Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER

Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER

Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER.

Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER
Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER

Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER Free Download in PDF Format.

Chapter # 7



Q.1 using kinetic molecular model of matter, explain these states of matter.

Ans. Kinetic Molecular Model of Matter:

According to this theory, the matter is made up of molecules which are always in motion. There are three states of matter i.e. solids, liquids and gases. There is a force of attraction between the molecules of solid, liquid and gas that depends upon the intermolecular spaces between the molecules.

The motion of molecules depends upon the temperature. By increasing the temperature, the kinetic energy of molecules also increases. So matter and its properties are based on the arrangement and motion of molecules. These three states of matter are explained on the basis of kinetic molecular theories which are as follow:

1. Solids:

i. Solids are made up of molecules which are arranged loosely in a fixed pattern (does not change their position).

ii. Solid has definite shape and volume.

iii. Molecules in solids vibrate about their mean positions.

iv. The attractive forces between the molecules are very strong.

v. They have minimum kinetic energy

2. Liquids:

i. In liquids, the molecules are close ogle her but the pattern of molecules are not fixed which means that the molecules in pattern keep changing their position.

ii. The liquids have no d finite shape but have volume which means that the molecules are able to move and change its shape.

iii. Liquids can adopt the shape of the container on which it is pour.

iv. The attractive forces between the molecules of liquid are less than the solid.

v. Their kinetic energy of molecules is little higher than that of solids.

3. Gases:

i. A gas is made up of molecules which are in constant random motion.

ii. The distance between molecules is larger as compared to size of molecules.

iii. The gases occupy no definite shape and no definite volume.

That’s why the molecules collide with each other and with the walls of the container elastically.

iv. The attractive forces between the molecules are negligible except during collisions.

v. Their volume depends upon temperature and pressure.

vi. The molecules of gases are having maximum kinetic energy.

 If force “F” is applied on is an “A” the pressure is

Pressure =

Mathematical Form:

Q.2 Define and explain density and pressure.

Ans. Density:

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

Density =

ρ =

Where “ρ” (rho) is the density and “m” is the mass and “V” is the volume of a substance.


Density tells us that how much mass exist in a given volume or how much an object is heavier than other. When the molecules are closed together, then there are more molecules in a unit volume. Therefore, its density will be greater. Solids are denser than liquids and liquids are denser than gases. The density of water is taken as standard.

In S.I unit, the density if water is 1000 kg/m3 or 1000 kgm-3. If density of an object is less than density of water (liquid) then object will float, but if density of an object is more than the density of water (liquid), then it will sink in water.

For Example:

Iron is denser (heavier) than air, because iron has more mass in the same volume as compared to air. We can also say that for some mass of iron and air, volume of iron (same mass) will be less than air. Also an iron nail is denser than water. That’s why it will sink in water because in iron more amount of mass present in unit Volume than water.


The S.I unit of density is kgm-3 or


Pressure is defined as force per unit area. It is denoted by “P”.

Mathematical Form:

P =      …….(i)

Relation between pressure and force:

Eq(i) clearly shows that pressure is directly proportional to the applied force. It means by applying greater force, pressure will be greater.

Relation between pressure and area:

Eq(i) also shows that pressure is inversely proportional to each other. It means that smaller the area for a given force, the greater will be the pressure. Therefore, a force exerted over a small area produces more pressure than the same amount force exerted over a large area.

For Example:

A girl wearing high heel pointed shoes exerts more pressure on the ground than the one wearing flat shoes.


The SI unit of pressure is Pascal (Pa) which is equal to Newton per square meter (Nm-2). i.e.

1 Pa = 1. Nm-2

So, one Pascal is the pressure when a force of “1N” acts normally on an area of “1m2”.

Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER
Physics Notes for Class 9 Chapter 7 PROPERTIES OF MATTER

Q.3. what is atmospheric pressure? How is it measured by using a mercury barometer?

Also disable how weather changes with atmospheric pressure?

Atmosphere Pressure:

A Thick layer of air around the earth is called atmosphere. The pressure exerted by this atmosphere on the surface of earth is called atmospheric pressure.


