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MORE ABOUT SOLIDS, LIQUIDS AND GASES

✔ ) the most appropriate answer. I. Tick (✔ 1. The fluid used in aneroid barometer is: (a) water (b) mercury (c) air (d) none of these 2. The normal atmospheric pressure is: (a) 70 cm of mercury (b) 76 cm of mercury (c) 67 cm of mercury (d) 80 cm of mercury 3. The intermolecular spaces in case of liquids is: (a) less than solids (b) more than gases (c) more than solids (d) more than solids and gases 4. The kinetic energy of molecules of a gas increases with: (a) fall in temperature (b) rise in temperature (c) decrease in pressure (d) increase in pressure 5. The pressure exerted at any point on an enclosed liquid is transmitted: (a) only towards the sides of vessel (b) only towards the base of vessel (c) in all directions of the vessel (d) none of these 6. When a body is fully immersed in liquid, the apparent loss in the weight of the body is equal to: (a) volume of liquid displaced by body (b) density of the liquid (c) weight of liquid displaced by body (d) none of these 7. When an ink fountain pen leaks on high altitude: (a) pressure of atmosphere is more than pressure inside the pen (b) pressure of atmosphere is less than pressure inside the pen (c) the density of ink on high altitude decreases (d) none of these 8. When the weight of a given body is less than weight of fluid displaced by it, then the body: Class-VIII Physics

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9.

10.

11. 12. 13.

14.

15.

16.

(a) sinks (b) floats (c) just keeps in line with the surface of fluid (d) none of these Buoyant force exerted by a fluid on a given body is: (a) equal to the volume of body (b) equal to the weight of body (c) equal to the volume of fluid displaced by body (b) equal to the weight of fluid displaced by the body Heat is transmitted in liquids by: (a) conduction (b) convection (c) both conduction and convection (d) none of these A device to measure the liquid pressure is: (a) barometer (b) thermometer (c) manometer (d) lactometer The liquid used in a simple barometer is: (a) water (b) alcohol (c) mercury (d) kerosene Liquid pressure with increasing depth: (a) decreases (b) remains the same (c) increases (d) none of these The pressure exerted at any point on an enclosed liquid is transmitted: (a) only at the bottom (b) only on the sides (c) in all directions (d) none of these Buoyant force experienced by a body partly or wholly immersed in a fluid is called: (a) upward force (b) horizontal force (c) downward force (d) none of these Buoyant force exerted by a fluid on a body is equal to the (a) weight of the body: (b) apparent loss of weight of the body (c) mass of the body (d) none of these

Class-VIII Physics

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17. If the weight of the body is more than the weight of fluid displaced by it, then the body: (a) float (b) first floats and then sinks (c) sinks (d) none of these Ans. 1. (d) 2. (b) 3. (c) 4. (b) 5. (c) 6. (c) 7. (b) 8. (b) 9. (d) 10. (b) 11. (c) 12. (c) 13. (c) 14. (c) 15. (a) 16. (b) 17. (c) II. Fill in the blanks. 1. Kinetic energy of the molecules ___________ with the increase in temperature. 2. With the increase in _____________ the intermolecular forces decrease. 3. The Phenomenon due to which exposed surface of a liquid behaves like stretched membrane is called _____________. 4. Mercury forms spherical droplets, because it has no force of _____________ with glass surface. 5. The meniscus formed by water in a test tube is of_____________ shape. 6. With the increase in the volume of an enclosed gas, its pressure _____________. 7. Convection in liquids takes place due to _____________ of its molecules. 8. The pressure in liquids is independent of _____________ of vessel. 9. The atmospheric pressure at sea level is 100,000 _____________. 10. When a solid floats in a liquid, then the weight of whole __________ is equal to the weight of liquid displaced by its immersed part. 11. All matter is made up of _____________. 12. Molecules of the same type _____________ each other. 13. A liquid droplet is in the shape of a _____________. 14. When a substance is heated, the kinetic energy of the molecules _____________. 15. When a gas is allowed to expand at a constant temperature its pressure ____________. Class-VIII Physics

