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Activity 1.1 : Question (as per the steps in the textbook):
1. Take a 100 mL beaker.
2. Fill half the beaker with water and mark the level of water.
3. Dissolve some salt/sugar with the help of a glass rod.
4. Observe any change in water level.
5. What do you think has happened to the salt?
6. Where does it disappear?
7. Does the level of water change?
Answer:
• When you stir salt (or sugar) in water, the salt particles spread out into the tiny spaces between the water particles.
• The salt seems to disappear because it breaks down into very small particles that mix with water.
• The level of water does not rise, showing that the salt particles fit into the empty spaces between water particles.
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Activity 1.2 : Question (as per the steps in the textbook):
1. Take 2–3 crystals of potassium permanganate and dissolve them in 100 mL of water.
2. Take out approximately 10 mL of this solution and put it into 90 mL of clear water.
3. Take out 10 mL of this new solution and put it into another 90 mL of clear water.
4. Keep diluting like this 5 to 8 times.
5. Is the water still coloured?
Answer:
• Even after diluting many times, the water remains slightly purple (coloured).
• This shows that each tiny crystal of potassium permanganate has millions of small particles.
• These particles keep spreading throughout the water, which is why we can still see some colour even after many dilutions.
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Activity 1.3 : Question (as per the steps in the textbook):
1. Put an unlit incense stick in a corner of your classroom. How close do you have to go near it to get its smell?
2. Now light the incense stick. What happens? Do you get the smell sitting at a distance?
3. Record your observations.
Answer:
• When the incense stick is unlit, its smell is weak, so you have to go closer to notice it.
• When it is lit, the scent spreads quickly and you can smell it from far away.
• This happens because the particles of the fragrance move and mix quickly with the air, showing that they are constantly moving.
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Activity 1.4 : Question (as per the steps in the textbook):
1. Take two glasses/beakers filled with water.
2. Put a drop of blue or red ink slowly and carefully along the sides of the first beaker and honey in the same way in the second beaker.
3. Leave them undisturbed in a corner.
4. What do you observe immediately after adding the ink drop and the honey drop?
5. How many hours or days does it take for the colour of ink to spread evenly throughout the water?
Answer:
• The ink spreads out more quickly in water, so the colour starts to move through the water soon after adding it.
• Honey spreads very slowly because it is thicker (more viscous).
• After some time (usually a few hours), the entire water in the first beaker becomes evenly coloured.
• This shows that particles in liquids move and mix on their own (diffusion), and it is faster for substances like ink than for thicker liquids like honey.
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Activity 1.5 : Question (as per the steps in the textbook):
1. Drop a crystal of copper sulphate or potassium permanganate into a glass of hot water and another containing cold water.
2. Do not stir the solution.
3. Allow the crystals to settle at the bottom.
4. What do you observe just above the solid crystal in each glass?
5. What happens as time passes?
6. Does the rate of mixing change with temperature? Why and how?
Answer:
• In hot water, the coloured region around the crystal spreads faster.
• In cold water, the colour still spreads but takes longer.
• Over time, both glasses become coloured, but the hot water does it much quicker.
• Yes, the rate of mixing (diffusion) increases with temperature because particles move faster when they have more energy (in hot water).
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Activity 1.6 : Question (as per the steps in the textbook):
1. Form three human chains:
o First group locks arms tightly (like dancers).
o Second group holds each other’s hands.
o Third group just touches each other with fingertips.
2. The fourth group runs around and tries to break these chains.
3. Which chain is easiest to break?
4. Which chain is the hardest to break?
5. If each student represents a particle, in which chain are the particles held together with the strongest force?
Answer:
• The chain of students just touching fingertips is the easiest to break.
• The chain with arms locked from the back is the hardest to break.
• This shows that when particles are held very tightly (like in a solid), it is difficult to break them apart. When they are loosely held (like in gases), it is easy to separate them.
