Heat (Std. 8)

Question 1:

A. Whom should I Pair with?

Group A	Group B
a. Temperature of a healthy human body	296 K
b. Boiling point of water	98.6∘F
c. Room temperature	0∘C
d. Freezing point of water	212∘F

B. Who is telling the truth?
a. The temperature of a substance is measured in joules.
b.Heat flows from an object at higher temperature to an object at lower temperature.
c. Joule is the unit of heat.
d. Objects contract on heating.
e. Atoms of a solid are free.
f. The average kinetic energy of atoms in a hot objects is less than the average kinetic energy of atoms in a cold objects.

C. You will find if you search.
a. A thermometer is  used to measure............... . 
b. The apparatus used to measure heat is called a..................... .
c. Temperature is the measures of the............................ Kinetic energy of the atoms in a substance.
d. The heat contained in a substannce is the measures of the....................... kinetic energy of atoms in the substance. 

ANSWER:

A. 

Group A	Group B
a. Temperature of a healthy human body	98.6∘F
b. Boiling point of water	212∘F
c. Room temperature	296 K
d. Freezing point of water	0∘C

B. 
a. Sentence a is lying as heat energy is measured in joules.
b. Sentence b is telling the truth as heat energy flows from an object at higher temperature to an object at lower temperature.
c. Sentence c is telling the truth as joule is the unit of heat.
d. Sentence d is lying as objects expand on heating.
e. Sentence e is lying as atoms of a solid are closely packed because of force attraction between them.
f.  Sentence f is lying as the average kinetic energy of atoms in a hot objects is greater than the average kinetic energy of atoms in a cold objects.

C.
a. A thermometer is  used to measure temperature.
b. The apparatus used to measure heat is called a calorimeter.
c. Temperature is the measures of the average kinetic energy of the atoms in a substance.
d. The heat contained in a substance is the measures of the total kinetic energy of atoms in the substance. 

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Question 2:

Nishigandha kept a vessel containing all the ingredients for making tea in a solar cooker. Shivani kept a similar vessel on a stove. Whose tea will be ready first and why?

ANSWER:

Shivani's tea will be prepared first.
In Shivani's case, the intensity of the flame in contact with the vessel is very high due to which the flow of heat will be faster. Thus, the time taken by the tea to reach its boiling point will be less. Hence, tea will be prepared fast.
In case of Nishigandha, the intensity of radiation reaching the vessel is not as high as compared to stove's flame. Thus, the flow of heat will be slower in this case due to which more time will be taken to reach the boiling temperature of the tea. Hence, tea will be prepared at slower rate.

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Question 3:

Write brief answers.
a. Describe a clinical thermometer. How does it differ from the thermometer used in laboratory?

b. What is the difference between heat and temperature ? what are their units.

c. Explain the construction of a calorimeter. Draw the necessary figure.

d. Explain why rails have gaps at specific distances.

e. Explain with the help of formulae the expansion coefficients of liquid and gas. 

ANSWER:

a. Clinical thermometer is used in homes. It has a glass tube with a bulb at one end and the other end closed. This bulb and some part of the tube is filled with thermometric liquid, such as mercury or alcohol. The rest of the volume of the tube has vacuum in it. There is a kink near the end of the glass tube which prevents the falling back of the thermometric liquid once it is removed from the hot body under observation.
It is basically used to measure the temperature of humans. A clinical thermometer has a temperature range of only 35 ºC to 42 °C. 


b.

Clinical thermometer	Laboratory thermometer
This thermometer is used in homes. It is basically used to measure the temperature of human body.	This thermometer is used to measure the temperature of all things, except the human body.
A clinical thermometer has a temperature range of only 35 ºC to 42 °C.	A laboratory thermometer has a temperature range of -10 ºC to 110 ºC.
It has kink.	It does not have kink.

b. 

Heat	Temperature
It is a form of energy which causes in us the sensation of hotness or coldness.	It is measure of the degree of hotness or coldness of an object.
It is measured in joules (J).	It is measure in kelvin (K), Celsius (oC) and Fahrenheit (oF)

c. Calorimeter

A device used for heat measurement is called a calorimeter.

Construction of a Calorimeter

• It consists of a metallic vessel and stirrers. They are made of copper or aluminium.

• The vessel is then kept inside a wooden jacket which contains heat-insulating materials.

