--- title: "Why isn't salt poisonous?" subtitle: "Year 8 Chemistry · Elements and compounds · Lesson 3" --- # Why isn't salt poisonous? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Sodium is a metal that fizzes and can burst into flame in water. Chlorine is a poisonous gas. Put them together and you can sprinkle the result on your chips. :::teacher-note **In one sentence:** Students define a compound (different atom types chemically joined in a fixed ratio, a new substance with new properties), tell it apart from a mixture, and read formulas (AC9S8U06, node 3, a hinge node). **Students should be able to say:** I can define a compound as two or more types of atom chemically joined in a fixed ratio. I can read a formula to find the atom types and how many of each. I can explain that a compound has different properties from the elements it is made from. **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 **Sodium is a soft metal that fizzes and can catch fire in water. Chlorine is a poisonous green gas. When they react they make sodium chloride. What do you predict sodium chloride is like?** ☐ A green gas that fizzes in water ☐ Something new, not like either of them ☐ A poisonous metal ☐ Half metal and half gas My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Table salt: a white solid you can eat. When different atoms join, the new substance has new properties. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: compounds and formulas :::keep-together ### Joined or mixed **Both boards had hydrogen and oxygen atoms. What made water a compound and not a mixture?** ☐ It has more atoms ☐ It is a liquid ☐ Its atoms are closer together ☐ Its hydrogen and oxygen atoms are chemically joined in each particle ::: :::teacher-note **Answer:** Its hydrogen and oxygen atoms are chemically joined in each particle In a compound, different types of atom are chemically joined. In a mixture they are just together, not joined. ::: :::keep-together ### Sort them **Element, compound or mixture?** Groups: Element, Compound, Mixture - O₂: {{field-15}} - H₂O: {{field-15}} - CO₂: {{field-15}} - He: {{field-15}} - Hydrogen gas and oxygen gas in one jar: {{field-15}} - NaCl: {{field-15}} - Air: {{field-15}} - N₂: {{field-15}} ::: :::teacher-note **Answer:** O₂ → Element; H₂O → Compound; CO₂ → Compound; He → Element; Hydrogen gas and oxygen gas in one jar → Mixture; NaCl → Compound; Air → Mixture; N₂ → Element ::: :::keep-together ### Reading a formula **Match each formula to the atoms in one particle.** A. 1 carbon, 2 oxygen B. 2 hydrogen, 1 oxygen C. 1 nitrogen, 3 hydrogen D. 1 carbon, 4 hydrogen {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `H₂O` | | | `CO₂` | | | `NH₃` | | | `CH₄` | | ::: :::teacher-note **Answer:** H₂O → B. 2 hydrogen, 1 oxygen; CO₂ → A. 1 carbon, 2 oxygen; NH₃ → C. 1 nitrogen, 3 hydrogen; CH₄ → D. 1 carbon, 4 hydrogen The small number belongs to the atom just before it. No number means one. ::: :::keep-together ### Fixed ratio **Water is always H₂O, never H₃O or HO. What does that tell you about compounds?** ☐ Water can be any mix of hydrogen and oxygen ☐ The atoms are joined in a fixed ratio ☐ Hydrogen is rarer than oxygen ::: :::teacher-note **Answer:** The atoms are joined in a fixed ratio Every water particle has exactly 2 hydrogen atoms for each oxygen atom. A mixture of hydrogen and oxygen gases can be in any ratio. ::: :::keep-together ### Why salt is safe **Sodium fizzes in water and chlorine is poisonous, yet sodium chloride is safe to eat. Why?** ☐ The sodium and chlorine atoms changed into other kinds of atom ☐ There is only a tiny amount of sodium and chlorine in it ☐ The sodium and chlorine are mixed in it, and cancel each other out ☐ It is a new substance: the atoms are joined, so it has new properties ::: :::teacher-note **Answer:** It is a new substance: the atoms are joined, so it has new properties A compound is a new substance. Its properties belong to the joined particles, not to the elements it was made from. ::: :::typical ## Elaborate: find the compounds :::keep-together ### Count atoms **How many atoms are in one particle of methane, CH₄?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 5 1 carbon + 4 hydrogen = 5 atoms. ::: :::keep-together ### A bigger formula **Glucose is C₆H₁₂O₆. How many atoms are in one glucose particle?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 24 6 carbon + 12 hydrogen + 6 oxygen = 24 atoms. ::: ::: :::wider ## Elaborate: making a compound :::keep-together ### New properties **Hydrogen and oxygen are gases that let things burn. Water puts fires out. What does that show?** ☐ A compound has different properties from the elements it is made from ☐ Water is a mixture of hydrogen and oxygen ☐ The atoms changed type ::: :::teacher-note **Answer:** A compound has different properties from the elements it is made from ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **compound** is two or more **types** of atom **chemically joined** in a **fixed ratio**. - A compound is a **new substance** with new properties (sodium chloride is not like sodium or chlorine). - In a **mixture**, substances are together but not joined, and each keeps its properties. - A **formula** gives the atoms in one particle: H₂O is 2 hydrogen and 1 oxygen. ## Exit ticket :::keep-together **Is carbon dioxide (CO₂) an element, a compound or a mixture? Why?** {{lines-3}} ::: :::teacher-note **Answer:** A compound: carbon and oxygen atoms are chemically joined in a fixed ratio, 1 carbon to 2 oxygen. :::