AMEND
EDUCATION ACADEMY
Matter
In Our Surroundings
Matter in our surrounding notes for class 9th
Matter in our surrounding Explanatory notes
Introduction:
Everything in this universe is made
up of material which scientists have named “matter”. The air we breathe, the
food we eat, stones, clouds, stars, plants and animals, even a small drop of
water or a particle of sand– each thing is matter.
Early Indian
philosophers classified matter in the form of five basic elements –air, earth,
fire, sky and water.
Modern day
scientists have evolved two types of classification of matter based on their
physical properties and chemical nature.
Characteristics
of Particles of Matter
1.
PARTICLES OF MATTER HAVE SPACE BETWEEN THEM- when we make tea, coffee or lemonade (nimbu paani ),
particles of one type of matter get into the spaces between particles of the
other. This shows that there is enough space between particles of matter.
Similarly particles of sugar, salt, Dettol, or potassium permanganate get
evenly distributed in water.
2.
PARTICLES OF MATTER ARE CONTINUOUSLY MOVING- Particles of matter are continuously moving, that is, they
possess what we call the kinetic energy. As the temperature rises, particles
move faster. So, we can say that with increase in temperature the kinetic
energy of the particles also increases.
3.
PARTICLES OF MATTER ATTRACT EACH OTHER-
Particles of matter have force acting between them. This force keeps the
particles together. The strength of this force of attraction varies from one
kind of matter to another.
States
of Matter
1.
THE SOLID STATE- all solids
have a definite shape, distinct boundaries and fixed volumes, that is, have negligible
compressibility. Solids have a tendency to maintain their shape when subjected
to outside force. Solids may break under force but it is difficult to change
their shape, so they are rigid.
2.
THE LIQUID STATE- Liquids have
no fixed shape but have a fixed volume. They take up the shape of the container
in which they are kept. Liquids flow and change shape, so they are not rigid
but can be called fluid. The rate of diffusion of liquids is higher than that
of solids. This is due to the fact that in the liquid state, particles move
freely and have greater space between each other as compared to particles in
the solid state.
3.
THE GASEOUS STATE- Gases are
highly compressible as compared to solids and liquids. The liquefied petroleum
gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied
to hospitals in cylinders is compressed gas. In the gaseous state, the
particles move about randomly at high speed. Due to this random movement, the
particles hit each other and also the walls of the container.
Matter
Can Change its State Water can exist in three states of matter–
· Solid, as ice,
· Liquid, as the
familiar water, and
· Gas, as water vapour.
EFFECT
OF CHANGE OF TEMPERATURE
On increasing
the temperature of solids, the kinetic energy of the particles increases. Due
to the increase in kinetic energy, the particles start vibrating with greater
speed. The energy supplied by heat overcomes the forces of attraction between
the particles. The particles leave their fixed positions and start moving more
freely. A stage is reached when the solid melts and is converted to a liquid.
The temperature at which a solid melts to become a liquid at the atmospheric
pressure is called its melting point.
The melting
point of ice is 273.16 K. The process of melting, that is, change of solid
state into liquid state is also known as fusion.
Solid
----heat----> Liquid -------------------heat--> Gas
<---cool----
<---cool---------------
So, we infer
that the state of matter can be changed into another state by changing the
temperature.
EFFECT
OF CHANGE OF PRESSURE
Increasing or
decreasing the pressure can change the state of matter. Applying pressure and reducing temperature can liquefy gases.
For an example,
Solid carbon dioxide (CO2) is stored under high pressure. Solid CO2 gets
converted directly to gaseous state on decrease of pressure to 1 atmosphere*
without coming into liquid state. This is the reason that solid carbon dioxide
is also known as dry ice.
Thus, we can
say that pressure and temperature determine the state of a substance, whether
it will be solid, liquid or gas.
Evaporation
Particles of
matter are always moving and are never at rest. At a given temperature in any
gas, liquid or solid, there are particles with different amounts of kinetic
energy. In the case of liquids, a small fraction of particles at the surface,
having higher kinetic energy, is able to break away from the forces of attraction
of other particles and gets converted into vapour. This phenomenon of change of
a liquid into vapours at any temperature below its boiling point is called
evaporation.
FACTORS
AFFECTING EVAPORATION
The rate of
evaporation increases with–
• An increase of
surface area: We know that evaporation is a surface phenomenon. If the surface
area is increased, the rate of evaporation increases. For example, while
putting clothes for drying up we spread them out.
• An increase
of temperature: With the increase of temperature, more number of particles get
enough kinetic energy to go into the vapour state.
