Answer: Sodium metaborate and boric anhydride (NaBO 2 and B 2 O 3 ).
- A Boron trioxide only
- B Elemental boron and sodium oxide
- C Sodium tetraborate decahydrate and some free boron
- D Sodium metaborate and boric anhydride (NaBO<sub>2</sub> and B<sub>2</sub>O<sub>3</sub>)
Correct answer: D. Sodium metaborate and boric anhydride (NaBO<sub>2</sub> and B<sub>2</sub>O<sub>3</sub>)
Explanation: On heating, borax loses water and then melts to a glassy bead of sodium metaborate and boric anhydride (NaBO<sub>2</sub>.B<sub>2</sub>O<sub>3</sub>), which dissolves coloured metal oxides to give characteristic colours.
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.