Question

Which of the following is incorrect? (1) Borax glass is the anhydrous form of borax. (2) Jeweller's borax is obtained by crystallizing the solution at \(25^{\circ} \mathrm{C}\). (3) Decahydrate form of borax is obtained by crystallizing the solution at \(60^{\circ} \mathrm{C}\). (4) Boric acid is less soluble in hot water but more soluble in cold water. (a) \(1,2,3\) (b) 3,4 (c) \(1,3,4\) (d) \(2,3,4\)

Short answer

The incorrect statements are (1), (2), (3) from option (a).

Step-by-step solution

Analyzing Statement 1

Borax glass is the anhydrous form of borax. This statement is incorrect because borax glass results from heating borax beyond certain temperatures, where it loses water, but it is not simply an anhydrous form like dehydrated crystals.

Analyzing Statement 2

Jeweller's borax is traditionally used in a purified and compact form for jewelry work. Crystallization at \( 25^{\circ} \text{C}\) is not specifically linked to the formation of jeweller's borax; this is more about refining purity than temperature-based crystallization. Thus, the connection made here is misleading.

Analyzing Statement 3

The decahydrate form of borax is obtained by cooling a concentrated solution of borax, typically below certain temperatures that are not necessarily \( 60^{\circ} \text{C}\). In fact, it forms more effectively at lower temperatures. Therefore, crystallizing it at \( 60^{\circ} \text{C}\) is incorrect for forming the decahydrate.

Analyzing Statement 4

Boric acid is less soluble in cold water and more soluble in hot water, contrary to what the statement suggests. Therefore, this statement is incorrect.

Identifying Incorrect Choice

The incorrect statements are (1), (2), (3), and (4). Thus, option (c) \(1,3,4\) misses statement 2, so the correct representation of incorrect statements is option (a) where it considers all incorrect statements including (1), (2), and (3).

Key concepts

Crystallization

Crystallization is a vital chemical process where a solid forms from a solution or melt, arranging itself naturally into a structured pattern. This process often separates substances based on differing solubility in certain conditions. In the context of borax, understanding temperature's role in crystallization is crucial. For example, the decahydrate form of borax is commonly achieved by cooling a concentrated solution to a certain threshold, typically below room temperature. Here, the idea is simple: a cooler temperature facilitates the alignment of borax molecules into a stable crystalline form. This understanding helps explain why the statement about crystallization at \(60^{\circ} \text{C}\) for decahydrate is incorrect, as higher temperatures generally keep borax in solution rather than promoting crystallization.

Anhydrous form

An "anhydrous form" refers to a substance that lacks water molecules. Especially for compounds capable of existing in hydrated forms, they can lose water upon undergoing specific processes. In the case of borax, heating it considerably results in the loss of water, eventually transforming it into borax glass rather than merely an anhydrous powder form. When borax is heated, various water molecules are gradually driven off until all that remains is the solid structure without water, often leading to glass-like characteristics. It's important to realize that anhydrous forms have different properties compared to their hydrated counterparts, including changes in color, structure, and behavior in chemical reactions.

Boric acid solubility

The solubility of boric acid in water is an intriguing phenomenon because it behaves oppositely compared to many other substances. Typically, solids are less soluble in cold water and more soluble in hot water due to increased molecular movement allowing better dissolution. However, boric acid dissolves more readily in hot water and less so as the temperature decreases.

• At higher temperatures, increased kinetic energy allows boric acid molecules to break intermolecular bonds more effectively, thus dissolving more easily.
• Conversely, in cold water, reduced kinetic energy means less dissolution, so boric acid appears less soluble.

This understanding helps clarify why the statement suggesting the opposite is incorrect. Recognizing this peculiar solubility trait is essential for correctly working with and studying boric acid in practical and theoretical applications.