• A decrease in
humidity: Humidity is the amount of water vapour present in air. The air around
us cannot hold more than a definite amount of water vapour at a given
temperature. If the amount of water in air is already high, the rate of
evaporation decreases.
• An increase
in wind speed: It is a common observation that clothes dry faster on a windy
day. With the increase in wind speed, the particles of water vapour move away
with the wind, decreasing the amount of water vapour in the surrounding.
HOW
DOES EVAPORATION CAUSE COOLING?
In an open
vessel, the liquid keeps on evaporating. The particles of liquid absorb energy
from the surrounding to regain the energy lost during evaporation. This
absorption of energy from the surroundings make the surroundings cold.
After a hot
sunny day, people sprinkle water on the roof or open ground because the large
latent heat of vaporisation of water helps to cool the hot surface.
Why
should we wear cotton clothes in summer?
During summer,
we perspire more because of the mechanism of our body which keeps us cool. We
know that during evaporation, the particles at the surface of the liquid gain
energy from the surroundings or body surface and change into vapour. The heat
energy equal to the latent heat of vaporisation is absorbed from the body
leaving the body cool. Cotton, being a good absorber of water helps in absorbing
the sweat and exposing it to the atmosphere for easy evaporation.
Why
do we see water droplets on the outer surface of a glass containing ice-cold
water?
Let us take
some ice-cold water in a tumbler. Very soon we see water droplets on the outer
surface of the tumbler. The water vapour present in air, on coming in contact
with the cold glass of water, loses energy and gets converted to liquid state,
which we see as water droplets.
Matter - Introduction
Anything that has both mass and volume is called matter.
For example – me, you, chair, table, computer, mouse, keyboard, dog, cat, cow,
ram, pen, pencil, tree, plant, building, stone, brick, mountain, river, etc.
Early Indian philosopher categorized matter in five types
and called them panch-tatwa (Five-elements) - Vayu (Air), Jal (Water), Agni
(Fire), Prithvi (Earth) and Aakash (Sky).
According to science matters are divided according to their
physical and chemical properties.
Physical Nature of Matter
• Matters are made of particles.
• The particles of matters are very small.
• The particles of matter are moving continuously.
• Particles of matter have space between them.
• Particles of matter attract each other because of force
of attraction.
Matter is made of particles –
All matters are made of particles. These particles are
very small and cannot be seen by naked eyes. For example a drop of water
contains about 1021 particles of water.
Particles of matter are very
small –
Particles of matters are very small. They cannot even
seen using a simple microscope.
When a tea spoon of salt is dissolved in one or two litre
of water, every drops of that water taste salty.
When few drops of dettol are mixed in water, whole water
gives the smell of dettol. Even after diluting this dettol mixed water, it
gives the smell of dettol.
This proves that particles of matter are very small.
Particles of matter are
moving continuously –.
This can be proved by putting a small crystal of
potassium permanganate in water. When you will put a small crystal of potassium
permanganate in water in a transparent glass, you will see that the purple
thread like structures start mixing with water and after some time all water
becomes purple. The thread like structures is the particles of potassium
permanganate, which start mixing with water because of the motion of particles
of water. Because of continuous motion, all particles start colliding and
finally potassium permanganate is mixed with water.
When you put a spoon of salt or sugar in a glass of
water, it dissolved in water after some-times. This happens because of the
motion of particles of sugar, salt and water.
Mixing of gases in nature is also the evidence of motion
of particles. We get the scent of our favourite food sitting in the drawing
room while food is being cooked in the kitchen. This happens because vapor
coming out from the hot food mixes with air comes to your nostril because of
the motion of their particles.
This proves that particles of matter are in motion
continuously.
Brownian Motion - Mr.
Robert Brown, a Scottish botanist observed the random movement of pollen grains
in water in 1827, it is called Brownian motion.
When pollen grains are kept in water, they start giggling
rapidly in random order. This is also an evident of motion of particles of
matter. Pollen grains moves randomly in water because the particles of water
hit the pollen grains from all sides in a random manner.
Pollen grains move faster in warm water, because
particles of warm water move more rapidly.
Brownian motion can be observed in the mixing of two
gases also having different colors. You can observe the dancing particle of
dust in the beam of sunlight which is coming through a slit in the room. The
particles of dust appear dancing because of its continuous motion along with
the particles of air.
Brownian motion proves that matters are made of very
small particles and they are moving continuously.
Particles of matter have space between them – Gas can be
compressed a lot because of the space between their particles.
