9th Class Physics Chapter 7 Exercise Short Questions with Answers

Properties of matter

9th Class Physics Chapter 7 Exercise Short Questions with Answers

Q 7.2 : How is kinetic molecular model of matter helpful in differentiating various states of matter ?

Kinetic molecular model is used to explain three states of matter . Solid , Liquid and Gases .

Solid : In solids , molecules are very close to one another , they passes greater attractive forces .

Liquids :In liquids , distance between molecules is greater as compared to solids .

Gases : In gases , distance between molecules is too much .

Q 7.3 : Does there exist a fourth state of matter ? What is that ?

Yes , there exists a fourth state of matter called plasma .

  • At very high temperature , the matter assumes the state of ions and electrons , This is called plasma .

Q 7.4 : What is meant by density ? what is its SI unit ?

Density :

” Density of a substance is defined as the mass per unit volume .”

Formula :

Unit : The SI unit of density is kilogram per cubic meter ( kgm-3 ).

Q 7.5 : Can we use a hydrometer to measure the density of milk ?

Yes , we can use hydrometer t measure the density of milk .

  • Hydrometer is partially immersed in the milk to determine its density .

Q 7.6 : Define the term pressure .

Pressure :

” The force acting normally on unit area at the surface of a body is called pressure .”

Formula :

Units : In SI , the unit of pressure is Newton per square metre ( Nm-2 ) or Pascal ( Pa ).

 → Pressure is a scalar quantity .

Q 7.7 : Show that atmosphere exerts pressure .

Experiment : ( crushing can )

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  • Boil an empty tin , half filled with water and with a cap / lid on the tin .
  • Let it cool under running tap water .
  • The tin will get crumpled / sqeezed as water cool down .
  • As the steam condenses , the pressure inside the metal tin decreases and the external atmospheric pressure that is higher , crushes the tin .

Q 7.8 : It is easy to remove air from a balloon but it is very difficult to remove air from a glass bottle . Why ?

Reason :

  • It is easy to remove air from a balloon . Because air pressure in the balloon is more than external pressure .
  • It is difficult to remove air from glass bottle . Because air pressure in the bottle is less than atmospheric pressure .

Q 7.9 : What is a barometer ?

Barometer :

→ ” The instrument used to measure atmospheric pressure is called barometer .”

  • One of the simple barometer is Mercury Barometer .

Q 7.10 : Why is water not suitable to be used in a barometer ?

Reason : Because water :

  • does not have required thermometric properties .
  • due to its low density .

Q 7.11 : What makes a sucker pressed on a smooth wall sticks to it ?

Air Pressure makes a sucker pressed on a smooth wall stick to it .

The atmospheric pressure is greater than the pressure in the sucker .This pressure presses sucker on the wall .

Q 7.12 : Why does the atmospheric pressure vary with height ?

As we go up Atmospheric Pressure decreases .

Reason : Because the density of air is not uniform in the atmosphere . It decreases continuously as we go up.

Q 7.13 : What does it mean when the atmospheric pressure at a place fall suddenly ?

  • A sudden fall in atmospheric pressure often followed by :

a storm ,

rain and

typhoon .

  • It occurs in few hours time .
  • It causes internal energy of air to decrease and coldness is produces .

Q 7.14 : What changes are expected in weather if the barometer reading shows a sudden increase ?

  • If a barometer reading shows a sudden increase / or a rapid increase in atmospheric pressure .

It means , soon there will be decrease in the atmospheric pressure indicating poor weather ahead .

Q 7.15 : State Pascal’s law .

Pascal’s Law :

” Pressure applied at any point of a liquid enclosed in a container , is transmitted without loss to all other parts of the liquid .”

Applications of Pascal’s Law :

  • Automobiles
  • Hydraulic break system
  • Hydraulic jacks
  • Hydraulic presses

Q 7.16 : Explain the working o f hydraulic press .

  • Hydraulic press works on the principle of Pascal’s Law .
  • It consists of two cylinders of different cross – sectional areas .

Q 7.17 : What is meant by elasticity ?

Elasticity :

” The property of a body to restore to its original size and shape as the deforming force ceases to act is called elesticity “

Examples : Rubber bands , elastic and other stretchy materials possess the property of elasticity .

Q 7.18 : State Archimedes principle .

Archimedes Principle :

” When an object is totally or partially immersed in a liquid , an upthrust acts on it equal to the weight of the liquid it displaces .”

This principle was given by a Greek scientist , Archimedes .

Q 7.19 : What is upthrust ? Explain the principle of flotation .

Upthrust :

” The fluids ( liquids ) exert force in the upward direction when some object is immersed into them . This is called upward thrust .”

Principle of flotation :

” The weight of fluid displaced is equal to weight of floating object inside liquid . Then object will not sink and keep floating . It is called Principle of flotation .”


” A floating object displaces a fluid having weight equal to the weight of the object .”

Q 7.20 : Explain hw a submarine floats the water surface and dives down into water .

  • A submarine floats on the surface of sea water . When it is not filled with sea water , then its weight is less than upward thrust .
  • But a submarine sinks into water , when it is filled with water , then its weight becomes larger as compared with upthrust of water .

Q 7.21 : Why does a piece of stone sink in water but a ship with a huge weight floats ?

  • A piece of stone sinks in water because , the weight of an equal volume of water is smaller than the weight of stone .

Stone has high density and small volume .

  • While ships floats on water because , the weight of an equal volume of water is greater than the weight of ships and boats .

Ships have less density and large volume .

Q 7.22 : What is Hooke’s law ? What is meant by elastic limit ?

Hooke’s Law :

” The strain produced in a body by the stress applied to it is directly proportional to the stress within the elastic limit of the body .'”

Elastic Limit :

Elastic Limit can be defined as a limit within which a body recovers its original length , volume or shape after the deforming force is removed .

Q 7.23 : Take a rubber band . Construct a balance of your own using a rubber band . Check its accuracy by weighing various objects .

  • Take a rubber band , hang it with a hook .
  • Then pointer is attached at the lower end of it with a scale in front of pointer .
  • Different known weights are suspended , one by one , at lower end of this rubber band .
  • The pointer position on the scale is marked for each different known weight suspended . It is called calibration of scale for weight measurements .
  • This makes a balance for weight measurement.

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