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Activity 1.7 : Question (as per the steps in the textbook):
1. Take an iron nail, a piece of chalk, and a rubber band.
2. Try breaking them by hammering, cutting or stretching.
3. In which of these substances do you think the particles are held together with greater force?
Answer:
• Iron nail is the hardest to break or change its shape.
• Chalk breaks more easily with a little force.
• Rubber band can be stretched.
• So, iron has the strongest force of attraction between its particles, chalk less so, and rubber band is flexible.
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Activity 1.8 : Question (as per the steps in the textbook):
1. Take some water in a container.
2. Try cutting the surface of water with your fingers.
3. Were you able to cut the surface of water?
4. What could be the reason behind the surface of water remaining together?
Answer:
• You cannot really “cut” the surface of water.
• Water molecules stay together because they have a force of attraction that holds them. This causes the surface to stay connected even if you try to slice through it gently.
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Activity 1.9 : Question (as per the steps in the textbook):
1. Collect a pen, a book, a needle, and a wooden stick.
2. Sketch their shape.
3. Do all these have a definite shape, distinct boundaries, and fixed volume?
4. What happens if you try to hammer or pull them?
Answer:
• All these objects (pen, book, needle, wooden stick) have definite shapes, clear boundaries, and a fixed volume.
• They do not change shape easily. If you apply a lot of force, they might break but will not change shape in a simple way.
• They are rigid and represent the solid state.
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Activity 1.10 : Question (as per the steps in the textbook):
1. Collect different liquids like water, cooking oil, milk, juice, a cold drink.
2. Take containers of different shapes.
3. Measure 50 mL of any one liquid and pour it into each container.
4. Does the volume remain the same?
5. Does the shape of the liquid remain the same in all the containers?
Answer:
• The volume remains 50 mL in all containers, so the amount of liquid does not change.
• The shape of the liquid changes according to the shape of each container.
• This shows that liquids have a fixed volume but no fixed shape.
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Activity 1.11 : Question (as per the steps in the textbook):
1. Take three 100 mL syringes and close their nozzles with rubber corks.
2. Remove the pistons.
3. Fill one syringe with air (no liquid, just air), the second with water, and the third with pieces of chalk.
4. Reinsert the pistons.
5. Try compressing the contents by pushing the piston in each syringe.
6. In which case was the piston easily pushed in?
7. What do you infer?
Answer:
• The syringe with air is easiest to compress.
• The syringe with water can be compressed only a little.
• The syringe with chalk pieces is almost impossible to compress.
• This tells us that gases are highly compressible, liquids are very slightly compressible, and solids are nearly incompressible.
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Activity 1.12 : Question (as per the steps in the textbook):
1. Take about 150 g of ice in a beaker. Suspend a thermometer in it so that its bulb touches the ice.
2. Start heating on a low flame.
3. Note the temperature when the ice starts melting and when it is fully melted.
4. Continue heating till the water starts boiling. Note the temperature when it starts boiling and when it has all turned to vapour.
Answer:
• Ice starts melting at 0°C (273 K). The temperature does not rise until all the ice has melted.
• After melting, the water temperature starts rising. It boils at 100°C (373 K). Again, the temperature stays at 100°C until all the liquid changes to vapour.
• The constant temperature during melting and boiling is due to latent heat (the hidden heat used for changing the state).
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Activity 1.13 : Question (as per the steps in the textbook):
1. Take some camphor and crush it. Put it in a china dish.
2. Cover it with an inverted funnel.
3. Put a cotton plug in the stem of the funnel.
4. Heat slowly.
5. Observe. What do you infer?
Answer:
• The camphor directly changes from solid to gas without first turning into a liquid.
• This process is called sublimation.
• Some solids like camphor and naphthalene show this property.
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Activity 1.14 : Question (as per the steps in the textbook):
1. Take 5 mL of water in a test tube and keep it near a window or under a fan.
2. Take 5 mL of water in an open china dish and keep it near a window or under a fan.
3. Take 5 mL of water in an open china dish and keep it inside a cupboard or in a corner of the room.
4. Record the time or days taken for the water to evaporate.
5. Repeat on a rainy day.
6. What do you infer about the effect of temperature, surface area, wind velocity, and humidity on evaporation?
Answer:
• Water evaporates faster when spread over a larger surface area (like in an open dish).