• The outer wooden jacket acts as a heat shield, and reduces the heat loss from the inner vessel.

• The outer jacket has an opening through which a mercury thermometer is inserted into the calorimeter.

​

The Universe

Question 1:

Name these –
(a) Birth place of stars
(b) Biggest planet in the solar system
(c) The galaxy which is our neighbour.
(d) Brightest planet in the solar system
(e) Planet with largest number of satellites
(f) Planets without a single satellite
(g) Planet with a rotation different from other planets.
(h) A celestial body that carries a tail along

ANSWER:

(a) Birth place of stars: Nebula
(b) Biggest planet in the solar system: Jupiter
(c) The galaxy which is our neighbour: Andromeda
(d) Brightest planet in the solar system: Venus
(e) Planet with largest number of satellites: Jupiter
(f) Planets without a single satellite: Mercury and Venus
(g) Planet with a rotation different from other planets: Venus and Uranus
(h) A celestial body that carries a tail along: Comets

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Question 2:

Fill in the blanks.
(a) The group of galaxies of which our Milky Way is a part is called ............. .
(b) Comets are made of ............. .
(c) The planet ............. appears as if it is rolling along its orbit.
(d) ............. is a stormy planet.
(e) The Pole Star is the best example of a ............. type of star.

ANSWER:

(a) The group of galaxies of which our Milky Way is a part is called 'Local Group'.
(b) Comets are made of ice and dust particles.
(c) The planet Uranus appears as if it is rolling along its orbit.
(d) Jupiter is a stormy planet.
(e) The Pole Star is the best example of a variable type of star.

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Question 3:

Say if the statements given below are right or wrong. Rewrite the statements after correcting them.
(a) Venus is the planet closest to the sun.
(b) Mercury is called a stormy planet.
(c) Jupiter is the biggest planet.

ANSWER:

(a) No, the statement is incorrect. Mercury is the planet closest to the Sun.

(b) No, the statement is incorrect. Jupiter is called a stormy planet.

(c) Yes, Jupiter is the biggest planet.

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Question 4:

Answer the following.
(a) What is a special characteristic of the planet Mars ?
(b) What are the types of galaxies ?
(c) Which celestial bodies does a galaxy include ?
(d) Name the different types of stars.
(e) What are the types of comets and on what basis are they classified ?
(f) What is the difference between meteors and meteorites ?
(g) What are the characteristics of the planet Neptune ?

ANSWER:

(a) The special characteristic of planet Mars is that it is red in colour. This is due to the presence of iron in its soil.

(b) Various types of galaxies which are identified according to their shapes are:

• Spiral galaxy: These are the spiral structures that extend from the centre into the disc. It consists of stars, gas and dust in a bulge at the centre and in the extended spiral arms.
• Elliptical galaxy: These galaxies have nearly ellipsoidal shape which consists of stars, gas and dust.
• Irregular galaxies: These galaxies don’t have a distinct regular shape.
• Barred spiral galaxies: These are spiral galaxies with a central bar-shaped structure composed of stars.

(c) A galaxy includes stars, cluster of stars, nebulae, clouds of gases and dusts, dead stars, newly born stars etc.

(d) The different types of stars in the universe are stated below:
Sun-like Stars: These are particularly similar to the Sun in size but there is a lot of variation in terms of temperature. Examples are Alpha Centurai, Tau Ceti etc.
Red Giants: These stars do not have as hot outer layer as that of the Sun with temperature ranging from 3000 ^oC to 4000 ^oC but they have a very high luminance than the Sun. They are red in color and their diameter is 10 to 100 times that of the Sun.
Super Nova: They are larger and even brighter as compared to the red giant stars and even the Sun. They are also considered as the primary source of heavy elements in the universe.
Binary or Twin Stars: A pair of two stars in which one revolves around the other or they both revolve around a common centre is known as binary or twin stars. 
Variable Stars: These stars have a variable shape and brightness i.e it keeps on changing whenever they expand or contract. Their brightness decreases when they expand and increases when they contract. Example is Pole Star.

(e) Comets are the solar system bodies that are usually spherical in shape and develop tail when they come near the Sun. Comets become visible when they are close to the Sun and the Earth.

