Section ll

f. Give reasons for the following

1. A spoonful of salt disappears in a glass of water.

Answer. When salt is added to water, its sodium and chloride ions separate due to water’s polarity. The water molecules surround these ions, breaking the ionic bonds and distributing them evenly—a process called dissolution. Since the particles are too small to see, the salt “disappears,” forming a homogeneous solution. This is a physical change because the salt can be recovered by evaporation. Temperature and stirring speed up this process by increasing particle movement.

2. Gases do not have a definite shape or definite volume.

Answer. Gases consist of widely spaced particles with weak intermolecular forces, allowing them to move freely in all directions. Unlike solids or liquids, gas particles are not tightly packed, so they expand to fill any container, taking its shape. Their volume changes with pressure and temperature (Boyle’s and Charles’s laws). For example, air fills a balloon but escapes if untied, showing no fixed boundaries.

3. When we put an inflated balloon in a refrigerator, its size decreases.

Answer. Cooling a balloon reduces the kinetic energy of gas particles inside, slowing their movement. According to Charles’s Law, gases contract at lower temperatures, decreasing volume. The rubber balloon’s elastic surface also tightens as the internal pressure drops. If heated again, the balloon re-expands as particles regain energy. This demonstrates gas thermal expansion/contraction.

4. Burning of a candle wick is a chemical change.

Answer. Burning involves combustion, where wax (hydrocarbon) reacts with oxygen to produce CO₂, water vapor, and heat (irreversible change). The wick turns to ash, a new substance with different properties. Unlike melting wax (physical change), burning releases energy and cannot be reversed, meeting all criteria for a chemical change: color change, gas production, and heat emission.

5. Gases expand to fill their container.

Answer. Gas particles move rapidly and randomly (Brownian motion) due to high kinetic energy. With negligible cohesive forces, they collide with container walls, creating pressure that pushes outward until the space is uniformly filled. For example, perfume spreads in a room, or helium fills any balloon shape. This property is utilized in pneumatic systems and gas storage.

G. Write any two salient  feature of each  of the following terms.

1. Matter

• Occupies Space – All matter has volume, whether solid, liquid, or gas.
  • Has Mass – Measurable quantity (e.g., grams or kilograms).
  • Three States – Exists as solids (fixed shape), liquids (fixed volume), or gases (no fixed shape/volume).
  • Particle Composition – Made of atoms/molecules with kinetic energy.
  • Physical/Chemical Properties – Can undergo changes like melting or burning.

2.Mass

• Measure of Quantity – Determines how much matter an object contains.
  • Constant Everywhere – Unaffected by location (unlike weight, which depends on gravity).
  • SI Unit: Kilogram (kg) – Standardized metric measurement.
  • Inertia – Greater mass = Greater resistance to motion change (Newton’s First Law).
  • Conserved in Reactions – Mass isn’t created or destroyed in physical and chemical changes.

3.Heat

• Energy Transfer – Flows from hotter to colder objects.
  • Changes States – Melts ice (solid → liquid) or boils water (liquid → gas).
  • Measured in Joules (J) – Or calories (cal).
  • Increases Particle Motion – Raises kinetic energy of molecules.
  • Thermal Expansion – Most matter expands when heated (e.g., mercury in thermometers).

4. Freezing

• Liquid → Solid Transition – Occurs at the substance’s freezing point (e.g., water at 0°C).
• Energy Release – Latent heat is emitted during the process.
• Particle Arrangement – Molecules form ordered, rigid structures.
  Reversible – Solids can melt back into liquids with heat.
• Examples – Water turning to ice, lava solidifying into rock.

5.Cohesive force

• Interparticle Attraction – Holds like molecules together (e.g., water droplets).
• Strongest in Solids – Maintains fixed shapes (e.g., diamond’s rigid structure).
• Weakest in Gases – Particles move freely with minimal attraction.
• Surface Tension – Causes liquids to minimize surface area e.g., water beads on leaves.
  
  
• Capillary Action – Helps water rise in narrow tubes against gravity.

H. Differentiate between the following.

1. Deposition and sublimation

Deposition	Sublimation
Gas → Solid (e.g., frost forming).	Solid → Gas (e.g., dry ice vaporizing).
Releases energy (exothermic).	Absorbs energy (endothermic).
Common in cold conditions.	Occurs at specific temps/pressures.
Used in semiconductor manufacturing.	Preserves food (freeze-drying).

2. Physical change and chemical change.

Answer.

Physical Change	Chemical Change
No new substance forms, e.g., ice melting.	New substances form (e.g., rusting).
Reversible (e.g., water → ice).	Irreversible (e.g., burning wood).
Alter shape/state: boiling, cutting.	Changes in composition digestion.
Energy changes are minor.	Often releases/absorbs heat/light.

3. Inter-particle force and interparticle space.

Inter-particle Force	Inter-particle Space
Attraction between particles.	Gaps between particles.
Strongest in solids, weakest in gases.	Largest in gases, smallest in solids.
Determines melting/boiling points.	Affects compressibility (e.g., gases).

4. Melting point and boiling point

Melting Point	Boiling Point
Solid → Liquid (e.g., ice at 0°C).	Liquid → Gas (e.g., water at 100°C).
Particles gain energy to vibrate freely.	Particles escape as vapor.
Unique to each substance.	Depends on atmospheric pressure.