When sugar is dissolved in water, the particles of sugar
disappear in water. This happens because particles of sugar get adjusted in the
spaces between the particles of water. Additionally you will notice that there
is no rise of water level takes place when one or two teaspoon of sugar is
added in a glass of water, this is because sugar particles get adjusted in the
space between the particles of water and no rise in the water level comes in
result.
Particles of matter attract
each other because of force of attraction.
Force of attraction between particles of matter keeps the
particles bonded together. The force of attraction between particles of solid
is greatest, between particles of liquid is moderate and between particles of
gas is lowest.
Because of the lowest force of attraction between the
particles of gas we can move our hand through air easily. To move our hand in
liquid, such as water, we have to apply some force, but from a solid such as
wood, we cannot move our hand. This is because the force of attraction between
particles of gas is almost negligible, in liquid the forces of attraction is
moderate but it is greatest in solid.
The force of attraction between particles of solid,
liquid and gas can be arranged in decreasing order as follows:
Matter In Our Surroundings
Change of States of Matter
Matters can be changed from one state to another state. A
solid can be changed into liquid and a liquid can be changed into gas. Most of
the metals, which are solid, turn into liquid on heating and turn into vapor on
further heating.
The change of state of matters depends upon mainly two
factors:
• Temperature
• Pressure
Effect of temperature on states of matter:
Solids change into liquid with increase in temperature.
A liquid changes into gas by increase in temperature.
Gas changes into liquid by decrease in temperature.
liquid changes into solid by decrease in temperature.
Example - Ice is solid, water is liquid and vapour is
gas.
On increase in temperature ice changes into water. On
further increase in temperature water changes into vapor.
On decrease in temperature vapor changes into water. On
further decrease in temperature water changes into ice.
On heating the particles of a solid get more kinetic
energy and they start moving rapidly. On the other hand heating increases the
space between the particles. When the kinetic energy of particles and space
between them become similar to the liquid, the matter fulfills the condition of
arrangement of particles. In this condition a solid changes into liquid.
On heating the particles of a liquid gets more kinetic
energy and they start moving more rapidly. The increase in temperature
increases the kinetic energy of particles and inter-space between them
increases. The increase in kinetic energy and space between the particles the
force of attraction between particles decreases. When the kinetic energy of
particles of liquid and space between the particles increases and force of
attraction between them decreases enough, the particles of liquid start moving
with great speed. In this condition the particles of liquid attain the
arrangement similar to gas and liquid changes into gas.
Water (a liquid) changes into ice (a solid) at 00C.
Water changes into vapour (a gas) at 1000C.
The change of solid to liquid is called melting. The
change of liquid to gas is called the vaporization. This happens because of
increase in temperature.
The change of gas into liquid is called condensation. The
change of liquid into solid is called freezing. These takes place because of
decrease in temperature.
Effect of pressure:
Solid –There is no effect of pressure on solids.
Solids are non compressible, i.e. solids cannot be
compressed as there is no space between their particles which could allow
compression.
When the pressure is increased on a solid, it is deformed
and finally broken.
Liquid – There is no effect on pressure on liquid.
Liquids are non compressible, i.e. liquids cannot be
compressed since there is not enough space between their particles to get
compressed.
Gas – The volume of gas decreases with increase in
pressure.
Since there is lot of space between the particles, gas is
highly compressible. Large volume of gas can be compressed to a small volume.
Because of this nature high compressibility, gas is transported easily after
compressed to a small volume in cylinders.
Natural gas is compressed to small volume and packed in
cylinders. It is used widely as fuel to running vehicles. Because of
compression it is called Compressed Natural Gas or simply known by CNG
(Compressed Natural Gas).
Oxygen is compressed and packed in small cylinders, which
is used to save life in hospitals.
On increasing more pressure gas turns into liquid.
The liquefaction of gas by increasing of pressure:
In general condition, the particles of gas are far from
each other. They move independently and rapidly in all directions. There is lot
of space between the particles of gas. The force of attraction is negligible
between the particles of gas. Because of these characters gas can be compressed
a lot.
When pressure is applied on the gas, its particles come
closer to each other. After applying high pressure, the particles of gas come
very close to each other that they start attracting each other. The space
between the particles of gas also decreases with increase in pressure. While
applying high pressure a lot of heat is evolved, which was using by the
particles of gas in movement. So, while applying high pressure to compress the
gas, temperature is keeping low to overcome with the heat released. As they
come closer, they lose kinetic energy. Finally they get closely packed at a
certain high pressure, at this stage they get bonded with each other and turned
into liquid.
