Problem 25 - Chemistry in 30 days

Question

In borax bead test, which compound is formed? (a) Orthoborate (b) Metaborate (c) Double oxide (d) Tetraborate

Step-by-Step Solution

Answer

(b) Metaborate.

Step 1: Understanding the Context

The borax bead test is a qualitative analytical test used to identify certain metal ions based on their color reaction. It involves heating borax (sodium tetraborate) and forming a bead on a platinum wire loop by heating it until it melts. The bead is then used to test various metallic compounds.

Step 2: Identifying the Chemical Reaction

When borax is heated, it decomposes to form metaborate and water vapor. This means that the borax composition changes in this heating process. The relevant reaction is as follows: \[\text{Na}_2\text{B}_4\text{O}_7 ightarrow 2 \text{NaBO}_2 + \text{B}_2\text{O}_3\]where sodium metaborate (NaBO2) and boric anhydride (\(B_2O_3\)) are formed.

Step 3: Choosing the Correct Option

From the chemical reaction above, the compound formed is sodium metaborate (NaBO2), which corresponds to the option (b) Metaborate.

Key Concepts

Qualitative Analytical Test

A qualitative analytical test is a method used in chemistry to determine the presence or absence of a certain chemical compounds or elements within a sample. Unlike quantitative methods, which measure the exact amount of a substance, qualitative tests focus on the types of substances.
Qualitative tests often involve observing a sample's reaction to various conditions, such as color changes or precipitate formation. These visual indicators can provide clues about the identity of the constituents. An example of this is the borax bead test, which uses changes in bead color to identify metal ions.

Sodium Tetraborate

Sodium tetraborate, commonly known as borax, is a crystalline compound often used in detergents, cosmetics, and insecticides. In chemistry, it serves as an important reagent for analytical tests, such as the borax bead test.
When borax is heated, it undergoes structural changes, decomposing into other compounds. Borax has the chemical formula \( ext{Na}_2 ext{B}_4 ext{O}_7\), and it is this decomposition process that plays a critical role in forming metaborate during the bead test.

Chemical Reaction

Chemical reactions are processes where substances, known as reactants, change into different substances, called products. In the borax bead test, a key reaction occurs when borax is exposed to high temperatures, causing it to break down.
The chemical reaction can be represented as: \[\text{Na}_2\text{B}_4\text{O}_7 \rightarrow 2 \text{NaBO}_2 + \text{B}_2\text{O}_3\]
Here, sodium tetraborate decomposes to form sodium metaborate (NaBO2) and boric anhydride (B2O3). This reaction is crucial because it prepares the conditions for identifying metal ions based on the resulting bead color.

Metal Ions Identification

Identifying metallic ions is important in both scientific studies and industrial applications. The borax bead test is a simple way to identify various metal ions. The method is based on the observation that different metal ions produce distinct colors when introduced to the hot borax bead.
For example, cobalt ions can produce a deep blue color, while chromium ions yield a green color. By comparing the observed color to known standards, it is possible to infer the identity of the metallic elements present.