Question

An aqueous solution of borax is (a) neutral (b) amphoteric (c) basic (d) acidic

Short answer

(c) basic

Step-by-step solution

Understanding Borax

Borax is chemically represented as Na₂B₄O₇·10H₂O. It is a salt made from sodium, boron, and oxygen.

Dissociation in Water

When borax is dissolved in water, it dissociates into sodium ions (Na⁺) and borate ions (B₄O₇²⁻). The borate ions further hydrolyze to form boric acid (H₃BO₃) and hydroxide ions (OH⁻): \( B_4O_7^{2-} + 7H_2O \rightarrow 4H_3BO_3 + 2OH^- \)

Effect on pH

The formation of hydroxide ions \((OH⁻)\) makes the solution basic because hydroxide ions neutralize any hydrogen ions \((H⁺)\). More \((OH⁻)\) ions than \((H⁺)\) ions results in a basic solution.

Conclusion

Since the solution contains more \((OH⁻)\) ions due to the dissociation of borate ions, it is basic, not neutral, acidic, or amphoteric.

Key concepts

Borate Ions

Borate ions, denoted by the chemical formula \( B_4O_7^{2-} \), are an essential component of borax solutions. Borax itself is a compound made up of sodium, boron, and oxygen, represented as \( Na_2B_4O_7\cdot10H_2O \). When borax dissolves in water, it releases borate ions along with sodium ions \((Na^+)\). This process allows borate ions to undergo hydrolysis, which means they react with water to form other compounds. One key reaction is the formation of boric acid \((H_3BO_3)\) and hydroxide ions \((OH^-)\). This transformation is crucial because it impacts the properties of the resulting solution. In summary, borate ions initiate a hydrolysis reaction that contributes to the solution's basic nature.

Hydroxide Ions

Hydroxide ions \((OH^-)\) play a crucial role in determining the properties of borax solutions. These ions are formed when borate ions hydrolyze in water, leading to an increase in the concentration of \(OH^-\) ions. The presence of these ions is significant because they are responsible for making the solution basic.Hydroxide ions can effectively neutralize hydrogen ions \((H^+)\), which is why their concentration indicates the basicity of a solution. - A higher concentration of \(OH^-\) means more basicity. - When more \(OH^-\) ions are present than \(H^+\) ions, the solution becomes basic.Understanding the role of hydroxide ions helps in predicting the nature of a solution's acidity or basicity, directly linked to the concept of pH.

pH

The pH of a solution is a measure of how acidic or basic the solution is. It is determined by the concentration of hydrogen ions \((H^+)\), typically ranging on a scale from 0 to 14. A neutral pH is around 7, which indicates equal concentrations of \(H^+\) and \(OH^-\) ions.In the case of a borax solution, the formation of additional hydroxide ions \((OH^-\)) due to borate ion hydrolysis results in a higher pH value, making the solution basic. Basic solutions have a pH greater than 7: - More \(OH^-\) ions shift the pH value upwards. - The balance tips towards basicity when there are more hydroxide ions than hydrogen ions.For students, it's important to remember that the pH scale is a way to indicate the solution’s nature, and in the context of borax, the solution is indeed basic.