This process is called liquidation or liquefaction of gas.
Petroleum Gas is compressed to small volumes. At more
pressure it turns into liquid, after that it is packed into cylinders. This is
used widely as fuel to cook food in households. Compressed petroleum gas is
commonly known as LPG (Liquefied Petroleum Gas) since it turns into liquid
form.
Oxygen, hydrogen, nitrogen, helium, etc. are compressed
to liquefy and used for many purposes. Liquefied oxygen is used as propellant
in spacecraft rockets. Liquefied oxygen is used for industrial and medical
purposes also. Liquefied oxygen is of blue color.
On further increase in pressure, gas turns into solid.
Carbon dioxide is gas turns into solid on high pressure.
Solid carbon dioxide is known as dry-ice. It is also known as ‘Cardice’ or
‘Card ice’.
Dry ice is used in the storage of food.
On decrease in pressure liquefied gas turns into gas.
Solid carbon dioxide (dry ice) turns into gas without changing into liquid on
decrease in pressure and increase of temperature.
Unit of Temperature:
Temperature is measured in Degree Celcius, Fahrenheit and
Kelvin.
Fahrenheit: Before the end of 20th century Fahrenheit was
used as unit to measure the temperature. The scale of Fahrenheit was named
after, the physicist Daniel Gabriel Fahrenheit. Fahrenheit is written as “0F”
and read as degree Fahrenheit.
320F (Thrity two degree Fahernheit) has been
defined for the freezing point of water.
2120F (Two hundred tweleve degree Fahrenheit)
has been defined for the boiling point of water. Fahrenheit was replaced by
Celsius.
Degree Celsius: The unit of Celsius is named after the
Swedish Astronomer Anders Celsius, who developed a similar scale. Untill 1948
Celsius was known as Centigrade. The word centigrade comes after the Latin word
‘Centum’.
Degree Celsius is written as “0C” and read as
degree Celsius.
00C (zero degree Celsius) has been defined for
the freezing point of water.
1000C (hundred degree Celsius) has been
defined for the boiling point of water.
Degree Celsius is replaced by Kelvin. Moreover, it is
used widely even today in clinical as well as laboratory thermometer.
Kelvin: Kelvin is the SI unit of temperature. The symbol
of Kelvin is ‘K’. The Kelvin scale is named after the physicist William
Thomson, 1st Baron Kelvin.
The scale of Kelvin is used widely because of its many
advantages over degree Celsius. In Kelvin temperatures are written in positive
usually.
273 K is defined as the freezing or melting point of ice.
373 K is defined as the boiling point of water.
At Kelvin scale -273 K is considered as the absolute zero
temperature.
Degree Celsius - Degree Fahrenheit - Kelvin
Matter In Our Surroundings
EVAPORATION
The change of liquid into vapor without reaching at its
boiling point is called Evaporation.
Evaporation takes place only at the surface of liquid
while vaporization takes place on the whole mass of liquid.
Evaporation takes place even at room temperature also.
Evaporation speeds up with rise in temperature.
When water or other liquid is left in open, gradually its
volume decreases. If you left some water in a pot in open, after two or three
days water disappears. If the water left in garden or in a open balcony, it
disappears quickly than kept in a room. This happens because of evaporation.
Process of Evaporation
- Molecules at the surface of water, when exposed some temperature, their
kinetic energy increases. Because of increase in kinetic energy those molecules
become able to overcome the force of attraction between the particles of
liquid. After getting required kinetic energy and decrease in force of
attraction, they escape in the air in the form of vapor. Additionally those
kinetic energy get some of the required kinetic energy from their neighboring
molecules also because of which the temperature of the adjacent molecules
decrease, which finally result in decrease of the temperature of surface of
liquid.
Factors affecting the
Evaporation:-
• Temperature
• Pressure
• Surface area
• Humidity in air
• Wind speed
We see that wet cloth is dried up quickly in a windy day
since the wind speeds up the rate of evaporation.
The wet clothes are given jerks before hanging them on
laundry line because by giving jerks some of the water droplets propelled out.
This reduces the presence of water in the wet cloths and they dried up quickly.
Evaporation in everyday life:
(a) Water from the surface of oceans, seas and other
large water bodies evaporate continuously as they are exposed to atmosphere.
The water vapor because of evaporation rises up in air and cumulates in the
form of cloud, which makes the rain. Hence, evaporation is one of the essential
parts of water cycle. Thus we can say that evaporation is one of the most
natural phenomena for us.
