Reactions that add up

how do you balance a chemical equation

You adjust the coefficients in front of each formula until the number of atoms of every element is the same on both sides of the arrow, without ever changing the formulas themselves. Working element by element and saving hydrogen and oxygen for last usually gets you there fastest. This course walks through that balancing process step by step.

why can't atoms be created or destroyed in a reaction

In a chemical reaction the atoms present at the start are simply rearranged into new combinations, so none appear from nowhere and none vanish. That is why the total mass of the products equals the total mass of the reactants. This principle, the conservation of matter, is the idea the whole course is built around.

what is the difference between a coefficient and a subscript

A subscript is part of a formula and tells you how many atoms of one element are in a single molecule, so changing it changes the substance itself. A coefficient sits in front of a whole formula and multiplies everything in it, and it is the only thing you are allowed to change when balancing.

how do you count atoms on each side of an equation

Multiply each element's subscript by the coefficient in front of its formula, then add up that element across every substance on one side. Do the same for the other side and compare the totals for each element one at a time. Keeping an orderly tally like this is the habit the course helps you build.

why does the mass stay the same before and after a reaction

Because the same atoms are present throughout and each atom keeps its mass, the reactants and products weigh exactly the same overall. A balanced equation is really a statement that this bookkeeping works out on both sides.