Properties of Matter - Physics For Class IX (Science Group) - Question Answers

Go To Index

Physics For Class IX (Science Group)
UNIT 7: PROPERTIES OF MATTER
Questions Answers

Q.1: Define matter? What are states of matter. Describe properties of each state in terms of their shape, volume, density and compressibility?
Ans: MATTER:
Anything which occupies space and having mass is called matter.

STATES OF MATTER:
There are three states of matter. These states are:

1. Solid
2. Liquid and
3. Gas

Example:
All the material objects around us belong to any one of these states.
(a) Water is the best example of three states of matter.

1. The solid state of water is ice.
2. The liquid state is water itself.
3. The gaseous state of water is steam.

(b) Human body is also an example of three states of matter.
 It consists of:

1. Solid in the form of organs.
2. Liquid in the form of blood.
3. Gas in the form of Oxygen and carbon dioxide for respiration.

PROPERTIES OF MATTER:

The properties of matter in these states can be described on the basis of the forces and distances between their molecules and energy of the molecules. These states have different properties which are:

States of Matter	Solid	Liquid	Gas
Shape	Fixed	Not fixed	Not fixed
Volume	Fixed	Not fixed	Not fixed
Density	High	High	Low
Compressibility	Incompressible	Incompressible	Compressible

Q.2: Water is the best example of three states of matter. Explain?
Ans: Water Is The Best Example Of Three States Of Matter:

• As Solid: The solid state of water is ice. Ice exists in many forms like, ice cubes, snow, glaciers and icebergs.
• As Liquid: The liquid state is water itself. Water is found in oceans, rivers and underground deposits.
• As Gasous: The gaseous state of water is steam. The “white smoke” in fact, a small cloud formed by water vapours in air above the cup.

Water is different from other substances because it is less dense in its solid state (ice), than its liquid state (water).

Q.3: How can a matter change its state? Define melting, boiling, condensing, freezing and evaporation in term of these changes?
Ans: MATTER CHANGE ITS STATE:
Addition or removal of a certain amount of energy can change the state of a matter. Conversion of matter between three states involves physical changes and not chemical changes.
The terms for these changes in the state are:

• Melting: conversion from solid to liquid.
• Boiling: conversion from liquid to gas.
• Condensing: conversion from gas to liquid.
• Freezing: conversion from liquid to solid.
• Evaporation: conversion from liquid to gas.

Q.4: Define evaporation?
Ans: EVAPORATION:
Evaporation is different from boiling. Evaporation is a process by which a liquid becomes a gas at temperatures below its boiling point.
For example, drying of wet clothes, drying of wet floor etc.

Q.5: State kinetic molecular model of matter. Explain the kinetic model in terms of forces between particles.

OR

Describe kinetic molecular theory or theory of matter. Also write down properties of solid, liquids and gases by considering the position and motion of molecules.
Ans: KINETIC MOLECULAR THEORY (MODEL) OR THEORY OF MATTER:
Kinetic molecular theory of matter explained the movement of particles in three states of matter. It is also explained the arrangement of the particles in these states.
Statement:
The kinetic molecular model of matter is,

"Matter is made up of tiny particles called atoms, or group of atoms called molecules. These molecules are always in continuous random motion."

The evidence of molecular motion is Brownian motion.
According to this model particles are in continuous motion. Thus an alternative name for model is 'The particle model of matter'.
The kinetic molecular theory explains the physical properties of solids, liquids and gases by considering the position and motion of molecules.

Properties Of Solids:
Position:

• The particles in solids have following features:
• The molecules are closely packed together and occupy minimum space.
• The molecules usually arranged in a regular pattern called lattice.
• There is a large number of particles per unit volume. That is why solids have the highest densities.

Movement Of Molecules:

• The movement of particles in solids have following features:
• The forces of attraction between particles are very strong.
• The particles are not able to change positions.
• The particles vibrate about fixed positions thus are not entirely stationary.
• This explains why solids have fixed shapes and volumes.

Properties Of Liquids:
Position:

• The particles in liquids have following features:
• The molecules are slightly further apart compared to that of solids.
• The molecules occur in clusters.
• There is slightly less number of particles per unit volume compared to solids.
• This is why liquids have relatively high densities.

