--- title: "Where does the mass go?" subtitle: "Years 9 and 10 Chemistry · Equations and conservation of mass · Lesson 9" --- # Where does the mass go? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Burn a pile of steel wool and it gets heavier. Burn a log and it gets lighter. Is mass appearing and vanishing? :::teacher-note **In one sentence:** Students explain why total mass is conserved in a closed reaction, account for apparent changes in open systems, and connect balanced equations to mass conservation (AC9S9U07, node 5, an integration node). **Students should be able to say:** I can explain why mass is conserved in a closed system. I can explain why mass seems to change in an open system. I can say what a balanced equation shows about mass. **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 **An antacid tablet is dropped into water in an open beaker on a balance. It fizzes and the balance reading falls. Where has the missing mass gone?** ☐ Into the carbon dioxide gas that escaped into the air ☐ It was destroyed in the reaction ☐ It turned into energy ☐ The balance is faulty My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Into the carbon dioxide gas that escaped. If the same reaction were done in a sealed flask, the reading wouldn't change. The mass is conserved: the gas just left the balance. ::: ## Explain: counting everything :::keep-together ### Closed system **Why is the total mass the same before and after a reaction in a sealed flask?** ☐ Any gas made has no mass, so only the solids and liquids count ☐ The flask absorbs the difference ☐ Atoms are only rearranged: none are made or destroyed, so the total mass stays the same ::: :::teacher-note **Answer:** Atoms are only rearranged: none are made or destroyed, so the total mass stays the same The same atoms are present before and after, just joined in new ways. Same atoms, same total mass. ::: :::keep-together ### Magnesium in a flask **In a sealed flask, 12.0 g of magnesium reacts with 8.0 g of oxygen. What mass of magnesium oxide forms (g)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 20 The total mass of products equals the total mass of reactants: 12.0 + 8.0 = 20.0 g. ::: :::keep-together ### Magnesium in air **In an open dish, 2.4 g of magnesium burns and the white ash weighs 4.0 g. What mass of oxygen from the air joined in (g)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 1.6 4.0 − 2.4 = 1.6 g. The ash is heavier because it contains oxygen from the air. ::: :::keep-together ### The log **A log burns in an open fire and the ash is much lighter than the log. Is mass conserved?** ☐ Yes: because the ash is the same thing as the log ☐ No: burning destroys mass ☐ Yes: most of the mass left as carbon dioxide and water vapour, which spread into the air ::: :::teacher-note **Answer:** Yes: most of the mass left as carbon dioxide and water vapour, which spread into the air Count the gases, and the total mass of reactants (wood and oxygen) equals the total mass of products (ash, carbon dioxide and water vapour). ::: :::keep-together ### Balanced equation **The balanced equation 2H₂ + O₂ → 2H₂O has 4 hydrogen atoms and 2 oxygen atoms on each side. What does this tell you about mass?** ☐ Water is lighter than hydrogen and oxygen together ☐ Nothing: equations don't show mass ☐ The same atoms are on each side, so the mass is the same before and after ::: :::teacher-note **Answer:** The same atoms are on each side, so the mass is the same before and after Balancing is the same idea as conservation of mass: each atom appears on both sides the same number of times. ::: :::typical ## Elaborate: predict the products :::keep-together **Iron (5.6 g) reacts completely with sulfur (3.2 g) in a sealed tube to make iron sulfide. What mass of iron sulfide forms (g)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8.8 5.6 + 3.2 = 8.8 g. ::: ::: :::wider ## Elaborate: waste :::keep-together **If mass is always conserved, why do chemists worry about waste?** ☐ Mass can be destroyed in a factory ☐ Waste is only a problem for the environment ☐ The atoms that end up in unwanted by-products are wasted for the purpose, even though they still exist ::: :::teacher-note **Answer:** The atoms that end up in unwanted by-products are wasted for the purpose, even though they still exist Conservation says nothing is lost, but atoms in by-products aren't in the product you wanted. Good reactions put more of the atoms into the product. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - In a chemical reaction atoms are **rearranged, not made or destroyed**, so the total mass is **conserved**. - In a **closed system** the mass stays the same. - In an **open system** it can seem to change because a **gas escapes** or **joins in from the air**; counting the gas shows the mass is still conserved. - A **balanced equation** has the same atoms on both sides. ## Exit ticket :::keep-together **A student weighs a candle before and after burning it in an open dish and finds it lighter. Is mass conserved?** {{lines-3}} ::: :::teacher-note **Answer:** Yes. The missing mass left as carbon dioxide and water vapour. Weighing the candle and the oxygen used against the gases would show the same total. :::