A balanced chemical equation obeys the law of

Analysis

The question asks for the fundamental scientific principle that necessitates balancing a chemical equation. In chemistry, we represent reactions using equations, and for these to be valid, they must account for every atom present before and after the reaction.

Concept Explanation

The Law of Conservation of Mass (formulated by Antoine Lavoisier) states that matter is neither created nor destroyed in a chemical reaction.

When you write a chemical equation, you are essentially creating a mathematical balance sheet. Because atoms cannot simply vanish or appear out of thin air, the total mass of the reactants must equal the total mass of the products. Balancing an equation ensures that the number of atoms of each element is the same on both sides of the arrow, which satisfies this law.

• Option B (Definite Proportions) refers to the fact that a chemical compound always contains the same elements in the same proportion by mass.
• Option C (Multiple Proportions) deals with two elements forming more than one compound.
• Option D (Conservation of Energy) relates to the First Law of Thermodynamics, which is important in thermochemistry but is not the primary reason for balancing the stoichiometry of an equation.

Step-by-Step Solution

1. Examine the requirement: A balanced equation requires that the number of atoms of each element on the left (reactants) equals the number of atoms on the right (products).
2. Apply the principle: Since atoms have mass, having an equal number of atoms on both sides ensures that the total mass remains constant throughout the reaction.
3. Conclusion: This direct correspondence between the number of atoms and the mass of the substances confirms that the equation is governed by the Law of Conservation of Mass.

Exam Tip

Whenever you see a question about "balancing equations," immediately associate it with "conservation of mass." Think of it like a scale: if you put 10g of reactants on one side, you must have 10g of products on the other side. If the numbers don't match, the equation isn't "balanced," and the law is being violated.