CBSE Class 9

Chapter 1 Matter in Our Surroundings Class 9 Science Important Questions with Answers PDF will help you in scoring more marks in your exams.

Matter in Our Surroundings Class 9 Important Questions Science Chapter 1

Question 1:
Answer the following questions.

1. Why is ice at 273 K more effective in cooling than water at the same temperature ?
2. Name the two gases which are supplied in compressed form in homes and hospitals.
3. What is dry ice ? (CBSE 2010, 2011)

Answer:

1. The three states of matter are : solid, liquid and gas. Out of these, the solid state is the most rigid since the particles present are very closely packed and interparticle forces are quite strong.
2. LPG (Liquid Petroleum Gas) is supplied in homes while liquefied oxygen in hospitals
3. Solid carbon dioxide (CO₂) is known as dry ice. It is so named as it does not wet a piece of paper or cloth.

More Resources

• Previous Year Question Papers for CBSE Class 9 Science
• NCERT Solutions for Class 9 Science
• NCERT Exemplar Solutions for Class 9 Science
• Value Based Questions in Science for Class 9
• HOTS Questions for Class 9 Science

Question 2:
CO₂ is a gas. Justify the given statement by two reasons. (CBSE 2011)
Answer:

1. CO₂ does not have a fixed volume. It can be compressed on applying pressure.
2. CO₂ does not have a fixed shape. It can take the shape of any container in which it is put.

Question 3:
What is a pure substance ? Give its one characteristic. (CBSE 2011)
Answer:
A pure substance is the one which cannot be separated into smaller parts by any physical methods. It may be either a pure element (e.g. sodium) or a pure compound (e.g. calcium carbonate).

Question 4:
What happens when you pour some acetone on your palm ? (CBSE 2010, 2011)
Answer:
Acetone is a very low boiling liquid. It immediately changes to vapours. The evaporation is an endothermic process. Acetone takes up heat energy from hand. Therefore, the palm immediately becomes cold or even numb.

Question 5:
Name SI unit of measuring temperature. The boiling point of water is 100°C under normal atmospheric pressure. Convert this temperature to SI Units. (CBSE 2011, 2016)
Answer:
SI units of temperature is Kelvin (K)
100°C = (100 + 273) = 373 K.

Question 6:
Give reasons for the following :
(a) Gases fill up completely the vessel in which they are kept.
(b) Gases exert pressure on the walls of the containing vessel. (CBSE 2011, 2012, 2014, 2016)
Answer:
(a) Particles constituting gases are very fast moving and diffuse at a fast speed. They therefore, readily fill up the vessel completely in which these are kept.
(b) Particles of gases collide against the walls of the containing vessel and impart momentum to them. This is responsible for the pressure of the gas.

Question 7:
Name three states of matter. Which state of matter is rigid and why ? (CBSE 2011, 2013)
Answer:
The three states of matter are : solid, liquid and gas. Out of these, the solid state is the most rigid since the particles present are very closely packed and interparticle forces are quite strong.

Question 8:
What is meant by particulate nature of matter ? List four characteristics of particle nature of matter.
Answer:
In order to illustrate the particle nature of matter, take some water in a glass beaker. To this add one spoon full of some salt (e.g., sodium chloride) and stir with a glass rod.
The important characteristics of the particulate nature of matter may be summed up as follows :

1. Every matter is made up of particles.
2. The particles constituting a matter are very small in size.
3. The particles have empty or vacant spaces in them known as interparticle spaces.
4. Particles are not stationary and are in a state of motion.
5. Attractive forces are present in the particles of a substance. These are called interparticle forces.
6. The particle motion increases with the rise in temperature.

Question 9:
Why do people sprinkle water on a roof after a hot summer day ? (CBSE 2011, 2016)
Answer:
Water has a large heat of vaporisation. In a hot summer day, the roof is quite hot. Water absorbs a large amount of heat from the roof for its vaporisation. The roof gets sufficiently cool and one can sleep comfortably.

Question 10:
List three characteristics of particles of matter. Describe one example for each characteristic to illustrate it.
Name the characteristics which are responsible for
(a) spreading of smell of scent in a room and
(b) water taking the shape of the vessel in which poured. (CBSE 2011)
Answer:
For the three characteristics of matter and their examples,
(a) Spreading of smell of scent in a room is because of diffusion of the gases. Scent consists of a number of sweet smelling gases or vapours.
(b) Water is a liquid and liquids take up the shape of any container in which these are kept.

Question 11:
The temperature-time graph given below shows the heating curve for pure wax. From the graph answer the following :

1. What is the physical state of the substance at the points A, B, C and D ?
2. What is the melting point of the substance ?
3. What is its boiling point ?
4. Which portions of the graph indicates that change of state is taking place.
5. Name the terms used for heat absorbed during change of states involved in above processes. (CBSE 2011)

Answer:

1. At point A : Wax is in the solid state.
   At point B : Wax has started melting and exists both in the solid as well as liquid states.
   At point C : Wax is in liquid state.
   At point D : Wax has started boiling. Therefore, it exists both in the liquid as well as gaseous states.
2. Melting point of wax = 15°C
3. Boiling point of wax = 110°C
4. The change of state (solid to liquid) is represented by the portion A₁ to B₂ (straight line).
   The change of state (liquid to gas) is represented by the portion D₁ to D₂ (straight line).
5. It is known as latent heat of fusion in case of solids and latent heat of vaporisation in case of liquids.