(b) In summer days sweats come out to regulate the
temperature of our body. The sweat evaporates because of increase in surface
area and getting the temperature from atmosphere. This is resulting in decrease
in temperature of skin and finally our body, which gives relief to us in hot
days. Sweating is a natural mechanism to keep cool the surface of our body in
hot days. This is the cause that with increase in temperature our body sweats a
lot especially in hot summer days.
(c) Water is kept in earthen pots to keep them cool.
Earthen pot has lot of pores. Water kept in earthen pots evaporates from the
pores of pots, which cools the neighboring molecules of water. This process
continues and whole of the water kept in the earthen pots become cooler. Hence,
water is kept in the earthen pot to keep them cool for drinking purpose.
(d) Wet clothes on the laundry lines are dried up because
of evaporation.
(e) Sea water has lot of salt. Sea water is left in
shallow pond. The water evaporates gradually because of heat of sun leaving the
salt in the shallow pond. These salts are collected and used for with food
after purification.
(f) Evaporative coolers are widely used in hot summer
days. In evaporative coolers, dry air is blow over husk saturated of water.
From the surface of husk water is evaporated resulted in cooling of husk. The
water particles at the surface of wet husk evaporate and cool the rest portion
of wet husk. Air blown from the cool husk is sent in the room, which cools the
room.
Matter In Our Surroundings
Diffusion in Matters
Diffusion: The
phenomenon of mixing of particles of different substances together is known as
diffusion. Diffusion takes place because of the movement of particles of
matters.
Diffusion is a natural process. The mixing of different gases
in air is a natural process that takes place continuously.
Diffusion takes place from higher concentration to lower
concentration.
Diffusion comes from Latin word ‘diffundere’. The meaning
of ‘diffundere’ is ‘to spread out’.
It was Robert Brown who discovered the random moment of
particles of matter. Albert Einstein developed the theory of Brownian motion
based on the discovery of Robert Brown.
Diffusion takes place in solid, liquid and gases.
Diffusion in gas:
Diffusion takes place most rapidly in gasses.
The particles of gas have more kinetic energy due to that
they move with high speed. Because of the movement of particles with high speed
gas diffuses most rapidly.
Example – Mixing of gases in air.
Smoke coming out of factories is seen only near the mouth
of chimneys. After rising high it mixes up with air and vanishes out. This
happens because of diffusion of gases.
Diffusion in liquid:
Diffusion takes place in liquids. The rate of diffusion
is less in liquid than that of gases.
The particles of liquid have enough movement because of
kinetic energy for diffusion. When two liquids are kept together they are mixed
because of the diffusion.
For example – When salt solution is kept in water this
makes whole the solution salty after some time. This happens because of
diffusion in liquids.
Diffusion in solid:
Diffusion takes place with slowest rate in the case of
solids. The rate of diffusion is almost negligible in solids.
Since the particles of solid have lowest kinetic energy
so they do not move. Hence, diffusion takes a long time in the case of solid.
Example – Some time we observe that the marks of chalk on
the black board could not be wiped out easily after a long time. This happens
because of the diffusion of particles of chalks with the particles of
black-board and marks of chalk could not be wiped out.
Alloys are the mixture of two or more than two metals.
Alloys are possible to get prepared because of the diffusion of solid into
solid. Bronze, steel, brass are the examples of alloy.
Diffusion in everyday life:
(a) We get the smell of our favourite food sitting in the
drawing room even when the food is being cooked in kitchen.
The vapor coming out from the food reaches to us because
of diffusion in air and we get the smell of food.
(b) We get the smell of burning incense stick sitting far
from it. When an incense stick is burnt, the incense present in it vaporizes
because of heat. The vapor coming out from the incense stick mixes with air and
reaches to us.
In similar way we sense the good or bad smell from a
distance.
(c) When sugar is added in water, it is mixed because of
the diffusion. The particles of sugar collide with the particles of water
because of their continuous movement. And finally they are mixed together
because of diffusion.
(d) Carbonated drinks are prepared by the diffusion of
gas in water. We notice a hiss sound when the cap of bottle of a carbonated
drink is open, the hiss sound comes because of the diffused gas coming out of
water. Soda, Soft drinks (coca-cola) etc. are the carbonated drinks and
prepared mainly because of the diffusion of carbon dioxide in water.
(e) Aquatic animals take dissolved oxygen in water while
breathing. Aquatic plants synthesize their food under water because of the
dissolved carbon dioxide in water. These gases dissolved in water because of
the diffusion.
Temperature and Rate of
Diffusion:
The rate of diffusion increases with increase in
temperature and decreases with decrease in temperature.