The air around us exerts a force in all directions and is opposed by an equal pressure inside our body, that’s why we do not feel atmospheric pressure. But if the internal pressure of our body is increase or decrease from atmospheric pressure then we feel difficulty in breathing.

At sea level, the density of air is maximum and the atmospheric pressure is also maximum. But as we go up from sea level (at high altitudes, the density of air decreases and the atmospheric pressure also decreases.

Measurement of Atmospheric Pressure by using mercury barometer:


It was first set up Torricelli, a pupil of Galileo in 1643.


Mercury barometer is an instrument that is used to measure the atmospheric pressure. This is mainly used in meteorology.


It consists of a long glass tube sealed at one end and the other end of tube is dipped in a mercury dish. The atmospheric possum process on the mercury in the dish and this keeps the column of mercury up in the glass tube. The greater the pressure, the higher will be the column of mercury.

Under normal atmospheric pressure, at sea level the column of mercury can rise to 76 cm or 760 mm in the glass tube. At higher altitudes, the atmospheric pressure decreases so column of mercury level also decreases.

Standard Atmospheric Pressure:

The pressure exerted by 760mm (or76cm) of mercury column at sea level is known is standard atmospheric pressure or 1 atmospheric. i.e.

1 atmosphere = 1.013 x 105 Pa

Or 1 atm = 1.013 x 105Pa

Atmospheric Pressure and weather:

Barometers kept in the same place at the same height above the sea level show some variations in atmospheric pressure from day to day. There pressure variations are shown on weather maps.


The lines in the map joining all those places with the same atmospheric pressure are called isobars.


The unit for pressure used in weather maps is called millibar (mbar) or bar. It is exactly equal to 100000 Pa or 100 kPa which is approximately equal to normal atmospheric pressure.


The strength of the wind is determined by the pressure gradient (slope). In weather maps, when the isobars are packed closely together, it indicates high pressure gradient. This means that strong winds may result and rainy weather is detected.

As wind moves from high pressure regions to low pressure regions. S , in the northern hemisphere, the wind moves anticlockwise around areas of low pressure and clock wise around area of high pressure.

Q.4 State Pascal’s principle and explain with example?

Ans. Pascal’s Principle:


Pascal’s Principle of fluid pressure states that whenever an external pressure is applied on a liquid (fluid) in a closed container the pressure is transmitted equally in all directions to every point of the liquid.


All the liquid exerts same pressure in all direction.


Pascal’s principle has discovered d by a French philosopher named Blasé Pascal. This principle is used in different hydraulic machines in our daily life i.e. hydraulic press, hydraulic brakes, hydraulic jacks and hydraulic lift etc.

Example of Hydraulic lift:

To understand the Pascal’s principle, consider an example of hydraulic lift. Hydraulic lift is a device or machine which works on Pascal’s principle and is used for lifting heavy loads like heavy vehicles easily. It consists of two cylinders “A” and “B” one of large cross-sectional area “A1” and the other of small cross Sectional area “A2”. i.e. A2 > A1.

Each of the cylinders is fitted with a piston which are connected by a tube and they are filled with a suitable fluid usually oil called Hydraulic fluid as shown in fig. Now, suppose if a force “F” is applied on the piston of cylinder “A” of cross sectional area “A1”, then the pressure “P1” exerted by this piston is:

P1 =    1          …..(i)              

In this way, the piston of cylinder “B” of cross sectional area “A2” moves upward and the vehicle is lifted upward by a force F2 which is placed at the platform of cylinder “B”. So, the pressure on the piston lifting vehicle is P2, which can be written as,

P2 =    2          …………(ii)               

Now, According to Pascal’s Principle,

P2 = P1 ………..(iii)

2 =  1

Putting values of P1 and P2 in eq……(iii),we get

A2 x   2 =

1 x A2

(F2) is lifted, the equation can be written as:                 

To calculate how much load 2




F2 =




F2 = ……….(i)

Equation (iv) shows the amount of force “F2” exerted on piston of cylinder “B” of Area “A2”.      

Also, we can find values of A1, A2 or F1 by this equation. Now, depending on the ratio of A2 / A1, the force “F2” can be as large as required. So, so lift more load, we should keep A2 > A1. Hence a relatively small force can be used to overcome a much larger load.