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16. Liquid pressure does not depend upon _____________. 17. For an object floating in water, its apparent loss of weight will be _____________ buoyant force. 18. The atmospheric pressure at sea level is ____________ mm – Hg. 19. Liquid pressure remains the _____________ in all directions at a given depth. 20. Pressure exerted by the liquids on the sides of the containing vessel is called _____________. 21. At high altitudes, the atmospheric pressure _____________. 22. The force of attraction between the molecules of different substances is called _____________ force. 23. The force of attraction between molecules of the same substance is called _____________. 24. If the weight of a body is equal to the buoyant force, it will _________ in the fluid. 25. A submarine can sink or rise up by filling or emptying its _________. 26. A brick feels _____________ in water than in air. 27. An iceberg is considered dangerous because it floats with only _____________ of its volume above water. 28. Buoyant force is equal to the loss of _____________ of the body when it is immersed in a fluid. 29. The upward force exerted by a liquid on an object immersed in it, is called _____________. 30. Mass per unit _____________ is called the _____________. 31. Larger the volume of a body immersed in a liquid the ____________ is the upthrust. 32. When the weight of a body is less than the weight of the liquid displaced by it, the body will_____________. 33. When the weight of a body is greater than the weight of the liquid displaced by it, the body will_____________. Ans. 1. increases 4. adhesion 7. motion Class-VIII Physics

2. intermolecular spaces 5. concave 8. area of cross-section 4

3. surface tension 6. decreases 9. Pascals Question Bank

10. 13. 16. 19. 22. 25. 28. 31.

soild 11. molecules sphere 14. increases shape of the vessel 17. equal to same 20. lateral pressure adhesion 23. cohesion ballast tanks 26. lighter weight 29. upthrust greater 32. float

12. 15. 18. 21. 24. 27. 30. 33.

attract decreases 760 decreases float a part volume, density sink

III. The statements given below are incorrect, write the correct statements. 1. When a solid floats in a liquid then the density of solid is always more than the density of liquid. Ans. When a solid floats in a liquid then the density of solid is always less than the density of liquid. 2. When the tube of a barometer is tilted, it records a different atmospheric pressure. Ans. When the tube of a barometer is tilted, it records the same atmospheric pressure. 3. Molecule is the smallest unit of matter, which may or may not have an independent existence, but always takes part in a chemical reaction. Ans. An atom is the smallest unit of matter, which may or may not have an independent existence, but always takes part in a chemical reaction. 4. Meniscus is formed in the liquids due to the force of gravity. Ans. Meniscus is formed in the liquids due to the strong force of cohesion. 5. Buoyant force of a liquid increases with the decrease in the density of liquid. Ans. Buoyant force of a liquid increases with the increase in the density of liquid. 6. The pressure of an enclosed gas increases with the increase in the volume of gas. Ans. The pressure of an enclosed gas decreases with the increases in the volume of gas. 7. Liquids exert same pressure in all directions at different depths. Class-VIII Physics

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Ans. 8. Ans. 9. Ans. 10. Ans. 11.

Ans.

12. Ans. 13. Ans. 14. Ans. 15. Ans. 16. Ans.

Liquids exert same pressure in all directions at same depths. Moisture in mercury increases the barometric height. Moisture in mercury decreases the barometric height. When a solid is completely immersed in a fluid, then the apparent loss of weight of solid is more than the weight of fluid displaced. When a solid is completely immersed in a fluid, then the apparent loss of weight of solid is equal to volume of fluid displaced. Solids can be compressed because the molecules are arranged close together. Solids cannot be compressed because the molecules are arranged close together. The relation between the upthrust and the weight of the liquid displaced by a submerged object was first discovered by a Greek mathematician and scientist named Aristotle. The relation between the upthrust and the weight of the liquid displaced by a submerged object was first discovered by a Greek philospher and scientist named Archimedes’. Objects appear heavier when kept immersed in a fluid. Objects appear lighter when kept immersed in a fluid. Buoyant force is experienced only when objects are immersed in liquids and not in gases. Buoyant force is experienced when objects are immersed in liquids and gases both. Buoyant force acts in all directions. Buoyant force acts in upward direction. The barometer reading changes for different positions at the same depth. The barometer reading is same for different positions at the same depth. Atmospheric pressure increases as we move from sea level to higher altitude. Atmospheric pressure decreases as we move from sea level to higher altitude.

