Balancing Chemical Equations

0:00Hydrogen burns in oxygen to make water. How do we write it as a chemical equation?

0:06Hydrogen is H two, oxygen is O two, and water is H two O.

0:13Reactants, what we start with, go left of the arrow. Products, what is made, go right.

0:21The small 2 is a subscript. It counts the atoms of that element in one molecule.

0:27Atoms are never created or destroyed, so the total mass stays the same: the law of conservation of mass.

0:35So each kind of atom must have the same count on both sides.

0:40Hydrogen: 2 on the left, 2 on the right. Oxygen: 2 on the left, only 1 on the right.

0:48One oxygen atom cannot just vanish, so this equation is unbalanced.

0:54Could we just write H two O two? No. That is hydrogen peroxide, not water.

1:02Never change a subscript. Change only the coefficient, the big number in front.

1:082 H two O means two water molecules. Oxygen now matches, but hydrogen is 4 on the right.

1:16So write 2 in front of H two. Now every atom matches: it is balanced.

1:23Now methane, C H four, burns in oxygen to make carbon dioxide and water.

1:30Left and right: carbon 1 and 1, hydrogen 4 and 2, oxygen 2 and 3, so it is unbalanced.

1:40A 2 in front of H two O balances hydrogen, but makes oxygen 4 on the right.

1:47A 2 in front of O two gives 4 oxygen atoms on the left. Now it is balanced.

1:54Change only the coefficients, never the subscripts, so atoms and mass stay the same on both sides.