All questions of Substance & Its Nature for UPSC CSE Exam

Sublimation
Sublimation is the process of converting a solid directly into a gas without passing through the liquid state. This phenomenon occurs when the solid is heated to a temperature where its particles have enough energy to break free from the solid structure and enter the gas phase.

Key Points:
- Sublimation is a phase transition from solid to gas.
- Some common examples of substances that undergo sublimation include dry ice (solid carbon dioxide), mothballs (paradichlorobenzene), and certain types of air fresheners.
- Sublimation is an endothermic process, meaning it requires energy input to break the intermolecular forces holding the solid together.
- The reverse process of sublimation, where a gas turns directly into a solid, is called deposition.
In conclusion, sublimation is a fascinating physical process that skips the liquid phase entirely and allows solids to transform directly into gases under the right conditions.

Understanding Non-Metals
Non-metals are elements that typically lack the characteristics of metals. They are generally poor conductors of heat and electricity, have higher electronegativities, and can form anions.
Elements in the Given Options
- Sodium (Na):
- Sodium is a metal, belonging to the alkali metals group. It is highly reactive and conducts electricity well.
- Iron (Fe):
- Iron is a transition metal known for its strength and ability to conduct electricity and heat. It is widely used in construction and manufacturing.
- Oxygen (O):
- Oxygen is a non-metal and a vital element for life. It is a gas at room temperature and is essential for respiration in living organisms. Oxygen is a poor conductor of heat and electricity, which is characteristic of non-metals.
- Calcium (Ca):
- Calcium is an alkaline earth metal. It is reactive, especially with water, and is important for biological functions, but it is still a metal.
Conclusion: Why Oxygen is the Correct Answer
Among the options, oxygen is the only non-metal. It is essential for combustion and respiration, and its gaseous state at room temperature further emphasizes its non-metallic nature. Understanding the differences between metals and non-metals is crucial for various scientific applications, including chemistry and biology.

Example of a Pure Substance: Salt
• Definition: A pure substance is a material that has a fixed chemical composition and distinct properties. It cannot be separated into other substances by physical means.
• Explanation: Salt (sodium chloride) is a pure substance because it is composed of only sodium and chlorine atoms in a fixed ratio. It has specific properties such as a high melting point and solubility in water.
• Comparison:
- Air is a mixture of gases like oxygen, nitrogen, and carbon dioxide, making it impure.
- Water is a compound consisting of hydrogen and oxygen atoms, but it can exist in different forms (liquid, solid, gas), so it is not a pure substance.
- Soil is a complex mixture of minerals, organic matter, and living organisms, thus not a pure substance.
• Characteristics: Pure substances have a sharp melting and boiling point, a specific crystalline structure, and homogeneous properties throughout the material.
• Applications: Salt is widely used in cooking, preserving food, and in various industries such as chemical manufacturing and water treatment. It showcases the importance of understanding pure substances in daily life.