Movement Of Molecules:

• The movement of particles in liquids have following features:
• The forces of attraction between particles are strong.
• The particles are free to move about within the liquid.
• These features explain why liquids have fixed volumes, but take the shape of the container.

Properties Of Gas:
Position:

• The particles in gases have following features:
• The molecules are very far apart.
• The molecules are arranged randomly and are free to move with very high speeds.
• There is small number of particles per unit volume.

Movement Of Molecules:

• The movement of particles in gases has following features:
• The forces of attraction between particles are negligible.
• The particles are able to move freely in random directions at very high speeds.
• The particles occupy any available space.

Q.6: Define the following:

• Brownian motion
• Lattice

Ans: BROWNIAN MOTION:
The evidence of molecular motion first discovered by the botanist Robert Brown in 1827. He observed the irregular motion of pollen grains suspended in water and deduced that the water molecules were in constant, random motion. This irregular motion caused by water molecules is called “Brownian motion” named after the scientist.

LATTICE:
The molecules are usually arranged in a regular pattern in solids, are called a lattice.

Q.7: Why some materials are solid and liquid while others are gases at room conditions?
Ans: The reason for the different states of matter is that they all have different elemental properties. Such as:
Gases Materials At Room Temperature:

• Forces between the molecules are responsible for the different states of matter as well as for the physical properties. 
• According to the Kinetic molecular model molecules of gases have large kinetic energy as a result there are no forces of attraction between them.
• So the molecules of gases can move freely and go farther apart.
• This is why gases can occupy any available space and can be compressed easily.
• Boiling and melting points of gases are also very low because of this reason.

Liquid Materials:

• The molecules of liquids as compared to that of the gases have less kinetic energy hence inter-molecular forces occur among them.
• That is why the molecules of liquids are very close to each other but still free to move.
• Therefore liquids do not have fixed shape but fixed volume.
• The melting and boiling points of liquids are also high as compared to gases.

Solid Materials:

• The molecules of solids have extremely lowest energies therefore experience strong attractive forces and can not move freely but only have small vibrations about mean positions.
• This gives solid a fixed shape and volume.
• That is why densities, melting and boiling points of solids are very high.

Conclusion:

• As a result we are able to convert water into ice,
• Cream into ice cream,
• Natural gas into compressed natural gas 'CNG' etc.

Q.8: The state of a substance can be changed either by heating or by cooling it. Explain?
Ans: Three States Of Matter Are Interchangeable:
The state of a substance can be changed either by heating or by cooling it.
By Heating:

• Solids To Liquids:
  When a solid substance is heated, the molecules start to vibrate more and more strongly. Eventually, the molecules vibrate more violently and inter molecular forces become weak. As a result 'material becomes a liquid.
  
  
• Liquids To Gases:
  If process of heating is continued further, then the molecules of liquid have sufficient energy to overcome all of the attractive forces as a result 'substance becomes a gas'.

By Cooling:

• Gases To Liquid:
  When a gas is cooled, the molecules move more slowly and collide with one another, may stick together and force of attraction between molecules increases. Keep cooling the gas and eventually all of the molecules stick together to form a liquid.
  
  
• Liquid To Solids:
  Further cooling will cause all of the liquid molecules to stick together to form a solid.

Example:
Helium has lowest boiling and melting points as compared to other substances. It solidifies only when it is cooled and compressed.
Mercury is the only metal that is not solid at room temperature.

Q.9: Describe the properties and behaviour of gases in the light of the kinetic theory?
Ans: BEHAVIOR GASES AND THE KINETIC THEORY:
Kinetic molecular theory clearly describes the properties and behavior of gases.
Hot air balloons are the practical applications of this behavior.
The behavior of gases

1. The molecules in the gases have relatively large distance between them.
2. The molecules in the gases move about very quickly.
3. A gas molecule moves in a straight line.
4. It changes its direction only when:
   (i) it collides with another gas molecule or
   (ii) with the walls of its container. After collision it moves away in a new direction.
5. Since gas molecules collide many times each second.

Therefore the motion of molecules is constant and random. The behavior of a gas can be described completely by its pressure, volume and temperature.

