Question 1. Which of the following things are matter?
Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, the smell of perfume.
Answer: Out of the list given in the question, chair, air, almonds, and a cold drink are matter.
Question 2. Give reasons for the following observation:
The smell of hot food reaches you even if you are several meters away. But to smell cold food, you have to go close to it.
Answer: The rate of diffusion of hot food is more because the particles have kinetic energy. This is why you can smell hot food from many meters away.
Question 3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer: The particles of water in the swimming pool have intermolecular space in them. As a result, the force of attraction between the water molecules is less. So, the diver can cut through the water in a swimming pool easily.
Question 4. What are the characteristics of the particles of matter?
Answer: The characteristics of the particles of matter are:
(1) Particles of matter have intermolecular space.
(2) Particles of matter have intermolecular force.
(3) Particles of matter are moving continuously.
Question 5. 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, and iron.
Answer: After arranging the list from the lowest density to the highest density we get:
air < exhaust from chimneys < cotton < water < honey < chalk < iron
Question 6. (a) Make a table to show the differences in the characteristics of different states of matter.
(b) Comment on the following characteristics of matter:
rigidity, compressibility, fluidity, when filling a gas container, shape, kinetic energy, and density.
Answer: (a) The differences in the characteristics of the three states of matter can be listed in a table as
|Shape and Volume||Fixed shape and volume.||Have a fixed volume but do not have a fixed shape.||Do not have a fixed shape or volume.|
|Rigidity||Cannot flow.||Can flow.||Can flow.|
|Intermolecular density||Solids have a very high intermolecular density.||Intermolecular density is less than solids.||Gases have very low intermolecular density.|
|Intermolecular space||Solids have very low intermolecular space.||Liquids have more intermolecular space than solids.||Very high.|
|Compressibility||Cannot be compressed.||Can be compressed.||Gases can be highly compressed.|
Characteristics of Solids - Have a fixed shape and volume. Solids are rigid and do not flow. The intermolecular space is less, so the intermolecular force is at maximum. Solids have negligible compressibility.
Characteristics of Liquids - Do not have a fixed shape but can take the shape of the container they are in. Liquids have a fixed volume, but they are not rigid and can flow. The intermolecular space in liquids is more than that of solids. But, the intermolecular force is less than that of solids. Liquids are compressible.
Characteristics of Gases - Gases do not have a fixed shape or a fixed volume. Gases are not rigid and can flow. The intermolecular space in gases is very high, so the intermolecular force is low. Gases are highly compressible.
(b) Comment on:
(i) Rigidity: This can be defined as the characteristic of a substance that allows it to maintain its shape, even when it is subjected to outside force.
(ii) Compressibility: All types of matter have some intermolecular space in them. When you apply an external force to the matter, you can bring these particles closer. The process of bringing particles closer with force is compressibility. Gases and liquids are compressible, but solids are not.
(iii) Fluidity: Liquids and gases do not have a fixed shape, so they can flow. This feature of particles that allow them to flow is called fluidity.
(iv) Filling of a gas container: Gases do not have a fixed shape or a fixed volume. Gas particles can vibrate in all directions, so you can fill up gas containers easily.
(v) Shape: The intermolecular space in solids is less, and the intermolecular force is more. This is the reason solids have a definite shape and volume.
(vi) Kinetic energy: This can be understood as the energy possessed by moving particles. As gas molecules vibrate in all directions, they have the highest kinetic energy.
(vii) Density: The density of matter can be calculated by checking the mass per unit volume. Solids have the highest density as their intermolecular space is less.
Question 7. Give reasons why
(a) A gas can completely fill any container in which it is kept.
(b) Gas exerts pressure on the container walls.
(c) We should call a wooden table solid.
(d) It is easy to move our hands in the air, but we cannot do the same through a solid block of wood.
(a) As gas molecules vibrate in all directions, their kinetic energy is high. As a result, gases can fill any vessel they are kept in.
(b) Gas exerts pressure on the container walls because it has high kinetic energy. The gas molecules vibrate and move in all directions. With this constant movement, the molecules hit the walls and exert pressure on them.
(c) If we take a wooden table, you will notice that the table has a solid shape and volume. This means that the molecules are tightly packed with each other. So, there is no intermolecular space and the table cannot be compressed. These characteristics are that of solid matter, so a wooden table should be called a solid.
(d) Air molecules have less force of attraction between them. So, you can separate the molecules with a small external force. When you move your hands, you are separating the air molecules and passing through them. But, if you want to move your hands through a block of wood, then it is not easy. This is because a block of wood is solid. The molecules in wood have less space between them, and the force of attraction is more. The molecules in the wood are bound together with a lot of force. So, to pass your hand through a block of wood, you need a large amount of external force.
Question 8. Liquids generally have a lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer: Although ice has a shape and volume, its density is lower than water. This low density is because of its structure. Ice molecules look like a cage and have lots of spaces in them. This structure creates low density and allows ice to float on water.
Question 9. Convert the following temperature to Celsius scale:
(a) 300 K (b) 573 K
(a) 300 – 273 = 27°C
(b) 573 – 273 = 300°C
Question. 10. What is the physical state of water at:
(a) 250°C (b) 100°C
(a) At 250°C water turns into gas.
(b) At 100°C some water remains as a liquid while some turns into gas (water vapour).
Question 11. For any substance, why does the temperature remain constant during the change of state?
Answer: To change the state of matter, you need to apply heat to it. Heat allows you to overcome the intermolecular attraction in the matter. The temperature should remain constant until the matter changes to the desired state. The extra heat needed by the molecules is called latent heat.
Question 12. Suggest a method to liquefy atmospheric gases?
Answer: To liquefy atmospheric gases, you should fill them in a cylinder fitted with a piston. The gases can then be liquefied by cooling and applying pressure.
Question 13. Why does a desert cooler cool better on a hot dry day?
Answer: For good results, you need to sprinkle the outer walls of the desert cooler. On a hot, dry day, this water evaporates and cools the air inside the cooler. This cool air is then sent into the room by the fan.
Question 14. How does the water kept in an earthen pot (matka) become cool during summer?
Answer: Earthen pots, or matkas, are porous and the surface of the pot has a lot of pores on it. As a result, the water oozes out through these pores from the pot. This water gets evaporated at the surface, thereby causing a cooling effect. This makes the water in the pot cool, even during the summer months.
Question 15. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer: When acetone, petrol, and perfume come in contact with air, they evaporate. This evaporation results in a cooling sensation in our palms.
Question 16. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer: When you pour tea into a saucer, it gets a larger surface area than in a cup. So, the rate of evaporation is faster due to the increase in surface area. As a result, tea cools down faster in a saucer than in a cup. This is why you can sip hot tea or milk faster from a saucer rather than a cup.
Question 17. What type of clothes should we wear in summer?
Answer: In the summer, we should wear light-coloured cotton clothes. This is because the pores in cotton clothes help to absorb sweat faster. Sweat evaporation also happens faster with cotton clothes. You should choose light-coloured dresses because they will reflect heat.