Final answer:
The student is ensuring that the number of atoms on each side of the reaction arrow is the same in her balanced chemical equation, as required by the law of conservation of matter.
Step-by-step explanation:
When a student is balancing a chemical equation in their chemistry class, they are using the fact that the number of atoms on each side of the reaction arrow must be the same. This reflects the answer to the question, which would be B). The number of atoms on each side of the reaction arrow must be the same. This requirement is due to the law of conservation of matter, which states that matter cannot be created or destroyed in a chemical reaction. To balance an equation, coefficients are used in front of reactants and products until the number and type of atoms are equal on both sides of the equation.
For instance, when a chemical equation is written as H₂(g) + O₂(g) → H₂O(l), it appears unbalanced because there are two oxygen atoms on the reactant side and only one on the product side. To balance this equation, we would write 2H₂O(l) on the product side, indicating that two water molecules are produced and therefore the equation would be balanced according to the conservation of mass. Additionally, the chemical symbols g (gas), l (liquid), s (solid), and aq (aqueous solutions) can be used to denote the states of compounds involved in the chemical reaction.