Probe and Ponder (Page 1)
Question: Why is it possible to pile up stones or sand, but not a liquid like water? Answer: Stones and sand are solids. Their particles are held together very tightly, so they have a fixed shape and can be piled up. Water is a liquid. Its particles are not held as tightly as in solids and can move around, so it flows and cannot be piled up.
Question: Why does water take the shape of folded hands but lose that shape when released? Answer: Water is a liquid, which means it does not have a fixed shape. It takes the shape of whatever container or space it is in (like folded hands) because its particles are free to move. When you open your hands, the water flows away because there is nothing holding it in that shape anymore.
Question: We cannot see air, so how does it add weight to an inflated balloon? Answer: Even though we cannot see air, it is made of matter. It consists of tiny particles that have mass. When you blow air into a balloon, you are adding many of these particles inside, which increases the weight of the balloon.
Question: Is the air we breathe today the same that existed thousands of years ago? Answer: The particles that make up the air (like nitrogen and oxygen) are recycled constantly in nature, but they are essentially the same matter. However, the mixture might have changed slightly due to things like pollution.
In-text Questions and Activities
Activity 7.1: Let us explore (Page 2) Question: What do you observe? (After grinding chalk) Answer: You observe a fine powder of chalk. Even though the piece is crushed, the powder is still made of white chalk particles.
Question: Is every speck of this fine chalk powder still composed of the same substance, or has it changed into something else on breaking or grinding? Answer: Every speck is still composed of the same substance (chalk). It has not changed into anything else.
Question: Is grinding chalk a physical change or a chemical change?
Answer: Grinding chalk is a physical change because only the size of the chalk particles has changed (they became smaller), but the substance is still chalk.
Text Question (Page 2): Is this grain the smallest unit of a bigger rock or can these grains of sand and clay be broken down further? Answer: The grains of sand and clay can be broken down further. They are made up of even smaller constituent particles.
Text Question (Page 3): Are the units of chalk obtained in this manner considered the smallest units of chalk? Answer: No, the tiny grains we see are not the smallest units. They are made up of millions of extremely small constituent particles that we cannot see with our eyes.
Activity 7.2: Let us perform (Page 3) Question: What happens to sugar when it is dissolved in water? Answer: When sugar dissolves, it breaks up into its tiny constituent particles. These particles separate and spread out into the spaces between the water particles.
Question: Does the water taste sweet? (Before stirring)
Answer: The water at the top might not taste very sweet immediately if the sugar has settled at the bottom and hasn’t dissolved yet.
Question: What difference in taste do you notice? Does it taste sweet? (After stirring) Answer: After stirring, the water tastes sweet. This confirms that sugar is present in the solution even though we cannot see it.
Question: Do you observe any sugar particles in the solution?
Answer: No, sugar particles cannot be observed because they have broken down into particles too small to be seen.
Text Question (Page 4): But, where did the sugar go?
Answer: The sugar particles separated and occupied the tiny empty spaces (interparticle spaces) between the water particles.
Text Question (Page 5): How are constituent particles held together in solids?
Answer: In solids, the constituent particles are held together by very strong forces of attraction called interparticle attractions.
Activity 7.3: Let us find out (Page 5) Question: In which of the above six objects do you think particles are strongly held together? Answer: In all the solid objects listed (Iron nail, Rock salt, Stone, Wooden block, Key, Aluminium), the particles are strongly held together. However, materials like iron and stone are generally harder, implying very strong forces.
Question: In the solid state, is there any way to move these particles apart? Answer: Yes, if we heat the solid, the particles vibrate more vigorously. If heated enough (to the melting point), they can overcome the strong forces and move apart, turning the solid into a liquid.
Text Question (Page 6): Solids have a definite volume; what about liquids and gases? Answer: Liquids have a definite volume but no fixed shape. Gases have neither a definite volume nor a definite shape.
Activity 7.4: Let us try and find out (Page 7) Question: Are you able to move your finger through the water?
Answer: Yes, you are able to move your finger through the water easily. This is because the forces holding water particles together are not as strong as in solids, allowing you to push them aside temporarily.
Text Question (Page 8): Do gases also have a fixed volume? Answer: No, gases do not have a fixed volume. They spread out to fill the entire space available to them.
Activity 7.6: Let us experiment (Page 10) Question: What do you observe? (When pushing the syringe plunger with air)
Answer: You observe that the plunger can be pushed in, and the volume of air inside the syringe decreases.
Question: What can we say about the behaviour of gas in the syringe? Answer: The gas is compressible. This shows that there is a lot of empty space between gas particles, which can be reduced by applying pressure.
Activity 7.7: Let us observe (Page 11) Question: Predict whether the water level will increase or decrease with respect to the mark B.
Answer: After the sugar dissolves completely, the water level will likely decrease slightly from mark B (the level immediately after adding sugar but before dissolving) or settle closer to the original water level A.
Question: What difference do you observe in the water levels? Answer: Initially, adding sugar raises the level. After dissolving, the level drops slightly. This is because the dissolved sugar particles fit into the empty spaces between the water particles, so they don’t add as much extra volume as solid sugar did.
Question: What do you observe in each case? Do the sand particles dissolve? Answer: Sand particles do not dissolve. They settle at the bottom. The volume of water increases and stays increased because the sand takes up its own space and does not fit into the spaces between water particles.
Question: Sugar and sand are both solids. Why does sugar dissolve in water but sand does not? Answer: Sugar dissolves because the water particles can pull the sugar particles apart and mix with them. Sand particles are held together too strongly for water to pull them apart, so they remain as solids.
