Introduction
Chemistry is often called the “central science” because it connects physics, biology, and environmental sciences. For aspirants of NDA and CDS examinations, a clear understanding of basic chemistry is essential—not just for scoring well in the exam but also for developing scientific reasoning. Among the important areas for revision are Physical and Chemical Changes, the Preparation and Properties of substances, and the study of Polymers. These topics form the foundation of applied chemistry and frequently appear in objective-type questions.
1. Physical and Chemical Changes
A physical change refers to a change in the form or state of a substance without altering its chemical composition. Examples include melting of ice, boiling of water, and dissolving sugar in water. These changes are usually reversible.
In contrast, a chemical change results in the formation of a new substance with different properties. Combustion of wood, rusting of iron, and souring of milk are common examples. Such changes are generally irreversible and involve energy transformations, like the release or absorption of heat and light.
Understanding the difference between these two types of changes helps in analyzing chemical reactions, balancing equations, and predicting products.
2. Preparation and Properties
Every compound or material has a specific method of preparation and distinctive properties that define its behavior in various conditions.
- Preparation: This involves the methods used to obtain a substance in pure form, such as laboratory synthesis or industrial manufacturing. For instance, oxygen can be prepared by the decomposition of potassium chlorate, while hydrogen is prepared by the action of dilute acids on metals like zinc.
- Properties: Once prepared, substances are studied based on physical properties (such as color, odor, density, and melting point) and chemical properties (such as reactivity with acids, bases, and other compounds). For example, oxygen supports combustion, while hydrogen burns with a blue flame and forms water when combined with oxygen.
These insights are crucial for solving application-based questions in the NDA/CDS examination, especially where one has to identify unknown substances or predict the outcome of reactions.
3. Polymers
Polymers are large molecules formed by the repeated linking of small units called monomers. They are of immense importance in everyday life as well as in defense applications.
- Natural Polymers: Such as cellulose, proteins, and natural rubber.
- Synthetic Polymers: Such as polythene, PVC, nylon, and bakelite.
Properties of Polymers include durability, flexibility, resistance to chemicals, and lightweight nature. These make them useful in packaging, textiles, transport equipment, and even in military gear.
For NDA/CDS preparation, focus should be on classification of polymers (addition and condensation), their uses, and common examples asked in exams.
Conclusion
The study of Physical and Chemical Changes, Preparation and Properties of substances, and Polymers not only sharpens fundamental concepts but also enhances practical understanding of chemistry in daily life and national defense applications. For aspirants of NDA CDS 2 2025, mastering these topics is vital for scoring well in the General Science section. Regular revision, supported with examples and applications, will ensure clarity and confidence during the examination.