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17. Conduction takes place due to molecular movement. Ans. Conduction takes place due to molecular vibration. 18. With the decrease in intermolecular space, the intermolecular forces decrease. Ans. With the decrease in intermolecular space, the intermolecular forces increase. 19. Aneroid barometer used pure mercury. Ans. Aneroid barometer uses no liquid. 20. With the decrease in the volume of gas its pressure decreases. Ans. With the decrease in the volume of gas its pressure increases. IV. Name the following. 1. The scientist who first proposed that molecules are constantly moving. 2. The force between the molecules of two different types of substances. 3. The state of matter in which it is easily compressible. 4. The curve surface of a liquid column. 5. A mode of heat transference accompanied by the physical movement of molecules. 6. The upward force experienced by an object when immersed in a fluid. 7. The instrument used to measure atmospheric pressure. Ans. 1. Robert Brown 4. Meniscus 7. Barometer

2. Adhesive force 5. Convection

3. Gaseous 6. Upthrust

V. Define these terms. 1. Surface tension Ans. Surface tension: The phenomenon due to which exposed (top) surface of a liquid contained in a vessel behaves like a stretched membrane is called surface tension. 2. Cohesion Ans. Cohesion: The force of attraction existing between the molecules of the same type is reduce called the cohesion. Class-VIII Physics

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3. Adhesion Ans. Adhesion: Adhesion is the force of attraction between the molecules of different substances. 4. Atmospheric pressure Ans. Atmospheric pressure: The thrust on the unit area due to the column of air on the earth’s surface is called atmospheric pressure. 5. Conduction Ans. Conduction: The process of transfer of heat in solids, from one particle to another without the actual movement of the particle is called conduction. 6. Convection Ans. Convection: The mode of heat transference in liquids and gases, taking place by the actual movement of molecules from one part to another is called convection. 7. Meniscus Ans. Meniscus: The formation of curved surface by the exposed surface of the liquid contained in a narrow vessel is called meniscus. VI. Match the following. 1. Intermolecular force (a) force of attraction between molecules of the substance 2. Cohesion (b) distribution of thrust exerted by a liquid due to its own weight 3. Adhesion (c) force of attraction between molecules 4. Liquid pressure (d) force of attraction between molecules of different substances Ans. 1. (c), 2. (a), 3. (d), 4. (b) VII. Differentiate between the following. 1. Solids and liquids on the basis of arrangement of molecules. Ans. In solids, there is very less space between the molecules and they are tightly packed. So they have a definite shape and definite volume. While in liquids intermolecular space is more than solid and they are less tightly packed. So they have a definite shape but no definite volume. Class-VIII Physics

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2. Conduction and convection. Ans. The mode of transmission of heat energy from one atom to another atom, in the direction of lower temperature without the actual movement of atoms from their mean position is called conduction. It takes place in solids. The phenomenon, due to which particles of a medium actually move towards the source of heat energy, and then on absorbing heat energy move away from it, thereby making a space for other particles of medium to move towards the source of heat is called convection. It takes place in liquids and gases. 3. Liquid pressure and atmospheric pressure. Ans. All liquids apply pressure on the walls and bottom of their container. The force applied by liquid per unit area is called liquid pressure. The force exerted by atmosphere per unit area on the surface of earth is called atmospheric pressure. VIII. Give reasons for the following. 1. Mercury does not “wet” glass. Ans. The cohesive forces between the molecules of mercury is greater than the adhesive forces between the molecules of mercury and glass. Thus mercury does not “wet” glass. 2. The sides of an empty can do not collapse inwards. Ans. The atmospheric pressure acts equally in all directions that is why the sides of an empty can are not pushed in as the air pressure inside the can is the same as the air pressure outside. 3. You will be able to lift a heavy object quite easily inside water. Ans. According to Archimedes’ Principle, when a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced by the body. Hence, the object feels much lighter inside water than its actual weights. So we are able to lift a heavy object quite easily in water. 4. If a thermometer is broken, small drops of mercury will roll like little balls. Ans. If a thermometer is broken small drops of mercury roll like little Class-VIII Physics

