Question

Borax on heating strongly above its melting point melts to a liquid, which then solidifies to a transparent mass commonly known as borax-bead. The transparent glassy mass consists of : (1) sodium pyroborate (2) boric anhydride (3) sodium meta-borate (4) boric anhydride and sodium metaborate

Short answer

The transparent glassy mass consists of boric anhydride and sodium metaborate.

Step-by-step solution

Understanding the Borax Bead Test

The borax bead test involves heating borax, which melts and then solidifies into a glassy, transparent mass. This test is used to identify the resulting compounds.

Chemical Composition of Borax

Borax (sodium borate) has the chemical formula Na2B4O7·10H2O. Upon heating, dehydration and decomposition occur.

Dehydration and Decomposition Process

When heated, borax loses water molecules and forms sodium metaborate (NaBO2) and boric anhydride (B2O3). The reaction can be written as: Na2B4O7·10H2O → 2 NaBO2 + B2O3 + 10 H2O

Resulting Compounds

The transparent glassy mass consists of both sodium metaborate (NaBO2) and boric anhydride (B2O3).

Final Answer

From the given options, the correct answer is (4) boric anhydride and sodium metaborate.

Key concepts

thermal decomposition

Thermal decomposition refers to the breakdown of a chemical compound when heated. In the case of the borax bead test, borax undergoes thermal decomposition. Borax, with the chemical formula Na2B4O7·10H2O, loses its water molecules when heated. This process is called dehydration. It then breaks down into simpler compounds. In the borax bead test, strong heating causes the borax to melt and then solidify into a glassy mass.
The reaction of thermal decomposition for borax can be written as:
\[ \text{Na}_2\text{B}_4\text{O}_7 · 10\text{H}_2\text{O} \rightarrow 2\text{NaBO}_2 + \text{B}_2\text{O}_3 + 10\text{H}_2\text{O} \] Here, borax (Na2B4O7·10H2O) is converted into sodium metaborate (NaBO2) and boric anhydride (B2O3), releasing water molecules (H2O) in the process. This reaction highlights the principle of thermal decomposition, where heat causes a compound to decompose into simpler substances.

sodium metaborate

Sodium metaborate (NaBO2) is one of the key products formed when borax is heated in the bead test. It is created through the process of thermal decomposition. It plays a significant role in the formation of the transparent glassy mass.
During the heating process of the borax bead test:

• Borax (sodium borate, Na2B4O7·10H2O) is initially heated.
• It loses water and breaks down into sodium metaborate (NaBO2).

Sodium metaborate is stable at higher temperatures and helps in forming the final glassy mass alongside boric anhydride. Its presence indicates the complete decomposition of borax. Thus, understanding the role of sodium metaborate is crucial to comprehending the borax bead test and its outcomes.
The chemical reaction showcasing the formation of sodium metaborate is:
\[ \text{Na}_2\text{B}_4\text{O}_7 · 10\text{H}_2\text{O} \rightarrow 2\text{NaBO}_2 + \text{B}_2\text{O}_3 + 10\text{H}_2\text{O} \] Here, NaBO2 is sodium metaborate which forms as borax undergoes thermal decomposition.

boric anhydride

Boric anhydride (B2O3) is another significant substance formed during the borax bead test. When borax is heated, it undergoes dehydration and decomposition. Besides forming sodium metaborate, it also produces boric anhydride.
Boric anhydride is a glassy, transparent substance and contributes to the final form of the bead in the test. Therefore, it is important to understand its formation. During the high-temperature heating of borax:

• Water molecules are released from the borax structure.
• The residue, post dehydration, further decomposes into simpler compounds, including boric anhydride (B2O3).

The presence of boric anhydride in the glassy mass confirms the decomposition process in the borax bead test. The balanced chemical reaction for this decomposition is:
\[ \text{Na}_2\text{B}_4\text{O}_7 · 10\text{H}_2\text{O} \rightarrow 2\text{NaBO}_2 + \text{B}_2\text{O}_3 + 10\text{H}_2\text{O} \] Here, B2O3, which is boric anhydride, coexists with sodium metaborate, forming the characteristic transparent mass in the bead test.