Matter in Our Surroundings
Class:9 | Chapter: 1
Extra Questions & Answer
1. What is Law of Conservation of Mass ?Answer: Law of conservation of Mass says that" The mass in an isolated system can neither be created nor be destroyed but can be transformed from one form to another". It means mass remains constant during any chemical reaction or physical change.
2. What is sublimation ?
Answer: When solids directly change into gaseous state without undergoing liquid state for example naphthalene balls disappear with time without leaving any solid.
Solid → Gas
3. Is dry ice the same thing as ordinary ice ?
Answer: No, dry ice is other name of solid carbon dioxide gas. It can be made by freezing liquified carbon dioxide. It can be used as a cooling agent.
4. Honey is more viscous than water.give reason.
Answer: Viscosity means resistance to flow and it depends upon intermolecular forces of attraction. Honey has more intermolecular intractions hence it is more viscous.
5. What is Brownian motion ?
Answer: It is zig - zag or random movement of a particle due to collisions with liquid or gas molecules surrounding it.
6. What is the effect of pressure on boiling point ?
Answer: Boiling point increases with increase in pressure. When pressuee is high more energy is required to boil that liquid, so boiling point will be more.
7. Name any two substances which sublime.
Answer: Carbon dioxide sublimes as dry ice, perfume tablets as perfume vapours.
8. Explain evaporation and its cooling effect in terms of kinetic energy of particles.
Answer: Evaporation : The process of change of a liquid into vapours at temperature below it's boiling point. When kinetic energy of surface liquid molecules is high , evaporation occurs. The remaining liquid molecules absorb energy from the surrounding to regain energy lost during evaporation and thus makes surrounding cool.
9. For any substance, why does the temperature remain constant during the change of state ?
Answer: This is due to latent heat (hidden heat) which we supply gets used up in changing the state by overcoming the forces of attraction . For melting the heat required is called latent heat of fusion and for boiling it is called latent heat of vapourisation.
10. Why do we see water droplets on the outer surface of a glass containing ice-cold water ?
Answer: The water vapour present in the air comes in contact with the glass containing ice-cold water.The vapour loses its energy to the cold water and gets converted to water droplets which we see on the outer surface of the glass. This process is known as Condensation.
11. Why do solids expand a bit on heating and contract a bit on cooling ?
Answer: This happens becouse exapansion and contraction depends on change in temperature. When there is increase in temperature, kinetic energy of solid particles increases and therefore they will vibrate over a larger distance and in case of cooling, kinetic energy and hence vibrations decrease.
12. Water as ice has a cooling effect, whereas water as steam may cause severe burns. Explain.
Answer: Water as ice melts, it absorbs the energy equal to the latent heat of fusion from the surroundings therefore causes cooling effect. But steam releases the extra heat equal to the latent heat of vaporization which it has absorbed when water was converted into steam.
13. What is a dry ice and what are its properties ?
Answer: Dry ice is solid carbon dioxide gas. It is colourless, odourless, white opaque solid. It is used as a cooling agent to slow down chemical reactions.
14. Why do surgeons often spray some ether on the skin before performing minor surgery ?
Answer: Surgeons spray ether on the skin before surgery becouse ether absorbs heat of vapourisation from that portion of skin and leaves it cool and numb. In this way surgery will be painless.
15. Name the change of state during the following changes:
(a) Drying of wet clothes (b) Melting of wax (c) Melting of ice (d) Formation of clouds
Answer: (a) liquid to gas
(b) solid to liquid
(c) solid to liquid
(d) liquid to gas
16. Is there any similarity in materials ?
Answer: Yes, They all belong to matter which has mass and occupies space.
17. When 50 g of sugar is dissolved in 100 mL of water, there is no increase in volume. What characteristic of matter is illustrated by this observation ?
Answer: Thus observation follows that the particles of matter have spaces between them so, there is no increase in volume.
18. What happens when an inflated air balloon is pricked with a pin ? Name the property of the gaseous state exhibited by this observation.
Answer: When an inflated balloon is pricked with a pin, it bursts with a popping sound. This observation indicates diffision is taking place.
19. Name the process which occurs when a drop of dettol is added to water.
Answer: When a drop of dettol is added to water, the molecules of dettol move from higher concentration region to lower concentration region and mix with water this is called diffusion.
20. To which physical state of matter do the following statements apply ?
(i) Incompressible, no fixed shape
(ii) Compressible, no definite volume
Answer: (i) liquid state
(ii) Gaseous state
21. Name the state of matter in which:
(i) Layers of particles can slip and slide over one another easily.
(ii) Particles just move around randomly because of very weak force of attraction.
Answer: (i) Liquid state
(ii) Gaseous state
22. Define density and give its SI unit.
Answer: Density : It is ratio of mass and volume of given substance. It's SI unit is Kg/L.
23. In which of the following, the particles have highest forces of attraction ?
Water, NaCl (solid), ice ,wax.