Diffusion takes place because of the movements of
particles of matters. The increase in temperature increases the kinetic energy,
which increase the speed of movement of particles of matter. The increase in
the speed of movement increases the rate of diffusion.
Example - We do not get the smell of cold food from a
distance while we get the smell of hot food from a distance. Because smell
coming out from the cold food in the form of gas do not have enough kinetic
energy to cover more distance. But when food is hot the vapor coming out from
it moves more rapidly because of more kinetic energy and reaches to us even if
we are at a distance from it. This shows that temperature increases the rate of
diffusion.
Matter In Our
Surroundings
Solution of NCERT
In Text Questions-Part-1
Question : 1 – Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold,
cold-drink, smell of perfume.
Answer :- Chair, air, almonds, and cold-drink are
matters.
Explanation: Things that occupy space and have some mass
are called matter. Since chair, air, almonds and cold-drink occupy some space
and have some mass, so these are matter.
Question: 2 - Give reasons for the following observation:
The smell of hot sizzling food reaches you several meters away, but to get the
smell from cold food you have to go close.
Answer: Smell of anything comes because of gases
emanating from the given thing. The smell reaches to us because of diffusion of
gas. The rate of diffusion increases with increase in temperature. This happens
because of higher kinetic energy due to higher temperature. That is why smell
of hot sizzling food reaches to us from several feet. On the other hand, the
kinetic energy of gases emanating from cold food is low because of lower
temperature. Due to this, we need to move closer to a cold food to take its
smell.
Question: 3 - A diver is able to cut through water in a
swimming pool. Which property of mater does this observation show?
Answer: Water is also a matter. We know that particles of
matter have space between them. In case of fluids, the space between particles
is large enough and due to this it becomes easier for a diver to cut through
water in a swimming pool.
Question : 4 - What are the characteristics of the
particles of matter?
Answer :- Characteristics of the particles of matter:
The particles of matter are very small.
The particles of matter have space between them.
The particles of matter are moving constantly.
The particles of matter attract one another.
Question : 5 - The mass per unit volume of a substance is
called density. (Density = mass / volume) Arrange the following in order of
increasing density
air, exhaust from chimneys, honey, water, chalk, cotton
and iron.
Answer :- Air < Exhaust from chimneys < cotton <
Water < Honey < Chalk < Iron
Explanation:
Air is the mixture of gases. Chimney exhaust is also a
mixture of gases; along with some heavier particles, such as ash. This makes
the density of chimney exhaust more than air.
Cotton is a porous solid and which has lot of air trapped
within pores. This makes its volume more than water. Therefore, it is less
dense than water.
Question : – 6 – (a) - Tabulate the differences in the
characteristics of states of matter.
Answer:
What's the difference between a solid, a
liquid, and a gas? You might think it's just a matter of temperature, but
there's more to it than that.In solids, atoms are
bonded fairly firmly together, though they do move about a bit. You don't need
to put a solid in a container; it stays where it is because its atoms are
locked tightly into a definite shape that, ordinarily, doesn't change. If a
solid is reasonably soft and you press it, you can make it change shape by
pushing its atoms into new positions. Heat a solid enough and you'll give its
atoms enough energy to break apart, forming a liquid. In liquids,
the atoms are more randomly arranged and a little bit further apart (but not
all that much). The forces between them are weaker and they can jiggle about
and flow past one another quite easily. That's why liquids pour. Take enough
heat away from a liquid and the atoms will slow down until they form a solid.
Add some more heat and some of the atoms can escape from it to form a gas.
Gases
have much more randomly arranged atoms than either liquids or solids. The
forces between the atoms are very weak, so the atoms can speed around freely
with lots of energy. A liquid can flow, but a gas goes one better and expands
to fill all the space available to it. If you squeeze a gas really hard or take
heat away from it, its molecules have to huddle together. Pretty soon they're
bonding to form a liquid. Keep squeezing or cooling and you'll lock them
together tightly to make a solid.
Question : – 7 – (b) - Comment upon the following:
rigidity, compressibility, fluidity, filling a gas container, shape, kinetic
energy and density
Answer: Rigidity: The greatest force of attraction
between particles and close packing of particles make solids rigid. Rigidity is
one of the unique properties of solids. Because of rigidity, a solid can resist
from getting distorted. Because of rigidity a solid has definite shape and
volume. Rigidity is negligible in fluid and gas.
Compressibility: Compressibility is one of the most
important characteristics of gas. Because of lot of space between particles, a
gas can be compressed to a great extent.