• Evaporation is faster under a fan or when wind speed is high.
• Evaporation is slower in a closed space or on a rainy (humid) day.
• Higher temperature and more wind help increase the rate of evaporation, while high humidity slows it down.
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Intext Questions (page 3)
1. Question: Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.
Answer: Matter: chair, air, almonds, lemon water
Not matter: love, smell, hate, thought, cold, smell of perfume
(Matter must have mass and occupy space.)
2. Question: Give reasons for the following observation: “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: Hot sizzling food gives off vapour particles that have more energy and move faster. They mix (diffuse) quickly with air and spread over a larger distance. Cold food releases fewer vapour particles with lower energy, so you have to be closer to smell it.
3. Question: A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer: This shows that particles of water have space between them and can be moved aside. The attractive force in liquids is not as strong as in solids, so the diver can move through water.
4. Question: What are the characteristics of the particles of matter?
Answer: a) They have spaces between them. b) They are constantly moving (they have kinetic energy). c) They attract each other with a force that varies from one kind of matter to another.
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Intext Questions (page 5)
1. Question: The mass per unit volume of a substance is called density (density = mass ÷ volume). Arrange the following in order of increasing density:
air, exhaust from chimneys, honey, water, chalk, cotton, iron.
Answer: Increasing order of density:
air < exhaust from chimneys < cotton < water < honey < chalk < iron
2. Question: (a) Tabulate the differences in the characteristics of states of matter.
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy, and density.
Answer (a): Solid : i) Definite shape and volume. ii) Particles are tightly packed. iii) Very hard to compress. iv) Does not flow.
Liquid : i) – No fixed shape but fixed volume. ii) Particles are less tightly packed compared to solids. iii) Slightly compressible. iv) Can flow.
Gas : i) Neither fixed shape nor fixed volume. ii) Particles are very far apart. iii) Highly compressible. iv) Flows and fills its container completely.
Answer (b):
• Rigidity: Solids are generally rigid, liquids and gases are not.
• Compressibility: Gases are highly compressible, liquids have very low compressibility, and solids are almost incompressible.
• Fluidity: Liquids and gases can flow; solids cannot.
• Filling a gas container: Gases spread and fill the container.
• Shape: Solids have definite shape, liquids take the shape of the container’s bottom, gases take the shape of the entire container.
• Kinetic Energy: Gases have the highest kinetic energy, liquids have moderate, solids have the least.
• Density: Solids have high density (generally), liquids have lower, and gases have the lowest density.
3. Question: Give reasons:
(a) A gas fills completely the vessel in which it is kept.
(b) A gas exerts pressure on the walls of the container.
(c) A wooden table should be called a solid.
(d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
Answer:
(a) Gas particles move in all directions with high speed and have large spaces between them, so they fill the container completely.
(b) The fast-moving particles of gas collide with the walls of the container, causing pressure.
(c) A wooden table has a definite shape, volume, and cannot flow, so it is a solid.
(d) Air is a gas with particles far apart, so you can easily move your hand through it. Wood is a solid with closely packed particles, so it needs a lot of force (like a karate expert) to break.
4. Question: Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer: In ice, water molecules form a cage-like structure with empty spaces, making it expand and become less dense than liquid water. Because it is less dense, ice floats on water.
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Intext Questions (page 7)
1. Question: Convert the following temperature to Celsius scale:
(a) 300 K = 300 – 273 = 27°C
(b) 573 K = 573 – 273 = 300°C
2. Question: What is the physical state of water at:
(a) 25°C (b) 100°C?
Answer: (a) At 25°C, water is a liquid (room temperature).
(b) At 100°C, water boils and starts changing into steam (gaseous state). However, right at 100°C, you can have both liquid and gas together if boiling is just starting.
