Matter in Our Surroundings- Solutions
Section Questions (Page 3)
Q1. Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.
Answer: Matter is anything that has mass and occupies space. So the following are matter:
Chair ✔ (has mass and volume)
Air ✔ (has mass and volume)
Almonds ✔ (has mass and volume)
Lemon water ✔ (has mass and volume)
Love, hate, thought, cold, smell, and smell of perfume are not matter — they are feelings, sensations or phenomena that do not have mass or occupy space.
Q2. Give reasons: The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
Answer: This is due to diffusion — the intermixing of particles of one matter into another on their own. At higher temperatures, the kinetic energy of particles increases, so particles of hot food move faster and diffuse quickly into the surrounding air, reaching us from far away. Cold food has particles with less kinetic energy, so diffusion is slow and we need to go close to smell it.
Q3. A diver is able to cut through water in a swimming pool. Which property of matter does this show?
Answer: This shows that particles of matter have spaces between them. Water particles have enough inter-particle space that allows the diver's hand (and body) to slip through, unlike a solid where particles are tightly packed and cannot be easily displaced.
Q4. What are the characteristics of the particles of matter?
Answer: The three main characteristics are:
Particles of matter have space between them – particles of one substance can get into the spaces of another (e.g., salt dissolving in water).
Particles of matter are continuously moving – they possess kinetic energy; this energy increases with temperature.
Particles of matter attract each other – there is a force of attraction between particles that holds them together; this force varies from substance to substance.
Section Questions (Page 6)
Q1. Arrange in order of increasing density: air, exhaust from chimneys, honey, water, chalk, cotton, iron.
Answer: Density order (lowest to highest):
Air < Exhaust from chimneys < Cotton < Water < Chalk < Honey < Iron
(Gases have the lowest density, followed by fluffy solids like cotton, then liquids, then dense solids.)
Q2(a). Tabulate the differences in characteristics of states of matter.
Q2(b). Comment upon: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
Answer:
Rigidity: Solids are rigid (maintain shape); liquids and gases are not.
Compressibility: Gases are highly compressible; liquids are slightly compressible; solids are negligibly compressible.
Fluidity: Liquids and gases can flow (fluids); solids cannot.
Filling a gas container: Gases have no fixed volume and fill the entire container due to high kinetic energy and negligible intermolecular forces.
Shape: Solids have a definite shape; liquids take the shape of the container; gases fill any container completely.
Kinetic energy: Solids have minimum KE (particles vibrate at fixed positions); liquids have intermediate KE; gases have maximum KE (particles move freely and randomly).
Density: Solids have the highest density; liquids are intermediate; gases have very low density.
Q3. Give reasons:
(a) A gas fills completely the vessel in which it is kept.
Gas particles have very high kinetic energy and move rapidly in all directions. There are negligible intermolecular forces, so particles spread throughout any available space, filling the entire vessel.
(b) A gas exerts pressure on the walls of the container.
Gas particles move randomly at high speed and collide with the walls of the container. The force exerted per unit area by these colliding particles is what we call gas pressure.
(c) A wooden table should be called a solid.
A wooden table has a definite shape, fixed volume, is rigid, and has negligible compressibility. Its particles are closely packed with strong forces of attraction — all properties of a solid.
(d) We can easily move our hand in air but not through a solid block of wood.
Air particles have large inter-particle spaces and weak forces of attraction, so our hand can displace them easily. In wood (a solid), particles are very closely packed with strong intermolecular forces, making it rigid and resistant to displacement.
Q4. Why does ice float on water?
Answer: Even though ice is a solid, it floats on water because ice is less dense than liquid water. Water has a unique property — when it freezes, its molecules arrange themselves in a hexagonal lattice structure that actually takes up more space than in liquid form. So ice has a lower density (≈ 0.917 g/cm³) than water (≈ 1 g/cm³), causing it to float.
Section Questions (Page 9)
Q1. Convert the following temperatures to the Celsius scale:
(a) 300 K
°C = K − 273 = 300 − 273 = 27°C
(b) 573 K
°C = K − 273 = 573 − 273 = 300°C
Q2. What is the physical state of water at:
(a) 250°C → Gas (Steam/Water vapour) — above boiling point (100°C)
(b) 100°C → Both liquid and gas — water is at its boiling point, so it exists as both liquid water and water vapour simultaneously
Q3. For any substance, why does temperature remain constant during change of state?
