1. Which of the following are matter?
Answer: Chair, air, almonds, lemon water, smell of perfume are matter because they have mass and occupy space.
Love, smell, hate, thought, cold are not considered matter because they do not have mass or occupy space.
2. Give reasons for the following observation: The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food, you have to go close.
Answer: The smell of hot sizzling food reaches you from a distance because the particles of hot food move faster due to higher kinetic energy, allowing them to spread over a greater distance. Cold food particles have less kinetic energy, so they spread more slowly, and you need to be closer to detect the smell.
3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer: This observation shows that particles of matter have spaces between them. The spaces allow the diver to move through the water.
4. What are the characteristics of the particles of matter?
Answer:
1. Particles of matter are very small.
2. They have spaces between them.
3. They are constantly moving.
4. They attract each other.
1. 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: Order of increasing density: Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron
2. (a) Tabulate the differences in the characteristics of states of matter.
Property Solid Liquid Gas
Shape Definite shape No definite shape No definite shape
Volume Definite volume Definite volume No definite volume
Compressibility Negligible Slightly compressible Highly compressible
Fluidity Cannot flow Can flow Can flow
Kinetic Energy Low Moderate High
Interparticle Space Very small Small Large
Force of Attraction Strong Moderate Weak
3. (b) Comment upon the following:
o Rigidity: Solids are rigid because their particles are tightly packed.
o Compressibility: Gases are highly compressible; liquids are slightly compressible, and solids are almost incompressible.
o Fluidity: Liquids and gases can flow; solids cannot.
o Filling a gas container: Gases fill the entire volume of the container they are in.
o Shape: Solids have a fixed shape; liquids take the shape of the container; gases have no fixed shape.
o Kinetic Energy: Gases have the highest kinetic energy, followed by liquids, and solids have the lowest.
o Density: Solids generally have the highest density, followed by liquids, with gases having the lowest density.
4. Give reasons:
(a) A gas fills completely the vessel in which it is kept.
Answer: Gases have particles that move freely and spread out to fill the entire space available, ensuring that the gas takes the shape of the container. This happens because the forces of attraction between gas particles are very weak, allowing them to move independently of each other.
(b) A gas exerts pressure on the walls of the container.
Answer: Gas particles are in constant motion and collide with the walls of the container, exerting pressure. These collisions with the walls create the force that we measure as pressure, and the greater the number of collisions, the higher the pressure exerted.
(c) A wooden table should be called a solid.
Answer: A wooden table has a definite shape, volume, and is rigid, which are characteristics of a solid. The particles in the wood are closely packed and arranged in a fixed pattern, giving the table its solid form and resistance to shape change.
(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: Air particles are far apart and can move freely, allowing easy movement. In contrast, solid particles are tightly packed in a fixed structure, which is why it requires a significant amount of force to penetrate or move through a solid object like wood.
5. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer: Ice floats on water because it has a lower density than liquid water. This is due to the structure of ice, where water molecules are arranged in a way that creates more space between them, making ice less dense than water.
6. Convert the following temperature to Celsius scale:
(a) 300 K = 300 – 373 = 27°C
(b) 573 K = 573 – 273 = 300°C
7. What is the physical state of water at:
(a) 250°C – Gas (Steam)
(b) 100°C – Liquid (Water) and Gas (Steam) depending on whether it is boiling or not.
8. For any substance, why does the temperature remain constant during the change of state?
Answer: The temperature remains constant during a change of state because the heat energy supplied is used to overcome the forces of attraction between particles, rather than increasing their kinetic energy. This energy is absorbed as latent heat.
9. Suggest a method to liquefy atmospheric gases.
Answer: Atmospheric gases can be liquefied by cooling them to very low temperatures and increasing the pressure.
10. Why does a desert cooler cool better on a hot dry day?
Answer: A desert cooler works better on a hot dry day because the dry air has low humidity, allowing more water to evaporate from the cooler’s pads. This evaporation process absorbs heat, making the air cooler.
11. How does the water kept in an earthen pot (matka) become cool during summer?
Answer: Water in an earthen pot cools due to evaporation. The tiny pores in the earthen pot allow water to seep out and evaporate. As the water evaporates, it absorbs heat from the remaining water, lowering its temperature.
12. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer: Our palm feels cold because acetone, petrol, or perfume evaporates quickly, absorbing heat from the skin, causing a cooling effect.
13. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer: We can sip hot tea or milk faster from a saucer because the larger surface area of the saucer allows the liquid to cool down more quickly by increasing the rate of evaporation.
14. What type of clothes should we wear in summer?
Answer: We should wear cotton clothes in summer because they absorb sweat and allow it to evaporate, helping to keep the body cool.
Exercise Questions:
1. Convert the following temperatures to the Celsius scale.
(a) 293 K = 293 K – 273 = 20°C
(b) 470 K = 470 K – 273 = 197°C
2. Convert the following temperatures to the Kelvin scale.
(a) 25°C = 25°C + 273 = 298 K
(b) 373°C = 373°C + 273 = 646 K
3. Give reason for the following observations.
(a) Naphthalene balls disappear with time without leaving any solid.
Answer: Naphthalene balls undergo sublimation, meaning they change directly from a solid to a gas without turning into a liquid. This is why they disappear over time without leaving any residue.
(b) We can get the smell of perfume sitting several meters away.
Answer: The particles of perfume diffuse rapidly in the air due to their high kinetic energy. This allows the smell to spread over a distance, enabling us to detect it even from several meters away.
4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Answer: Order: Oxygen < Water < Sugar
5. What is the physical state of water at—
(a) 25°C : Liquid
(b) 0°C : Solid (Ice) or Liquid (Water), depending on the specific conditions.
(c) 100°C : Liquid (Water) or Gas (Steam), depending on whether the water is boiling.
6. Give two reasons to justify—
(a) Water at room temperature is a liquid.
Answer: Water has a fixed volume but no fixed shape, allowing it to flow and take the shape of its container, which are properties of a liquid.
(b) An iron almirah is a solid at room temperature.
Answer: An iron almirah has a definite shape, a fixed volume, and is rigid, all of which are characteristics of a solid.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer: Ice at 273 K is more effective in cooling because it absorbs more heat from the surroundings during the process of melting, whereas water at the same temperature does not undergo any phase change.
8. What produces more severe burns, boiling water or steam?
Answer: Steam produces more severe burns than boiling water because steam contains more energy in the form of latent heat of vaporization, which it releases upon contact with the skin.