Select Page

Short Questions

7.2.     How kinetic energy molecular model of matter helps differentiate various states of matter?

Ans:    Kinetic Molecular Model of Matter:

• The matter is made up of particles called molecules.
• The molecules remain in continuous motion.
• Molecules attract each other.

Characteristics of Molecular Model of Matter:

Kinetic molecular model is used to explain the three states of matter – solid, liquid and gas.

1. Solids:
• Solids such as a stone, metal spoon, pencil, etc. have fixed shapes and volume.
• Their molecules are held close together by strong forces of attraction.
• Molecules of solids vibrate about their mean positions but do not move from place to place.

1. Liquids:
• The distances between the molecules of a liquid are more than in solids. Thus, attractive forces between them are weaker.
• Like solids, molecules of a liquid also vibrate about their mean position but are not rigidly held with each other.
• Due to the weaker attractive forces, they can slide over one another. Thus, liquids can flow.
• The volume of a certain amount of liquid remains the same but because it can flow hence, it attains the shape of a container to which it is put.

1. Gases:
• Gases such as air have no fixed shape or volume. They can be filled in any container of any shape.
• Their molecules have random motion and move with very high velocities. In gases, molecules are much farther apart than solids or liquids. Thus, gases are much lighter than solids and liquids.
• They can be squeezed into smaller volumes.
• The molecules of a gas are constantly striking the walls of a container. Thus, a gas exerts pressure on the walls of the container. (P ∝E)

1. Plasma – The fourth state of matter:
• Production/Formation of Plasma:

The kinetic energy of gas molecules goes on increasing if a gas is heated continuously. This causes the gas molecules to move faster and faster. The collisions between atoms and molecules of the gas become so strong that they tear off the atoms. Atoms lose their electrons and become positive ions. This ionic state of matter is called plasma. Plasma is also formed in gas discharge tubes when an electric current passes through these tubes.

(ii)     Plasma is called the fourth state of matter in which a gas occurs in its ionic state. Positive ions and electrons get separated in the presence of electric or magnetic fields.

(iii)    Plasma also exists in neon and fluorescent tubes when they glow.

(iv)    Most of the matter that fills the universe is in the plasma state.

(v)     In stars such as our Sun, gases exist in their ionic state.

(vi)    Plasma is highly conducting state of matter. It allows electric current to pass through it.

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

Ans:   Yes, the fourth state of matter is called plasma.

Plasma:

At very high temperature, the collision between atoms a molecule tears off their actions. Atoms become positive ions. The ionic state of matter is called the plasma-the fourth state of matter.

7.4     What is meant by density? What is its SI unit?

Ans:   Density:

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

Density =   mass/volume OR D = m/v

Unit of Density:

SI unit of density is kilogramme per cubic metre (kgm – 3).

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

Ans:   Lactometer is used to measure the density of milk. Whereas hydrometer is used to measure the concentration of acid in a battery. It is called acid meter.

7.6     Define the term pressure.

Ans:   Pressure:

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

Pressure  P       = Force / Area

Or                                  P       =  F / A

Unit of Pressure:

In SI units, the unit of pressure is Nm – 2 also called pascal (Pa). Thus

1 Nm – 2     =      1 Pa

7.7      Show that atmosphere exerts pressure.

Ans:    Atmospheric Pressure:

The Earth is surrounded by a cover of air called the atmosphere. It extends to a few hundred kilometers above sea level. Just as certain sea creatures live at the bottom of the ocean, we live at the bottom of a huge ocean of air. Air is a mixture of gases. The density of air in the atmosphere is not uniform. It decreases continuously as we go up. Atmospheric pressure acts in all directions.

Experiment:

Take an empty tin can with a lid. Open its cap and put some water in it. Place it over the flame. Wait till water begins to boil and the steam expels the air out of the can. Remove it from the flame. Close the can firmly by its cap. Now place the can under tap water. The can will squeeze due to atmospheric pressure.

When the can is cooled by tap water, the steam in it condenses. As the steam changes into water, it leaves a space behind it. This lowers the pressure inside the can as compared to the atmospheric pressure outside the can. This will cause the can to collapse from all directions. This experiment shows that atmosphere exerts pressure in all directions.

The fact can also be demonstrated by collapsing of an empty plastic bottle when air is sucked out of it.

7.8     It is easy to remove air from a balloon but it is very difficult to remove air from a glass bottle why?

Ans:    This is because the air inside the balloon is a fairly high pressure than the atmospheric pressure air outside the balloon. On the other hand air pressure inside the glass bottle is already equal to the atmospheric pressure so it is difficult to remove air from a glass bottle.