Activity 7.8: Let us experiment (Page 12) Question: What do you observe? (Potassium permanganate in water) Answer: You see streaks of pink colour spreading out from the grain. Eventually, the entire water turns pink.
Question: Do you know why this happens? Answer: This happens because water particles are constantly moving. They hit the potassium permanganate particles and spread them throughout the glass.
Think like a scientist (Page 13) Question: Drop a small grain of potassium permanganate into each of them (hot, room temp, ice-cold water). Watch carefully and compare. What do you observe?
Answer: The colour spreads the fastest in hot water and the slowest in ice-cold water. This is because particles move faster when they are hot (they have more thermal energy).
Activity 7.9: Let us find out (Page 13) Question: Do you notice the fragrance from a distance?
Answer: Yes, after a few minutes, you can smell the incense stick from a distance. This shows that gas particles (fragrance) move freely and mix with air particles to reach you.
Text Question (Page 14): Can you share a few other real-life situations where you have experienced the movement of gas particles? Answer:
- Smelling food being cooked in the kitchen from another room.
- Smelling perfume worn by someone walking past you.
- The smell of garbage spreading in the air.
Discover, design, and debate (Page 15) Question: Fix a balloon over the neck of a bottle and put the bottle in hot water. Explore what will happen? Answer: The balloon will inflate (get bigger). The heat from the water warms up the air inside the bottle. As air particles get hotter, they move faster and spread out (expand), filling the balloon.
Question: Is this property of gases (spreading to fill space) beneficial or harmful? Answer: It is both.
- Beneficial: It allows oxygen to spread in the atmosphere so we can breathe, and helps fragrances or pleasant smells reach us.
- Harmful: It allows harmful gases, smoke, or bad smells (pollution) to spread quickly to clean areas.
Keep the curiosity alive (Exercises – Page 16, 17)
- Choose the correct option. The primary difference between solids and liquids is that the constituent particles are: Answer: (i) closely packed in solids, while they are stationary in liquids. (Incorrect) (ii) far apart in solids and have fixed position in liquids. (Incorrect) (iii) always moving in solids and have fixed position in liquids. (Incorrect) (iv) closely packed in solids and move past each other in liquids. (Correct)
- Which of the following statements are true? Correct the false statements. (i) Melting ice into water is an example of the transformation of a solid into a liquid.
Answer: True.
(ii) Melting process involves a decrease in interparticle attractions during the transformation. Answer: True. (The heat overcomes the strong attractions, making them weaker) .
(iii) Solids have a fixed shape and a fixed volume.
Answer: True.
(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.
Answer: True.
(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room.
Answer: True (similar to the incense stick activity).
(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell. Answer: False. Correct statement: On heating, the particles of camphor gain energy and move faster, turning into gas (vapour) which spreads in the air. The “smell” is the particles themselves reaching your nose, not released energy.
- Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen? Answer: (iii) Nothing of the chair will remain. Justification: Matter is made up entirely of particles. If you remove all the particles, there is no matter left, so the chair simply vanishes.
- Why do gases mix easily, while solids do not?
Answer: Gases mix easily because there is a lot of empty space between their particles, and the particles move very freely and quickly. Solids do not mix easily because their particles are packed tightly together and held in fixed positions by strong forces, so they cannot move into each other.
- When spilled on the table, milk in a glass tumbler flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement. Answer: Milk is a liquid. Its particles are held together loosely enough to slide past each other, so it flows and has no fixed shape. The glass tumbler is a solid. Its particles are held very tightly in fixed positions, so it keeps its rigid shape and does not flow.
- Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour. Answer: (Since I cannot draw, I will describe the diagrams you should draw based on Figures 7.4 and 7.12)
- Ice (Solid): Draw particles (circles) packed very closely together in a neat, regular pattern. They are touching each other.
- Water (Liquid): Draw the particles slightly less organised. They are still close but not in a perfect pattern, showing they can move around.
- Water Vapour (Gas): Draw the particles far apart from each other, spreading out to fill the box, with arrows showing they are moving freely in all directions.
- Draw a picture representing particles present in the following: (i) Aluminium foil: Draw particles packed tightly in a regular pattern (Solid). (ii) Glycerin: Draw particles close together but slightly disordered (Liquid). (iii) Methane gas: Draw particles far apart with plenty of space between them (Gas).
- Observe Fig. 7.16a which shows the image of a candle that was just extinguished… Identify the different states of wax… and match them with Fig. 7.16b. Answer:
- Solid Wax (The main body of the candle): Matches the box with tightly packed, orderly particles (Top Left box in Fig 7.16b).
- Liquid Wax (The melted pool near the wick): Matches the box with particles close but slightly disordered (Top Right box in Fig 7.16b).
- Gaseous Wax (The smoke/vapour rising from the wick): Matches the box with particles spread far apart (Bottom box in Fig 7.16b).
- Why does the water in the ocean taste salty, even though the salt is not visible? Explain. Answer: Ocean water tastes salty because salt is dissolved in it. When salt dissolves, its particles break down into extremely small units that are too small to see. These tiny salt particles spread into the spaces between the water particles, so the water tastes salty everywhere.
- Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain. Answer: They are solids. Each individual grain of rice or particle of flour has a fixed shape and volume. They do not flow like water. The reason they seem to take the shape of the container is that they are a collection of many tiny solids poured together, similar to a pile of stones.