Answer: Here NaCl, ice and wax are in solid state at room temperature and water is in liquid state. As we know solids have more forces of attraction than liquids. Now, NaCl has ionic bonding, Ice (Solid form of water) has H - bonding and wax has weak Van der waals forces of attraction. Ionic bonding is stronger than H - bonding and Van der waals forces Therefore, NaCl has highest forces of attraction.
24. Why do the gases exert more pressure on the walls of the container than the solids ?
Answer: It is due to their continous motion in random directions. They collide with walls of container and hence exert pressure but solid particles don't move they have fixed position.
25. Which of the following diffuses faster ?
Water vapour, wax , ethyl alcohol.
Answer: Water vapour diffuses faster becouse they are in gaseous state. In gaseous state particles have more kinetic energy.
26. Can materials exist in all the three states ?
Answer: Yes , They exist in solid, liquid and gaseous state.
27. When a crystal of potassium permanganate is placed in a beaker containing water, its purple colour spreads throughout the water. What do you conclude from this observation about the nature of potassium permanganate and water ?
Answer: When a crystal of potassium permanganate is placed in a beaker, it breaks into smaller particles and move in random directions and intermixes with water molecules. Therefore, it shows diffusion of potassium permanganate with water.
28. Why do solids have a regular geometrical shape ?
Answer: Solids have highest intermolecular intractions so, they can't move from their fixed position. They are in close packed structure and becouse of this close packing they have regular geometrical shape.
29. Why are gases compressible but not liquids ?
Answer: Gases have least intermolecular forces and largest intermolecular space So, By applying pressure on them, they can be compressed easily and in large extent but in liquids, they have measurable forces.
30. Can a rubber band change its shape on stretching? Is it a solid ?
Answer: Yes, Rubber band changes it's shape becouse of it's elasticity. It expands when we apply some force and regains its original shape and size after removing external force. Yes it is a solid.
31. Why steam at 100°C is better for heating purposes than water at 100°C ?
Answer: Steam at 100 °C is better for heating becouse it has absorbed more heat (latent heat of vapourisation) So, it will provide more heat.
32. Give two ways in which melting points and boiling points can be useful.
Answer: 1. In identification of substances.
2. To test the purity of given substance.
33. What do you understand by the term ‘latent heat of fusion’?
Answer: Latent heat of fusion : It is the the amount of heat required to convert a unit mass of the substance from the solid state to the liquid state without changing the temperature.
34. How much is the latent heat of fusion of ice ?
Answer: 333.55 KJ/Kg
35. Why do the doctors advise to put strips of wet cloth on the forehead of a person having high fever ?
Answer: It is advisable to put strips of wet cloth on the forehead of a person having high fever becouse these strips will take heat from the body and help to lower down the temperature by evaporation.
36. Why do wet clothes dry quickly in the sun than in the shade ?
Answer: Rate of evaporation will be more in higher temperature. Sun produces heat and raises temperature and hence wet clothes dry quickly in sun than in the shade.
37. Why do trees acquire more leaves during summer ?
Answer: During summer, trees need to keep them cool by more transpiration. Leaves are the Location for transpiration that's why trees aquire more leaves for more transpiration.
38. Why do we feel comfortable under a fan when we are perspiring ?
Answer: Wefeel comfortable under a fan because air from fan helps in evaporation of sweat from our body and we feel cool and comfortable.
39. Why do people sprinkle water on the roof after a hot sunny day ?
Answer: People sprinkle water on the roof becouse it provides a cooling effect by causing evaporation. The water molecules absrorb heat from surrounding and changes to vapour and hence cause cooling effect.
40. Sponge, though compressible, is a solid ?
Answer: Solids have less compressibility but sponge can be compressed easily due to presence of air spaces. In these air spaces, air is filled .When we apply force, air is expelled out and sponge can be compressed and again after removal of force it regain its shape.
41. Under what conditions gases can be liquefied ? In which form LPG is filled in gas cylinder ?
Answer: Gases can be liquified by applying pressure and by reducing temperature.
LPG is filled in liquid state.
42. Liquid generally have lower density as compared to solids, but ice floats on water. Find out, why.
Answer: This is due to open cage like structure of ice . When water freezes to ice volume increases so, density decreases and hence ice floats.
43. Give reason to explain why it takes longer time to dry wet clothes in humid weather ?
Answer: In humid weather, there is already plenty of water present in air so, air will not take as much water as it takes on hot day Therefore, rate of evaporation will slow down and hence it takes longer time to dry wet clothes in humid weather.
44. Why mixture does not have a fixed melting point or a fixed boiling point? Give reason.
Answer: Mixtures don't have fixed composition of various components So, they don't have sharp or fixed melting and boiling points.
45. On suffering from fever which will lower down your body temperature, more ice or ice cold water ?
Answer: Ice will lower down body temperature more because of latent heat of fusion . Ice will absorb more heat from the body and hence can be used effectively.