7.9     What is a barometer?

Ans:   Measuring Atmospheric Pressure:

At sea level, the atmospheric pressure is about 101,300 Pa or 101,300 Nm – 2.

Barometer:

The instruments that measure atmospheric pressure are called barometers. One of the simple barometers is a mercury barometer. It consists of a glass tube 1meter long closed at one end.

Construction:

After filling it with mercury, it is inverted in a mercury trough. Mercury in the tube descends and stops at a certain height. The column of mercury held in the tube exerts pressure at its base. At sea level, the height of the mercury column above the mercury in the trough is found to be about 76 cm. The pressure exerted by 76 cm of the mercury column is nearly 101,300 Nm – 2 equals to atmospheric pressure. It is common to express atmospheric pressure in terms of the height of the mercury column. As the atmospheric pressure at a place does not remain constant, hence, the height of the mercury column also varies with atmospheric pressure.

7.10    Why water is not suitable to be used as the barometer?

Ans:     Mercury is 13.6 times denser than water. Atmospheric pressure can hold a vertical column of water about 13.6 times the height of the mercury column at a place. Thus, at sea level, the vertical height of the water column would be 0.76 m x 13.6 = 10.34 m. Thus, a glass tube more than 10 m long is required to make a water barometer.

Following are the reasons why mercury and not water is used in a barometer:

• Mercury is relatively denser than water, consequently, the length of the column of water would have to be about 34 feet high to exert pressure equal to that of the atmosphere while the column of mercury needs to be only 30 inches to exert pressure equal to that of pressure.
• Mercury ha a very low vapour pressure when compared to that of water. So, it is more sensitive than water to change in the atmospheric pressure and rises more quickly to record the changes in atmospheric pressure.
• Mercury’s freezing point is much lower than that of water’s so it can record the atmospheric pressure at temperatures below that of 0 degrees centigrade.
• Mercury does not evaporate easily so very little mercury vapour enters the vacuum above the mercury in the tube.
• Mercury is a metal shines brightly and so can be used to read the markings on the tube easily.

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

Ans:    The sucker is dish-shaped, when pressed against a smooth surface the air is forced beneath the sucker. The rubber makes an airtight seal and the air pressure outside is greater than the air pressure beneath the sucker, thus forcing rubber sucker to ‘stick’

7.12    Why does atmospheric pressure vary with height?

Ans:    Atmospheric pressure reduce with altitude for two reasons-both related to gravity.

• The gravitation attraction (g) between the earth and air molecule is greater for those molecules nearer to earth than those further away – they have more weight – dragging them close together and increasing the pressure (force per unit area) between them.
• Molecules further away from the earth have less weight (because gravitational attraction is less) but they are also ‘standing’ on the molecules below them, causing compression. The lower down have to support more molecules above them and are further compressed (pressurized) in the process.

Note:  It is gravitational force minus the effect of the Earth’s spin (an effect that is greatest at the equator)

7.13     What does it mean when the atmospheric pressure at a place falls suddenly?

Ans:     The changes in atmospheric pressure at a certain place indicate the expected changes in the weather conditions of that place. For example, a gradual and average drop in atmospheric pressure means a low pressure in a neighbouring locality. Minor but rapid fall in atmospheric pressure indicates a windy and showery condition in the nearby region. A decrease in atmospheric pressure is accompanied by breeze and rain. Whereas a sudden fall in atmospheric pressure often followed by a storm, rain and typhoon to occur in a few hours’ time.

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

Ans:     On the other hand, increasing atmospheric pressure with a decline, later on, predicts an intense weather condition. A gradual large increase in the atmospheric pressure indicates a long spell of pleasant weather. A rapid increase in atmospheric pressure means that it will soon be followed by a decrease in the atmospheric pressure indicating poor weather ahead.

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.

Experiment:

It can be demonstrated with the help of a glass vessel having holes all over its surface.

Fill it with water. Push the piston. The water rushes out of the holes in the vessel with the same pressure. The force applied to the piston exerts pressure on the water. This pressure is transmitted equally throughout the liquid in all directions.

In general, this law holds good for fluids both for liquids as well as gases.

Applications of Pascal’s law:

Pascal’s law finds numerous applications in our daily life such as automobiles, hydraulic brake system, hydraulic jack, hydraulic press and another hydraulic machine.

7.16    Explain the working of a hydraulic press.

Ans:    Hydraulic Press:

A hydraulic press is a machine which works on Pascal’s law. It consists of two cylinders of different cross-sectional areas. They have fitted with pistons of cross-sectional areas a and A.

The object to be compressed is placed over the piston of large cross-sectional area A. The force F1 is applied to the piston of the small cross-sectional area a. The pressure P produced by the small piston is transmitted equally to the large piston and a force F2 acts on A which is much larger than F1.