Liquid and solid cannot be compressed because of the
least space between their particles.
Fluidity: The ability to flow is called fluidity. The
less force of attraction and more space between particles make liquid and gas
to flow. That’s why liquid and gas are called fluid.
Filling of a gas container: Liquids do not fill a gas
container completely, while gases fill the gas container completely in which it
is kept. This is because the particles of gas can move in all the directions.
Shape: Solids have fixed shape. Liquid and gas take the
shape of the container in which they are kept. This happens because of less
force of attraction and more kinetic energy between particles of liquids and
negligible force of attraction and highest kinetic energy between particles of
gas.
Kinetic energy: The kinetic energy of particles of solid
is the minimum. They only vibrate at their fixed position. The kinetic energy
of particles of liquid is more than that of solid. But they can slide above one
another. The kinetic energy of particles of gas is the maximum.
Density: The mass per unit volume of a substance is
called density. The density of solid is highest, of liquid is less than solid
and of gas is minimum.
Matter In Our Surroundings
Solution of NCERT In Text Questions - part-2
Question : – 8 - Give reasons
(a) A gas fills completely the vessel in which it is
kept.
Answer: The force of attraction between particles of gas
is negligible. Because of this, particles of gas move in all directions. Thus,
a gas fills the vessel completely in which it is kept.
(b) A gas exerts pressure on the walls of the container.
Answer - Because of negligible force of attraction
between particles of gas, the particles of gas have the highest kinetic energy.
These properties enable the particles of gas to move in all directions and hit
the walls of container from all sides. Because of this a gas exerts pressure on
the walls of the container in which it is kept.
(c) A wooden table should be called a solid.
Answer:- A wooden table has fixed shape and fixed volume,
which are the main characteristics of solid. Thus a wooden table should be
called a solid.
(d) We can easily move our hand in air but to do the same
through a solid block of wood we need a karate expert.
Answer:- Since, air is gas, so its particles are loosely
packed and there is negligible force of attraction between its particles.
Because of that we can easily move our hand in air.
But wood is a solid, so the force of attraction between
its particles is greatest. The particles of wooden block are closely packed.
That’s why we cannot move our hand through a solid block of wood. However a
karate expert can exert required pressure to break the great force of
attraction of the particles of a solid wooden block.
Question – 9 - Liquids generally have lower density as
compared to solids. But you must have observed that ice floats on water. Find
out why.
Answer: During freezing of water, some space between the
particles of water is left vacant with some air trapped between them. These
empty spaces having air in them makes the density of ice; lower than that of
water. That’s why ice floats on water.
Question : – 10 - Convert the following temperature to
Celsius scale.
(a)300 K (b) 573 K
Answer :- (a) Temperature on Kelvin scale = Temperature
on Celsius scale + 273
Therefore,
Temperature on Celsius scale = Temperature of Kelvin
scale – 273
Hence, Temperature on Celsius scale = 300K – 273 = 27⁰C
Answer (b) Temperature on Kelvin scale = Temperature on
Celsius scale + 273
Therefore, Temperature on Celsius scale = Temperature of
Kelvin scale – 273
Hence, Temperature on Celsius scale = 573K – 273 = 300⁰C
Question : – 11 - What is the physical state of water at
(a) 250⁰C (b) 100⁰C
Answer: (a) Since, water boils at 100⁰C,
thus it is in gaseous state at 250⁰C.
Answer: (b) Since, water boils at 100⁰C,
thus at 100⁰C water is in transition phase, i.e. in both liquid and
gaseous states.
Question : – 12 - For any substance why does the
temperature remain constant during the change of state.
Answer: During the change of state of any substance, the
heat supplied or released is utilised in phase change. Such heat is called
latent heat. So, the temperature of any substance remains constant during the
change of state.
Question: 13 - Suggest a method to liquefy atmospheric
gas.
Answer - Atmospheric gas is liquefied by increasing
pressure and decreasing temperature.
Question : – 14 - Why does a desert cooler cool better on
a hot dry day?
Answer - Desert cooler works on the basis of evaporation.
In hot and dry days the moisture level is very low in atmosphere which
increases the rate of evaporation. Because of faster evaporation, cooler works
well. That’s why desert cooler cool better on a hot dry day.
Question : – 15 - How does the water kept in an earthen
pot (matka) become cool during summer?
Answer: Water from porous wall of earthen pot evaporates
continuously, which lowers the temperature of water kept in the earthen pot.
