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 in hot.

Step 1: Identify the Compounds

We need to identify what each of the given compounds are. (a) \( \mathrm{ZnS} \) is Zinc Sulfide. (b) \( \mathrm{ZnCl}_{2} \) is Zinc Chloride. (c) \( \mathrm{ZnSO}_{4} \) is Zinc Sulfate. (d) \( \mathrm{ZnO} \) is Zinc Oxide.

Step 2: Recall Color Properties

Some zinc compounds are known for their particular color-changing properties upon heating.- \( \mathrm{ZnO} \) is known to change color from white when cold to yellow when hot due to its crystal structure.

Step 3: Verify with Known Information

Review known chemical properties of these compounds:- \( \mathrm{ZnS} \), \( \mathrm{ZnCl}_2 \), and \( \mathrm{ZnSO}_4 \) do not change color with temperature change.- \( \mathrm{ZnO} \) is specifically noted for turning yellow when heated.

Key Concepts

Color change in Zinc Oxide

Zinc Oxide (ZnO) displays a fascinating property of changing color with temperature shifts. At room temperature, Zinc Oxide appears white. However, when heated, it adopts a yellow hue. This change occurs because of alterations in its crystal lattice as heat affects how light interacts with it.
This phenomenon is attributed to a small loss of oxygen from the structure when heated, causing changes in light absorption and dispersion patterns. When cooled down, the Zinc Oxide returns to its original white color. This color shift is unique among zinc compounds and is not shared by Zinc Sulfide (ZnS), Zinc Chloride (ZnCl₂), or Zinc Sulfate (ZnSO₄). The properties of ZnO make it useful in applications that require precise temperature-sensing materials.

Zinc compounds

Zinc in its compound forms is prevalent across various chemical and industrial applications due to its versatile properties. Some common zinc compounds include:

• Zinc Oxide (ZnO): Frequently used in the production of rubber, glass, and pharmaceuticals. It's also notable for its color-changing property with temperature.
• Zinc Sulfide (ZnS): Best known for its application in luminescent materials and pigments.
• Zinc Chloride (ZnCl₂): Utilized in galvanizing and soldering processes.
• Zinc Sulfate (ZnSO₄): Often used in agriculture as a soil amendment and in healthcare products.

Each of these compounds possesses different physical properties and usages based on their chemical structure and reaction with other substances. Knowing these properties helps in selecting the right compound for the desired application.

Temperature effects on compounds

Temperature can dramatically impact the properties of chemical compounds. This includes changes in color, state, chemical reactivity, and more. In the case of Zinc Oxide, as the temperature increases, the change in its structure affects the way light interacts with the material, leading to a visible color change from white to yellow. This is a reversible reaction and showcases the principle of thermochromism, where materials change color with temperature changes.
However, not all zinc compounds exhibit such changes. Compounds like Zinc Sulfide, Zinc Chloride, and Zinc Sulfate remain stable in color irrespective of the temperature. Understanding these temperature effects is essential for industries that rely on the consistent performance of materials under varying environmental conditions. It also illustrates the important aspect of chemical behavior and the interaction of thermal energy with molecular structures.