Question 12:
Explain which one will cause more severe burns—boiling water at 100°C or steam at 100°C. (CBSE 2011, 2012)
Answer:
In order to illustrate the particle nature of matter, take some water in a glass beaker. To this add one spoon full of some salt (e.g., sodium chloride) and stir with a glass rod.
The important characteristics of the particulate nature of matter may be summed up as follows :

1. Every matter is made up of particles.
2. The particles constituting a matter are very small in size.
3. The particles have empty or vacant spaces in them known as interparticle spaces.
4. Particles are not stationary and are in a state of motion.
5. Attractive forces are present in the particles of a substance. These are called interparticle forces.
6. The particle motion increases with the rise in temperature.

Question 13:
On suffering from high fever, which will lower your body temperature more; ice or ice cold water ?
(CBSE 2012, 2014)
Answer:
Ice will lower the body temperature more than ice cold water because latent heat of fusion of ice is quite high (335 kj kg^-1). Ice is therefore, expected to absorb more heat energy from the body and will lower the body temperature more than ice cold water.

Question 14:
Flow will you change water from gaseous state to liquid state ? Suggest a simple activity. (CBSE 2011)
Answer:
Pass the water vapours or steam through a water condenser as used in case of simple distillation. It gets condensed to form liquid water.

Question 15:
Justify that melting of wax is a physical change. (CBSE 2011)
Answer:
It can be justified in two ways :

1. No new substance is formed and there is no change in the chemical properties of wax as a result of melting.
2. When the liquid wax is cooled for sometime, it again gets solidified.

Question 16:
Ramesh took two beakers A and B containing hot water and cold water respectively. In each beaker, he dropped a crystal of copper sulphate. He kept the beakers undisturbed. After sometime what did he observe and why ? (CBSE 2011, 2012)
Answer:
In both the beakers, the solutions became blue. However, this happened at a faster rate in the beaker B which contains hot water. This change has taken place because of the process of diffusion which proceeds at a faster rate in hot water as compared to cold water.

Question 17:
Archit dropped a crystal of potassium permanganate into two beakers A and B containing hot water and cold water respectively. After keeping the beakers undisturbed for some time what did he observe and why ?
(CBSE 2012)
Answer:
Potassium permanganate crystals have purple colour. In a beaker B containing hot water, the purple colour would spread more readily as compared to the beaker A which contains in it cold water. Actually the kinetic energy of the particles increases with the rise in temperature.

Question 18:
Account for the following ;

1. When sugar crystals dissolve in water, the level of water does not rise appreciably.
2. Naphthalene balls disappear with time without leaving any solid residue.
3. A wooden table should be called a solid.
4. Dogs generally hang out their tongue in summer. (CBSE 2012)

Answer:

1. Since water is a liquid, there are intermolecular spaces. Sugar particles occupy these spaces. As a result, water
   level does not rise appreciably,
2. A pure substance is the one which cannot be separated into smaller parts by any physical methods. It may be either a pure element (e.g. sodium) or a pure compound (e.g. calcium carbonate).
3. For the three characteristics of matter and their examples,
   1. Spreading of smell of scent in a room is because of diffusion of the gases. Scent consists of a number of sweet smelling gases or vapours.
   2. Water is a liquid and liquids take up the shape of any container in which these are kept.
4. In summer, dogs get exhausted due to the loss of persipiration from their body. Since they are always running most of the time, they hang out their tongue.

Question 19:
(a) 5 mL of water was taken in a test tube and china dish separately. These samples were then kept under different conditions as below :
(i) Both the samples are kept under a fan.
(ii) Both the samples are kept inside a cup board.
State in which case evaporation will be faster ? Give reason to support your answer.
(b) How will the rate of evaporation change if above activity is carried out on a rainy day. Justify your answer.
(CBSE 2012)
Answer:
(a) Evaporation will be faster under a fan. Since fan helps in the fast movement of the air, water will get more opportunity to evaporate under a fan than inside a cup board. In this case, the outside air will not come in contact with water.
(b) On a rainy day, there is humidity in air. As a result, evaporation of water will slow down.

Question 20:
A rubber band can change its shape on stretching; will you classify it as solid or not ? Justify your answer.
(CBSE 2012)
Answer:
Yes, it can be classified as a solid. It is an elastic solid which changes its shape on stretching and regains the same when the stretching force is removed.

Question 21:
When a solid melts, its temperature remains constant. Explain. (CBSE 2012)
Answer:
The heat energy supplied once the solid starts melting is used up as latent heat of fusion. The melting point temperature of a solid gives an idea of the interparticular forces which bind the constituents in a solid. Thus, greater the melting point temperature, more will be the magnitude of intermolecular forces. For example,
Melting point temperature of sodium = 370 K
Melting point temperature of potassium = 336 K
This means that the attractive forces in atoms of sodium in the solid state of the metal are more than the forces that are present in the atoms of potassium also in the same state.

Question 22:
State one similarity and two differences between boiling and evaporation. (CBSE 2012, 2014, 2015)
Answer:
Similarity : In both evaporation and boiling, liquid changes to vapour state.
Differences :

1. Evaporation takes place from the surface while boiling occurs throughout the liquid.
2. Liquid can evaporate at all temperatures while boiling occurs only at a fixed temperature known as the boiling point temperature.

