Q.Describe the structure of the tetraborate ion present in borax, $\text{Na}_2\text{B}_4\text{O}_7\!\cdot\!10\text{H}_2\text{O}$, in terms of triangular and tetrahedral boron centres. What happens when an aqueous solution of borax is heated, and what is this solution's approximate pH character?

[!TLDR] Borax's tetraborate ion has 2 triangular (sp2, BO3) and 2 tetrahedral (sp3, BO4) boron centres linked by bridging oxygens; heated borax solution hydrolyses to give an alkaline solution. [!ANSWER] The $[\text{B}_4\text{O}_5(\text{OH})_4]^{2-}$ ion contains two three-coordinate and two four-coordinate borons joined by shar …

Borax, conventionally written $\text{Na}_2\text{B}_4\text{O}_7\!\cdot\!10\text{H}_2\text{O}$, is more structurally informatively written as $\text{Na}_2[\text{B}_4\text{O}_5(\text{OH})_4]\!\cdot\!8\text{H}_2\text{O}$, which makes explicit that the anion of interest is the tetraborate ion $[\text{B}_4\text{O}_5(\text{OH})_4]^{2-}$. This ion contains four boron atoms in total, but not all in the same coordination environment: two of the four borons are three-coordinate, planar, $sp^2$-hybridised $\text{BO}_3$ triangular units (structurally just like the boron centres in boron oxide), while the other two are four-coordinate, tetrahedral, $sp^3$-hybridised $\text{BO}_4$ units. These four boron-oxygen units are linked together by sharing bridging oxygen atoms into a single closed, ring-containing framework, with hydroxyl groups and the two compensating sodium ions completing the overall salt. When an aqueous solution of borax is heated (or simply dissolved), the tetraborate ion undergoes hydrolysis: because borax is effectively the salt of a strong base (sodium hydroxide) and a weak acid (boric acid), the borate anion reacts with water to regenerate some …

Break down the tetraborate ion's boron centres by coordination number/hybridisation, then apply the weak-acid/strong-base salt-hy …

Assuming all four boron atoms in borax are identical (all triangular or all tetrahedral); predicting an acidic rather than alkaline solution b …