Question

A white substance, 'X' evolves on warming with ethanol and conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\), a vapour which burns with a green-edged flame. \(X\), when mixed with solid \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and heated on a cavity on charcoal gives a white luminous mass which, when moistened with a drop of \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) solution and heated further yields a green mass. The substance, \(X\) is expected to be (a) \(\mathrm{ZnSO}_{4}\) (b) \(\mathrm{HgCl}_{2}\) (c) \(\mathrm{AlPO}_{4}\) (d) \(\mathrm{Zn}\left(\mathrm{BO}_{2}\right)_{2}\)

Short answer

The substance X is Zn(BO2)2 (option d).

Step-by-step solution

Identify the clues from the gas evolution and flame color

The problem states that substance X evolves a vapor with a green-edged flame when warmed with ethanol and concentrated H2SO4. This is a characteristic of borates, which are known to produce a green flame due to the presence of boron.

Analyze the reaction with Na2CO3 and charcoal

The substance X, when mixed with Na2CO3 and heated on charcoal, gives a white luminous mass. This indicates the formation of a metal oxide residue. Among the options, Zn(BO2)2 would decompose to form ZnO, a white solid.

Consider the reaction with Co(NO3)2

When the white luminous mass is moistened with Co(NO3)2 and heated, it forms a green mass. This is a cobalt test for zinc, confirming the presence of ZnO when it forms a green-colored compound with cobalt nitrate upon heating, known as Rinnmann's Green.

Finalize the identification of X based on evidence

Based on the green flame test indicative of borates and the formation of a green mass on reaction with cobalt nitrate, the compound X is most consistent with Zn(BO2)2 as it satisfies both conditions.

Key concepts

Boron

Boron is an essential element often discussed in the realm of inorganic chemistry. It is a metalloid, which means it has properties that are intermediate between metals and nonmetals. Boron is notable for forming borates, which are compounds where boron is combined with oxygen. These borates are crucial in various applications, especially because they exhibit unique behaviors when heated. Boron compounds are also known for producing specific flame colors due to the excitement of electrons.
One such interesting characteristic of boron is that its compounds, like borates, produce a green-edged flame during combustion. This is because the energy absorbed by boron electrons is released as light in the green region of the spectrum. Hence, the presence of boron can often be inferred using simple visual tests in a laboratory setting.

Flame test

The flame test is a qualitative analysis technique used to identify the presence of certain metal ions based on the color they emit when heated. This method is particularly useful in determining elements like boron, copper, sodium, and potassium.
When a compound containing boron is exposed to a flame, the electrons in the boron atoms absorb energy and jump to higher energy levels. As these excited electrons return to their ground state, they release energy in the form of visible light. For boron, this light appears with a distinctive green edge. It’s an easy and effective way for chemists to identify boron-containing compounds without the need for complicated instruments.
Flame tests are not only simple but also provide quick information about the presence of certain elements, making them valuable in both educational and professional laboratory settings.

Cobalt nitrate test

Cobalt nitrate test is a fascinating method predominantly used to confirm the presence of certain metal oxides. This test involves applying a solution of cobalt nitrate to a substance, followed by heating. It is especially known for its ability to identify zinc oxide.
When zinc oxide is mixed with cobalt nitrate and subjected to heat, it forms a distinctive green compound known as Rinnmann's Green. This test is widely used because it offers a visually striking confirmation of zinc, differentiating it from other metal oxides that may not display such vivid color changes.
In the context of identifying compound X, observing a green mass upon application of cobalt nitrate provided critical evidence pointing towards the presence of zinc oxide, which eventually helped deduce the identity of the initial substance.

Metal oxides

Metal oxides are compounds characterized by their metal cation bonds to oxygen anions. These compounds are quite stable and are frequently encountered in a variety of environments, from Earth's crust to industrial applications.
When certain metal oxides are heated in the presence of specific reagents, they undergo color changes that can help identify the metal in question. For example, zinc oxide appears as a white substance at room temperature but interacts with cobalt nitrate to produce a distinctive green color upon heating.
Understanding metal oxides is essential in inorganic chemistry because their physical and chemical properties, such as conductivity and reactivity with other compounds, provide insight into the nature of the metals themselves. They are key players in reactions involving heat and are fundamental components of more complex industrial processes.

Zinc compounds

Zinc is a versatile element, playing a critical role in both biological and industrial processes. In inorganic chemistry, zinc compounds exhibit unique properties that make them pivotal across various applications, from galvanization to medicine.
Zinc oxide, in particular, is a well-known compound due to its utility and role in chemical analysis. When used in reactions, it often serves as a precursor or reactant leading to significant products, like Rinnmann's Green during the cobalt nitrate test. This interaction is key to identifying zinc presence in unknown substances.
Moreover, zinc compounds like zinc borate are involved in ``boron-flame`` reaction tests due to their boron content. Such compounds display amazing properties that are harnessed in flame retardants and ceramics, demonstrating the broad applicability of zinc in technology and industry.