The Amazing World of Matter – Long Answer Questions (Class 9 Science: Chemistry)

Medium Level (Application & Explanation)

Q1. Explain the Particle Theory of Matter and how it accounts for the properties of solids, liquids, and gases.

Answer:

The particle theory states that all matter is made of tiny particles (atoms or molecules) which are in constant motion.
In solids particles are closely packed and vibrate about fixed positions; this explains definite shape and volume and rigidity.
In liquids particles are close but can move past one another, giving liquids definite volume but no fixed shape — they take the container’s shape.
In gases particles are far apart and move freely, which explains no definite shape or volume, compressibility, and ability to expand.
The strength of forces between particles and their kinetic energy determine state; higher kinetic energy (temperature) makes particles move more freely, causing changes of state like melting and evaporation.

Q2. Differentiate between physical and chemical changes with two clear examples of each and reasons why they are classified so.

Answer:

Physical changes alter appearance but not the composition. Examples:
- Melting ice → water (H₂O): only state changes; molecules remain H₂O.
- Tearing paper: size and shape change; chemical identity unchanged.
Chemical changes form new substances with different properties. Examples:
- Burning wood → ash, CO₂ and gases: new substances produced by chemical reactions.
- Rusting of iron → iron oxide (Fe₂O₃·xH₂O): iron reacts with oxygen and water to form a new compound.
Key signs of chemical change: color change, gas evolution, heat/light, precipitate formation, and irreversibility compared to many physical changes.

Q3. Why are gases easily compressible but solids are not? Explain using particle arrangement and forces.

Answer:

Gases have particles that are far apart with large empty spaces between them. Under pressure, these particles can be forced closer, reducing volume—hence high compressibility.
Solids have particles tightly packed in fixed positions with very little empty space. Strong forces keep them close, so applying pressure barely reduces volume—thus low compressibility.
In gases, intermolecular forces are weak compared to particle kinetic energy, allowing particles to be squashed together. In solids, strong bonding (ionic, covalent, metallic) resists compression.
Compressibility reflects particle spacing and strength of attractions between particles.

Q4. Describe two simple experiments to demonstrate diffusion in a liquid and in a gas, and explain the observations using particle motion.

Answer:

Liquid experiment: place a crystal of potassium permanganate in still water. The purple spreads slowly without stirring. Observation: colour gradually fills the water. Explanation: particles of KMnO₄ move randomly and spread into water — diffusion due to particle motion.
Gas experiment: spray perfume at one corner of a room. After some time, the scent is noticed far away. Observation: smell reaches distant parts. Explanation: perfume molecules move randomly through air, colliding and dispersing — faster diffusion in gas because particles are more energetic and farther apart.
Both show particles move from regions of higher concentration to lower concentration until uniform.

Q5. Differentiate evaporation and boiling. List factors that increase the rate of evaporation.

Answer:

Evaporation is a surface phenomenon occurring at any temperature where molecules at the liquid surface with sufficient energy escape into vapor; it is slow and cools the remaining liquid.
Boiling is a bulk phenomenon occurring at a specific boiling point when vapor pressure equals external pressure; bubbles form throughout the liquid and escape.
Factors increasing evaporation rate: higher temperature (more particles with enough energy), larger surface area, lower humidity (less vapor in surrounding air), and air movement/wind (removes vapor molecules).
Evaporation depends on molecular energy distribution, while boiling depends on external pressure and the liquid’s vapor pressure.

High Complexity (Analytical & Scenario-Based)

Q6. A sealed, insulated container holds ice which melts and then the water slowly evaporates. Explain the energy changes, particle behaviour, and whether total mass of the system changes.

Answer:

In the insulated sealed container, heat flows into ice from surroundings (if allowed) causing melting; energy absorbed is the latent heat of fusion and increases particle kinetic energy, weakening bonds so ice becomes liquid.
For evaporation, molecules at the surface with enough energy escape into gas phase; energy required is the latent heat of vaporization taken from the liquid, lowering temperature unless additional heat enters.
Particle behaviour: ordered solid → disordered liquid → free-moving gas with increasing average kinetic energy.
In a sealed container, the total mass remains constant (conservation of mass): molecules only change state but are contained, so no mass is lost.

Q7. Two cubes of equal volume are made of iron and wood. Explain why the iron cube feels heavier and whether it will sink or float on water. Use density concept and include the formula.

Answer:

Density (ρ) = mass / volume. For equal volumes, the object with greater mass has higher density. Iron has a much higher density (~7.8 g·cm⁻³) than typical wood (~0.6–0.9 g·cm⁻³).
The iron cube therefore has greater mass and higher ρ, so it feels heavier.
Whether an object sinks or floats depends on comparison of its density with water (1.0 g·cm⁻³). Iron’s density > water → iron cube sinks. Wood’s density < water → wood floats.
Floating also involves buoyant force equal to weight of displaced water; if object density < water, buoyant force supports it.

Q8. Explain why the boiling point of water decreases at high altitudes and how this affects cooking time for rice or vegetables.

Answer:

Boiling occurs when a liquid’s vapour pressure equals external (atmospheric) pressure. At high altitude, atmospheric pressure is lower, so water reaches vapour pressure at a lower temperature, hence boils at a lower temperature than 100°C.
Lower boiling temperature means water is cooler when boiling, so cooking slows down; foods like rice or vegetables take longer to cook because heat transfer to the food is at a lower temperature.
To compensate, one can increase cooking time, use a pressure cooker (raises pressure and boiling point), or pre-boil longer.
This is important for practical cooking and explains why recipes may vary with altitude.

Q9. Describe the chemical process of rusting of iron. State conditions required, the chemical change involved with formula, and why it is not reversible by simple physical means.

Answer:

Rusting is a chemical change where iron reacts with oxygen and water to form hydrated iron(III) oxide, commonly written as Fe₂O₃·xH₂O.
Conditions required: presence of oxygen (air) and moisture (water); salts or acidic conditions accelerate rusting.
Chemical change: iron atoms lose electrons (oxidation) and oxygen is reduced; new compound has different properties (brown, flaky) from iron.
Rusting is not reversible by physical methods because the process forms new chemical bonds and substances. To convert rust back to iron requires chemical reduction (electrolysis or strong reducing agents), not merely reshaping or heating.

Q10. You have a mixture of salt (sodium chloride) and sand. Propose a step-by-step separation procedure, explain the principle behind each step, and write any chemical formulas used.

Answer:

Step 1 — Dissolve: Add water to the mixture. Salt (NaCl) dissolves (forms Na⁺ and Cl⁻ ions), sand (SiO₂) remains insoluble. Principle: solubility difference.
Step 2 — Filter: Use filtration to separate the insoluble sand (residue) from the salt solution (filtrate). Principle: physical separation by particle size/solubility.
Step 3 — Evaporate: Heat the salt solution to evaporate water; as water evaporates, NaCl crystals form. Principle: evaporation/crystallization.
Step 4 — Collect: Cool, collect and dry NaCl crystals and dry the sand. Result: pure sand (SiO₂) and sodium chloride (NaCl) separated.

keep practising these concepts and remember the key words: particles, density, diffusion, evaporation, boiling point, chemical change, and conservation of mass!