Answer: Na₂B₄O₇·10H₂O.
- A Na₂B₄O₇·10H₂O
- B H₃BO₃ crystals
- C solid B₂O₃
- D gaseous BF₃
Correct answer: A. Na₂B₄O₇·10H₂O
Explanation: Borax is sodium tetraborate decahydrate, Na₂B₄O₇·10H₂O.
Diamond's rigid 3D tetrahedral network makes it the hardest natural material, while graphite's flat hexagonal sheets are held together only by weak forces, letting them slide past each other (used as a lubricant and in pencils).
Concept context
Covers the Boron family (Group 13: B, Al) and the Carbon family (Group 14: C, Si, Ge, Sn, Pb). Key topics include Lewis acid behaviour of BF₃, borax bead test, silicates, silicones, allotropes of carbon, and trends in properties down the group.