Q.5: How pressure varies with depth in liquids? Explain  

Ans. We know that P = gh. It is clear from the eq that pressure varies with depth and density of liquid.


Let us take a contain r full of water, there are three holes A, B and C in the container.

  1. From first whole “A” water exert with low speed because of less depth from top of container.
  2. It means on whole “A” due to less depth, pressure “P” is low
  3. From second whole “B” water exert with greatest speed. It means that pressure “P2” at whole “B” is greater than whole “A”.
  4. From third whole “C” water exert with much fast speed than whole “A” and “B” because its depth is greater than hole “A” and “B”. It means that at whole “C” the pressure “P3” is maximum.

Thus, pressure varies directly with depth below the surface of a liquid in any container.

Q.6 what is meant by buoyant force or up thrust in fluids?

Buoyant Force or Up thrust:

The upward force exerted by the fluid on the object which is immersed in it is called up thrust or buoyant force. It is denoted by “FB”


When an object is immersed in a fluid, the pressure on the lower surface of the object is higher than the pressure on the upper surface. The difference in pressures leads to an upward net force acting on the object due to the fluid pressure called buoyant force and this phenomenon is called buoyancy. So, the buoyant force depends on the density and volume of an object that is immersed in a fluid.

For Example:

If we try to push a piece of crock underwater, we feel an up award, force that is pushing the cork back up. So when we release the cork, it will rise to the surface and floats due to up thrust or buoyant force. Here, the buoyant force is greater than the weight of the cork, so it will move upward and float.

Q.7 State and explain Archimedes principle.

Ans. Archimedes Principle:

Archimedes principle states that “The buoyant force acting on an object fully or partially submerged in a fluid (liquid) is equal to the weight of the fluid displaced by the object. Or

Buoyant force = weight of fluid displaced

FB = W


According to A chimes principle, every object experiences a buoyant force. When a solid object is dipped in a liquid, an upward buoyant force acts on the object. The magnitude of this force “FB” is given by Archimedes principle. For example, if we dipped a brick in a container of water, the brick sinks and water level rises in it.

The total displaced volume of water is equal to the volume of the brick that is underwater. So, the magnitude of the “FB” acting on the brick is equal to the weight of water displaced by brick. The buoyant force will decrease the weight of the object which is known as apparent loss of weight. That loss in weight of brick is equal to the weight of the liquid displaced.

Principle of Floatation:

A floating object always displaces its own weight of liquid (Fluid) in which it is immersed. The Archimedes principle applies to both floating and submerged bodies and to all fluids i.e. liquids and gases. Whether an object floats or sinks can be explained by using this principle. The floatation or sinking of an object depends on the density of an object relative to the fluids and the buoyant force.

According to law of floatation:              

  1. When buoyant force “FB” is greater than objects weight (W) i.e. FB > W, the object will float in liquid or if B >o, it will float.
  2. When buoyant force “FB” is smaller than object’s weight i.e. FB < W the object will sink. Or we can say if density of liquid (   ) is lesser than density of object (  o) i.e.   B  <   o, it will
  3. When FB = W, the object will float in the liquid in which it is immersed or we can say, if B = o then object will fully immersed but not sink. Sink.


The criteria for sinking or floating of an object depend on the net force acting on it. This net force can be calculated as follows F net = FB – W (object) ……….(i)

Now we can apply Archimedes principle, FB is equal to the weight of fluid (liquid) displaced (WF)

i.e.       FB = W f

Now eq …..(i) Becomes

F net = W f – W …… (ii)

As we know that, W = mg

So W f = mfg. and Wo = mog

Where “mo” represents the mass of submerged object and “mf” is the mass of fluid displaced and “g” is the acceleration due to gravity. So put values of Wf and Win eq …..(ii)

F net = mfg. – mog …… (iii)

As we know that, m = ρ V,

So, mf = ρf Vf  &  mo = ρo Vo the eq ….(iii) becomes

F net =ρ V  g – ρoVog

Or        F net = (ρ V  – ρoVo) g


Now, a relationship between the weight “W” of submerged object of mass “m0” and density

“ρ0” and the buoyant force “FB” on the object by fluid displaced of mass ‘    ’ and density ‘’ = B can be found by considering their ration as follows: equal to the volume of liquid displaced i.e.            

