Question

Borax-bead test is responded by (a) divalent metals (b) heavy metals (c) light metals (d) metals which form coloured metaborates.

Short answer

The borax bead test is responded by (d) metals which form coloured metaborates.

Step-by-step solution

Understanding the Borax Bead Test

The borax bead test is a qualitative test used in chemistry to detect the presence of certain metals. It is based on the observation that when borax is heated, it swells and loses water of crystallization to form a colorless, glassy bead of sodium metaborate. When the hot bead comes in contact with a metal compound, it can form a colored metaborate if the metal is receptive to this reaction.

Analyzing the Options

It is important to consider the characteristics of the metals in each option and how they react in the borax bead test. Divalent metals (option a) suggests a specific oxidation state. Heavy metals (option b) refers to density and/or propensity to form complexes, and light metals (option c) are typically low in density and not known for forming colored compounds in such tests.

Determining the Correct Group of Metals

Metals that form colored metaborates when heated in the presence of borax will result in a bead test that changes color, indicating a positive result. This is the fundamental principle of the borax bead test. Thus, we are looking for metals that specifically form colored compounds when reacting with borax.

Selecting the Correct Answer

Based on the principle of the borax bead test—metals forming colored metaborates when reacted with borax—the correct answer is (d) metals which form coloured metaborates. These metals show a positive result in the borax bead test by altering the color of the bead, which is the basis for detecting their presence.

Key concepts

Qualitative Test in Chemistry

When we talk about a qualitative test in chemistry, we're referring to a method used to determine the presence or absence of certain substances, without necessarily quantifying them. The Borax Bead Test is an example of such a qualitative test. It's a classic analytic technique that helps chemists recognize and classify certain metal ions based on color changes.

Chemists use qualitative tests for preliminary analysis, creating a basis for further, more quantitative experiments. At the heart of qualitative analysis lies the observation of specific reactions—like the creation of colored compounds when a sample is tested against a reagent, in this case, borax, which is used to deduce the presence of various metals. Understanding these tests is fundamental for students and professionals alike, as they lay the groundwork for deeper research into the properties and reactions of elements and compounds.

Sodium Metaborate

Sodium metaborate, formed during the Borax Bead Test, is a chemical compound that results from the dehydration of borax. When borax (Na₂B₄O₇·10H₂O) is heated, it loses its water of crystallization and fuses into this glassy substance. What's crucial here is how sodium metaborate can act as a flux, meaning it can dissolve other metal oxides at high temperatures, which wouldn't otherwise dissolve in water.

The bead of sodium metaborate created in the test serves as an investigative tool because of its ability to interact with metal ions. The behavior of this compound—with its clear glassy appearance when pure—gives scientists a 'clean slate' to observe color changes when metals in question are introduced. It is a compelling example of how a simple compound can open up a world of chemical investigation.

Colored Metaborates Detection

The detection of colored metaborates is the culmination of the Borax Bead Test, hinging on the compound's ability to reveal the presence of certain metals through distinct coloration. A metal's ability to form a colored metaborate upon reaction with borax provides a distinctive visual cue that can point to the metal's identity.

For example, cobalt can form a blue bead while chromium might produce a green one. It is this characteristic color change that indicates the presence and, sometimes, the identity of a metal. Not all metals will cause the sodium metaborate bead to change color—only those with specific properties. Manganese can yield beads ranging from green to violet depending on its oxidation state, whereas iron can appear as green or brown. Learning to recognize these colors and associating them with specific metals is an instrumental skill in qualitative chemistry.