Year 9 Science · Unit 2 · Lesson 8
Challenge Worksheet
Learning Goals
Evaluate the claim
Someone claims…
"Silicon dioxide (SiO₂) is a covalent compound made of non-metals, just like carbon dioxide (CO₂). Both have the general formula XO₂ where X is a non-metal. Therefore, SiO₂ should be a gas at room temperature like CO₂, and it should dissolve in water since CO₂ does. A student is confused because sand (SiO₂) is clearly a solid that doesn't dissolve in water, but the same student insists both compounds must have the same properties because they are both covalent."
(a) What error is the student making? Using lesson concepts, explain why CO₂ and SiO₂ have very different physical properties despite both being covalent compounds containing non-metals.
(b) Silicon in SiO₂ forms four bonds (like carbon in diamond). Based on this, predict the structure of SiO₂ and explain why it has a melting point of 1713 °C.
1. CO₂ fire extinguishers are used to protect electronic equipment. A technician says this is because "CO₂ doesn't dissolve ionic compounds, doesn't conduct electricity, and doesn't damage circuit boards." Using your knowledge of CO₂'s covalent bonding and molecular shape, explain why each of these three claims is correct.
2. CSIRO uses synthetic diamond in drill bits for mineral exploration. Diamond is a giant covalent structure where every carbon atom bonds to four others in a continuous 3D network. Explain why this structure makes diamond (a) the hardest natural substance, and (b) an electrical insulator, even though it is made entirely of carbon atoms.
Wrap Up
In one sentence, what was the main idea of this lesson?