Class 9 Science Chapter 1 Question Answers - Matter in Our Surroundings (Not in 2021-22 Syllabus)

Ques1. We can smell the perfume sitting several meters away. Why?
Ans. This is because perfumes contain volatile solvent which carries pleasant smelling vapours. They diffuse quite fast and can reach people sitting several meters away.

Ques2. Why can you smell the perfume of incense sticks?
Ans. The particles of the perfume (matter) are not stationary, but are continuously moving. They drift through the air, and hence, we can smell the perfume.Incense Stick

Ques3. Why can not you smell its perfume at a short distance when the incense stick is not lighted ?
Ans. The particles of the perfume (matter) do not have sufficient energy to drift through the air. Thus, a few steps away from incense sticks, we cannot smell it.

Ques4. Why is the smell of the perfume of incense stick fill the whole room in a few minutes, when lighted?
Ans. When the incense stick is lighted, the heat energy makes the particles of the perfume to move rapidly. Thus, they easily drift through the air in the room and hence we can smell it anywhere in the room.

Ques5. A rubber band is a solid, but it can change its shape. Why ?
Ans. We can regard a rubber band as a solid because it regains its shape when the stretching force is removed from it.

Ques6. When salt or sugar are poured into different kinds of vessels, why do they take the shape of vessels ?
Ans. Salt or sugar takes the shape of a containing vessel, but does not change its individual shape. For example, sugar crystals are cubical, and they remain cubical in any vessel.

Ques7. Sponge is a solid, yet we are able to compress it. Why?
Ans. Sponge has very small holes throughout its structure. These holes are filled with air. When it is compressed, the air in the holes is squeezed out. Thus, we are able to compress it.Sponge

Ques8. Arrange the following substances in the increasing order of forces of attraction between the particles water, sugar and oxygen.
Ans. Oxygen > water > sugar.

Ques 9. What is the physical state of water at:
(a) 25°C
(b) 0°C
(c) 100°C
Ans. 
(a) At 25°C, water is in a liquid state.
(b) At 0°C, water is in solid state, provided heat is removed from it.
(c) At 100°C, water is in a gaseous state, provided heat is supplied to it.

Ques10. Give two reasons to justify.

(a) Water at room temperature is a liquid.
(b) An iron almirah is solid at room temperature.

Ans. 

(a) 

• Intermolecular forces are less.
• Intermolecular spaces and kinetic energy is more.
• Thus, the molecules of water can interchange their spaces, and hence, water is in a liquid state at room temperature.

(b)

• Intermolecular forces are very large.
• Intermolecular spaces, as well as, kinetic energy are very small.Iron almirah
• Thus, the molecules are held very, very tightly, with the result, the iron almirah has a definite shape and definite shape and definite volume, and hence, is a solid.
  
  

Ques11. State your observation immediately after adding the blue ink drop.
Ans. As the drop of ink trickles along the sides of the beaker, the blue colour of the ink starts diffusing in water, which appears like wavy blue streaks in water.Diffusion of ink in water

Ques12. State your observation immediately after adding the honey drop.
Ans. The honey drop continues travelling along the side of the beaker and there is no visible diffusion of it in water.

Ques13. How much time does it take for the colour of ink to spread evenly?
Ans. The ink spreads evenly in the water in about two hours.

Ques14. How does the diffusion of honey vary with the diffusion of ink and why ?
Ans. The diffusion of honey is very slow as compared to the diffusion of ink. It is because the honey is a dense liquid. Its particles have strong intermolecular forces as compared to water. Thus, it diffuses slowly in water.

Example: 

•  Add few crystals of sugar to water they intermix (dissolve) with water spontaneously.
• When we add a few drops of ink to water, the colour of the ink gets dispersed evenly in the entire liquid.

The gases also diffuse into liquids:

• Aqueous solution of ammonia contains ammonia diffused in water.
• The gases from the atmosphere diffuse and dissolve in water especially O₂, CO₂ are essential for the survival of aquatic animals and plants.
• The fish and other aquatic animals can utilize the dissolved oxygen for producing energy form food.
  Aquatic Life

Ques15. What happens around each crystal of solid on introducing it to water ?
Ans. Dense and deep violet colour is formed around each crystal. However, the size of deep violet colour in hot water is larger than in the cold water.

Ques16. What happens as the time passes, and why?
Ans. The dense violet colour starts diffusing slowly into cold water in the form of coloured streaks. Gradually, the solution changes to pink colour, which is darker near the base of the beaker. In case of hot water, the dense violet colour rapidly diffuses to form pink colour, which is more homogeneous as compared to the cold water.

Ques17. Does the rate of diffusion change with temperature? If so, why?
Ans. The rate of diffusion increases with the temperature. It is because the boiling hot water molecules have more kinetic energy and there are larger intermolecular spaces. Thus, the particles of solid potassium permanganate rapidly diffuse, and hence, the rate of diffusion increases.Potassium permanganate crystals in water

Ques 18. What do you observe on the sides of the glass beaker?
Ans. Tiny bubbles of gas cling to the sides of the beaker.

Ques 19. Give an explanation to your above observation.

Ans. 

• The tiny bubbles are of air (especially carbon dioxide and oxygen) which got dissolved in water naturally. 
• These gases are expelled when water is warmed.
• The gases like oxygen and carbon dioxide diffuse and hence dissolve in water. The dissolved oxygen in water is essential for the respiration of water animals.
• The dissolved carbon dioxide helps the water plant to synthesize their food by the process of photosynthesis.
  Photosynthesis

Ques 20. What do you observe when force is applied and then removed on the plunger of the syringe containing water ? Give a reason for your answer.
Ans. The plunger does not move inward on the application of force. When the force is removed, the plunger does not move backward.
Reason: The liquids have small intermolecular spaces. Thus, they cannot be compressed.