Question 23:
Draw a flow sheet diagram to illustrate interconversion of three states of matter. Name the process of each interconversion. (CBSE 2012)
Answer:
It is because of the change in interparticle spaces and inter particle forces. If a solid is to be converted into liquid, the interparticle spaces have to be increased. Similarly, if a liquid is to change to the gas, these spaces have to be further increased.

In other words, we can say that one state of a substance can be converted into the other by changing interparticle spaces.They actually change interparticle forces of attraction. Please note that the process can be reversed also under suitable conditions. This is known as interconversion of states of matter. The obvious question which strikes the minds of everybody is to how to bring about the change of state. There are two ways to achieve this.
By changing the temperature
By changing thé pressure.

Question 24:

1. You want to wear your favourite shirt in a party, but the problem is that it is still wet after a wash. Mention three steps with reason that you would take to dry it faster ?
2. It is a hot summer day. Priyanshi and Ali are wearing cotton and nylon clothes respectively. Who do you
   think would be more comfortable and why ? (CBSE 2012, 2013, 2014, 2016)

Answer:

1. 1. Squeeze the shirt with force. By doing so, some of the moisture is removed.
   2. Spread the shirt on a stand. It provides greater surface area for evaporation.
   3. Iron the shirt. Increase in temperature helps in drying the shirt.
2. Cotton clothes would be more comfortable than the nylon clothes since these are porous. Persipiration sticking to the skin can escape from the pores. Priyanshi would feel more comfortable.

Question 25:
Explain how three states of matter arise due to the variation in the characteristics of the particles.
(CBSE 2013)
Answer:
The three states of matter arise because of two characteristics

1. Interparticle spaces : These are minimum in the solid state and maximum in the gaseous state.
2. Interparticles forces of attraction : These are maximum in the solid state and minimum in the gaseous state.

Question 26:
List any two properties of liquids which are common to gases. (CBSE 2013)
Answer:

1. Both of them do not have any definite shape. They can take up the shape of the container in which these are kept,
2. Both of them show the property of diffusion. However, gases diffuse faster.

Question 27:
How is heating of sugar different from heating of ammonium chloride ? Explain your answer. (CBSE 2013)
Answer:
Sugar melts upon heating whereas ammonium chloride sublimes upon heating without leaving behind any residue.

Question 28:
To which physical state of matter, do the following statements apply ? (CBSE 2013)

1. incompressible, no fixed shape
2. incompressible, high melting point
3. compressible, no definite volume
4. incompressible, highly fluid.

Answer:

1. Liquid state
2. Solid state
3. Gaseous state
4. Liquid state.

Question 29:
A drop of ink and a drop of honey are placed in watel in different beakers. Which of the two will spread faster ? Give reason for your answer. (CBSE 2013)
Answer:
Ink drop will spread faster as compared to honey. The density of honey is more than that of ink. Therefore, particles in ink diffuse faster as compared to the particles in honey.

Question 30:
Suppose you want to convert a gas into a liquid. Which two methods can you apply ? (CBSE 2013, 2016)
Answer:

1. By increasing the pressure or by compressing the gas.
2. By lowering the temperature or by cooling the gas.

Question 31:
Describe an activity to determine the melting point of ice with a diagram. (CBSE 2013, 2016)
Answer:
We know that the melting point of ice and the freezing point of water in pure states are both zero. This means that at this temperature, both are present. Upon heating, the temperature would actually not change but heat energy supplied would be absorbed by the ice as latent heat of fusion. This would result in the melting of ice.

The temperature would remain zero degree till the whole of ice has melted. Further heating would increase the temperature of water till it starts boiling at 100°C. The curve (d) gives the correct representaton.

Question 32:
Explain why do wet clothes dry faster when we spread them out. (CBSE 2013)
Answer:
By spreading wet clothes, the surface area available for evaporation increases. Therefore, wet clothes dry faster.

Question 33:
Describe an activity to show that particles of matter have spaces between them. (CBSE 2014, 2015)
Answer:
The important characteristics of the particulate nature of matter may be summed up as follows :

1. Every matter is made up of particles.
2. The particles constituting a matter are very small in size.
3. The particles have empty or vacant spaces in them known as interparticle spaces.
4. Particles are not stationary and are in a state of motion.
5. Attractive forces are present in the particles of a substance. These are called interparticle forces.
6. The particle motion increases with the rise in temperature.

Question 34:
How will you separate a mixture of naphthalene balls powder and common salt ? Draw a neat and labelled diagram to show the process. (CBSE 2015)
Answer:
We have so far studied that upon heating, a solid initially changes to the liquid state and then to the gaseous state when the temperature is increased. The process or the change of state can be reversed when the temperature is decreased. However, there are some exceptions. Certain solids directly change to the gaseous state upon heating without passing through the liquid state. This is called sublimation. The substance obtained on cooling the vapours is known as sublimate. Sublimation may be defined as :
the change of solid directly into the gaseous state without passing through the liquid state upon heating.

Naphthalene balls, camphor, iodine, ammonium chloride are some common examples of the substances which undergo sublimation.

Question 35:
Define (a) Compressibility (b) Rigidity (c) Fluidity (CBSE 2015)
Answer:
(a) Compressibility: It is the property as a result of which the particles of any matter come closer on applying pressure.
(b) Rigidity: It is the capacity of the particles of a matter to resist a change in shape and size on applying stress.
(c) Fluidity: It is the property as a result of which particles of a matter have tendency to flow.

