Case 1: Riya kept a few ice cubes on a plate in her room. After some time, she noticed water droplets forming on the plate.

Q1. Identify the process taking place.

Melting — the ice cubes changed from solid to liquid state due to heat absorption from the surroundings.

Q2. What does this process demonstrate about the nature of matter?

It shows that matter can change its state when energy is added or removed — a reversible physical change.

Case 2: A balloon filled with air bursts when pressed too hard.

Q1. What property of gases is demonstrated here?

Gases are highly compressible; applying force reduces the space between particles causing pressure buildup until it bursts.

Q2. Why are gases compressible but solids are not?

Gas particles are far apart with large empty spaces, whereas in solids particles are tightly packed with negligible space.

Case 3: During summer, wet clothes dry faster in the sun than in the shade.

Q1. Name the process responsible for drying of clothes.

Evaporation — the process by which water from wet clothes changes into vapour.

Q2. State two factors that increase the rate of evaporation.

Higher temperature and increased surface area (like spreading clothes) speed up evaporation.

Case 4: When perfume is sprayed in a corner of a room, its smell spreads throughout quickly.

Q1. Which property of matter is shown here?

Diffusion — spontaneous mixing of particles from high to low concentration due to random motion.

Q2. Why is diffusion faster in gases than in liquids?

Gas particles move freely at high speed with larger interparticle spaces, allowing faster mixing.

Case 5: An earthen pot cools water naturally on hot summer days.

Q1. Explain the principle involved.

Evaporation — water seeps through the pores and evaporates from the outer surface, taking latent heat from the water and cooling it.

Q2. What is the role of latent heat here?

Latent heat of vaporization is absorbed from water during evaporation, reducing its temperature.

Case 6: Dry ice (solid CO₂) is used to keep ice cream cold during transport.

Q1. What special property of dry ice makes it useful here?

Sublimation — dry ice directly changes from solid to gas without becoming liquid, absorbing heat in the process.

Q2. Why is it called “dry” ice?

Because it does not melt into liquid water but directly turns into carbon dioxide gas.

Case 7: When a glass of cold water is left in open air, droplets appear on its outer surface.

Q1. Which process is responsible for droplet formation?

Condensation — water vapour in air cools and converts into liquid droplets on the cold surface.

Q2. What does this indicate about the presence of water vapour in air?

It proves that air always contains moisture in the form of water vapour.

Case 8: A pressure cooker cooks food faster than an open pot.

Q1. Why does this happen?

Inside a pressure cooker, pressure increases, which raises the boiling point of water, allowing food to cook faster.

Q2. Which concept of matter is involved here?

Boiling point depends on external pressure — an increase in pressure raises the boiling point.

Case 9: A student observed camphor disappearing when left in an open dish.

Q1. What is this process called?

Sublimation — camphor changes directly from solid to gas without becoming liquid.

Q2. Mention another substance that sublimes.

Naphthalene or ammonium chloride.

Case 10: When steam comes into contact with a metal surface, droplets of water form.

Q1. What change of state occurs here?

Gas (steam) to liquid (water) — condensation.

Q2. Why does condensation release heat?

Because molecules lose energy while changing from a high-energy gaseous state to a low-energy liquid state.

Case 11: A bottle of cold drink is kept in sunlight and gas bubbles form rapidly inside it.

Q1. Why do bubbles form on heating?

Gas dissolved in the liquid escapes due to reduced solubility at higher temperature, forming bubbles.

Q2. What does this show about solubility and temperature?

Gas solubility in liquids decreases as temperature increases.

Case 12: A student noticed that salt does not change temperature when it dissolves in water.

Q1. What kind of change is this — physical or chemical?

Physical change — no new substance forms; salt only disperses as ions in water.

Q2. Which property of matter allows salt to dissolve?

The presence of interparticle spaces in liquids allows small particles of salt to spread uniformly.

Case 13: Two perfumes are sprayed at opposite ends of a room; both scents mix after some time.

Q1. Name the process involved.

Diffusion of gases — both vapours intermingle spontaneously.

Q2. Which factor affects this rate most?

Temperature — higher temperature increases particle motion and diffusion rate.

Case 14: A steel bridge expands slightly during summer and contracts in winter.

Q1. Which property of matter is responsible?

Thermal expansion — particles move farther apart when heated.

Q2. How is this managed in bridge design?

Expansion joints are left to accommodate changes without damage.

Case 15: A pressure cooker lid has a safety valve that releases steam if pressure gets too high.

Q1. What scientific principle justifies the safety valve?

Increased heat increases vapour pressure; the valve releases excess steam to prevent explosion.

Q2. How does vapour pressure relate to boiling point?

A liquid boils when its vapour pressure equals external pressure.

Case 16: A perfume bottle kept open for long loses its fragrance.

Q1. Which process causes the fragrance to fade?

Evaporation — perfume molecules escape into air over time.

Q2. Why does it happen faster in warm conditions?

Because higher temperature increases molecular kinetic energy, accelerating evaporation.

Case 17: Naphthalene balls kept in clothes disappear over time.

Q1. Which process is responsible?

Sublimation — naphthalene changes directly from solid to gas.

Q2. What does this show about the strength of molecular forces?

Weak intermolecular forces allow easy transition between solid and gaseous states.

Case 18: During winter, water vapour condenses into dew drops on grass.

Q1. What change of state is this?

Gas to liquid — condensation.

Q2. Why does condensation occur more in early morning?

Because temperature drops at night, cooling air and reducing its capacity to hold moisture.

Case 19: Ice cubes stick together when taken out from the freezer.

Q1. What causes the cubes to stick?

Surface melts slightly; water refreezes between cubes, joining them — an example of freezing.

Q2. What type of change is involved?

Physical change — reversible on melting again.

Case 20: A student feels cool after applying spirit on the skin.

Q1. Why does this happen?

Spirit evaporates quickly, absorbing heat from skin during evaporation and causing a cooling sensation.

Q2. Which property of liquids does this show?

Evaporation produces cooling — a common physical property of volatile liquids.