Year 8 Science · Unit 2 · Lesson 9
Challenge Worksheet
Learning Goals
Evaluate the claim
Someone claims…
"Sodium is a soft metal that explodes when it touches water, and chlorine is a poisonous green gas. So table salt, which is sodium chloride, must be dangerous to eat."
(a) What part of this claim is supported by science? (Hint: are sodium metal and chlorine gas themselves dangerous?)
(b) Table salt is made of sodium ions (Na⁺) and chloride ions (Cl⁻), not sodium atoms and chlorine atoms. Explain why these ions behave so differently from the dangerous elements they formed from.
(c) Use the idea of a full, stable outer shell to explain why Na⁺ and Cl⁻ are stable and unreactive, unlike the sodium metal and chlorine gas they came from.
1. Your body needs sodium, potassium and calcium as ions for nerves, muscles and bones, and we get them safely from food and salt. Explain, using the idea of ions, why we take in these elements as ions rather than as the pure, highly reactive metals.
2. A student claims a sodium ion (Na⁺) is "just a sodium atom that is now positive, so it must still explode in water like sodium metal does." What is wrong with this reasoning? Explain using protons, electrons and stability.
Wrap Up
In one sentence, what was the main idea of this lesson?