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# Why isn't salt poisonous?

Year 8 Chemistry · Elements and compounds

<!-- Speaker note: 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). About 25 minutes (Engage and Explore 7 min · Explain 10 min · Evaluate 3 min · Typical adds 5 min, Wider about 5 min). -->

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## Today

**We are learning to** a compound is two or more types of atom chemically joined in a fixed ratio. It is a new substance with new properties, and a formula like H₂O tells you which atoms and how many.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace models or demonstrations. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is students treating a compound as a mixture of its elements ("water is hydrogen and oxygen mixed"). The two Explore boards hold the atom types constant so that only joining differs. Accuracy note: sodium chloride is not made of NaCl molecules; it is a lattice of ions (Year 10). The lesson uses its formula only as a ratio and never draws it as a particle. Teacher: In your own words: the salt paragraph is the best evidence for the node. -->

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## Why it matters

> 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.

<!-- Speaker note: Last lesson: atoms come in types, one for each element. This lesson: what happens when different types join. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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

<!-- Speaker note: What happens: Table salt: a white solid you can eat. When different atoms join, the new substance has new properties. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: if you can, show a short video of sodium in water, then a salt shaker. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fcompounds%2Fstep-2&code=widget%3A+atoms%0Ashow%3A+H2+x8%2C+O2+x4%0Acaption%3A+A+mixture+of+hydrogen+and+oxygen.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/compounds/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask 'what is the same on both boards?' (atom types) and 'what is different?' (joined or not). -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Table salt: a white solid you can eat. When different atoms join, the new substance has new properties.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: Its hydrogen and oxygen atoms are chemically joined in each particle

<!-- Speaker note: In a compound, different types of atom are chemically joined. In a mixture they are just together, not joined. -->

<!-- Speaker note: Teacher: the first question is the checkpoint. The sort mixes formulas and descriptions on purpose. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Sort them

**Element, compound or mixture?**

Groups: Element · Compound · Mixture

O₂ · H₂O · CO₂ · He · Hydrogen gas and oxygen gas in one jar · NaCl · Air · N₂

* Answer: O₂ → Element; H₂O → Compound; CO₂ → Compound; He → Element; Hydrogen gas and oxygen gas in one jar → Mixture; NaCl → Compound; Air → Mixture; N₂ → Element

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Reading a formula

**Match each formula to the atoms in one particle.**

| | |
|---|---|
| `H₂O` | A. 1 carbon, 2 oxygen |
| `CO₂` | B. 2 hydrogen, 1 oxygen |
| `NH₃` | C. 1 nitrogen, 3 hydrogen |
| `CH₄` | D. 1 carbon, 4 hydrogen |

* 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

<!-- Speaker note: The small number belongs to the atom just before it. No number means one. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: The atoms are joined in a fixed ratio

<!-- Speaker note: Every water particle has exactly 2 hydrogen atoms for each oxygen atom. A mixture of hydrogen and oxygen gases can be in any ratio. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: It is a new substance: the atoms are joined, so it has new properties

<!-- Speaker note: A compound is a new substance. Its properties belong to the joined particles, not to the elements it was made from. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: find the compounds

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fcompounds%2Fstep-4&code=widget%3A+atoms%0Ashow%3A+H2O+x3%2C+CO2+x3%2C+NH3+x3%2C+CH4+x3%2C+O2+x2%0Acaption%3A+Five+gases.+Find+the+compounds.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/compounds/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Reading formulas from particle pictures, and back, builds both directions of the representation. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Count atoms

**How many atoms are in one particle of methane, CH₄?**

* Answer: 5

<!-- Speaker note: 1 carbon + 4 hydrogen = 5 atoms. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## A bigger formula

**Glucose is C₆H₁₂O₆. How many atoms are in one glucose particle?**

* Answer: 24

<!-- Speaker note: 6 carbon + 12 hydrogen + 6 oxygen = 24 atoms. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: making a compound

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fcompounds%2Fstep-5&code=widget%3A+reaction%0Ashow%3A+H2+x12%2C+O2+x6%0Areact%3A+2+H2+%2B+O2+-%3E+2+H2O%0Aea%3A+1.5%0Arelease%3A+1%0Agoal%3A+react%0Acaption%3A+%22Hydrogen+and+oxygen.+Watch+for+the+yellow+rings%3A+each+one+is+a+reaction.%22%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/compounds/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Seeing elements join into a compound links this node to chemical change: the compound is a new substance made by rearranging atoms. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: A compound has different properties from the elements it is made from

<!-- Speaker note:  -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Is carbon dioxide (CO₂) an element, a compound or a mixture? Why?**

* A compound: carbon and oxygen atoms are chemically joined in a fixed ratio, 1 carbon to 2 oxygen.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: chemical change (AC9S8U07), where compounds are made and broken: a new substance is the test. In class: build H₂O, CO₂, NH₃ and CH₄ with a model kit, then pull apart a water model to show that the elements' properties do not survive inside the compound. -->