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balls due to surface tension. 5. A gas can be easily compressed. Ans. The molecules of a gas are very far apart and they have very large intermolecular spaces. So a gas can be easily compressed by applying pressure. 6. Water has a tendency to form spherical droplets. Ans. Water has tendency to form spherical droplets due to surface tension which is mainly caused due to unbalanced cohesive force acting near the top surface of a liquid. 7. Hydrogen-filled balloons tend to float upwards. Ans. Hydrogen is lighter than air, so hydrogen filled balloons tend to float upwards. 8. Giant hot-air balloons rise up. Ans. When air is heated, it gets lighter and tends to rise upward, so giant hot air balloons rise up. 9. A piece of iron floats in mercury but sinks in water. Ans. Density of mercury (13.6 g cm–3) is higher than that of water (1 g cm–3), thus mercury exerts more upthrust on iron piece. So a piece of iron floats in mercury and sinks in water. 10. A balloon filled with helium gas rises up. Ans. Since, helium gas is lighter than air, so a balloon filled with helium gas rises up. 11. One find easier to swim in sea water as compared to pure swimming pool water. Ans. Since density of sea water is more than pure swimming pool water. So sea water exerts more upthrust on the body, and hence, one find easier to swim in sea water as compared to pure swimming pool water. 12. Only a tip of iceberg visible outside the water surface. Ans. As the density of ice is slightly less than sea water, therefore only a tip of iceberg is visible outside the water surface. IX. Answer the following questions. 1. State three factors which determine pressure in liquids. Class-VIII Physics

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Ans. The three factors which determine pressure in liquids are: (a) Pressure in a liquid is directly proportional to its height (depth) (b) Pressure in a liquid is directly proportional to its density. (c) Pressure in a liquid is directly proportional to the acceleration due to gravity. 2. Describe an experiment to prove that lateral pressure in liquids is directly proportional to the depth of liquid. Ans. Experiment to prove that lateral pressure in liquids is directly proportional to the depth of liquid: We take a metallic cylinder with a small opening near the base tied with a small rubber balloon. Now gradually pour water into the cylinder. We observe that with the increase in the level of water, the balloon starts bulging, i.e. more the depth of water the more is the lateral pressure. 3. Define the following terms: (a) element (b) atom (c) molecule. Ans. (a) Element: An element is defined as a substance, which cannot be subdivided into two or more simple substances by any chemical means. (b) Atom: An atom is defined as the smallest unit of an element which may or may not have an independent existence, but always takes part in a chemical reaction. (c) Molecule: Molecule is defined as the smallest unit of matter, which has an independent existence and can retain complete physical and chemical properties of matter. 4. State five important points of kinetic theory of matter. Ans. The five important points of kinetic theory of matter are: (i) Molecules are in a state of continuous motion, which does not stop over any length of time. (ii) The kinetic energy of the molecules increases with the increase in temperature and vice versa. Class-VIII Physics

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(iii) The molecules always attract each other. The force of attraction between similar kind of molecules is called force of cohesion while the force of attraction between different kinds of molecules is called force of adhesion. (iv) The empty space between the molecules is called intermolecular space. (v) The intermolecular force of attraction increases if the intermolecular space between the molecules decreases and vice versa. 5. On the basis of kinetic theory of matter explain. (a) Why the solids have a definite volume and definite shape? (b) Why the liquids have a definite volume but no definite shape? (c) Why the gases have no definite volume and no definite shape? Ans. (a) Intermolecular spaces between the molecules of a solid are very small, so the molecules attract one another with a strong force which holds the molecules at one particular place. Because of the fixed position of the molecules, the solids have a definite shape and definite volume. (b) Intermolecular spaces between the molecules of a liquid are large, so the molecules attract each other with a lesser force. Due to this, the molecules do not stay at one place. Instead, they interchange their position and take the shape of the containing vessel. So liquids have a definite volume but no definite shape. (c) In gases, the molecules are very much far apart, and move independently. So, the gases have no definite volume and no definite shape. 6. (a) What do you understand by the term surface tension? (b) Name the force responsible for surface tension. (c) Why does a small amount of mercury on placing over a glass plate form a spherical ball? (d) Why does a small amount of water on placing over a glass place form an oval shaped drop? Class-VIII Physics