Answer: During a change of state, the heat supplied is used to overcome the forces of attraction between particles rather than increasing the kinetic energy of particles. This heat is called latent heat (hidden heat). Since temperature depends on kinetic energy, and kinetic energy is not changing, the temperature remains constant until the entire substance changes its state.
Q4. Suggest a method to liquefy atmospheric gases.
Answer: Atmospheric gases can be liquefied by:
Applying high pressure — bringing particles closer together, and
Reducing temperature (cooling) — decreasing kinetic energy so particles cannot overcome intermolecular forces.
Both methods together are used industrially (e.g., liquid nitrogen, liquid oxygen in cylinders).
Section Questions (Page 10 — Evaporation)
Q1. Why does a desert cooler cool better on a hot dry day?
Answer: On a hot dry day, humidity (water vapour in air) is low. Since the surrounding air can absorb more water vapour, evaporation rate from the cooler pads is higher. More evaporation means more heat absorbed from the surroundings, resulting in better cooling.
Q2. How does water in an earthen pot (matka) become cool in summer?
Answer: An earthen pot is porous and has tiny holes. Water seeps out through these pores and evaporates from the outer surface. During evaporation, water absorbs latent heat from the remaining water inside the pot, cooling it down.
Q3. Why does our palm feel cold when we put acetone/petrol/perfume on it?
Answer: Acetone, petrol, and perfume are highly volatile liquids. When placed on the palm, their particles absorb heat energy (latent heat of vaporisation) from our palm to evaporate quickly. This loss of heat from the skin makes the palm feel cool.
Q4. Why can we sip hot tea faster from a saucer than a cup?
Answer: A saucer has a larger surface area than a cup. Larger surface area leads to faster evaporation of hot tea. More evaporation causes faster cooling. So tea in a saucer cools down more quickly and can be sipped sooner.
Q5. What type of clothes should we wear in summer?
Answer: We should wear cotton clothes in summer. Cotton is a good absorber of sweat (perspiration). When the sweat is exposed to the atmosphere, it evaporates quickly, absorbing heat from the body and keeping us cool. Synthetic fabrics do not absorb sweat well and feel uncomfortable.
End Exercises (Page 12–13)
Q1. Convert to Celsius scale:
(a) 293 K → 293 − 273 = 20°C
**(b) 470 K** → 470 − 273 = 197°C
Q2. Convert to Kelvin scale:
(a) 25°C → 25 + 273 = 298 K
**(b) 373°C** → 373 + 273 = 646 K
Q3. Give reasons:
(a) Naphthalene balls disappear with time without leaving any solid.
Naphthalene undergoes sublimation — it changes directly from the solid state to the gaseous state without passing through the liquid state. So it disappears into the air without leaving any liquid or solid residue.
(b) We can get the smell of perfume sitting several metres away.
Perfume is a volatile liquid. Its particles evaporate and diffuse rapidly into the surrounding air due to high kinetic energy. Since gas particles move fast and have large spaces between them, they spread over large distances quickly, reaching our nostrils from far away.
Q4. Arrange in increasing order of forces of attraction: water, sugar, oxygen.
Answer:
Oxygen < Water < Sugar
Oxygen is a gas → weakest intermolecular forces
Water is a liquid → intermediate forces
Sugar is a solid → strongest forces of attraction
Q5. Physical state of water at:
(a) 25°C → Liquid (between 0°C and 100°C)
(b) 0°C → Both solid and liquid (melting point — both ice and water coexist)
(c) 100°C → Both liquid and gas (boiling point — water and steam coexist)
Q6. Give two reasons to justify:
(a) Water at room temperature is a liquid:
Water has no fixed shape — it takes the shape of its container.
Water has a fixed volume and flows easily (has fluidity), unlike solids.
(b) An iron almirah is a solid at room temperature:
It has a definite shape and fixed volume that does not change.
It is rigid and hard — it cannot be compressed or made to flow.
Q7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer: Ice at 273 K (0°C) absorbs extra heat called the latent heat of fusion (334 J/g) while melting into water, in addition to absorbing heat to raise its temperature. Water at 273 K can only absorb heat to raise temperature. Since ice absorbs much more heat from its surroundings during melting, it is a more effective coolant.
Q8. What produces more severe burns — boiling water or steam?
Answer: Steam produces more severe burns. Steam at 100°C has more energy than boiling water at the same temperature because steam contains additional energy in the form of latent heat of vaporisation (2260 J/g). When steam condenses on the skin, this extra latent heat is released, causing more severe burns.
Q9. Name A, B, C, D, E and F in the state-change diagram:
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