--- title: "Balancing Equations and Conservation of Mass" description: "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 l" canonical: https://lightmysky.com/learn/science/balancing-equations-and-conservation-of-mass-mt_KUaPCEIA5f source: https://lightmysky.com/learn/science/balancing-equations-and-conservation-of-mass-mt_KUaPCEIA5f.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Balancing Equations and Conservation of Mass 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. Subject: Science · Area: Matter & Materials · Ages 15 to 16 Page: https://lightmysky.com/learn/science/balancing-equations-and-conservation-of-mass-mt_KUaPCEIA5f ## Ready when they can - Turns a word equation into a balanced symbol equation with correct formulas - Adds state symbols and says what each one tells a reader - Explains an apparent mass gain when a metal burns and an apparent loss when gas escapes an open flask - Checks a proposed equation and says which element is not balanced ## Lesson: Balancing equations and conservation of mass 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. *(drawing: Only the numbers in front may move. Doubling MgO fixes the oxygen, and then the magnesium has to be doubled to match.)* **Example.** 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. *(drawing: Four short labels that say what a reader would actually see in the flask.)* **Example.** 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. **Recap.** 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](https://creativecommons.org/licenses/by/4.0/) ## Practice 28 questions on this page, each with its working shown. ## Needs first - [Relative Formula Mass and the Mole](https://lightmysky.com/learn/science/relative-formula-mass-and-the-mole-mt_-SiMKJK4rH) - [Conservation of Mass](https://lightmysky.com/learn/science/conservation-of-mass-mt_ehC1wsdmUz) - [Naming Ionic and Simple Molecular Compounds](https://lightmysky.com/learn/science/naming-ionic-and-simple-molecular-compounds-mt_GJ267pzxCI) ## Opens up - [Reacting Masses, Limiting Reactants and Yield](https://lightmysky.com/learn/science/reacting-masses-limiting-reactants-and-yield-mt_C2K6kp5Chv)