Pressure on the piston of the small area a is given by

= F1 / a

Apply Pascal’s law, the pressure on the large piston of area A will be the same as on small piston.

= F2 / A

Comparing the above equations, we get

F1 / a = F2 / A

F2 = A x F1 / a

Or    F2 = F1 A/ a    ………………..(i)

Notes:   Since the ratio A/a is greater than 1, hence the force F2 that acts on the larger piston is greater than the force F1 acting on the smaller piston. Hydraulic systems working in this way are known as force multipliers.

7.17     What is meant by elasticity?

Ans:     The property of a body to restore its original size and shape as the deforming force ceases to act is called elasticity.

Due to elasticity, we can determine the strength of a material and the deformation under the action of a force.

7.18     State Archimedes Principle.

Ans:     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.

Explanation:

Consider a solid cylinder of cross-sectional area A and height h immersed in a liquid.

Let h1 and h2 be the depths of the top and bottom faces of the cylinder respectively from the surface of the liquid.

Then          h2 – h1              =          h

If P1 and P2 are the liquid pressures at depths h1 and h2 respectively and r is its density, then according to the equation

P1    =    p g h1

P2     =    p g h2

Let the force F1 is exerted at the cylinder top by the liquid due to pressure P1 and the force F2 is exerted at the bottom of the cylinder by the liquid due to P2.

\                  F1          =       P1 A            =   p g h1A

And              F2          =       P2 A            =   p g h2A

F1 and F2 are acting on the opposite faces of the cylinder. Therefore, the net force F will be F2 – F1 in the direction of F2. This net force F on the cylinder is called the upthrust of the liquid.

\                   F2 – F1            =        p g A (h2 – h1)

Or     Up thrust of liquid       =      p g (A h)

(  Ah = V)

Or                                          =      p g V

=      (pV) g        (pV = m )

=       mg                   ……………..(i)

Here Ah is the volume V of the cylinder and is equal to the volume of the liquid displaced by the cylinder. Therefore,  rgV is the weight of the liquid displaced. Equation (i) shows that an upthrust acts on the body immersed in a liquid and is equal to the weight of the liquid displaced, which is the Archimedes principle.

7.19     What is upthrust? Explain the principle of floatation.

Ans:     Upthrust:

Upthrust is a force that pushes an object up and makes it seem to lose weight in a fluid. (Remember, a fluid means a liquid or a gas).

The upthrust, or buoyancy, keeps ships afloat. The upthrust, or buoyancy, keep swimmers on top of the water.

Principle of floatation:

An object sinks if its weight is greater than the upthrust acting on it. An object floats if its weight is equal or less than the upthrust. When an object floats in a fluid, the upthrust acting on it is equal to the weight of the object.

In the case of a floating object, the object may be partially immersed. The upthrust is always equal to the weight of the fluid displaced by the object. This is the principle of floatation. It states that:

A floating object displaces a fluid having a weight equal to the weight of the object. Archimedes principle is applicable to liquids as well as gases.

7.20     Explain how a submarine moves up the water surface and down into the water.

Ans:     Ships and submarines:

A submarine can travel over as well as underwater. It also works on the principle of floatation. It floats over water when the weight of water equal to its volume is greater than its weight. Under this condition, it is similar to a ship and remains partially above water level. It has a system of tanks which can be filled with and emptied from seawater. When these tanks are filled with seawater, the weight of the submarine increases. As soon as its weight becomes greater than the upthrust, it dives into the water and remains underwater. To come upon the surface, the tanks are emptied from seawater.

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

Ans:    it is due to Archimedes principle. The density of ship is less, it displaces more liquid, experience more upward thrust and floats whereas the density of stone is more, it displaces less liquid experience less upward thrust and sinks.

7.22    What is Hook’s law? What is meant by elastic limit?

Ans:    Hook’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.

Thus                   stress      ∝        strain

Or                      stress      =      constant  x  strain

Or                      stress / strain =   constant

Elastic Limit:

The greatest stress that can be applied to a material without causing permanent deformation is called the elastic limit.

The stress point at which a material, if subjected to higher stress, will no longer return to its original shape. Brittle materials tend to break at or shortly past their elastic limit, while ductile materials deform at stress levels beyond their elastic limit.

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

Ans:     We know that the length of a rubber band increases on stretching it. Similarly, the pointer of a spring balance is lowered when a body is suspended from it. It is because the length of the spring inside the balance increases depending upon the weight of the suspended body.

A rubber band scale will be fairly accurate, but only for a short time. Eventually, the rubber band will begin to stretch and wear out. A better scale may be made by substituting a metal spring for the rubber band. Such a scale will be just as accurate, and because the spring is made of metal, it will last much longer.