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Ans. (a) The phenomenon due to which exposed (top) surface of a liquid contained in a vessel behaves like a stretched membrane is called surface tension. (b) The surface tension is mainly caused due to unbalanced cohesive force. (c) The unbalanced cohesive force acting on the molecules of liquid at the surface tends to pull them towards the centre of the liquid. It reduces the surface area and cause droplets of mercury to take a spherical shape. (d) When a small amount of water is poured on a glass plate, it takes an oval shape. It is because the force of attraction between the molecules of water i.e., the force of cohesion is far less than the force of adhesion between the molecules of water and the glass plate. 7. What do you understand by the term meniscus? Name the kind of meniscus formed (a) in case of water (b) in case of mercury. Ans. The formation of curved surface by the exposed surface of a liquid contained in a narrow vessel is called meniscus. (a) Concave meniscus is formed in case of water and (b) convex meniscus is formed in case of mercury. 8. Liquids exert pressure. Explain on the basis of kinetic model. Ans. The molecules of a liquid in a closed vessel are not stationary but move about randomly in all possible directions. In doing so, the molecules of liquid strike against the side of container, and hence, exert some force. This force exerted per unit area on the sides of containing vessel is the cause of liquid pressure. 9. Gases exert pressure. Explain on the basis of kinetic model. Ans. The molecules of a gas in a closed vessel are not stationary but move about randomly in all possible directions. In doing so, the molecules of a gas strike against the side of container and hence exert some specific force. This force exerted per unit area on the sides of containing vessel is the cause of gas pressure. 10. How does the pressure of an enclosed gas change with the decrease Class-VIII Physics

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Ans.

11. Ans.

12. Ans.

13.

Ans.

14.

in volume? Explain it on the basis of kinetic model of matter. With the decrease in volume of an enclosed gas, its pressure increases. When the gas is compressed to half its volume, the number of molecules per unit volume of the gas becomes double. So the force exerting against the side of container is also doubled. Therefore, the pressure of the enclosed gas is doubled. Solids get heated by conduction. Explain conduction on the basis of kinetic model of matter. When a solid is heated from one end, its atom gain heat energy due to which their kinetic energy increases and hence they vibrate about their mean positions with more amplitude. Thus these energetic atoms strike against the neighbouring atoms and transfer a part of kinetic energy to them. Thus, the temperature of the conductor rises. Fluids get heated by convection. Explain convection on the basis of kinetic model of matter. When a fluid is heated from the base of the container, then the particles in that region absorb heat energy which increases kinetic energy of the molecules. Hence, they move more vigorously. These energetic particles rise upward which results in fall in pressure. To equalise the pressure, the particles of liquid from the colder regions sink down. This in turn gives rise to convection current. How does liquid pressure, depends upon the following? (a) depth of liquid (b) density of liquid (c) acceleration due to gravity at a given place (d) area of cross-section of the containing vessel (a) Liquid pressure is directly proportional to the depth of liquid. (b) Liquid pressure is directly proportional to its density. (c) Liquid pressure is directly proportional to the acceleration due to gravity at a given place. (d) Liquid pressure is independent of the area of cross-section of the containing vessel. Why are the walls of a hydroelectric dam made thick at the base of dam?

Class-VIII Physics

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Ans. The walls of the dam are made very thick at the base, so that they can withstand lateral pressure which increases with increase in depth of liquid. 15. Why do the deep sea divers wear specially designed suits? Ans. Pressure of the liquid increases with increase in depth. As sea divers go deep down into the sea, the liquid pressure exerted on them would be so great that would even crushed. So deep sea divers wear a specially designed suits to withstand this pressure. 16. What do you understand by the term atmospheric pressure? What is its average value at sea level? Ans. The force exerted by atmosphere per unit area on the surface of earth is called atmospheric pressure. The average value of atmospheric pressure at sea level is 76 cm of vertical height of mercury. 17. The atmospheric pressure at sea level is 76 cm of mercury. What do you understand by the statement. Ans. The atmospheric pressure at sea level is 76 cm of mercury. It means air exerts as much pressure at sea level as is exerted by 76 cm of vertical height of mercury column. 18. Describe briefly the construction of simple barometer? Ans. A simple barometer is obtained by inverting a mercury filled tube about 100 cm in length and closed at one end in a trough filled with mercury, taking care not to let in air by inverting the tube. 19. What do you understand by the term buoyancy and buoyant force? State two factors which determine the buoyant force exerted by a liquid. Ans. Buoyancy: The phenomenon due to which a body experiences an upward force when partly or wholly immersed in a fluid is called buoyancy. Buoyant force: An upward force experienced by a body when partly or wholly immersed in a fluid is called buoyant force. Factors determining buoyant force: (i) Buoyant force is directly proportional to the volume of the fluid displaced by solid. Class-VIII Physics

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20. Ans.

