1. Matter in Our Surroundings Class 9 Science Chapter 1 NCERT Solutions (CBSE)
CBSE Class 9 Science Chapter 1: Matter in Our Surroundings – Full NCERT Textbook Solutions
Chapter 1: Matter in Our Surroundings
Internal Question :
1. Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.
Answer: The items that are considered matter are chair, air, almonds, cold, and lemon water. Matter refers to anything that has mass and occupies space. Love, smell, hate, thought and the smell of perfume are not considered matter as they do not have mass or occupy physical space.
2. 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: The smell from hot sizzling food can travel far because its tiny particles move around quickly due to the heat, reaching us from a distance. Cold food doesn't release as many particles, and they move slower, so we have to get closer to smell it.
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 space between them. The diver can move through the water because there is space between water molecules, allowing the diver to cut through it.
4. What are the characteristics of the particles of matter?
Answer: The characteristics of the particles of matter are
(i) Particles of matter have space between them .
(ii) Particles of matter are continuously moving .
(iii) Particles of matter attract each other .
Internal Question :
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: Arranging the substances in order of increasing density: Air < Cotton < Chalk < Water < Honey < Exhaust from chimneys < Iron
2. (a) Tabulate the differences in the characterisitcs of states of matter.
(b) Comment upon the following:
rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
Answer: (a) Tabulated differences in characteristics of states of matter:
|
Characteristics |
Solid |
Liquid |
Gas |
|
Rigidity |
High |
Moderate |
Low |
|
Compressibility |
Low |
Moderate |
High |
|
Fluidity |
No |
Yes |
Yes |
|
Filling a gas container |
No |
No |
Yes |
|
Shape |
Definite |
No |
Indefinite |
|
Indefinite Kinetic Energy |
Low |
Moderate |
High |
|
Density |
High |
Moderate |
Low |
(b) Comments on the properties of matter :
Rigidity: Solids are rigid because their particles are closely packed and can only vibrate at fixed positions.
Compressibility: Gases are highly compressible as there is large space between particles. Solids are least compressible, while liquids are slightly compressible.
Fluidity: Liquids and gases can flow, so they are called fluids. Solids cannot flow.
Filling a gas container: Gases completely fill the container because their particles move freely in all directions.
Shape: Solids have a fixed shape. Liquids take the shape of the container, while gases have no fixed shape.
Kinetic Energy: Gas particles have maximum kinetic energy, liquids have moderate, and solids have minimum kinetic energy.
Density: Solids have the highest density, liquids have less, and gases have the lowest density due to the arrangement of particles.
3. 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) A gas fills completely the vessel in which it is kept because gas particles are in constant random motion and have negligible intermolecular forces. This allows them to spread out and occupy the entire available space within the container.
(b) A gas exerts pressure on the walls of the container because gas particles collide with the walls, transferring momentum and exerting a force on the walls due to their high kinetic energy.
(c) A wooden table should be called a solid because it has a definite shape and volume. Wood is composed of tightly packed molecules held together by strong intermolecular forces, giving it a rigid and fixed structure.
(d) We need a karate expert or an extraordinary amount of force to move our hand through a solid block of wood because the particles in solids are tightly packed, and intermolecular forces between them create resistance to movement. In contrast, air has low density and its particles are widely spaced, allowing easy movement .
4. 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 its density is lower than that of liquid water. When water freezes, it forms a crystalline structure with more space between molecules, making it less dense and causing it to float.
Internal Question :
1. Convert the following temperature to celsius scale:
(a) 300 K (b) 573 K.
Answer: (a) To convert 300 K to the Celsius scale, we subtract 273.15 from the given temperature:
300 K – 273.15 = 26.85°C
Therefore, 300 K is equivalent to 26.85°C.
(b) To convert 573 K to the Celsius scale, we again subtract 273.15 from the given temperature:
573 K – 273.15 = 299.85°C
Therefore, 573 K is equivalent to 299.85°C.
2. What is the physical state of water at:
(a) 250°C (b) 100°C ?
Answer: (a) At 250°C, water is in the gaseous state. It exists as steam or water vapor. The boiling point of water is 100°C, and at 250°C, water is well above its boiling point, causing it to be in the form of a gas.
(b) At 100°C, water is in the liquid state. This is the boiling point of water at standard atmospheric pressure. Water reaches its boiling point and undergoes a phase change from a liquid to a gas (steam) at this temperature.
