Class 9 Chemistry - Chapter 1: States of Matter and Phase Changes Short Question Answers

Short Answer Questions with Explanations

1. Why is there a need to divide Chemistry into many branches? Give three reasons.

   • Chemistry covers a wide range of topics, so dividing it helps in better understanding.
   • Each branch focuses on a specific area, such as organic chemistry for carbon compounds and physical chemistry for energy changes.
   • It helps scientists specialize and solve real-world problems efficiently.

2. Reactions may take place due to electrons outside the nucleus or inside the nucleus. Which branches of Chemistry cover these reactions?

   • Electrons outside the nucleus: This involves chemical reactions studied in Physical Chemistry and Organic Chemistry.
   • Inside the nucleus: These reactions are studied in Nuclear Chemistry.

3. What types of problems are solved in Analytical Chemistry?

   • Identifying unknown substances in food, medicines, or forensic investigations.
   • Checking the purity of water, air, and industrial chemicals.
   • Measuring the amount of a substance in a sample (e.g., glucose in blood tests).

4. Both graphite and graphene have hexagonal layered structures. What is the difference?

   • Graphite: Has many layers of carbon atoms stacked together, making it soft and slippery.
   • Graphene: Has only a single layer of carbon atoms, making it much stronger and a good conductor of electricity.

5. Why are supercritical fluids important?

   • They have properties of both liquids and gases, making them useful in extracting essential oils, decaffeinating coffee, and cleaning delicate materials.

6. In which state does matter exist in the Sun?

   • Matter in the Sun exists in the plasma state, which is a superheated gas with charged particles.

7. What is the importance of graphene?

   • It is the thinnest and strongest material known.
   • It conducts electricity better than copper.
   • It is used in making fast-charging batteries and flexible electronic devices.

8. Which form of matter do most material things in the world belong to?

   • Most things around us exist in the solid state, such as wood, metals, and plastics.

Constructed Response Questions with Explanations

1. How does a supercritical state look like?

   • A supercritical fluid appears like a gas but has the ability to dissolve substances like a liquid. It forms when a substance is heated and pressurized beyond its critical point. For example, carbon dioxide in the supercritical state is used in decaffeinating coffee.

2. In what way is plasma created in a fluorescent tube?

   • Inside a fluorescent tube, electricity passes through a gas like mercury vapor, exciting its atoms. This produces plasma, which emits ultraviolet light. The tube's coating converts this into visible light.

3. Most molecules studied in biochemistry are organic in nature. Why?

   • Biochemistry deals with living organisms, and most life-related molecules (such as proteins, carbohydrates, and fats) are made of carbon, which is the basis of organic chemistry.

4. What is the difference between organic and biochemistry?

   • Organic Chemistry studies all compounds containing carbon, including synthetic ones like plastics.
   • Biochemistry focuses only on carbon compounds found in living organisms, such as DNA and proteins.

5. Why does diamond shine? Can we improve its brilliance?

   • Diamond shines due to total internal reflection, where light gets trapped and bounces inside.
   • Its brilliance can be improved by cutting it in precise angles to reflect more light.

6. Explain the dissolution of NaCl in water.

   • When NaCl (salt) is added to water, its Na+ (sodium) and Cl- (chloride) ions separate and mix with water molecules due to their attraction to the partial charges of water.

7. Why do different compounds have different solubilities in water at a particular temperature?

   • The solubility of a compound depends on temperature, pressure, and the nature of the solute and solvent.
   • For example, sugar dissolves more in hot water than in cold water, while gases dissolve better in cold water.

8. Why can NaCl be crystallized from water just like KNO₃?

   • When NaCl solution is evaporated, water molecules leave, allowing salt particles to come together and form crystals. The same happens with KNO₃.

9. Why is graphite slippery to touch? What property makes it a good lubricant?

   • Graphite is slippery because its carbon layers slide over each other easily.
   • This property makes it a good lubricant in machines where oil cannot be used.

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Descriptive Questions with Explanations

1. Mention the branch of Chemistry for each topic:

   • Rate of a reaction → Physical Chemistry
   • Digestion of food in the human body → Biochemistry
   • Properties of plasma → Physics & Physical Chemistry
   • Ecosystem study → Environmental Chemistry
   • Reactions during fireworks → Inorganic Chemistry
   • Absorption of UV waves using a spectrometer → Analytical Chemistry

2. What are allotropes? Explain carbon and sulfur allotropes.

   • Allotropes are different forms of the same element with different structures.
   • Carbon allotropes: Graphite (soft, used in pencils), Diamond (hard, used in jewelry), and Graphene (strongest material).
   • Sulfur allotropes: Rhombic sulfur (stable at room temperature) and Monoclinic sulfur (stable at high temperatures).
   • Coal vs. Diamond: Both are made of carbon, but diamond has a rigid structure while coal has an irregular structure.

3. What are supercritical fluids? How are they different from ordinary liquids?

   • Supercritical fluids have properties of both gases and liquids. They flow like gases but dissolve substances like liquids.
   • Example: Supercritical CO₂ is used to extract caffeine from coffee.

4. Define solubility. How does it change with temperature?

   • Solubility is the maximum amount of solute that can dissolve in a solvent at a given temperature.
   • Increase in temperature usually increases solubility for solids but decreases solubility for gases.

5. What types of movements are present in gaseous and liquid molecules?

   • Gases: Molecules move randomly and rapidly in all directions.
   • Liquids: Molecules slide past each other, allowing flow but keeping them close together.

6. Differentiate between inorganic and organic chemistry.

   • Organic Chemistry: Studies carbon-based compounds like sugars, oils, and plastics.
   • Inorganic Chemistry: Studies minerals, metals, and non-carbon compounds like salts and acids.

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Investigative Questions with Explanations

1. Describe a process in which potassium nitrate is purified by crystallization in water.

   • Crystallization is a technique to purify substances.
   • Dissolve potassium nitrate in hot water → Filter impurities → Cool the solution → Pure potassium nitrate crystals form.

2. Why is graphene useful in electronics?

   • High conductivity: Conducts electricity better than metals.
   • Strength and flexibility: Stronger than steel but very thin.
   • Uses: Used in touchscreens, batteries, and microchips for future technology.