Q.10: Define pressure? Describe behaviour of gas by its pressure. OR Describe pressure of a gas?
Ans: PRESSURE:
Pressure is defined as the force per unit area.
Pressure Of A Gas:
During Collision Of molecules:
All the gases exert pressure on the walls of their container. This pressure is the total force exerted per unit area by the gas molecules during collision. The gas molecules exert pressure only when they collide with the walls. The number of collisions is proportional to the number of molecules. If the number of molecules is doubled then number of collisions will also be doubled. Hence the pressure is also doubled.

Example:
Blowing up a balloon is an example of pressure.
If more air is pushed into the balloon it will be inflated more. Because air molecules apply pressure on the rubber walls of balloon hence it gets inflated.

By Compressing A Gas:
Pressure of a gas can also be increased by compressing it. This is done by reducing the size of the gas container. The gas molecules have been compressed into a smaller volume so they will collide more frequently with the walls of container and creates more pressure. If the gas is compressed to half its original volume its pressure will be doubled.

Q.11: Define volume? Describe the volume of a gas OR Describe behaviour of gas by its Volume?
Ans: VOLUME:
The space occupied by substance is known as volume.
Volume Of A Gas:
No Fixed Volume:
The gas has no definite volume because the molecules of the gas are far away from each other and can move freely at high speeds. Therefore gas always takes up the shape and volume of its container.
For example:
The smell of a perfume quickly spreads through the room as soon you spray it at your body or clothes. Because, the molecules move freely and randomly at high speeds through out the room.
Decrease Volume By Compression:
Volume of a gas can also be increased by decreasing its pressure. This could be done by reducing the load on the piston of the gas container. As the gas molecules are in random motion, they quickly covers the whole space and the volume increases. If the gas is compressed to half its original volume its pressure will be doubled.

Q.12: Describe temperature of a gas? OR Describe behaviour of gas by its temperature? OR What is the effect of temperature on the average translational kinetic energy of molecules?
Ans: TEMPERATURE:
The temperature of a gas is determined by the average translational kinetic energy of its molecules. If a gas is heated the average translational kinetic energy of its molecules increases and temperature of the gas rises. If a gas is cooled down the average translational kinetic energy of its molecules decreases and temperature of the gas falls.

Q.13: State and explain Boyle's law? OR Pressure-Volume relationship in gases.
Ans: BOYLE'S LAW:
(Pressure - volume relationship in gases)
Robert Boyle, an English physicist and chemist in 1662, studied the relationship between pressure and volume of a gas.
Statement:
Robert Boyle conclude his law known as 'Boyle's law' which states that;
"The volume of a fixed mass of a gas is inversely proportional to its pressure, provided its temperature remains constant."
Mathematically,
The results of a Boyles experiment are shown below:

P ∝ ¹/_V Or

V ∝ ¹/_P Or

PV = Constant

where,
P = Pressure of a gas
V = Volume of a gas

Expalanation:

1. If pressure of the gas is double its volume becomes half. If pressure increases by three times then volume becomes one-third and so on.
2. The graph between “p” and “V” is in the shape of hyperbola, while the graph between “p” and 1/V” is in straight line.
3. The graph between “p” and “V” shows that if pressure increases then volume decreases and vice-versa, i.e.
4. The graph between “p” and “1/V” shows a straight line passing through the origin.
   
   
   
   
5. At constant temperature the product of pressure and volume is constant. i.e.
   
   pV = constant ............. (i)
6. Using above result, at constant temperature, we can write:
   
   Initial pressure x initial volume = Final pressure x final volume
   P₁V₁ = P₂2_V ................ (ii)

In above equation,we use the same units for both values of p ( for example Pa, kPa or atmosphere etc), and the same units for both values of V ( for example m³, dm³ or cm³ etc)
Thus, Robert Boyle conclude his law known as 'Boyle's law'.

Q.14: Write down the applications of pressure-volume (p-V) relationship of a gas OR Boyle's law?
Ans: Applications of (p-V) relationship of a gas OR “Boyle's law”:
Some applications of pressure-volume (p-V) relationship of a gas i.e. Boyle's law are:

1. Aerosols, such as spray paints, use the Boyle’s law in their working mechanism.
2. A bicycle pump is good example of Boyle’s law.
   As the volume of the air trapped in the pump is reduced, its pressure goes up, and air is forced into the tyre.
3. Syringe: When fluid are drawn into a syringe, The volume inside the syringe is increased, the pressure decreases on the inside of syringe. The pressure n the outside of the syringe is greater, therefore fluids are forced into the syringe.
4. Lungs: When we breathe, our diaphragm moves downward, increasing the volume of the lungs. this cause the pressure inside the lungs to be less than the outside pressure so air rushes in.