Question 36:
(a) Define the process of vaporisation.
(b) List four factors which affect the rate of evaporation. (CBSE 2015)
Answer:
(a) vaporisation and may be defined as the amount of heat energy that is needed to convert one kg of a liquid at its boiling point temperature into its vapour state without any rise in temperature.
Latent heat of vaporisation of water is 226 kj kg^-1. It is the amount of heat that is absorbed when one kilogram of water at its boiling point temperature (100°C) changes to vapour state without any further rise in temperature.
(b)

1. Surface area available for evaporation
2. Increase in temperature
3. Decrease in humidity
4. Increase in the speed of wind

Question 37:
State the physical state of water as the following temperatures.
(a) 373 K
(b) 300 K
(c) 200 K (CBSE 2015)
Answer:
(a) Gas or Vapours
(b) Liquid
(c) Ice.

Question 38:
Why does perspiration keep our body cool ? (CBSE 2015)
Answer:
Actually, we perspire a lot in the hot and humid weather. Since cooling is caused during evaporation, the body temperature gets lowered. We feel more comfortable. Now, cotton is of porous nature and is a good absorber of water coming out of the pores as sweat. The synthetic clothes are less porous and donot absorb sweat so quickly. As the sweat evaporates, it absorbs some energy from the body since the clothes are in contact with our skin. The temperature of the body gets lowered and we feel more comfortable. We feel less comfortable in nylon and terylene clothes during summer.

Question 39:
Tabulate three differences between boiling and evaporation. (CBSE 2015)
Answer:

Question 40:
Give reasons for the following observations

1. The smell of lighted incense stick spreads several metres away.
2. A liquid has a fixed volume but not a fixed shape.
3. Ice floats over water.
4. A wooden table is called a solid at room temperature. (CBSE 2015)

Answer:

1. This happens because of diffusion. For example, the smell of food particularly of fish being cooked in the kitchen spreads in the lobby and even in different rooms because of diffusion.
2. Liquids do not have fixed shapes and take up the shape of any container in which these are put. A liquid cannot be compressed on applying pressure. Actually, the interparticle forces in the liquids are so strong that the pressure which is applied is not in a position to overcome these. Liquids therefore keep their volume.
3. Ice (solid state) floats over water (liquid state). Both are chemically same and are made from H2O molecules. Actually, the structure of ice is more porous* as compared to that of water. Therefore, for a given mass, the volume of ice is more than that of water and its density is comparitively less. As a result, ice floats over water.
4. different containers. For example, blue crystals of copper sulphate have needle like shape which they retain whether kept in a beaker or in a china dish or placed on the palm of our hand.

Question 41:
(a) When common salt is added to water, will there be any change in volume ? Give reason.
(b) Write any one similarity between three states of matter. (CBSE 2015, 2016)
Answer:
(a) No, there will be no change in volume. The particles of common salt will occupy inter particle empty spaces present in the molecules of water. The salt will dissolve in water and there will not be any change in the level of the solution thus formed.
(b) All the three states of matter consist of particles which have specific mass.

Question 42:
Out of boiling and evaporation, which is a surface phenomenon ? Explain. (CBSE 2015)
Answer:
Evaporation is a surface whereas boiling once started occurs throughout the liquid. In a liquid, the particles or molecules experience mutual forces of attraction. However, these are not stationary and have some kinetic energy at all temperatures. The particles of a liquid are also colliding with one another and exchanging energy during the collisions. Above the liquid surface, atmosphere or air is present which is a mixture of several gases. The particles of the liquid present on the surface have a tendency to come out from the surface so that they may acquire more freedom to move and become part of the atmosphere. This is also known as randomness. To overcome the interparticle forces of attraction, they need some energy which they take up from the rest of the particles or molecules of the liquid. As a result, their temperature gets lowered and cooling is caused.

Question 43:
Define boiling point. Write down the boiling point of water on Celcius scale and Kelvin scale. (CBSE 2016)
Answer:
The boiling point temperature of a liquid may be defined as :
the temperature at which a liquid starts boiling or the liquid state of a substance changes into gaseous/vapour state.
For example, the boiling point temperature of water is 100°C or 373 K. It is interesting to note that what we have noticed in the melting of a solid, also happens in the boiling of a liquid. It means that once the liquid starts boiling, its temperature does not change although it is still being heated. The explanation is also similar. As long as the liquid has not boiled, the heat energy which is supplied increases the kinetic energy of the particles (or H2O molecules) present in the liquid. As a result, the temperature rises. Once the liquid starts boiling, the heat energy is now being used to bring about a change in state from liquid to gas or vapours. It is known as latent heat of vaporisation and may be defined as :
the amount of heat energy that is needed to convert one kg of a liquid at its boiling point temperature into its vapour state without any rise in temperature.
Latent heat of vaporisation of water is 226 kj kg-1. It is the amount of heat that is absorbed when one kilogram of water at its boiling point temperature (100°C) changes to vapour state without any further rise in temperature.

Question 44:
Liquids and gases are commonly known as fluids. Compare their properties and show that they can flow easily.
(CBSE 2016)
Answer:
Unlike solids, the liquids have fluidity and not rigidity i.e., they have tendency to flow. This is due to lesser interparticle or intermolecular forces that are present in the liquid state as compared to the solid state. However, the liquids differ in their relative fluidity. For example, water flows at a faster rate than honey because in honey, the particles are heavier and also more closely packed.
Gases have maximum fluidity and least rigidity.
Since the interparticle spaces are the maximum in the gaseous state, the attractive forces are the least. As a result, the fluidity is very large while rigidity is negligible.

