Problem 86 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Which of the following is a ferromagnetic substance? (a) \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) (b) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{Fe}_{3} \mathrm{O}_{4}\) (d) \(\mathrm{CrO}_{2}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{Fe}_{3} \mathrm{O}_{4}\) is the ferromagnetic substance.

Step 1: Understanding Ferromagnetism

Ferromagnetism is a property of materials that allows them to become magnets or be attracted to magnets. Such materials have unpaired electrons and magnetic domains that can be aligned in the same direction, resulting in a strong magnetic effect.

Step 2: Identifying Common Ferromagnetic Substances

Some elements are typically associated with ferromagnetism, including iron (Fe), cobalt (Co), and nickel (Ni). Compounds and alloys containing these elements can also exhibit ferromagnetic properties.

Step 3: Evaluating the Given Substances

Among the choices given, (a) \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) is hematite which is antiferromagnetic,(b) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) is also antiferromagnetic,(c) \(\mathrm{Fe}_{3} \mathrm{O}_{4}\) or magnetite contains iron in both divalent and trivalent forms, exhibiting ferromagnetism, and(d) \(\mathrm{CrO}_{2}\) is a chromoxide and shows some ferromagnetic-like properties only at temperatures below the Curie temperature.

Step 4: Select the Correct Answer

Based on the known ferromagnetic properties of certain elements and compounds, the correct answer for the ferromagnetic substance is (c) \(\mathrm{Fe}_{3} \mathrm{O}_{4}\) or magnetite.

Key Concepts

Ferromagnetic Substances

Ferromagnetic substances are materials that demonstrate a strong magnetic response. These materials contain magnetic domains where the magnetic moments of atoms are aligned in the same direction, producing a net magnetic field. The most familiar examples of ferromagnetic substances include iron, cobalt, nickel, and certain alloys and compounds comprising these elements.

When these materials are placed within an external magnetic field, the domains align in parallel to the field, enhancing the material's overall magnetism. This ability to be magnetized and retain magnetization even after the external magnetic field is removed is a distinctive feature of ferromagnetic materials.

Ferromagnetic substances are utilized in a variety of applications such as in creating permanent magnets, magnetic recording media, and in transformers and electric motors. Understanding their properties is critical in the field of material science and for the development of technological devices.

Magnetic Properties of Materials

Materials exhibit varying degrees of magnetic properties based on their atomic structure and the behavior of their electrons. There are several classifications of magnetism which include diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism, and ferrimagnetism.

Diamagnetic materials, like copper and bismuth, are weakly repelled by magnetic fields. Paramagnetic materials, such as aluminum and platinum, are weakly attracted to magnetic fields. Ferromagnetic materials show strong attraction to magnetic fields, which is the concept under consideration with ferromagnetic substances.

Antiferromagnetic materials have magnetic moments that are aligned in opposite directions, canceling each other out. Ferrimagnetic materials, similar to ferromagnetics, have their magnetic moments pointing in the same direction but differ in that they possess two different ionic species, which leads to incomplete cancellation and a net magnetic moment.

Curie Temperature

The Curie temperature, or Curie point, is a critical temperature above which ferromagnetic materials lose their permanent magnetic properties to become merely paramagnetic. Paramagnetism is a form of magnetism where materials are only attracted when in the presence of an externally applied magnetic field.

For example, iron becomes paramagnetic when heated above its Curie temperature of 770°C. When ferromagnetic materials are cooled back down below the Curie point, they regain their ferromagnetic properties. This concept is named after Pierre Curie, who discovered that magnetism of materials was temperature-dependent.

Knowledge of the Curie temperature is essential when working with magnetic materials in technology applications, as operating these materials above their Curie temperatures can lead to a loss of desired magnetic functionalities. For instance, magnetic recording devices must be designed considering the Curie temperatures of the materials to ensure longevity and reliability of data storage.