Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compound of zinc is white in cold and yellow in hot? (a) \(\mathrm{ZnS}\) (b) \(\mathrm{ZnCl}_{2}\) (c) \(\mathrm{ZnSO}_{4}\) (d) \(\mathrm{ZnO}\)

Step-by-Step Solution

Answer

The compound that is white in cold and yellow in hot is \( \mathrm{ZnO} \).

Step 1: Identify the Compound Descriptions

Let's look into the properties of each compound. We know that - \( \mathrm{ZnS} \) is zinc sulfide, typically a white powder.- \( \mathrm{ZnCl_2} \) is zinc chloride, which appears as a white crystalline solid.- \( \mathrm{ZnSO_4} \) is zinc sulfate, a white crystalline powder.- \( \mathrm{ZnO} \) is zinc oxide, described as being white at room temperature and turning yellow when heated, then reverting to white when cool.

Step 2: Determine Color Change Behavior

Among the listed compounds, only \( \mathrm{ZnO} \), zinc oxide, exhibits the specific behavior of changing color from white when cool to yellow when heated. This property is due to the changes in its crystal lattice structure which affects how it reflects light.

Step 3: Conclusion Based on Observations

Given the above information, \( \mathrm{ZnO} \) matches the description of being white in cold and yellow in hot. The other compounds do not demonstrate this color-changing behavior.

Key Concepts

Zinc Oxide (ZnO)

Zinc Oxide, commonly referred to as ZnO, is a compound that has intriguing properties, especially its notable color-changing behavior. At room temperature, ZnO appears as a white powder. However, upon heating, ZnO undergoes a distinct transformation. It changes color to a yellow hue. This color change is due to alterations in its crystal lattice structure. When cooled, ZnO reverts back to its original white color. This interesting change in appearance makes ZnO unique amongst zinc compounds.

Zinc Oxide is widely used in various applications from cosmetics to manufacturing. Its ability to change colors when heated can be a valuable property in certain temperature-sensitive applications. Moreover, ZnO is also known for its semiconducting properties, which make it useful in electronics as well. For students studying chemistry, understanding Zinc Oxide’s behavior offers insight into how structural changes at the microscopic level can influence the physical properties of materials.

Color Change in Chemistry

In chemistry, color change is a visual indicator that a chemical reaction or a physical change has occurred. The color change in zinc compounds, particularly in ZnO, is a result of a unique phenomenon that involves alterations in the material's crystal lattice. When ZnO is heated, the energy provided allows for defects known as "oxygen vacancies" to develop within the crystal lattice.

These vacancies affect how light interacts with the material, changing the color it reflects. Initially, ZnO reflects white light when cool. Upon heating, the vacancies cause ZnO to absorb more light, resulting in a yellow appearance. As the lattice structure cools, these vacancies diminish, and ZnO returns to its original white color.

Understanding color change in chemistry is not only fascinating but also serves practical purposes. It can help in identifying chemical substances, understanding reaction conditions, and it is even used in creating temperature indicators. This concept demonstrates how changes at the atomic level can give rise to visible changes that we can easily observe.

Crystal Lattice in Solids

The crystal lattice is the orderly structure that makes up the solid form of certain compounds, like Zinc Oxide. It's an arrangement where particles, like atoms or molecules, are repeated at regular intervals in three dimensions. This medical arrangement contributes significantly to the physical properties of the material.

In the case of ZnO, the crystal lattice is responsible for its change in color with temperature. This structure is sensitive to changes in temperature, affecting how it reflects and absorbs light. The defects, such as vacancies within the lattice, can significantly alter the compound's physical characteristics like color. By understanding how the crystal lattice functions, we gain insights into why certain materials behave differently under varying conditions.

The crystal lattice also plays a critical role in conductivity and strength of materials. It can affect everything from how a material looks to how it conducts electricity. This is why materials science and solid-state chemistry are keen on studying lattice structures to engineer materials with desired properties.