Balance a symbol equation so every element has the same atom count on both sides, add state symbols, and use the balance to explain why mass is conserved even when a reaction looks like it gained or lost some.
You can weigh out a mole now, which means you can count particles with a balance. The next thing to get right is what a reaction does to those particles. Conservation of mass is the rule you already have: matter is neither created nor destroyed by a change. In a reaction that means every atom that goes in comes out again, attached to something else. The club is about to burn a strip of magnesium ribbon, and the ash will weigh more than the ribbon did, which looks like the rule failing.
A symbol equation names what goes in on the left, the reactants, and what comes out on the right, the products. Written in names alone it is a word equation. Since no atom is created or destroyed, every element must appear in the same numbers on both sides, and making those counts match is balancing. One hard restriction: change only the large numbers in front of a formula. A front number multiplies the whole formula, so 3SO₂ means 3 sulfur atoms and 6 oxygen atoms. Never touch a subscript, because it is part of the substance: H₂O and H₂O₂ are different things.
Magnesium plus oxygen gives magnesium oxide. Swap each name for its formula and the word equation becomes Mg + O₂ → MgO. Now count. The left has 1 magnesium and 2 oxygens. The right has 1 of each, so the oxygen is short by one. Write a 2 in front of MgO and the right now has 2 magnesium and 2 oxygen. That leaves the magnesium short on the left, so write a 2 in front of Mg as well. The finished line is 2Mg + O₂ → 2MgO, with 2 magnesium and 2 oxygen on each side.
State symbols go in brackets straight after each formula and turn the equation into a description of the bench. (s) is solid, (l) is liquid, (g) is gas, and (aq) is aqueous, meaning dissolved in water. The magnesium line becomes 2Mg(s) + O₂(g) → 2MgO(s). The front numbers now do a second job as well. They are counts of particles, so 2Mg + O₂ also reads as two moles of magnesium reacting with one mole of oxygen molecules.
Now the two cases that look like the rule failing. Burn 48 g of magnesium ribbon and the ash weighs 80 g. Nothing was created: 32 g of oxygen came out of the air and joined it, and 48 + 32 = 80. Drop marble chips, which are calcium carbonate, into acid in an open flask and the balance reading falls, because CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ and the carbon dioxide leaves the flask. Seal it and the reading holds steady. Mass is conserved in both. A balance only ever sees what is still in the container.
No atom is created or destroyed, so every element has to appear in the same numbers on both sides. Balance by changing only the large numbers in front of a formula, never a subscript, then count everything again at the end. State symbols say what a reader would see: (s), (l), (g) and (aq). When a balance reading rises or falls during a reaction, something has crossed the edge of the container, not been made or destroyed.
Adapted from OpenStax Chemistry 2e (CC-BY 4.0), openstax.org · license
Last week you could turn a mass into a count of particles. This week the club burns something and the balance argues back. A strip of magnesium ribbon goes on: 48 g. It gets burned, and the white ash left behind reads 80 g.
Where did the extra 32 g come from?
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