--- marp: true theme: default html: true --- # Where does the mass go? Years 9 and 10 Chemistry · Equations and conservation of mass --- ## Today **We are learning to** in a chemical reaction atoms are rearranged, not made or destroyed, so the total mass of the reactants equals the total mass of the products. In a closed system the mass reading stays the same. In an open system it can seem to change because a gas escapes (mass falls) or a gas from the air joins in (mass rises); counting the gas shows the mass is still conserved. - 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. --- ## Why it matters > Burn a pile of steel wool and it gets heavier. Burn a log and it gets lighter. Is mass appearing and vanishing? --- 💬 Discussion ## Engage: predict **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 --- ✅ Check ## What happened? * 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. --- ✅ Check ## 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 * Answer: Atoms are only rearranged: none are made or destroyed, so the total mass stays the same --- ✅ Check ## 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: 20 --- ✅ Check ## 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: 1.6 --- ✅ Check ## 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 * Answer: Yes: most of the mass left as carbon dioxide and water vapour, which spread into the air --- ✅ Check ## 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 * Answer: The same atoms are on each side, so the mass is the same before and after --- ✅ Check ## Elaborate: predict the products **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: 8.8 --- ✅ Check ## Elaborate: waste **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 * Answer: The atoms that end up in unwanted by-products are wasted for the purpose, even though they still exist --- ## 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. --- ✅ Check ## Exit ticket **A student weighs a candle before and after burning it in an open dish and finds it lighter. Is mass conserved?** * 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.