--- title: "Is it a new substance?" subtitle: "Year 8 Chemistry · Chemical change · Lesson 7" --- # Is it a new substance? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** You can melt ice and freeze it again. You can't unburn toast. Is that how to tell a chemical change? Dissolved salt comes back when the water dries up… :::teacher-note **In one sentence:** Students state the new substance test for chemical change, explain it as atoms rearranging into new particles, and use it in place of the reversibility rule (AC9S8U07, node 2, a hinge node). **Students should be able to say:** I can state that a chemical change makes a new substance. I can classify changes as physical or chemical with the new substance test. I can explain why "can it be reversed?" is not a reliable test, with a counterexample. **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 **Someone says: "If a change can be reversed it is physical, and if it can't be reversed it is chemical." Is that a reliable test?** ☐ Yes, it always works ☐ No, it never works ☐ It works for most changes but there are exceptions both ways My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It has exceptions both ways. Dissolving salt can be reversed and is physical, but some chemical changes can be reversed too (recharging a battery). The reliable test is whether a new substance is made. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the new substance test :::keep-together ### The reaction **On the reaction board, what happened to the atoms?** ☐ They rearranged into new particles: hydrogen chloride ☐ New atoms were made ☐ Some atoms were destroyed ☐ They melted together ::: :::teacher-note **Answer:** They rearranged into new particles: hydrogen chloride The atoms are the same; they are arranged into different particles. A new particle means a new substance. ::: :::keep-together ### The change of state **When the ice melted and boiled, what happened to the water particles?** ☐ They stayed water particles; only their spacing and motion changed ☐ They became steam particles, a new substance ☐ They turned into hydrogen and oxygen ::: :::teacher-note **Answer:** They stayed water particles; only their spacing and motion changed A change of state is physical: the same particles, arranged and moving differently. ::: :::keep-together ### Physical or chemical **Physical or chemical? Use the new substance test.** Groups: Physical, Chemical - Ice melting: {{field-15}} - Salt dissolving in water: {{field-15}} - Paper burning: {{field-15}} - An iron nail rusting: {{field-15}} - Cooking an egg: {{field-15}} - Water boiling: {{field-15}} - Baking soda fizzing in vinegar: {{field-15}} - Chocolate melting: {{field-15}} ::: :::teacher-note **Answer:** Ice melting → Physical; Salt dissolving in water → Physical; Paper burning → Chemical; An iron nail rusting → Chemical; Cooking an egg → Chemical; Water boiling → Physical; Baking soda fizzing in vinegar → Chemical; Chocolate melting → Physical ::: :::keep-together ### When the old rule fails **Dissolving salt can be reversed, and it is physical. Which of these breaks the old "can it be reversed?" rule the other way?** ☐ Cutting paper ☐ Freezing water ☐ Recharging a battery: a chemical change that can be reversed ::: :::teacher-note **Answer:** Recharging a battery: a chemical change that can be reversed Charging and using a battery runs chemical reactions forwards and backwards. Reversibility is a clue, not the test. ::: :::typical ## Elaborate: is dissolving chemical? :::keep-together ### Disappear, or spread out? **The salt disappeared. Is dissolving a chemical change?** ☐ No: the salt particles are still there, spread between the water particles ☐ Yes: the salt turned into water ☐ Yes: the salt was destroyed ::: :::teacher-note **Answer:** No: the salt particles are still there, spread between the water particles No new substance: physical. The salt particles spread out between the water particles. ::: ::: :::wider ## Elaborate: burning :::keep-together ### Where did the atoms go? **When methane burns, what happens to its atoms?** Groups: True, False - The carbon atoms end up in carbon dioxide: {{field-15}} - The hydrogen atoms end up in water: {{field-15}} - The methane is destroyed and turns into energy: {{field-15}} - The gas gets hotter: {{field-15}} ::: :::teacher-note **Answer:** The carbon atoms end up in carbon dioxide → True; The hydrogen atoms end up in water → True; The methane is destroyed and turns into energy → False; The gas gets hotter → True ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **chemical change** makes a **new substance**: atoms separate and join into new particles. - A **physical change** (melting, boiling, dissolving) keeps the same particles. - **"Is there a new substance?"** is the test. "Can it be reversed?" is only a clue: dissolving can be reversed and is physical. - In a chemical change no atoms are made or destroyed; they are rearranged. ## Exit ticket :::keep-together **Is rusting physical or chemical? How do you know?** {{lines-3}} ::: :::teacher-note **Answer:** Chemical: rust (iron oxide) is a new substance with different properties from iron. :::