Question

A certain salt (X) gives the following tests: On strongly heating it swells to give glassy material. When concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is added to a hot concentrated solution of \((\mathrm{X})\), white crystals of a weak acid separate out. Identify \((\mathrm{X}) ?\) (a) \(\mathrm{NaBO}_{2}\) (b) \(\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7}\) (c) \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{Ca}_{2} \mathrm{~B}_{6} \mathrm{O}_{11}\)

Short answer

The salt X is \( \text{Na}_2 \text{B}_4 \text{O}_7 \), which is borax.

Step-by-step solution

Understanding the Swelling Effect

The first clue given is that upon strong heating, the salt (X) swells to give a glassy material. This property is characteristic of borate compounds, which expand and form a glass-like structure when heated.

Analyzing the Reaction with Sulfuric Acid

The second clue states that when concentrated sulfuric acid (23 yphen{3}4 yphen{4}5 yphen{5}) is added to a hot concentrated solution of (X), white crystals of a weak acid separate out. This is typical for borate compounds, especially borax (23 yphen{2}) which reacts with sulfuric acid to form boric acid, a weak acid that crystallizes out.

Identifying the Borate Compound

Considering the properties discussed, the salt must be a borate that swells to form a glassy material and produces boric acid when treated with sulfuric acid. 4 yphen{4}) fits all these characteristics, as it forms boric acid when reacted with 23 yphen{3}4 yphen{4}5 yphen{5}.

Conclusion

Based on the reactions and characteristics provided, the compound X is identified as 4 yphen{4}). It is the only option that satisfies both the swelling to a glassy material and the formation of white crystals of a weak acid upon reaction with sulfuric acid.

Key concepts

Borate Compounds

Borate compounds are essential in many chemical contexts. They contain boron, oxygen, and usually, another element such as sodium or calcium. A very well-known borate compound is Borax, scientifically referred to as sodium borate or \( \text{Na}_2\text{B}_4\text{O}_7 \).Borates have interesting properties:

• They often occur as colorless or white crystals and are generally water-soluble.
• When heated strongly, borate compounds often swell and form glass-like materials. This glass formation is due to the fusion of borate compounds, which then form a glassy surface.
• Borate compounds are widely used for laundry, as fluxes in metallurgy, and as additives in the glass and ceramic industries.

These compounds are also vital in laboratory experiments and reactions, making them a crucial topic of study.

Thermal Decomposition

Thermal decomposition refers to the process in which a chemical compound breaks down into simpler substances when heated. This decomposition is an important concept to understand as it reveals how substances transform under heat. For borate compounds:

• When heated strongly, they often undergo a transformation where they swell before melting. This is a unique trait where the compound expands due to loss of bound water or other gases from the structure.
• For compounds like borax, heating can lead to the formation of glassy materials. The borax loses its crystal water and may form anhydrous salts, eventually resulting in a glassy material upon cooling.

Understanding thermal decomposition helps in predicting the behavior of materials at high temperatures, crucial for industrial and experimental purposes.

Reaction with Sulfuric Acid

When borate compounds react with sulfuric acid, it leads to fascinating results. This reaction is important for identifying substances and understanding their properties.Specifically:

• Addition of concentrated \(\text{H}_2\text{SO}_4\) to a solution of a borate compound, like borax, results in the precipitation of boric acid \((\text{H}_3\text{BO}_3)\).
• Boric acid crystallizes out as white, flaky crystals. These are indicative of the weak acid nature of the product.
• This type of reaction is typical in qualitative analysis to identify borate compounds among other salts.

Such reactions are integral not only academically for learning chemical properties but also practically in industry for synthesizing acid derivatives or in analytical chemistry for compound identification.