Ques 21. What do you observe when force is applied and then removed on the plunger of the syringe containing air ? Give a reason for your answer.
Ans. The plunger moves downward on the application of force to a considerable length. When the force is removed, the plunger moves backward and takes its original position.
Reason: The gases have large intermolecular spaces. Thus, they easily get compressed on the application of force. The compressed gases are under high pressure. When the force is removed, this high pressure forces the plunger back to its original position.

Ques 22. Give reasons:
(a) A gas fills the vessel in which it is kept completely.
Ans. The molecules of a gas have large intermolecular spaces and kinetic energy, but extremely small intermolecular forces. Thus, the molecules of the gas spread in the entire space of the containing vessel on account of high kinetic energy and practically to intermolecular forces, hence filling entire space of the vessel.

(b) A gas exerts pressure on the walls of the container.
Ans. The molecules of a gas have very large kinetic energy. When these molecules strike against the walls of a containing vessel, they exert a certain average force per unit area. As the force per unit area is known as pressure, therefore, the gases exert pressure on the sides of the containing vessel.

(c) A wooden table should be called a solid.
Ans. Solids are rigid, incompressible, and have definite shape and volume. Since the table has all the above mentioned properties, therefore, it it solid.

Particles in solids

(d) We can easily move our hand in the air, but to do the same through a solid block of wood, we need a karate expert.
Ans. The intermolecular forces between the molecules of a gas are almost negligible and intermolecular spaces are very large. Thus, we can easily move our hand in air, without any appreciable force.
The intermolecular forces between the molecules of a solid are very large and intermolecular spaces are very small. Thus, a lot of force is required to separate the molecules of a solid. It is for the same reasons that we need karate expert to break a block of wood.

Ques 23. The mass per unit volume of a substance is called density. (Density = Mass / Volume). Arrange the following in the order of increasing density: air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Ans. Exhaust from chimneys, air, cotton, water, honey and iron.
Example:
(i) CNG (compressed Natural gas) is used as fuel in internal combustion engines.
(ii) Oxygen in compressed form is supplied to hospitals for serious patients in cylinders.
(iii) LPG (Liquefied petroleum gas) which is used in home for cooking.

Ques 24. We can easily move our hand in the air, but to do the same through a solid block of wood, we need a karate expert.
Ans. In air the inter particle attractive forces are negligible and hence, it is easy to separate the particles in air and we can easily move our hand through it. In a solid block of wood, the inter particle forces are very strong and hence, it is not easy to separate the particles. Therefore it is not easy to move our hand through a solid block of wood (only a karate expert can do it). Due to this property, a large volume of gas can be compressed into a small cylinder and transported easily.

Ques 25. Arrange the following substances in increasing order of forces of attraction between the particles -water, sugar, oxygen.
Ans. Oxygen < water < Sugar

Ques 26. Give two reasons to justify:
(a) Water at room temperature is a liquid.
(b) An iron almirah is solid at room temperature.
Ans. 
(a) 
(i) Intermolecular forces are less.
(ii) Intermolecular spaces and kinetic energy is more.
Thus, the molecule of water can interchange their spaces and hence water is in liquid state at room temperature.

(b) 
(i) Intermolecular forces are very large.
(ii) Intermolecular spaces, as well as kinetic energy, are very small
Result: Thus, the molecules are held very, tightly, with the result, the iron almirah has a definite shape and definite volume, and hence, is a solid.

Ques 27. The diver is able to cut through water in a swimming pool.
Ans. The diver is able to cut through water in the swimming pool because matter is not continuous, but it is made up of particles which have vacant spaces between them moreover, the attractive forces between molecules of water are not very strong. The diver can easily cut through the water by applying force to displace water and occupy its place.

Ques 28. Why does ice float on water?
Ans. Solids generally have higher density than the liquids but ice due to its specific structure has larger inter particle spaces and hence has lower density than liquid water. As a result ice floats on water.
Ice floats on water

Ques 29. Convert the following temperatures to the celsius scale.
(a) 300 K
(b) 573 K
Ans.
(a) (300 - 273) = 27°C. Temperature in °C = Temperature in K -273
(b) (573 - 273) = 300°C. Temperature in °C = Temperature in K -273

Ques 30. Convert the following temperature to the Kelvin scale.
(a) 25°C
(b) 373°C
Ans.
(a) 25 + 273 = 298 K
(b) 373 + 273 = 646 K

Ques 31. Ice is at 273 K more effective in cooling, than water at the same temperature, why?
Ans. Ice at 273 K will absorb heat energy or latent heat from the medium to overcome the fusion to become water. Hence, the cooling effect of ice is more than the water at the same temperature because water does not absorb this extra heat from the medium.

Ques 32. What produces more severe burns, boiling water or steam?
Ans. Steam will produce more severe burns than boiling water. It is because, 1 g of steam at 373 K (100°C) contains 2260 J of heat energy more in the form of latent heat of vaporization as compared to water at 373 K (100°). Thus, steam produces more severe burns.

Ques 33. What is the physical state of water at:
(a) 25°C
(b) 0°C
(c) 100°C
Ans. 
(a) 25°C - Water is in a liquid state.
(b) 0°C - Water is in solid state.
(c) 100°C - Water is in a gaseous state.

Ques 34. Naphthalene balls disappear with time without leaving any solid why
Ans. Naphthalene is volatile, solid, and has a tendency to sublime.Naphthalene Balls

Therefore, it changes into vapors completely, which disappear into the air, and no solid is left.