--- title: "Where do the atoms go?" subtitle: "Years 9 and 10 Chemistry · Equations and conservation of mass · Lesson 7" --- # Where do the atoms go? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A log burns down to a handful of ash. Where did the rest of the log go? :::teacher-note **In one sentence:** Students model a reaction as atoms rearranging, count atoms on both sides, and connect the unchanged atoms to conservation of mass (AC9S9U07, node 2, a hinge node). **Students should be able to say:** I can use a model to show a reaction with the same atoms before and after. I can count the atoms on each side of an equation and check they match. I can explain that atoms are rearranged, not made or destroyed, so mass is conserved. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **Methane (natural gas, CH₄) burns in a gas stove. What happens to the carbon and hydrogen atoms in the methane?** ☐ They turn into heat and light ☐ They float away as smoke, still as methane ☐ They join oxygen atoms to make new particles ☐ They are destroyed by the flame My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** They join oxygen atoms: carbon ends up in carbon dioxide (CO₂) and hydrogen in water (H₂O). Every atom is still there. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: rearranged, not destroyed :::keep-together ### The atom count **What happened to the number of each type of atom during the reaction?** ☐ It stayed the same ☐ It went up as new particles formed ☐ It went down as the methane was used up ::: :::teacher-note **Answer:** It stayed the same Particles change; atoms don't. Bonds break and new bonds form. ::: :::keep-together ### Count one side **In CH₄ + 2O₂ → CO₂ + 2H₂O, how many oxygen atoms are on the left side?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 4 2O₂ is two oxygen particles, each with 2 atoms: 4 oxygen atoms. ::: :::keep-together ### Count the other side **How many oxygen atoms are on the right side?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 4 CO₂ has 2, and 2H₂O has 2 (one in each water): 4. ::: :::keep-together ### Why mass is conserved **Why must the mass of the products equal the mass of the reactants?** ☐ The same atoms are there, just rearranged, and each atom keeps its mass ☐ The products are the same substances as the reactants ☐ Energy is conserved ::: :::teacher-note **Answer:** The same atoms are there, just rearranged, and each atom keeps its mass Same atoms, same mass. This is the law of conservation of mass. ::: ## balanced or not? :::keep-together ### Balanced or not **Is each equation balanced (the same atoms on both sides)?** Groups: Balanced, Not balanced - H₂ + Cl₂ → 2HCl: {{field-15}} - H₂ + O₂ → H₂O: {{field-15}} - 2H₂ + O₂ → 2H₂O: {{field-15}} - C + O₂ → CO₂: {{field-15}} - N₂ + H₂ → NH₃: {{field-15}} - CH₄ + O₂ → CO₂ + H₂O: {{field-15}} ::: :::teacher-note **Answer:** H₂ + Cl₂ → 2HCl → Balanced; H₂ + O₂ → H₂O → Not balanced; 2H₂ + O₂ → 2H₂O → Balanced; C + O₂ → CO₂ → Balanced; N₂ + H₂ → NH₃ → Not balanced; CH₄ + O₂ → CO₂ + H₂O → Not balanced ::: :::keep-together ### Changing the formula **To balance H₂ + O₂ → H₂O, a student changes it to H₂ + O₂ → H₂O₂. What is wrong?** ☐ Nothing: it balances now ☐ You can't put a 2 after O ☐ H₂O₂ is a different substance (hydrogen peroxide), not water ::: :::teacher-note **Answer:** H₂O₂ is a different substance (hydrogen peroxide), not water Balance by changing how many particles (the big numbers in front), never the formulas: 2H₂ + O₂ → 2H₂O. ::: :::typical ## Elaborate: making water :::keep-together ### Predict the amount **In 2H₂ + O₂ → 2H₂O, 12 hydrogen particles react with 6 oxygen particles. How many water particles can form?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 12 Every 2 H₂ and 1 O₂ make 2 H₂O. Six times over: 12 H₂O, with 24 H and 12 O atoms, the same as at the start. ::: ::: :::wider ## Elaborate: open and closed :::keep-together ### An open fire **Wood burned on an open fire leaves ash that weighs much less than the wood. Does that break conservation of mass?** ☐ Yes: the energy given out had mass ☐ Yes: the burnt atoms were destroyed ☐ No: carbon dioxide and water vapour went into the air, carrying atoms away ::: :::teacher-note **Answer:** No: carbon dioxide and water vapour went into the air, carrying atoms away In an open container, gaseous products escape. In a sealed one, the mass doesn't change. ::: :::keep-together ### A rusting nail **An iron nail gets heavier as it rusts. Where does the extra mass come from?** ☐ Water makes it heavier ☐ Rust is a heavier kind of iron ☐ Oxygen atoms from the air join the iron ::: :::teacher-note **Answer:** Oxygen atoms from the air join the iron Iron + oxygen → iron oxide. The mass gained is the mass of the oxygen atoms. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - In a chemical reaction, **atoms are rearranged**: bonds break and new bonds form. No atom is made or destroyed. - The same atoms means the **same mass**: the law of **conservation of mass**. - A **balanced equation** has the same number of each type of atom on both sides. Balance it by changing the numbers in front, never the formulas. - Mass seems to change only when a gas escapes or joins from the air. ## Exit ticket :::keep-together **When charcoal burns in a sealed jar, does the total mass change? Why?** {{lines-3}} ::: :::teacher-note **Answer:** No: the carbon and oxygen atoms are rearranged into carbon dioxide, which stays in the jar. Same atoms, same mass. :::