VIII (NCERT 2025 ed.), Exploring Forces, p.75

7. Solving the Original PYQ (exam-level)

Having mastered the fundamentals of chemical combinations, you can now see how the Law of Conservation of Mass acts as the cornerstone of stoichiometry. This law specifically dictates that in any chemical reaction, the total mass of the reactants must be equal to the total mass of the products, as mass is neither created nor destroyed. As highlighted in Science, class X (NCERT 2025 ed.), this principle is the very reason we must balance chemical equations to reflect the reality of atomic rearrangement during a transformation.

To arrive at the correct answer, you must look for a chemical transformation where different substances combine to form a new product while maintaining a constant mass. In Option (D), we see a precise quantitative demonstration: 12 gm of Carbon reacting with 32 gm of Oxygen to produce exactly 44 gm of Carbon Dioxide. The simple arithmetic (12 + 32 = 44) perfectly mirrors the theoretical requirement of the law, making Option (D) the most robust and scientifically accurate example because it illustrates the transition of matter from one chemical species to another without any loss in total mass.

UPSC often includes "distractor" options like (A), (B), and (C) which describe physical stability or physical changes rather than the specific law governing chemical reactions. While it is true that heating a platinum wire results in no mass change, this is an observation of physical properties and inert conditions. Similarly, the thermal expansion of air is a physical change involving volume and density, not a chemical reaction. The "trap" here is to confuse the general observation of mass remaining constant with the best example of a fundamental law of chemical combination.