Question

Borax is used as a cleansing agent because on dissolving in water it gives: (a) Alkaline solution (b) Acidic solution (c) Bleaching solution (d) Basic solution

Short answer

Borax gives a basic solution when dissolved in water.

Step-by-step solution

Understanding Borax Structure

Borax is chemically known as sodium borate, with the formula \( \text{Na}_2\text{B}_4\text{O}_7\cdot 10 \text{H}_2\text{O} \). It has sodium, boron, and oxygen in its structure.

Dissolution of Borax in Water

When borax dissolves in water, it dissociates into sodium ions (\( \text{Na}^+ \)), borate ions (\( \text{B}_4\text{O}_7^{2-} \)), and water. The dissociation process can be represented as: \( \text{Na}_2\text{B}_4\text{O}_7 ightleftharpoons 2\text{Na}^+ + \text{B}_4\text{O}_7^{2-} \).

Understanding the Reaction with Water

In water, the \( \text{B}_4\text{O}_7^{2-} \) ions undergo hydrolysis, reacting with water to form \( \text{OH}^- \) ions (making the solution basic) and \( \text{H}_3\text{BO}_3 \) (boric acid): \( \text{B}_4\text{O}_7^{2-} + 7\text{H}_2\text{O} ightarrow 4\text{H}_3\text{BO}_3 + 2\text{OH}^- \).

Determining the Nature of the Solution

The production of \( \text{OH}^- \) ions makes the solution basic because \( \text{OH}^- \) ions increase the pH of the solution. Therefore, when borax dissolves in water, it results in a basic solution.

Key concepts

Sodium borate

Sodium borate, more commonly known as borax, is a fascinating compound. It has the chemical formula \( \text{Na}_2\text{B}_4\text{O}_7\cdot 10 \text{H}_2\text{O} \). This formula tells us that borax is made up of two sodium ions, four boron atoms, and seven oxygen atoms linked together.
The term "sodium borate" might sound complex, but it essentially refers to the presence of sodium ions and borate ions in its structure. Borax can appear as either a white powder or as crystalline particles, with a slightly glassy texture.

• Sodium ions are positively charged.
• Borate ions carry a negative charge.

This unique combination of ions gives rise to some interesting properties in borax, especially when it is dissolved in water.

Hydrolysis

Hydrolysis is a key chemical process that describes how compounds break down when they interact with water. In the context of borax, hydrolysis is responsible for rendering the solution basic when borax is dissolved in water.
When the borate ion \( \text{B}_4\text{O}_7^{2-} \) encounters water, it undergoes a chemical reaction. This reaction forms boric acid \( \text{H}_3\text{BO}_3 \) and hydroxide ions \( \text{OH}^- \). To put it simply:

• Hydrolysis leads to the formation of hydroxide ions.
• These ions are responsible for the increase in pH.
• The overall reaction is: \( \text{B}_4\text{O}_7^{2-} + 7\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{BO}_3 + 2\text{OH}^- \).

The presence of these \( \text{OH}^- \) ions is crucial as they play a significant role in determining the basic nature of the borax solution.

Basic solution

A basic solution, often referred to as an alkaline solution, is defined by its pH level, which is greater than 7. The production of hydroxide ions \( \text{OH}^- \) is what primarily makes a solution basic.
When borax dissolves in water, the hydrolysis reaction releases these hydroxide ions, thus increasing the solution's pH level. Consequently, unlike acidic or neutral solutions, a basic solution exhibits the following traits:

• Has a bitter taste.
• Feels slippery to the touch.
• Primarily due to the presence of hydroxide ions, it can conduct electricity.

These properties are common among substances that form basic solutions, and they are significantly enhanced in borax due to the ample production of hydroxide ions via hydrolysis.