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

(d) \(\mathrm{ZnO}\) is white in cold and yellow when hot.

Step 1: Identify the compounds

Examine the given list of compounds to determine which one has the property of changing color with temperature. The compounds are zinc sulfide (\(\mathrm{ZnS}\)), zinc chloride (\(\mathrm{ZnCl}_2\)), zinc sulfate (\(\mathrm{ZnSO}_4\)), and zinc oxide (\(\mathrm{ZnO}\)).

Step 2: Evaluate the properties of each compound

Research and evaluate the characteristics of each compound:- \(\mathrm{ZnS}\) (Zinc sulfide) is typically white.- \(\mathrm{ZnCl}_2\) (Zinc chloride) is colorless or white.- \(\mathrm{ZnSO}_4\) (Zinc sulfate) is also generally white.- \(\mathrm{ZnO}\) (Zinc oxide) is known to change color from white to yellow when heated.

Step 3: Determine the correct compound

\(\mathrm{ZnO}\) is the only compound among those listed that is known for changing color when heated. It is white at room temperature, and turns yellow when heated, returning to its original white color upon cooling.

Key Concepts

Color change with temperature

In the fascinating world of chemistry, some compounds exhibit a magical-like transformation known as thermochromism. This is the ability of a substance to change color with varying temperatures. One such captivating example is zinc oxide (\(\mathrm{ZnO}\)). At room temperature, zinc oxide appears white. However, when it is heated, it undergoes a remarkable transformation—it turns a striking yellow. This change is caused by small changes in the arrangement of oxygen atoms within the zinc oxide lattice when it is heated. As the heat affects the compound's structure, it temporarily changes the way that light is absorbed, leading to the observed color shift. What's truly intriguing is that upon cooling, zinc oxide reverts back to its initial white color. This reversible behavior not only fascinates those in the field of science but also has practical applications in creating temperature-sensitive materials used in various industries.

Zinc oxide properties

Zinc oxide (\(\mathrm{ZnO}\)) is an amazing compound with versatile properties that make it very useful in everyday life. Apart from being thermochromic, it has a number of other interesting characteristics.

• Safe and non-toxic: It is considered safe and non-toxic, making it an excellent ingredient in cosmetics and sunscreen due to its ability to reflect ultraviolet (UV) light.
• Semi-conducting material: This compound is also used in electronic devices, as it exhibits semiconductor properties.
• Antibacterial effects: Zinc oxide is valued for its antibacterial properties, helping in the treatment and prevention of infections.

These properties make zinc oxide a valuable resource across various fields, from healthcare to technology, showcasing the myriad ways in which it can be applied.

Chemical properties of zinc compounds

Zinc compounds, such as zinc oxide, zinc sulfide (\(\mathrm{ZnS}\)), and zinc sulfate (\(\mathrm{ZnSO_4}\)), each have distinct chemical properties that define their uses and behaviors.

• Zinc oxide (\(\mathrm{ZnO}\)): Known for its color-changing ability when heated, it is also a stable compound that reacts with acids to form other zinc salts and water.
• Zinc sulfide (\(\mathrm{ZnS}\)): Commonly found in nature as the mineral sphalerite, it has luminescent properties and is often used in phosphorescent materials.
• Zinc chloride (\(\mathrm{ZnCl_2}\)): This compound is versatile, being used as a flux in metallurgical processes and as a dehydrating agent due to its affinity for water.
• Zinc sulfate (\(\mathrm{ZnSO_4}\)): It is commonly used in agriculture as a fertilizer additive to prevent zinc deficiency in crops.

By understanding these chemical properties, we can better appreciate the unique applications and roles that zinc compounds play across different sectors. Each compound possesses traits that make it suitable for specific industrial, technological, or ecological applications.