Give Reasons:

Q.1) A balloon kept under sunlight shattered, why?
When the balloon is kept in the sun, due to Sun's heat, the kinetic energy of gaseous particles inside the balloons also gets increased and the balloon expands. This will increase the pressure on the walls of the balloon. It continues to expand and comes to a stage when the balloon bursts.

Q.2: Why a hot coffee or tea in a cup became cold as the time passes?
The heat from the tea is transferred to the surroundings through a process called convection. The nearby air molecules will collide with the tea in the cup and take away its heat energy and transfer it through the surrounding air. This is the reason hot tea becomes cold after some time.

Q.3: Clothes dry up quickly under sunlight?
Warm water evaporates much faster than cool water. So, the heat from the sun dries the clothes quickly.

Q.4: Honey is thicker than water, why?
In water, only water molecules are there. So, the attraction between the molecules (inter-molecular force) is not much high. Honey contains many types of molecules. Therefore they have more inter-molecular forces between the molecules, so that the layers of liquid cannot move freely upon the other layers. That results in the resistance to the deformation in the fluid. So, we can say that due to more inter-molecular forces, honey is more thicker than water.

Q.5: Why do water and milk or other liquids boil at different temperatures?
Different liquids have different boiling points depending on the strength of bonding between the particles and the mass of the particles. The heavier the particles in the liquid, and the stronger the bonding, the higher the boiling point will be. This is the reason water and milk or other liquids boil at different temperatures.

Q.6: Why do water and milk take the shapes of the container in which they are poured? OR Why do different substances boil and melt at different temperatures?
water and milk are liquid state of matter. As the particles in a liquid are close together, but they are not bound to fixed positions; they can slide past and around each other. This enables liquids to take the shape of their container and to flow when they are poured.

Q.7: Why liquids and gases take the shapes of their containers while solids have definite shapes?
In liquids, the forces of attraction between particles are strong and these particles are free to move about within the liquid. So, the liquids have fixed volumes, but take the shape of the container. Similarly in gases, the forces of attraction between particles are negligible and these particles are able to move freely in random directions at very high speeds. thus particles occupy any available space.
But in solids, the forces of attraction between particles are very strong. and they are not able to change positions. Their particles vibrate about fixed positions thus are not entirely stationary. That's why solids have fixed shapes and volumes.

Q.8: Explain why:
(i) Solids have the highest densities.
Ans: There are a large number of particles per unit volume. That is why solids have the highest densities.

(ii) Solids have fixed shapes and volume.
Ans: in solids, the forces of attraction between particles are very strong. and they are not able to change positions. Their particles vibrate about fixed positions thus are not entirely stationary. That's why solids have fixed shapes and volumes.

(iii) Liquids have relatively high densities.
Ans: There is slightly less number of particles per unit volume compared to solids. This is why liquids have relatively high densities.

Q.9: Why dose a gas has neither a fixed shape nor a fixed volume?
Ans: The molecules of a gas keep moving very fast in all directions. They are far from one another. Thus the cohesive forces are insignificant and the gases do not have a fixed shape or a fixed volume. They occupy all the space available to them.

Q.10: Its is easy to compress air as compared to water. Why?
In gases the molecules are far apart from each other. There are a lot of empty spaces between gas molecules. As air is a mixture gases so it can be compressed quite easily . Whereas water, is a liquid and its molecules are closer then air. Liquid are incompressible. That is why it is easy to compressed air as compared to water.

Q.11: Why can gases be compressed easily while solids and liquids?
In gases the molecules are far apart from each other. There are a lot of empty spaces between gas molecules. So it can be compressed quite easily. While solids and liquids,have high force of attraction among molecules so they are closer then gas molecules and incompressible. That is why it is easy to compressed gas as compared to solids and liquids.

Q.12: Why did the smell of perfume quickly spread through the room.
Ans: The smell of perfume quickly spreads through the room as soon we spray it at our body or clothes because the molecules move freely and randomly at high speeds throughout the room.