Comets can be categorized into two groups depending on how long they take to complete one revolution around the sun. These are:

• Long Period Comets: These comets complete one revolution around the Sun in 200 and more years. Great Comet of 1843 is one of the examples of this type.
• Short Period Comets: These comets complete one revolution around the Sun in years less than 200. One of the examples is Halley's comet that takes 76 years to complete the revolution. It last appeared in 1985.

(f) Meteors: These are rocky pieces originating from the asteroid belt and enter the Earth’s atmosphere as a result of its gravitational pull. On entering the Earth’s atmosphere, they acquire very high speeds. These high speeds of meteors cause friction with the Earth’s atmosphere and thus the meteors heat up and evaporate completely. Meteors are also known as shooting stars.

Meteorites: These are the unburnt part of meteors which fall to the surface of Earth on ocassions when the meteors do not burn completely in the atmosphere itself.

(g) Following are the characteristics of planet Neptune:

• A season on this planet lasts for about 41 years.
• Wind blows with extremely high speed on this planet.

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Question 5:

Match the following

Group A	Group B
(1) Galaxy	(a) From east to west
(2) Comet	(b) 33 satellites
(3) Sun-like star	(c) Spiral
(4) Saturn	(d) Sirius
(5) Venus	(e) Halley

ANSWER:

Group A	Group B
(1) Galaxy	(c) Spiral
(2) Comet	(e) Halley
(3) Sun-like star	(d) Sirius
(4) Saturn	(b) 33 satellites
(5) Venus	(a) From east to west

our skeletal system

Question 1:

Fill in the blanks with the proper word.
(a) The place where two or more bones are connected is called a .............. .
(b)Cells of the epidermis contain a pigment called .............. .
(c) ...................... and ...................... are the two layers of the human skin.
(d)The human skeletal system is divided into ...................... parts.

ANSWER:

(a) The place where two or more bones are connected is called a joint.
(b)Cells of the epidermis contain a pigment called melanin.
(c) Epidermis and dermis are the two layers of the human skin.
(d)The human skeletal system is divided into two parts.

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Question 2:

Match the pairs.

'A'	'B'
(1) Ball and socket joint	(a) Knee
(2) Hinge joint	(b) Wrist
(3) Gliding joint	(c) Shoulder

ANSWER:

'A'	'B'
(1) Ball and socket joint	(c) Shoulder
(2) Hinge joint	(a) Knee
(3) Gliding joint	(b) Wrist

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Question 3:

Right or wrong? If wrong, write the correct sentence.
(a) Bones are soft.
(b)The human skeleton protects the internal organs.

ANSWER:

(a) Bones are soft- Wrong
Bones are hard and non flexible.

(b)The human skeleton protects the internal organs- Right

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Question 4:

Put a  mark at the proper places.
(a) The system which gives a definite shape to our body.
□$\mathbf{\square }$ Excretory system
□$\mathbf{\square }$ Respiratory system
□$\mathbf{\square }$ Skeletal system
□$\mathbf{\square }$ Circulatory system

(b) The ................ joint is seen in fingers and toes.
□$\mathbf{\square }$ Hinge joint
□$\mathbf{\square }$ Ball and socket joint
□$\mathbf{\square }$ Immovable joint
□$\mathbf{\square }$ Gliding joint

ANSWER:

(a) The system which gives a definite shape to our body.
□$\mathbf{\square }$ Excretory system
□$\mathbf{\square }$ Respiratory system
 Skeletal system
□$\mathbf{\square }$ Circulatory system

(b) The ................ joint is seen in fingers and toes.
 Hinge joint
□$\mathbf{\square }$ Ball and socket joint
□$\mathbf{\square }$ Immovable joint
□$\mathbf{\square }$ Gliding joint

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Question 5:

Answer the following questions in your words.
(a)What are the functions of your skin?
(b)What should you do to keep your bones strong and healthy?
(c)What are the functions of the human skeletal system?
(d)Which are the various reasons due to which our bones might break?
(e) What are the different types of bones? How many types are there?

ANSWER:

 a. The most important functions of the skin are:

• Protection − The skin provides protection to the body from mechanical injury, ultraviolet rays, from disease-causing microbes, and prevents desiccation.

• Temperature regulation − The skin helps in the maintenance of body temperature at a constant.

• The skin contains numerous sense receptors, which help in perceiving the surroundings.