Question 45:
Tabulate the differences in the states of matter on the basis of the following characteristics :

1. Rigidity
2. Kinetic energy
3. Density. (CBSE 2016)

Answer:

1. Rigidity: We have seen that rigidity is maximum in the solid state and fluidity or particle motion is negligible. In the liquid state of a substance, both these characters are
   different. The liquids are less rigid than the solids and the molecular motion is also comparatively more.
2. Kinetic energy: The kinetic energy is linked with movement of the particles from one place to the other. Since the constituents in the solid state are very closely packed, they have negligible kinetic energy. That is why solids do not flow.
3. Density: Mass occupied by a solid per unit volume and is obtained by dividing the mass of a particular solid by the volume occupied by that mass of the solid.
   The unit of density : kg/L or kg/dm³.

Hope given Previous Year Question Papers for CBSE Class 9 Science Chapter 1 Matter in Our Surroundings are helpful to complete your science homework.

If you have any doubts, please comment below. Learn Insta try to provide online science tutoring for you.

Chapter 4 Structure of the Atom Class 9 Science Important Questions with Answers PDF will help you in scoring more marks in your exams.

Structure of the Atom Class 9 Important Questions Science and Answers Chapter 4

Question 1.
J. Chadwick discovered a sub-atomic particle which has no charge but has mass nearly equal to that of a proton. Name the particle and give its location in an atom. (CBSE 2011)
Answer:
The information suggests that the particle discovered by Chadwick is neutron. It is denoted by the symbol ¹₀n. It is located in the nucleus of an atom along with proton.

More Resources

• Previous Year Question Papers for CBSE Class 9 Science
• NCERT Solutions for Class 9 Science
• NCERT Exemplar Solutions for Class 9 Science
• Value Based Questions in Science for Class 9
• HOTS Questions for Class 9 Science

Question 2.
If ‘K’ and ‘L’ shells of an atom are completely filled, then what would be

1. the total number of electrons in an atom and
2. its valency ? (CBSE 2011)

Answer:
According to Bohr Bury scheme, ‘K’ and ’L’ shells of an atom can have maximum of 2 and 8 electrons respectively. If these are completely filled, this means that :

1. Total number of electrons in the atom = 2 + 8 = 10
2. Valency of the atom = zero.

Question 3.
For chlorine, Z = 17, A = 35. Give the number of protons, electrons and neutrons in

1. chlorine atom
2. chloride ion. (CBSE 2011)

Answer:

1. In chlorine atom (Cl)
   Number of protons (Z) = 17
   Number of neutrons = A- Z = 35-17= 18
2. In chloride ion (CF)
   Number of protons (Z = 17)
   Number of electrons = (Z+1)=17+1 = 18
   Number of neutrons = A – Z = 35 – 17 = 18

Question 4.
List three observations of the experiment performed by Rutherford for his model of an atom.
Answer:
From the scattering experiment, Rutherford made the following observations :

1. Most of the alpha particles were able to pass through the gold foil undeflected.
2. Some of these particles were deflected by small angles.
3. A very few (one out of approximately 12000) alpha particles suffered major deflections and even came back in the same direction.

Question 5.
An atom of an element has two electrons in outermost M-shell. State its
(a) Electronic configuration
(b) Number of protons
(c) Atomic number
(d) Nature whether metal or non-metal
(e) Valency
(f) Name (CBSE 2011)
Answer:
(a) Since the atom has two electrons in outermost M-shell, this means that K and F shells are already filled. Therefore, electronic configuration is 2(K) 8(F) 2(M)
(b) Number of protons = Number of electrons = 12
(c) Atomic number = Number of protons =12
(d) The elements with two valence electrons is a metal
(e) Valency of the element = Number of outermost electrons = 2
(f) The element is magnesium (Mg)
Hydrogen has three isotopes which are written as :

Question 6.
Explain why :
(a) These isotopes have identical chemical properties.
(b) These isotopes are electrically neutral
(c) These isotopes differ in their masses. (CBSE 2011, 2016)
Answer:
(a) The isotopes have identical chemical properties because all of them have one electron in the only shell (K-shell)
(b) The isotopes are electrically neutral because each one has one proton and one electron.
(c) The isotopes differ in their masses because they differ in their mass numbers (1,2 and 3 respectively).

Question 7.
In the following table, the mass number and the atomic number of certain elements are given :

(a) Select the pair of isobars from the above table.
(b) What would be the valency of the element C listed in the above table ?
(c) Which two sub-atomic particles are equal in number in a neutral atom ? (CBSE 2011)
Answer:
(a) Elements D and E are pair of isobars since they have same mass no. = 40
(b) The electronic configuration of the element C with Z = 7 is 2, 5.
It has five valence electrons. Its valency can be either 5 or 3 (8 – 5) = 3.
(c) In a neutral atom, the number of electrons in the extra-nuclear portion is equal to the number of protons in the nucleus.

Question 8.
(a) Why are the chemical properties of the isotopes same ?
(b) Draw Bohr model for helium atom.
(c) What are the number of protons, neutrons and electrons in ⁵⁹₂₇Co and ¹⁰⁸₄₇Ag ? (CBSE 2011)
Answer:
(a) Isotopes have the same atomic number as well as the same electronic configuration. Therefore, their chemical properties are the same.
(b)

(c)

Question 9.
(a) Which popular experiment is shown in the figure ?