5.Gas and Vapour

Gas	Vapor
Naturally gaseous (e.g., oxygen).	Gaseous form of solids/liquids (e.g., steam).
Exists at room temp (e.g., helium).	Forms via heating/evaporation.
Fixed boiling point.	Condenses into liquid when cooled.

l. Short answer questions

1. Why does chocolate (solid) melt when heated but does not change its state when cooled?

Answer. Chocolate melts when heated because the thermal energy breaks the bonds between its solid cocoa butter and sugar particles. As temperature rises, particles vibrate faster, transitioning from an ordered solid to a free-flowing liquid. Cooling reverses this, but recrystallization requires sustained low temps—hence, it doesn’t instantly resolidify.

This physical change retains chocolate’s composition, unlike a burning chemical change.

2. What happens to the air inside a balloon (gas) when it is placed in the freezer?

Answer. The balloon shrinks because cooling reduces air particles’ kinetic energy, slowing their movement. According to Charles’s Law, gases contract in cold temps, decreasing volume. The rubber also tightens as pressure drops. Heating reverses this—particles regain energy, re-expanding the balloon.

3. How does the smell of perfume (liquid) spread in a room?

Answer. Perfume spreads via diffusion: liquid particles evaporate into gas, moving randomly (Brownian motion) and colliding with air molecules. Over time, they disperse evenly, filling the room. Heat accelerates this by increasing particle speed.

4. State an example of Brownian motion.

Answer. Dust particles dancing in sunlight demonstrate Brownian motion. Air molecules (invisible) collide with dust, causing erratic movement. This proves gas particles are in constant, random motion.

5. What are two main effects of heat on the states of matter?

1. Answer. State Change: Solid → Liquid → Gas (e.g., ice → water → steam).
2. Expansion: Particles move apart (e.g., hot air balloons rise).

6. On what basis does the particulate theory of matter explain the three states ?

Answer. The theory explains states via particle arrangement:

• Solids: Tight, vibrating particles.
• Liquids: Loosely packed, flowing.
• Gases: Far apart, chaotic motion.

7. Sarah conducted an experiment where she mixed iron filings with sulphur powder in a test tube . Then she heated the mixture over a Bunsen burner. Do these two acts lead to the same kind of changes or different? Justify your answer.

• Answer. Mixing: Physical change (no new substance).
• Heating: Chemical change (forms iron sulphide, a black solid). Heat triggers irreversible reactions, unlike mixing.

J. Long answers questions.

1. a. Give a simple example that gases expand on heating

Answer.When a gas is heated, its particles absorb thermal energy and move faster, increasing collisions with container walls. This raises internal pressure, forcing the gas to expand and occupy more space. A common example is a hot-air balloon: heating the air inside reduces its density, making it rise. Conversely, cooling contracts gases, as seen when an inflated balloon shrinks in a freezer. This behavior follows Charles’s Law, which states gas volume is directly proportional to temperature (at constant pressure). Such expansion principles are vital in engines, weather systems, and industrial applications like gas storage and pneumatic tools.

b. What happens when solids are heated? Explain with the help of an experiment.

Answer. Take an ice cube (solid) and heat it gently. As temperature rises, the ice absorbs energy, causing its rigid molecular structure to weaken. At 0°C, it melts into water (liquid), showing particle mobility. Further heating to 100°C boils the water into steam (gas), where particles escape freely. This demonstrates how heat disrupts intermolecular forces, changing states.

The reverse processes—cooling steam to water or freezing water to ice—highlight energy release.

Such experiments confirm kinetic theory: higher energy increases particle motion, altering matter’s physical form without chemical change.

2.a. Explain the interconversion of states of matter.

Answer. Matter transitions between states via energy changes:

• Melting (Solid → Liquid): Heat breaks rigid bonds (e.g., ice → water).
• Freezing (Liquid → Solid): Cooling forms ordered structures (e.g., water → ice).
• Vaporization (Liquid → Gas): Particles gain energy to escape (e.g., boiling water).
• Condensation (Gas → Liquid): Cooling reduces particle motion (e.g., dew formation).
• Sublimation (Solid → Gas): Particles skip liquid phase (e.g., dry ice → CO₂ gas).
• Deposition (Gas → Solid): Gas particles settle directly (e.g., frost formation).
  These changes are reversible and governed by temperature and pressure.

2b. Define the process that takes place during the interconversion of states of matter.

Answer. State transitions involve energy exchange:

• Endothermic Processes: Require heat absorption (melting, vaporization, sublimation). For example, sweating cools the body as water absorbs heat to evaporate.
• Exothermic Processes: Release heat (freezing, condensation, deposition). Frost forming on grass releases latent heat.
• Critical Points: At specific temperatures/pressures (e.g., water’s boiling point), phases coexist. These principles underpin technologies like refrigeration (coolant phase changes) and weather cycles (water’s natural state shifts).

3. a. With the help of a diagram, show the arrangement of the particles in the three states of matter.

• Solids: Particles are tightly packed in fixed positions, vibrating slightly (e.g., diamond’s rigid lattice).
• Liquids: Particles are close but slide past each other, taking the container’s shape (e.g., water in a glass).
• Gases: Particles are far apart, moving rapidly and randomly (e.g., oxygen in air).
  Visual Aid: A labeled diagram showing particle density and motion in each state clarifies their properties.

3b. Distinguish between the three states of matter – solid, liquid, and gas.

Answer. Solid vs. Liquid vs. Gas

Property	Solid	Liquid	Gas
Shape	Fixed (e.g., iron rod)	Takes container’s shape (e.g., juice)	No fixed shape (e.g., perfume vapor)
Volume	Fixed	Fixed	Expands infinitely
Density	High (tight packing)	Moderate	Very low
Energy	Low particle motion	Moderate flow	High-speed collisions
Example: Ice (solid) → Water (liquid) → Steam (gas) shows progressive energy increase and order loss.