• The skin is also involved in the synthesis of vitamins such as Vitamin D.

• The skin also helps in disposing off the excess water and salt by evaporation.​

Ans b. The following things should be done in order to keep our bones healthy and strong:

• Have calcium rich foods like milk, yoghurt as calcium is an important component of bones.
• We need to take enough vtiamin D as it helps in the absorption of calcium present in foods.
• Some other important nutrients like potassium, vitamin K and magnesium also help the body to absorb and use calcium. 
• Daily exercise is required to keep our bones in a state of good health.

Ans c. The skeletal system includes all of the bones and joints in the body. It performs the following functions:

• gives a definite shape to the body
• provides support and protection for the soft tissues that make up the rest of the body
• provides attachment points for muscles to allow movements at the joints

Ans d. Bones are structurs which are hard and non flexible. The chances of breaking of bones occurs, if the bones are weak. There are various reasons due to which bones may become weak and may break:

• certain diseases can result in weakening of the bones and cause them to break. For example, osteoporosis and arthirits are diseases which result in the weakening of bones
• defeciency of nutrients like calcium and vitamin D 
• certain stress or injury may result into fracturing of bones
• bone loss also occurs with age and thus making them more fragile
• alcohol and smoking also have a negative impact on bones and bone density

Ans e. There are 4 types of bones:

1. Flat bones: They are thin shaped and, in some cases, provide mechanical protection to soft tissues beneath or enclosed by the flat bone.
2. Irregular bones: They have complicated shapes which cannot be classified as 'long', 'short' or 'flat'. Their shapes are due to the functions they fulfill within the body.
3. Long bones: These type of bones are longer than wider and they function to support the weight of the body and facilitate movement
4. Short bones: They are almost cube shaped bones as they are equal in their length and width.

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Question 6:

What will happen if -?
(a)There are no joints in our body.
(b)There is no melanin pigment in our skin.
(c) Instead of 33 vertebrae in our body, we had one single and straight bone.

ANSWER:

a. Joints are the places where 2 bones are joined together. They permit the various kinds of movement which we carry out in order to fulfill our requirements. In absences of these joints our body would become stiff and we would not be able to carry out our daily tasks.

b. Melanin is a pigment which is found in skin and hair as well. It is important for two reasons, firstly it imparts colour to our skin and the tone of our skin depends on the amount of melanin pigment, secondly it protects our skin and the inner parts from ultravoilet rays. If the melaning pigment is absent, all the humans will have the same skin colour and we will become susceptible to the UV rays present in our atmosphere.

c. The 33 vertebrae together form the structure which we know as spine. They permit the movement of our spine so that we can bend it easily and carry out other functions. In other words we can say that they provide flexibilty to the spine. If in place of them, only one single and straight bone will be present then we would loose our flexiblity and remain in fixed positions. We would always stand straight like a pole and would not be able to sit down or perform other activities.

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Question 7:

Draw diagrams.
(a) Types of joints
(b) Structure of the skin

ANSWER:

(a) Types of joints

1. Gliding joint




2. Ball and socket joint


3. Hinge joint




(b.) Structure of skin

Std. 8 Light

Question 1:

Fill in the blanks
i. The perpendicular to the mirror at the point of incidence is called....................... .
ii. The reflection of light from a wooden surface is.................. reflection.
iii. The working of Kaleidoscope is based on the properties of................... .

ANSWER:

i. The perpendicular to the mirror at the point of incidence is callled normal.
ii. The reflection of light from a wooden surface is diffused reflection.
iii. The working of Kaleidoscope is based on the properties of reflection of light.

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Question 2:

Draw a figure describing the following.
The reflecting surface of two mirrors make an angle of 90^∘ with each other. If a ray incident of one mirror has an angle of incidents of 30^∘, draw the ray reflected from the second mirror. What will be its angle of reflection? 

ANSWER:

Let r₂ be the angle of reflection from the second mirror.

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Question 3:

How will you explain the statement 'we cannot see the objects in a dark room?

ANSWER:

Light enables us to see objects around us. We see an object when light falls on it and gets reflected from its surface and enters our eyes. In a dark room, there is no source of light. Thus, no light falls on the surface of objects and hence we do not see them. This is why we cannot see the objects in a dark room.

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Question 4:

Explain the differences between regular and irregular reflection light.