(b) List three observations of this experiment.
(c) State conclusions drawn from each observation of this experiment.
(d) State the model of atom suggested on the basis of the above experiment. (CBSE 2011)
Answer:
(a) Rutherford  Model
(b)  Rutherford made the following observations :

1. Most of the alpha particles were able to pass through the gold foil undeflected.
2. Some of these particles were deflected by small angles.
3. A very few (one out of approximately 12000) alpha particles suffered major deflections and even came back in the same direction.

(c) Conclusions

1. As most of the alpha particles passed through undeflected, this means that they did not come across any obstruction in their path. Thus, most of the space in an atom is expected to be empty.
2. As a few alpha particles suffered minor deflections and a very few major deflections, this means that these must have met with some obstructions in their path.
3. This obstruction must be :
   1. Very small : Only a few particles were obstructed by it.
   2. Massive : Each alpha particle has 4u mass and is quite heavy. It could easily pass through a light obstruction by pushing it aside.

(d) The main features are listed as follows :

1. An atom consists of two parts. These are nucleus and extra nuclear portion.
2. Nucleus is present in the centre of the atom and is surrounded by extra nuclear portion.
3. The radius of the nucleus of an atom is nearly 10^-15m while that of the atom is about 10^-10m. Thus, nucleus is very small in comparison to the atom.
4. The mass of the atom is mainly of the nucleus. All the protons and neutrons (discovered later on by Chadwick) are present in the nucleus.
5. The positive charge on the nucleus is because of protons present (each proton has one unit positive charge).
6. All the electrons are present in the extra-nuclear space around the nucleus.
7. The total positive charge of the nucleus due to the presence of protons is the same as that of the electrons present in the extra nuclear space. Therefore, the atom as a whole is electrically neutral.
8. Electrons present in the extra nuclear portion are not stationary. These are revolving around the nucleus at high speed following a circular path.
9. The revolving electrons do not come close to the nucleus or drawn towards the nucleus because their force of attraction towards the nucleus is balanced by the centrifugal force which is of the same magnitude. It is directed away from the nucleus

Question 10.
There are two elements ²⁶₁₃A and ²⁶₁₄B. Find the number of sub-atomic particles in each of these. What is the relation between these atoms ?
Answer:

Since the two elements have same mass number hut different atomic numbers, these are related to each other as isobars.

Question 11.
(a) In the gold foil experiment, what observations led Rutherford to conclude that
(i) Most of the space inside the atom is hollow.
(ii) The central portion of the atom is positively charged.
(iii) Volume occupied by the nucleus is very small as compared to the total volume of the atom.
(iv) Almost the entire mass of the atom concentrated at its centre.
(b) If bromine atom is available in the form of two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom. (CBSE 2014, 2016)
Answer:
(a) (i) Most of the α-particles passed through the foil undeflected.
(ii) Some α-particles (positively charged) were deflected and a few came back in the same direction. This means that there is very small, positively charged and massive portion present in the centre of the atom. This is known as nucleus.
(iii) The number of α-particles which were deflected back was very small.
(iv) Since the mass of the atom is mainly due to positively charged protons (neutrons were not known at that time), this means that the entire mass of the atom is concentrated in the centre of the atom called nucleus.
(b) % of Br isotope with mass number 79 = 49.7 % of Br isotope with mass number 81 = 50.3

Question 12.
State the postulates stated by Neils Bohr in order to overcome the objection as raised against Rutherford’s model of atom.
Answer:
The main postulates of the theory are listed :

1. In the extra nuclear portion of an atom, the electrons revolve in well defined circular paths known as orbits.
2. These circular orbits are also known as energy levels or energy shells.
3. These have been designated as K, L, M, N, O, … (or as 1, 2, 3, 4, 5, …) based on the energy present.
4. The order of the energy of these energy shells is :
   K<L<M<N<0 <…. or 1< 2< 3 < 4<5 <….
5. While revolving in an orbit, the electron is not in a position to either lose or gain energy. In other words, its energy remains stationary. Therefore, these energy states for the electrons are also known as stationary states.

Question 13.
Which of the following are isotopes and which are isobars ?
Argon, Protium, Calcium, Deuterium. Explain why the isotopes have similar chemical properties but they differ in physical properties ? (CBSE 2012)
Answer:
The symbol notations for elements are represented as follows :
¹₁H and ²₁H are isotopes while ⁴⁰₁₈Ar and ⁴⁰₂₀Ca are isobars. Since the isotopes of an element have same atomic number, they have same electronic configuration and also similar chemical properties. They have different physical properties since their mass numbers are different.

Question 14.
Explain Bohr and Bury rules for distribution of electrons into different shells.
Answer:

1. The maximum number of electrons which can be present in a particular energy shell of an atom is given by 2n². Here ‘n is the number of the energy shells or energy levels.
2. The outermost energy shell in an atom cannot have more than eight electrons even if it has a capacity to take up more electrons according to first rule.
3. It is not necessary for a given shell to complete itself before another shell starts forming. As a rule, the new shell is formed as soon as the outermost shell, acquires eight electrons.

