Question

in a chemical equation, is it true that :
the reactant side and product side can have different numbers of atoms for each element
there can be different elements on the reactant side and the product side
the coefficients indicate the number of molecules of each reactant used and the number of molecules of each product made
an element or compound must be accompanied by a phase change

Explanation:

To determine the correct statement about chemical equations, we analyze each option:

1. The first option is false because of the Law of Conservation of Mass, which states that the number of atoms of each element must be the same on both the reactant and product sides.
2. The second option is false. In a balanced chemical equation, the elements on the reactant side and product side must be the same (since atoms are conserved, just rearranged).
3. The third option is true. In a chemical equation, the coefficients (the numbers in front of the chemical formulas) represent the number of molecules (or moles, in a macroscopic sense) of each reactant used and each product formed. For example, in the equation \(2H_2 + O_2

ightarrow 2H_2O\), the coefficient 2 in front of \(H_2\) means 2 molecules (or moles) of hydrogen gas react, 1 molecule (or mole, since the coefficient 1 is implied) of oxygen gas reacts, and 2 molecules (or moles) of water are produced.

1. The fourth option is false. While phase changes (like (s), (l), (g), (aq)) can be included to show the physical state of substances in a chemical equation, they are not mandatory. Substances can be written without indicating their phase, especially in some simplified or general chemical equations.

Answer:

The correct option is the third one: "the coefficients indicate the number of molecules of each reactant used and the number of molecules of each product made"