ANSWER:

Regular reflection of light	Irregular reflection of light
This type of reflection occurs on smooth surfaces such as a plane mirror.	This type of reflection occurs on rough surfaces such as a black board.
In this type of reflection, the reflected rays are parallel to each other.	In this type of reflection, the reflected rays are non-parallel to each other.
In this type of reflection, the reflected rays are in one direction.	In this type of reflection, the reflected rays are in different direction.

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Question 6:

Study the following incident.
Swara and Yash were looking in a water filled vessel. They could see their images clearly in the still water. At that instant, Yash threw a stone in the water. Now their images were blurred. Swara could not understand the reason for the blurring of the images. 

Explain the reason for blurring of the images to swara by answering the following questions.

i. Is there a relation between the reflection of light and the blurring of the images.?

ii. Which types of reflection of light can you notice from this?

iii. Are laws of reflection followed in these types of reflection? 

ANSWER:

i. Yes, there is a relation between the reflection of light and blurring of the images.

ii. Initially, the water was still and its surface was smooth. Thus, regular type of reflection was taking place from its surface i.e. all the light rays coming from Swara's body or Yash's body reflected in the same direction on hitting the water surface and reached our eyes. Because of this, we saw a clear image. 
But when Yash threw a stone in the water, the water surface became rough due to which irregular type of reflection of light occurred on its surface i.e all the light rays coming from Swara's body or Yash's body reflected in different directions on hitting the water surface. Thus, only some part of the reflected rays reached our eyes. Because of this, we see a blurred image.

iii. Yes, laws of reflection are followed in both the types of reflection.

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Question 7:

Solve the following examples.
a. If the angle between the plane mirror and the incident ray is 40^∘, what are the angles of incidence and reflection?
b. If the angle between the mirror and reflected ray is 23^∘, what is the angle of incidence of the incidence ray? 

ANSWER:

a
 

 

Std 7 Light

Question 1:

Fill in the blanks .

(a) When the beams from the headlights of  a car fall on an object in the night , the shadows called ............. and  ..................... can be seen.

(b) During a lunar eclipse the shadow of the ............. falls on the ......................... .

(c) During a solar eclipse the shadow of the .............. falls on the ................ .

(d) Various shades of colour are seen in the sky at sunrise and sunset due to ........................ .

ANSWER:

(a) When the beams from the headlights of  a car fall on an object in the night , the shadows called umbra and penumbra can be seen.

(b) During a lunar eclipse the shadow of the Earth falls on the Moon.

(c) During a solar eclipse the shadow of the Moon falls on the Earth.

(d) Various shades of colour are seen in the sky at sunrise and sunset due to scattering of sunlight.

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Question 2:

Give reasons.

(a) Space beyond the earth's atmosphere appears dark.

(b) We are able to read while sitting in the shade.

(c) We should not observe the solar eclipse with naked eyes.

ANSWER:

(a) When sunlight is scattered by the particles present in the atmosphere, the surrounding gets illuminated and appears bright. In space, there is no atmosphere i.e. no source for scattering the sunlight. Thus, as sunlight is not scattered in different directions in the space, it appears dark.

(b) Light, which is required for seeing and reading, will reach us even when we are sitting under the shade. This light comes to us upon reflecting from various objects around us.

(c) We should not observe solar eclipse with naked eyes because by doing so we might damage our eyesight. We may suffer from permanent vision impairment or serious eye injuries. Thus, we should always use some device such as binoculars while watching the sun during solar eclipse.

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Question 3:

Give some example of scattering of light that we come across in day-to-day life.

ANSWER:

Some example of scattering of light that we come across in day-to-day life are:

• Blue colour of the sky: Out of the seven components present in sunlight, blue colour is scattered the most by the particles present in the atmosphere and hence, the sky appears blue.
• Reddish colour of the sun during sunrise and sunset: At sunrise or sunset, the sun is located near the horizon of the Earth. Hence, light has to travel a long distance through the Earth’s atmosphere. At the time of sunrise or sunset, when white sunlight falls on suspended atmospheric particles, blue colour light scatters out in deep space, while red colour light scatters less, and reaches the observer on the surface of the Earth. Hence, when this less scattered red light reaches our eyes, the sun and its surroundings appear to be reddish.

