A chemical reaction is balanced when the number of atoms of each element is the same on both sides of the equation. This can be determined by inspecting the coefficients in front of each molecule or compound in the equation. For example, consider the following balanced chemical equation:

$$CH_4 + 2O_2 → CO_2 + 2H_2O$$

In this equation, there is one carbon atom on both sides of the equation, four hydrogen atoms on both sides, and two oxygen atoms on both sides. Therefore, the equation is balanced.

If an equation is not balanced, it can be balanced by adjusting the coefficients in front of the molecules or compounds. For example, consider the following unbalanced chemical equation:

$$CH_4 + O_2 → CO_2 + H_2O$$

In this equation, there is one carbon atom on both sides of the equation, four hydrogen atoms on both sides, but only two oxygen atoms on the product side. To balance the equation, we need to add a coefficient of 2 in front of the O2 molecule:

$$CH_4 + 2O_2 → CO_2 + 2H_2O$$

Now the equation is balanced, with one carbon atom, four hydrogen atoms, and two oxygen atoms on both sides.

Balancing chemical equations can be a challenging task, but it is essential to ensure that the law of conservation of mass is upheld.