Question 15.
The atomic number and mass number of an element are 16 and 32 respectively. Find the number of protons, electrons and neutrons in it. State its valency. Is this element a metal or a non – metal. Justify your, answer.
(CBSE 2012, 2014, 2016)
Answer:
No. of protons = Atomic number =16
No. of electrons = Atomic number =16
No. of neutrons = Mass number – Atomic number = 32-16 = 16
Electronic configuration of the element = 2, 8, 6
Valency of the element = (8 – 6) = 2
The element is a non – metal since it tends to gain electrons and not lose electrons.

Question 16.
The K and L shells of an atom are completely filled. Find the number of electrons present in it. State the name of the element. . (CBSE 2012, 2013)
Answer:
Number of electrons present : K(2), L(8) = 10. The element is neon (Ne).

Question 17.
You are given an element X. Find out
(a) Number of protons, electrons and neutrons in ‘X’.
(b) Valency of ‘X’ ,
(c) Write the chemical formula of the compound formed when ‘X’ reacts with
(i) hydrogen,
(ii) carbon.
(CBSE 2012)
Answer:
(a) Number of protons = 8
Number of electrons = 8 Number of neutrons = 16 – 8 = 8
Electronic configuration = 2, 6
(b) Valency of ‘X’ =8-6 = 2
(c) The element ‘X’ is ‘O’. The formula of the compounds with hydrogen (H) and oxygen (O) are :
(i) H₂O (ii) CO₂.

Question 18.
Define the terms (a) isotope, (b) isobar giving one example in each case. Name the element whose isotope is used in
(i) nuclear reactor,
(ii) treatment of cancer. (CBSE 2012)
Answer:
(a) Most of the elements exist in nature. An investigation of some of these have revealed that the atoms of a particular element may have different mass numbers. However, their atomic numbers are the same. These are called isotopes. Thus, isotopes may be defined as : the different atoms of the same element having same atomic number but different mass numbers.
(b)  Isobars may be defined as : the atoms belonging to the different elements with same mass numbers but different atomic numbers.
(i) U-235 isotope is used in nuclear reactor as a fuel.
(ii) Co-60 isotope is used in the treatment of cancer.

Question 19.
(a) What are canal rays ? Who discovered them ? What is the charge and mass of canal rays ?
(b) How are canal rays different from electrons in terms of charge and mass ? (CBSE 2012)
Answer:
(a) Canal rays are the rays which originate in the discharge tube experiment and move away from the anode. These are called canal rays since they seem to be flowing like a stream of water in a canal.
(b) Canal rays are basically anode rays which consist of protons. For the charge and mass of protons. For the comparison of canal rays and electrons in terms of charge and mass.

Question 20.
A certain particle X has 17 protons, 17 electrons and 18 neutrons
(i) What is the mass number of X ?
(ii) What is atomic number of X ?
(iii) What is valency of X ?
(iv) Identify the element.
Answer:
(i) Mass no. of X = No. of p + No. of n = 17 + 18 = 35
(ii) Atomic no. of X = No. of p = 17
(iii) Electronic configuration of X = 2, 8, 7
Valency of X = (8 – 7) = 1
(iv) Name of element X = Chlorine (Cl).

Question 21.
Who discovered protons ?
Answer:
Protons were discovered by Goldstein.

Question 22.
(a) The element helium has 2 electrons in its valence shell but its valency is not 2. Explain.
(b) Choose the isotopes from the following nuclei :
(i) 8p + 8n
(it) 8p + 9n
(iii) 18p + 22n
(iv) 20p + 20n (CBSE 2013)
Answer:
(a) The element helium (He) is a noble gas element (Z = 2). It has only one shell (K shell) which can have maximum of two electrons only. It therefore, does not take part in chemical combination and its valency is zero.
(b) Elements (i) and (ii) represent pair of isotopes since they have 8 protons (Atomic no. = 8)

Question 23.
What are valence shell and valence electrons ? (CBSE 2013)
Answer:
Valence shell is the outermost shell in an atom. The electrons present in it are called valence electrons.

Question 24.
Complete the following table :

Answer:

Question 25.
The total number of nucleons in the atoms of calcium and argon is 40 and the atomic numbers of calcium and argon are 20 and 18 respectively. Name the pair of these two elements and also find out the number of neutrons present in the nucleus of argon atom. (CBSE 2013)
Answer:
The elements which have same no. of nucleons but different atomic numbers are called isobars. Therefore, calcium (Ca) and argon (Ar) represent a pair of isobars.
No. of neutrons in the nucleus of Ar = 40 – 18 = 22.

Question 26.
Atom of an element has one proton, one electron and no neutron. Name the element. How will you represent it ? (CBSE 2013)
Answer:
The element is called protium or hydrogen. It is represented as j H

Question 27.
An atom has 2 electrons in M-shell. What is the atomic number of the element ? (CBSE 2014)
Answer:
K and L shells of the atom are filled and M shell has two electrons. Therefore,
Total number of electrons in the atom = 2 + 8 + 2= 12
Atomic number (Z) of the element = 12.

Question 28.
An element is represented as ¹⁶₈X. Find :
(a) The number of electrons in element X. .
(b) Mass number of an element X.
(c) The number of neutrons in element X. (CBSE 2014, 2016)
Answer:
(a) The number of electrons in element X = 8
(b) Mass number of element X = 16
(c) The number of neutrons in element X = 16-8 = 8.

Question 29.
The electronic configuration of potassium (K) is 2,8,8, 1 instead of 2,8,9 though the M shell can accommodate up to 18 electrons. Explain. (CBSE 2014)
Answer:
M-shell cannot have more than 8 electrons in case it happens to be outermost or valence shell in an atom. Therefore, the correct electronic configuration of the element potassium (K) is 2,8,8,1.