3. For any substance, why does the temperature remain constant during the change of state?
Answer: During the change of state, such as from a solid to a liquid or from a liquid to a gas, the temperature remains constant because the heat energy being supplied or released is used to overcome the intermolecular forces holding the particles together, rather than increasing the temperature.
4. Suggest a method to liquefy atmospheric gases.
Answer: One method to liquefy atmospheric gases is through a process called cryogenic liquefaction. This involves compressing the gases to high pressures and then cooling them to extremely low temperatures using refrigeration systems or by utilizing cryogenic substances such as liquid nitrogen or helium.
Internal Question :
1. Why does a desert cooler cool better on a hot dry day?
Answer: A desert cooler cools better on a hot dry day because the dry air allows more water to evaporate. During evaporation, water absorbs heat from the air, making the air cool.
2. How does the water kept in an earthen pot (matka) become cool during summer?
Answer: The water kept in an earthen pot (matka) becomes cool during summer because water seeps through the tiny pores of the pot and evaporates. During evaporation, it absorbs heat from the water inside the pot, making the water cool.
3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer: Our palm feels cold when we put acetone, petrol, or perfume on it because these liquids evaporate quickly. During evaporation, they absorb heat from our palm, making it feel cold.
4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer: We are able to sip hot tea or milk faster from a saucer than from a cup because a saucer has a larger surface area. This allows the tea or milk to cool faster as more heat escapes to the air.
5. What type of clothes should we wear in summer?
Answer: We should wear cotton clothes in summer because cotton absorbs sweat and allows it to evaporate easily. During evaporation, sweat takes heat from our body, making us feel cool.
Exercises :
1. Convert the following temperatures to the celsius scale.
(a) 293 K (b) 470 K
Answer: (a) To convert 293 K to the Celsius scale, we subtract 273.15 from the given temperature:
293 K – 273.15 = 19.85°C
Therefore, 293 K is equivalent to 19.85°C.
(b) To convert 470 K to the Celsius scale, we again subtract 273.15 from the given temperature:
470 K – 273.15 = 196.85°C
Therefore, 470 K is equivalent to 196.85°C.
2. Convert the following temperatures to the kelvin scale.
(a) 25°C (b) 373°C
Answer: (a) To convert 25°C to the Kelvin scale, we simply add 273.15 to the given temperature:
25°C + 273.15 = 298.15 K
Therefore, 25°C is equivalent to 298.15 K.
(b) To convert 373°C to the Kelvin scale, we again add 273.15 to the given temperature:
373°C + 273.15 = 646.15 K
Therefore, 373°C is equivalent to 646.15 K.
3. Give reason 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 disappear without leaving any solid residue because they undergo sublimation. Sublimation is the process in which a substance changes directly from a solid to a gas without passing through the liquid state.
(b) We can smell perfume from a distance because perfume molecules are volatile and can evaporate into the air. These molecules move through the air and reach our nose, allowing us to detect the scent even when sitting several meters away.
4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Answer: Oxygen < Water < Sugar
5. What is the physical state of water at—
(a) 25°C (b) 0°C (c) 100°C ?
Answer: (a) At 25°C, water is typically in a liquid state. It is within the temperature range at which water commonly exists as a liquid.
(b) At 0°C, water undergoes a phase change and transitions from a liquid to a solid state. It freezes and becomes ice. The melting point of water is precisely 0°C (32°F), so at this temperature, water exists as a solid.
(c) At 100°C, water undergoes another phase change and transitions from a liquid to a gaseous state. It boils and becomes steam. The boiling point of water is precisely 100°C (212°F), so at this temperature, water exists as a gas.
6. 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 is a liquid at room temperature because its melting point is 0°C (32°F), which is above typical room temperatures, and its boiling point is 100°C (212°F).
(b) An iron almirah is a solid at room temperature because iron's melting point and boiling point are significantly higher than room temperatures.
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 than water at the same temperature because ice needs extra heat (called latent heat) to melt into water. This heat is taken from the surroundings, causing more cooling.
8. What produces more severe burns, boiling water or steam?
Answer: Steam produces more severe burns than boiling water. This is because steam at 100°C contains extra heat called latent heat of vaporisation. When steam comes in contact with the skin, it condenses into water and releases this extra heat, causing deeper and more painful burns than boiling water.
9. Name A,B,C,D,E and F in the following diagram showing change in its state .
Answer: A → Fusion (melting) ; B → Vaporisation (boiling/evaporation) ; C → Condensation ; D → Solidification (freezing) ; E → Sublimation (solid to gas) ; F → Deposition (gas to solid) .
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