In summer moisture level is very low in the atmosphere,
which increases the rate of evaporation as evaporation is inversely
proportional to the moisture level in atmosphere. That is why in summer water
kept in earthen pot becomes cool.
Question : – 16 - Why does our palm feel cold when we put
some acetone or petrol or perfume on it?
Answer - When acetone or petrol or perfume is poured over
palm, it evaporates quickly as these are volatile liquids. The evaporation
lowers the temperature of palm and our palm feels cold.
Question : – 17 – Why are we able to sip hot tea or milk
faster from a saucer rather than a cup?
Answer - When hot tea or milk is kept in a saucer, the
liquid is exposed over a larger surface area as compared to in case of the
liquid being kept in a cup. The larger surface area enables the faster cooling.
That’s why we are able to sip hot tea or milk faster from a saucer rather than
from a cup.
Question : – 18 - What type of clothes should we wear in
summer?
Answer - We should wear cotton cloths in summer. Because
cotton cloths are good absorbent and they absorb sweat excreted in summer.
Because of evaporation of sweat from the surface of cotton cloth, we feel
colder and comfortable.
Exemplar Problem and Solution
Long Answer Type Questions
Q -24. 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?
Answer:
Sweating is a natural mechanism to keep our body cool.
Our body sweats a lot in the hot summer days. A cotton cloth absorbs sweat
easily, while a nylon cloth does not.
Evaporation of sweat absorbed by cotton cloth resulted in
cooling effect to our body. The rate of evaporation also increases with the
increase in surface area.
When one wear cotton cloth in hot summer days it absorbs
the sweat coming out from our body easily. The surface area of sweat also
increases as it is absorbed by the cotton cloth which increases the rate of
evaporation and one who wears cotton cloth feels more comfortable because of
cooling effect due to evaporation of sweat.
Hence, Priyanshi who wearing cotton cloth will feel more
comfortable, while Ali does not.
Q - 25. You want to wear your favourite shirt to a party,
but the problem is that it is still wet after a wash. What steps would you take
to dry it faster?
Answer:-
Wet cloths dried up because of evaporation. The rate of
evaporation increases with the increase in temperature, surface area and wind
speed.
Hence, keeping the conditions to increase the rate of
evaporation in mind, following steps should be taken to get the favourite shirt
dried up quickly-
(a) We shirt should be spread up on the wire. This will
increase the surface area of wet shirt which will increase the rate of
evaporation.
(b) Speedy wind should be supplied to the shirt using a
fan which will increase the rate of evaporation.
(c) If it is a day time, arrange the above combination in
the sun, so that increase in temperature would increase the rate of
evaporation.
(d) If it is not a day time, a hot iron can be used also
before hanging the shirt on a wire.
Q - 26 - Comment on the following statements:
(a) Evaporation produces cooling.
Answer:
In the process of evaporation, water molecules get some
of the required kinetic energy to overcome the force of attraction between its
particles to get escaped from atmospheric temperature and some of the kinetic
energy from its neighbouring molecules. Because of getting some of the kinetic
energy from neighbouring molecules, the temperature of neighbouring molecules
of water decreases which is resulted in cooling effect.
Hence, evaporation produces cooling.
(b) Rate of evaporation of an aqueous solution decreases
with increase in humidity.
Answer:
The moisture present in the atmosphere is called
humidity. A dry atmosphere can hold more water molecules than a humid atmosphere.
Hence, the rate of evaporation of aqueous solution decreases with the increase
of humidity and vice versa.
That’s why increase in humidity in the atmosphere
decreases the rate of evaporation of an aqueous solution.
(c) Sponge though compressible is a solid.
Answer:
There are lots of pores found in a sponge which are
filled with air. When a sponge is compressed the air trapped in the pores
expelled out, and when it is released, pores again filled with the air and a
sponge comes in its original shape.
Therefore, sponge is compressible because of air trapped
in its pores and not because of the space between its particles as happens in
the case of gas.
Hence, sponge is a solid instead of it is compressible.
Q - 27 - Why does the temperature of a substance remain
constant during its melting point or boiling point?
Answer:
When heat is supplied to a solid, say ice, it starts
melting. But temperature does not rise even after continuous supply of heat
until all ice melts. The extra heat supplied, is used in melting of ice, which
is called latent heat of fusion.
Similarly, when a liquid, say water, starts boiling the
temperature of water does not rise instead of continuous supply of heat. The
extra heat supplied to the boiling water is used in the vapourisation of water,
which is known as latent heat of vapourisation.
That’s why, the temperature of a substance remains
constant during its melting or boiling point.