Question 30.
Show the electron distribution in magnesium atom and magnesium ion diagrammatically and also give their atomic numbers. (CBSE 2014)
Answer:
Atomic number (Z) of magnesium is 12. Its electronic distribution is 2,8,2. Magnesium ion (Mg²⁺) is formed by the loss of two electrons from the valence shell of the atom. Therefore, the electronic distribution * in the ion is 2, 8. These are shown diagrammatically as follows :

Question 31.
The composition of two atoms A and B is given :

(a) What are the mass numbers and atomic numbers of A and B ?
(b) What is the relation between the two chemical species ?
(c) Which element or elements do they represent ?
Answer:
(a) Mass number of A = 17 + 18 = 35 u
Mass number of B = 17 + 20 = 37 u
(b) The two chemical species exist as pair of isotopes since they have the same number of protons and electrons.
(c) They represent the element chlorine (Cl).

Question 32.
Write the names of three elementary particles which constitute an atom. (CBSE 2015)
Answer:
Three elementary particles which constitute an atom are ; electron (e), proton (p) and neutron (n).

Question 33.
(a) What is the relationship between two elements X and Y whose atomic numbers are 18 and 20 respectively but their mass numbers remain the same as 40 ?
(b) Are their chemical properties same or different ? Explain and support your answer.
(c) Which has more number of electrons Na or Na⁺ ? Why ? (CBSE 2015)
Answer:
(a) The elements X and Y are related to each other as isobars.
(b) Since the atomic numbers of the elements are different, their chemical properties also differ.
(c) The electronic configurations of Na and Na⁺ ion are given as follows :
Na (Z =11), K(2), L(8), M(l)
Na⁺ (Z = 11), K(2), L(8), M(-)
Na⁺ ion is formed when Na atom loses one electron. Therefore, Na atom has more electrons (11) than Na⁺ ion (10).

Question 34.
Write the electron distribution of oxygen atom. How many valence electrons does it have ? (CBSE 2015)
Answer:
Atomic number (Z) of oxygen is 8.
Electronic distribution = K(L), L(6)
Valence electrons = 6.

Question 35.
(a) Why does helium have zero valency ?
(b) Name the scientist and his experiment to prove that nucleus of an atom is positively charged.
(CBSE 2015)
Answer:
(a) Atomic number (Z) of helium is 2. Its electronic configuration is K(2). This means that its atom has completely filled shell which is the only shell it has. Therefore, valency of the element is zero.
(b) The scientist is Rutherford and the experiment is known as α-particle scattering experiment.

Question 36.
List the observations in α-particle scattering experiment which led Rutherford to make the following conclusions :
(i) Most of the space in an atom is empty,
(ii) Whole mass of an atom is concentrated in its centre.
(iii) Centre is positively charged. (CBSE 2015)
Answer:
(i) As most of the alpha particles passed through undeflected, this means that they did not come across any obstruction in their path. Thus, most of the space in an atom is expected to be empty.
(ii)  As a few alpha particles suffered minor deflections and a very few major deflections, this means that these must have met with some obstructions in their path.
This obstruction must be :

• Very small : Only a few particles were obstructed by it.
• Massive : Each alpha particle has 4u mass and is quite heavy. It could easily pass through a light obstruction by pushing it aside.

(iii) Positively charged : Alpha particles have positive charge. Since they were repelled or deflected back, the obstruction must also carry same charge i.e., positive charge, (similarly charged particles always repel each other).

Question 37.
Define isotopes. Why do isotopes have same atomic number but different mass numbers ? Explain with the help of an example. (CBSE 2015)
Answer:
Most of the elements exist in nature. An investigation of some of these have revealed that the atoms of a particular element may have different mass numbers. However, their atomic numbers are the same. These are called isotopes. Thus, isotopes may be defined as :
The different atoms of the same element having same atomic number but different mass numbers.
Please note that the difference in the mass numbers of the isotopes is because of the difference in the number of neutrons present. For example,
Hydrogen (H) exists in the form of three isotopes. These are named as :

Question 38.
Write the two postulates of Thomson’s model of an atom. What were the drawbacks in this model ?
(CBSE 2015)
Answer:

1. An atom may be regarded as a positively charged sphere in which protons are present.
2. The negatively charged electrons may be regarded as studded or embedded in this sphere.

Question 39.
(a) How many neutrons are present in C-14 isotope of carbon ?
(b) How many protons does He²⁺ ion possess ?
(c) How many electrons can be filled in the third orbit of an atom at the maximum ? (CBSE 2015)
Answer:
(a) C-14 isotope of carbon has mass number 14 and atomic number 6. In this isotope ;
No. of protons = Atomic no. = 6.
No. of electrons = Atomic no. = 6.
No. of neutrons = Mass no. – Atomic no.
= 14 – 6 = 8.
(b) He²⁺ ion (z = 2) has two protons,
(c) The third orbit of an atom can have maximum of 8 electrons.

Question 40.
Identify the most stable atom from the following. Also give the reason for your answer :

Answer:
Argon Ar₁₈⁴⁰ is the most stable atom. It is a noble gas atom with completely filled shells (2, 8, 8).

Hope given Previous Year Question Papers for CBSE Class 9 Science Chapter 4 Structure of the Atom are helpful to complete your science homework.

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