• Danger signals in red colour:​ Red colour is scattered least by the atmospheric particles and it can travel longer distance as compared to other colours. Thus, it alarms a person of the potential danger from far distance.

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Question 4:

Why is the shadow of bird flying high not seen on the earth?

ANSWER:

When a bird flies high up in the sky, the darkest part of its shadow, called umbra, does not reach the Earth. Hence, the shadow of bird flying high is not seen on the Earth.

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Question 5:

Why is a penumbra not obtained from a point source?

ANSWER:

We do not obtain penumbra from a point source because all the light rays falling on the shadowing object are completely blocked by it. This is illustrated in the figure below.

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Question 6:

Answer the following questions in your own words.

(a) What is meant by scattering of light ?

(b) Does the shadow really vanish in the zero shadow condition?

(c) Will the laser beam be seen if it passes through a glass box which contains a lighted incense stick?

ANSWER:

(a) The ability of light to spread in various directions is known as scattering of light.

(b) In the zero shadow condition, the shadow does not really vanishes. In fact, it is formed just beneath the object because of which we are not able to see it.

(c) Yes, the laser beam will be seen if it passes through a glass box which contains a lighted incense stick. This is because the laser beam will be scattered by the smoke particles formed in the glass box due to burning of incense stick. Due to this scattering, the path of laser beam will become visible.

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Question 7:

Discuss and write.

(a) Write a science based paragraph on 'What if the sun did not rise?'

(b) What efforts will you make to remove the misconceptions about eclipses?

(c) Various eclipses and the conditions during the period.

ANSWER:

(a) If the Sun did not rise, the Earth will not get illuminated. Due to this, we won't be able to see objects around us if any other temporary source of light is not used. Also, we know, Sun is the main source of energy to all living beings. So, plants won't be able to prepare their food because of absence of sunlight and soon they will die. This will create imbalance in the food chain which in turn will result in the destruction of all kinds of organisms on Earth.  Also, all the production of electrical energy will get terminated because of absence of this solar energy. Thus, it is true to day that Sun is main source of our survival.

(b) To remove the misconceptions about eclipses, such as we should not eat during eclipses etc., following efforts can be made:

• Social media such as Newspapers, T.V., radio,etc. should launch some programmes and debates discussing the myths existing related to eclipses. These mediums should highlight the superstitious talks prevailing from the past and how we can remove this. These mediums should discuss why science do not support these misconceptions.
• At the school level, discussion should be held in classrooms regarding these misconceptions. They should be asked whether they have seen such thing existing in their home. If yes, they should be taught how to change the mindset of their parents and grandparents to eradicate these misconceptions.

(c) There are two types of eclipses known as

• Solar eclipse: It occurs when the Moon comes in between the Earth and the Sun and the Moon's shadow falls on the Earth. The solar eclipse occurs only on the new moon day. Two types of solar eclipse are:
  Total Solar eclipse: When the solar disc is completely covered by the Moon. 
  Partial Solar eclipse: When the solar disc is partially covered by the Moon. 

 

• Lunar eclipse: It occurs when the Earth comes in between the Moon and the Sun and the Earth's shadow falls on the Moon. The lunar eclipse occurs on the full moon night. Two types of lunar eclipse are:
  Total lunar eclipse: When the total Moon comes under the shadow of the Earth. 
  Partial lunar eclipse: When the partial Moon or some part of the Moon comes under the shadow of the Earth.

 

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Question 8:

Explain the difference:

(a) Point sources and extended sources of light.

(b) Umbra and penumbra.

ANSWER:

(a)  

Point Source of Light	Extended Source of light
It is that source of light which forms only the umbra region on the screen of the shadowed object.	It is that source of light which forms umbra as well as penumbra region on the screen of the shadowed object.
eg: A pin hole through which sunlight is entering is a point source of light	eg: Sun is an extended source of light

(b)

Umbra	Penumbra
It is the part of the shadow where all of the light from the source is blocked by the shadowing object.	It is the region around the umbra where the light from the source is partially blocked by the shadowing object.
It is formed using an extended source of light.	It is formed using point source of light.

 

Std. 6 Sound

13.Changes- Physical and Chemical

Question 1:

 Distinguish between the following.
 (a) Physical change and chemical change
 (b) Periodic change and non-periodic change
 (c) Natural change and man-made change

ANSWER:

a. 

