Conservation of mass: atoms are rearranged, never created or destroyed

1. The law of conservation of mass states that the total mass of the products equals the total mass of the reactants.
   1. No atoms are created and no atoms are destroyed during a chemical reaction.
   2. The reaction only rearranges the existing atoms into new substances.
2. The mass stays the same because every atom present at the start is still present at the end, just joined together differently.
3. Burning 48 g of magnesium in 32 g of oxygen produces exactly 80 g of magnesium oxide.

Symbol equations: the same atoms appear on both sides of the arrow

1. A symbol equation shows the reactants and products using their chemical formulae.
2. The reactants go on the left of the arrow and the products go on the right.
3. An equation is balanced when each element has the same number of atoms on both sides.
4. The balanced equation for magnesium burning is $2Mg+O_2\to 2MgO$.

Multipliers and subscripts: two numbers that count very different things

1. A multiplier is the full-size number written in front of a formula, and it multiplies every atom in that whole formula.
   1. A formula written with no multiplier counts as one.
2. A subscript is the small number inside a formula, and it counts only the atoms of the element written immediately before it.
   1. An element symbol with no subscript counts as one atom.
3. So $3H_{2}O$ means three water molecules, which is six hydrogen atoms and three oxygen atoms in total.

Balancing an equation: adjust the multipliers, then count again

1. Write the correct chemical formula for every reactant and product first.
2. Count the atoms of each element on both sides.
3. Put whole-number multipliers in front of formulae until each element matches on both sides.
4. Count every element one last time to confirm the equation balances.

Checking your answer: count each element separately, every time

1. Count each element on its own, because matching the total number of atoms is not enough.
2. Multiply every subscript in a formula by the multiplier in front of it to get the true atom count.
3. If one element does not match, change a multiplier and then recheck the whole equation from the start.

State symbols show the physical state of each substance

1. You can add state symbols in brackets after each formula to show its physical state.
   1. $(s)$ is solid, $(l)$ is liquid and $(g)$ is gas.
   2. $(aq)$ means aqueous, which is a substance dissolved in water.
2. Adding state symbols does not change the balancing, as it only describes each substance.