As the volume of submerged object is Hence, above equation … (iii) shows the relation between weight of the object immersed and the buoyant force. Also by using this equation we can find out the density of solid object (ρ ) and density of liquid displaced (           B) or the buoyant force.

Q.8 what is elasticity? Explain.


The property of solid materials to return to their original shape and size after removal of deforming force is called elasticity.


A force is required to change the shape of a solid. Rubber band, spring the bow and tennis ball are examples of elastic bodies. When deforming force is stored in it that enables the body to regain its original shape. For example, if we apply a force to strength a rubber band, its length increases. When the force is removed, the rubber band will return to its original position.

The solids may or may not come to its original state. If a body returns to its original state, the material is said to be elastic. Greater is the opposing force, greater will be the elasticity. But if a body does not return to its original state after applying force the material is said to be inelastic. Examples of inelastic materials are plasticize, clays, dough etc.

Elastic Limit:

Elastic limit is the limit in an elastic body up to which elasticity exists. After crossing this limit means increasing the force the body will break or permanently deforms when the deforming force is removed.

For example, by hanging a weight to a spring it stretches. If we continuously increase the weight, a point comes when it cannot gain its original shape and break; it means that the elastic limit has been crossed. So different materials have different elasticity depending upon the nature of the material.

Q.9 State and explain Hooke’s law.

Hooke’s Law:

This law states that “Within elastic limit the extension or compression in a body is directly proportional to the restoring force”


“With in elastic limit, stress is directly proportional to strain produced in a body”.


When a body (spring) is stretched or compressed this extension (x) or compression is directly proportional to the applied force (F). This relationship is known as Hooke’s law.


Mathematically it can be written ∝ as

Fres    – x


Fres = – k x

 Where “k” is known as the force constant or the “modulus of elasticity.” Its value depends upon the nature of material and system of units. The negative sign shows that force is directed against displacement. The unit of force constant “k” is Nm-1.

Q.10 Define and explain stress strain and young’s modulus?


The stress is defined as the force applied per unit area of cross section on an elastic body to produce deformation. It is denoted by sigma “ ”.

Mathematical Form:

Stress =

Mathematically it can be written as



The SI unit of stress is Nm-2 or Pascal (Pa).


The stress is defined as the extension per unit length.


It is the ratio of charge in length o he original length of a body. Strain is denoted by epsilon ( ).

Mathematical Form:

Strain =                     

Mathematically, it can be written as;


=  ℎ                                         

As strain is the ratio of two lengths, so, it does not have a unit.

Suppose a wire of length “” and area “A” is stretched by applying force (stress), then the extension (change in length) is produced in a wire is “x” means its length increases that causes strain. Simply when stress is applied on a wire, strain is produced in it and the length of a wire changes “x” from the original length “” as shown in figure.

Young’s Modulus:


“The strain produced in an elastic body is directly proportional to the stress with in the limit of proportionality.”


“It is the ratio of stress to the linear strain”.

Mathematical Form:


Stress    strain

Stress = Young’s Modulus x Strain ……….(i)

Now, rearrange the eq….(i), we get

Young’s modulus =                        


Y =

,we get          

Whereas stress = F/A and strain = x/                               

Y=/      …..(ii)  /

Rearrange the eq,

Y = x


Y =

As young’s modulus is constant of proportionality, so within elastic limit, the ratio is constant where value depends on the nature of materials.


The unit of young’s modulus is N/m Or Nm-

Stress strain curve:

Stress and strain curve show the relationship between them. When stress increases, then strain also increases as shown in figure. When the stress r a h s to the elastic limit of the material, then due to urethra stress rapid changes occur in shape of material.

This region is shown between “A” and “B”. At point “C” due to maximum stress, material can withstand without breaking. But if the stress is further increased up to point “D” then the material breaks.

Q.11 Define Plasma State.

Ans: Plasma State:

Plasma is the fourth state of matter. It consists of free electrons and atoms from which the electrons have been removed. Plasma exists in the sun, where thermonuclear reactions take place at very high temperatures.

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