Physical Change	Chemical Change
The chemical composition of a substance does not change	The chemical composition of a substance does not remain same
Easily reversible	Not easily reversible
No new substance is formed	New substances are always formed
Melting of ice is an example of physical change	Burning of paper is an example of chemical change

b.

Periodic changes	Non periodic changes
1. Events that repeat themselves after a fixed period of time and whose ocurence can be predicted are called periodic changes.	The events that do not repeat themselves after regular interval of time are called non periodic changes.
2. Example- rotation of earth, revolution of earth, day and night, seasons etc.	Example- natural disasters like earthquake, cyclones etc.

c.

Natural changes	Man made changes
1. The changes which are brought about by nature itselfand are not under the control of humans are called natural changes.	The changes which are brought about by humans are called man made changes.
2. Examples- change of season, tides in sea, ageing of plants etc.	Examples- burning of fuels, cutting hair, switching on lights etc.

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Question 2:

Under which different types can the following changes be classified?
(a) Conversion of milk into yoghurt.
(b) Bursting of a cracker.
(c) Occurence of an earthquake.
(d) Revolution of the earth around the sun.
(e) Stretching of a spring.

ANSWER:

(a) Conversion of milk into yoghurt- Chemical change
(b) Bursting of a cracker- Chemical change
(c) Occurence of an earthquake- Natural change
(d) Revolution of the earth around the sun- Periodic change
(e) Stretching of a spring- Physical change

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Question 3:

Give reasons.
(a) While purchasing canned food, its expiry date should be checked.
(b) An iron article should be given a coat of paint
(c) A wooden article should be polished.
(d) Copper and brass utensils should be tinned.
(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.

ANSWER:

a. While purchasing canned food their expiry date should be checked because an expiry date indicates that for how long a canned food is safe for consumption. Canned foods with expired dates are not considered worth consumption as they could affect your health. They might get contaminated with microorganisms like bacteria or may have no nutritional value left after its expiration.

b. Iron articles often undergo rusting resulting in the formation of a reddish brown layer on its surface. Rusting is caused by the exposure of iron to the air and water in the surroundings. An iron article which is coated with a paint prevents the interaction of iron with the surrounding air and water and thus protects it from rusting.

c. A wooden article should be polished because it protects the wooden article from sunlight and all other external conditions and also it reduces the probability of wooden mites damaging the furniture. 

d. Copper and brass materials also undergo corrosion resulting in the formation of a greenish layer on this surface. When the surface of copper/ brass comes in contact with air or water, it results in corrosion and to prevent this, the process of tining is used. In this process copper and brass articles are coated with tin.

e. A dry handkercheif gets wet at once because it quickly absorbs the water in which it is dipped. However, it takes longer for a wet handkerchief to dry because the water needs to evapourate from the handkerchief. Evaporation is a slow process which is dependant on other factors as well like relative humidity.

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Question 4:

What will you take into account while identifying the following?
(a) A physical change in a substance.
(b) A chemical change in a substance.

ANSWER:

a. Physical changes are those changes in which only the forms of substances get modified; the chemical natures and compositions of the substances involved are not altered.

Characteristics:

• No new substance is formed during a physical change.
• Most physical changes can be reversed easily.
• The chemical composition of a substance undergoing physical change remains the same.

b. Chemical changes are changes that involve reaction of substances with one another. Such reactions result in alterations in the chemical compositions of the substances involved. These changes lead to the formation of new substances.

Characteristics:

• One or more new substances are formed during a chemical change.
• Most chemical changes cannot be reversed easily.
• The chemical composition of a substance undergoing chemical change does not remain the same.
• A chemical change is always accompanied by a change in energy.

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Question 5:

Read the paragraph and note down the various types of changes it mentions.
It was nearing six o'clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light.

ANSWER:

It was nearing six o'clock in the evening (Periodic change). The sun was setting (Periodic change). A breeze was blowing (Natural change). Leaves on the tree were shaking (Physical change). Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys (Physical change). Then he felt hungry (Chemical change) and went into the house. Mother made a dough from wheat flour and fried purees (Irreversible change). While eating hot purees, his attention was drawn outside the window. It had started raining (Natural change). There was lightning (Natural change), too. Sahil was enjoying his dinner in the dim light.