21. Ans.

(ii) Buoyant force is directly proportional to the density of fluid displaced by solid. How does the atmospheric pressure decrease due to change in altitude? As we go up in the atmosphere, the atmospheric pressure decreases. At a height of 1 km above sea level, the height of mercury column in the barometer falls to 73 cm. At a height of about 40 km above the sea level the air pressure is only about 1 cm of mercury. (a) State Archimedes’ Principle. (b) Briefly describe, how will you verify Archimedes’ Principle. (a) Archimedes’ Principle:- “When a body (solid) is partly or wholly immersed in a fluid, it experiences an upward force which is equal to the weight of the fluid displaced by the immersed part of the body.” (b) Verification of Archimedes’ Principle: (i) Take a glass stopper and tie it to a spring balance. Let its weight be W1g. (ii) Take an overflow jar filled with beaker and place in a clean beaker. Weigh the empty beaker. Let it be W3g. (iii) Now lower the stopper carefully in the overflow jar. Note the weight of the stopper when it is completely immersed. Let it be W2g. As the stone is immersed into the overflow jar, it displaces water which is collected in the beaker. Let the weight of beaker along with water displaced be W4g. Weight of stopper in air = W1 g Weight of stopper when fully immersed in water = W2 g Weight of empty beaker = W3 g Weight of beaker plus displaced water = W4 g Apparent loss in weight of water = (W1– W2) g Let us say a1 g. Weight of water displaced = (W4 – W3) g Let us say a2 g. We will see that a1 = a2

Class-VIII Physics

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22. Ans.

23. Ans. 24. Ans. 25. Ans.

26. Ans.

27. Ans.

i.e. (W1 – W2) = (W4 – W3) The apparent loss of weight of the stone or the buoyant force on it will be found to be equal to the weight of the water displaced, thus verifying Archimedes’ Principle. (a) State the law of floatation. (b) Why does ice float on the surface of water? (a) Law of floatation: It states that when a body is floating in a liquid, then the weight of whole body acting vertically downward is equal to the weight of the liquid displaced by the immersed part of the body, acting vertically upward. (b) Density of ice is around 0.91 g cm–3 at 0°C which is less than density of fresh water 1 g cm–3. When ice is dropped in water, it experiences a greater buoyant force than its own weight and thus ice floats on the surface of water. Why do ink pens start leaking on high altitude? On higher altitudes fountain pens start leaking because air inside the pen is at higher pressure than air outside. Why is the hearing affected while coming down from hills? There is less atmospheric pressure at the hills, so the hearing is affected while coming down from hills. State Pascal’s law? Pascal’s law,” It states that when pressure is applied to any part of an enclosed liquid it is transmitted equally in all directions with undiminished force and acts at right angles to the surface of vessel. A piece of iron sinks in water but large ships made of iron float on water. Explain why? According to law of floatation, those objects which are less dense than water will float in water and the objects whose density is greater than water will sink. The average density of ship is less than that of water whereas that of piece of iron more than that of water. Hence, larger ship floats and an iron piece sinks. Why is mercury used in a barometer? Mercury is used in a barometer because

Class-VIII Physics

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28. Ans.

29. Ans.

30. Ans.

(a) density of mercury is very high and a short length of mercury can counterbalance normal atmospheric pressure. (b) it does not wet the sides of glass tube, and hence, helps in recording true atmospheric pressure. (c) it is easily seen in the glass tube. What are the defects of a simple mercury barometer? A simple mercury barometer has following defects: (a) As the apparatus is all glass, therefore, there is chance of its breakage. (b) The level of mercury in the glass trough increases or decreases with the change in atmospheric pressure. Thus a permanent scale cannot be marked on the glass tube. (c) It is neither compact, nor portable, thus it cannot be carried to different places. When a solid substance is heated, what happens to its molecule? When a solid is heated, kinetic energy of its molecules increases and hence they vibrate about their mean positions with more amplitude and after some time, the solid begins to change into liquids. Why are liquids and gases called fluids? Liquids and gases have the property to flow. Liquids flow from upward to downward direction while gases flow in all directions. So, liquids and gases are called fluids.

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