A balanced chemical equation is based on the law of?

Step 1: Analyze the question The question asks which law a balanced chemical equation is based on. Balancing an equation means making the number of atoms of each element equal on both sides.

Step 2: Explain the concept Let's check each option:

• A. Periodicity: The periodic law says the properties of elements repeat periodically when they are arranged by atomic number. It deals with the periodic table, not equations.
• B. Constant composition (definite proportions): A given compound always contains the same elements in the same proportion by mass, whatever its source.
• C. Multiple proportion: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios (e.g. CO and CO₂).
• D. Conservation of mass: Matter is neither created nor destroyed in a chemical reaction, so the total mass of the reactants equals the total mass of the products.

Step 3: Show the solution Consider the formation of water:

Unbalanced: H₂ + O₂ → H₂O

• Left: 2 O atoms. Right: 1 O atom. This is not balanced, so an oxygen atom has "disappeared".

Balanced: 2H₂ + O₂ → 2H₂O

• Left: 4 H and 2 O. Right: 4 H and 2 O.

Since the atoms on both sides are equal, the mass on both sides is equal. This is exactly the law of conservation of mass (Lavoisier).

Step 4: Answer The correct option is D. Conservation of mass.

Step 5: Exam tip Remember: "Balance = Mass stays the same." Whenever you see "balanced equation" in a question, think of conservation of mass. Also remember that:

• Constant composition → same compound, same ratio.
• Multiple proportion → different compounds of the same elements, simple whole-number ratios.