3. Question: For any substance, why does the temperature remain constant during the change of state?
Answer: During the change of state, the heat supplied is used as latent heat to overcome the forces of attraction between particles. Because this heat is used for the change of state (not for raising temperature), the temperature stays constant until the entire substance has changed its state.
4. Question: Suggest a method to liquefy atmospheric gases.
Answer: By cooling (lowering the temperature) and compressing (increasing the pressure), we can bring the particles of gas close enough to form a liquid.
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Intext Questions (page 9 – Evaporation)
1. Question: Why does a desert cooler cool better on a hot dry day?
Answer: On a hot dry day, the rate of evaporation is faster because of high temperature and low humidity. More evaporation means more cooling, so a desert cooler works best under these conditions.
2. Question: How does the water kept in an earthen pot (matka) become cool during summer?
Answer: The earthen pot has tiny pores. Water seeps out through these pores and evaporates from the outer surface of the pot. Evaporation requires heat, which is taken from the water inside the pot, thus cooling it.
3. Question: Why does our palm feel cold when we put some acetone, petrol, or perfume on it?
Answer: These liquids evaporate very quickly. During evaporation, they absorb heat from our palm, making the palm feel cold.
4. Question: Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer: A saucer has a larger surface area, so the tea or milk spreads out and cools faster due to quicker evaporation. Once cooled, it is easier to sip.
5. Question: What type of clothes should we wear in summer?
Answer: We should wear cotton clothes. Cotton absorbs sweat from our body, and when the sweat evaporates, it cools us down.
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★ EXERCISE QUESTIONS AND ANSWERS ★
1. Question: Convert the following temperatures to the Celsius scale:
(a) 293 K = 293 – 273 = 20°C
(b) 470 K = 470 – 273 = 197°C
2.
3. Question: Convert the following temperatures to the Kelvin scale:
(a) 25°C (b) 373°C
Answer: (a) 25°C = 25 + 273 = 298 K (b) 373°C = 373 + 273 = 646 K
4. Question: Give reasons for the following observations:
(a) Naphthalene balls disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several metres away.
Answer:
(a) Naphthalene balls sublime, meaning they change directly from solid to vapour without becoming liquid, so they disappear over time.
(b) Perfume particles mix with air and diffuse quickly. This allows the smell to travel to people even if they are far away.
5. Question: Arrange the following substances in order of increasing forces of attraction between the particles: water, sugar, oxygen.
Answer: Oxygen (gas) < water (liquid) < sugar (solid)
6. Question: What is the physical state of water at:
(a) 25°C (b) 0°C (c) 100°C?
Answer: (a) Liquid at 25°C (room temperature).
(b) Solid or liquid at 0°C (this is the melting point; it can be ice or water).
(c) Liquid or gas at 100°C (this is the boiling point; water can be liquid or can turn to steam).
7. Question: Give two reasons to justify:
(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.
Answer:
(a) Water has no fixed shape but has a fixed volume; it flows easily, so it is a liquid.
(b) An iron almirah has a rigid shape and a fixed volume. It cannot flow or be compressed easily, so it is a solid.
8. Question: Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer: Ice at 273 K can absorb extra heat from the surroundings when it melts (latent heat of fusion). Water at 273 K does not need latent heat for melting (it is already in liquid form). Hence, ice at 273 K cools more effectively.
9. Question: What produces more severe burns, boiling water or steam?
Answer: Steam causes more severe burns because it has extra energy due to the latent heat of vaporization. When steam condenses on the skin, it releases this extra heat, making the burn more severe.
10. Question: Name A, B, C, D, E, and F in the diagram showing change of its state:
(Typically, the diagram has arrows showing: Solid ⇄ Liquid ⇄ Gas)
Answer:
o A: Sublimation (Solid to Gas)
o B: Deposition (Gas to Solid)
o C: Fusion or Melting (Solid to Liquid)
o D: Freezing (Liquid to Solid)
o E: Vaporization or Boiling (Liquid to Gas)
o F: Condensation (Gas to Liquid)