Motions of the Earth

Earth has two types of motions:

Rotation - Rotation can be defined as the movement of the Earth on its axis.

Revolution- Revolution can be defined as the movement of the Earth on its orbit around the Sun

This rotation and revolution are responsible for causing day and night, and change in seasons and climate on the Earth.

ROTATION

We know that the Earth spins on its axis, an imaginary line running north to south across its center. The Earth rotates from west to east and completes one rotation in 24 hours. This rotation leads to day and night on the planet. We all know that the Sun rises in the east and sets in the west. It appears to move from east to west in the sky. But in reality, the Sun is stationary and it is the Earth which is rotating from west to east. For example, when we travel in a car, things outside the car like buildings, trees, etc., appear to be moving backwards. But we know that they are actually stationary and it is we who are moving forward in the car.

 

When the Earth rotates on its axis, only one half of its surface faces the Sun at a time. This half of the Earth receives the Sun's light and heat and experiences day time. The part of the Earth that faces away from the Sun experiences darkness and night time. The circle of illumination is an imaginary line that divides the half of the Earth that experiences day from that which experiences night.

As the Earth rotates, different regions experience the phenomena of day and night.

Look at the image given below.

Consider a point on the Earth's surface, say X. We know that the Earth rotates from west to east. This means, the point X will at some time move east to position R. At this position, it receives diffused light from the Sun. This time of the day when the Sun is not visible and yet there is some light is called dawn.

In time X will reach position P and experience sunrise. This denotes the time when the Sun appears on the eastern horizon. When X reaches N, it will directly face the Sun. This time of the day when sunlight falls directly on X is called noon or mid-day.

After N, X will begin to move away from the Sun. At E, the Sun will appear to go down on the western horizon. This is the time of sunset. At G, X will receive some diffused light for a while after which it will experience darkness. This time of the day is called dusk or twilight. At H, X will be in complete darkness. This time is called midnight. As the Earth spins on its axis, these positions will keep repeating every 24 hours.

If the Earth did not rotate it would always be day in the part of the Earth facing the Sun. This part would experience continuous warmth. It would always be night in the half facing away from the Sun. This half would remain freezing cold all the time. Life would not have been possible in such extreme conditions.

 

REVOLUTION

The Earth revolves around the Sun on a fixed path or orbit. This orbit is not circular, but elliptical in shape. The rotation as well as the revolution of the Earth takes place in the same direction. The Earth completes one revolution around the Sun in 365 days and 6 hours. Therefore, revolution is also called annual motion of the Earth. This motion causes change of seasons on the Earth. As the Earth's orbit is elliptical, the Earth is closer to the Sun at certain points on the orbit and far from the Sun at certain points. The Earth is said to be in perihelion when its distance from the Sun is minimum. When the distance between the Earth and the Sun is maximum, it is said to be in aphelion.

Perihelion and aphelion

Think and Answer

Does the Earth experience any other motion apart from rotation and revolution? It is believed that every point in the entire universe is in motion relative to every other point, because the entire universe is rapidly expanding. Besides the Earth's movement around the Sun, the Earth along with the entire Solar  System is moving around the center of the Milky Way Galaxy. It takes our Solar System 225 million years  to make one trip around the galaxy.

 

LEAP YEAR

The Earth completes one revolution in 365 days and 6 hours. For our convenience, we consider one year to have 365 days and ignore 6 hours. Every 4th years, the 6 hours add up to become 24 hours i.e., one day. The extra day is added to the month of February. Therefore, every four years, February has 29 days instead of 28 days. So every fourth year has 366 days. Such a year is known as a leap year.

A leap year is always a multiple of 4. The Romans first designated February 29 as leap day. A person born on February 29 is called a "leapling" or a 'leaper".

INCLINATION OF THE EARTH'S AXIS

While it revolves around the Sun, the Earth remains tilted to one side. We study the tilt of the Earth through its axis. The axis is an imaginary line passing through the centre of the Earth around which the Earth rotates.

The axis makes an angle of 66½° with the orbital plane of the Earth. The orbital plane can be defined as the plane formed by the Earth's orbit. The axis makes an angle of 23/½° with the plane perpendicular to the orbital plane. The two ends of the axis are called the North Pole and the South Pole.

 

EFFECTS OF TILTED AXIS AND REVOLUTION

(a) Unequal length of days and nights

Due to the inclination of the Earth's axis, the length of days and nights varies from season to season and from the Equator to the Poles. For most part of the year, only one Pole of the Earth is tilted towards the Sun. When the North Pole is tilted towards the Sun, the Northern Hemisphere receives more heat from the Sun. The Southern Hemisphere faces away from the Sun and receives less heat. Therefore, the Northern Hemisphere is said to be experiencing summer season, while the Southern Hemisphere is said to have winter season. During summer season, days are longer than nights.

 

Days are longer in places nearer to the North Pole than the South Pole. At the Equator, the length of the illuminated and dark parts of the circle is equal. That means any point on the Equator has almost equal length of day and night all through the year. As we go north, the length of the illuminated part of the parallel is more than the darkened part. It means that days become longer and nights become shorter when we move from the Equator towards the North Pole. 

In fact, the North Pole experiences six months of daylight. When the South Pole is tilted towards the Sun, the Southern Hemisphere receives more heat in comparison to the Northern Hemisphere. Therefore, the Southern Hemisphere experiences summer, while the Northern Hemisphere experiences winter. As seen in the figure, the Northern Hemisphere has longer nights and shorter days. Places near the North Pole have six months of darkness.

(b) Change in temperature

Summers are hot and winters are cold. This is because the hemisphere facing the Sun receives more direct rays of the Sun, while the hemisphere facing away receives more slanting rays of the Sun. Direct rays have more heating capacity than slanting rays. This is because vertical rays get concentrated on a smaller area whereas slanting rays get spread over a wider area. A change in temperature affects the weather conditions of a place. A change in weather conditions leads to a change of seasons.

 

Think and Answer Why is the Equator hot and wet throughout the year? The Earth bulges at the Equator, so this part is always close to the Sun and therefore, extremely hot. Warm air can hold more water than cold air. Warm air is also less dense than cold air. As warm air rises, it cools and the water vapour condenses and falls as rain. At the Equator, there is an overall upward movement of air. This causes more condensation and hence more rain. Think and Answer What is the average length of day and night at the Equator? At the Equator, every day and every night is 12 hours long.

 

(c) Changing seasons

The term 'season' refers to a period of the year when general weather conditions remain the same. As the season changes, the weather conditions also change consecutively. The four major seasons experienced in most parts of the world are summer, autumn, winter, and spring. The interval between the seasons is about three months.

 

CHANGE IN SEASONS

The change in the position of the Earth as it moves around the Sun bring and the tilt in it's axis about a change in seasons. This phenomenon is best understood by studying the four positions of the Earth during its revolution around the Sun.

The position on June 21

On this date, the North Pole is tilted towards the Sun. The rays of the Sun fall vertically on the Tropic of Cancer (23/½°N). It is summer season in the Northern Hemisphere and winter season in the Southern  Hemisphere. The position of the Earth on June 21 is also called 'summer solstice' in the Northern Hemisphere, and the 'winter solstice' in the Southern Hemisphere.

The term solstice refers to that time when the Sun is directly  overhead one of the tropics. Because a large portion of the Northern Hemisphere receives sunlight, regions lying north of the equator experience the longest day and the shortest night on June 21. The nights are longer than the days in areas in the Southern Hemisphere on this day.

The position on December 22

On this date, the South Pole is tilted towards the Sun and the rays of the Sun fall vertically on the Tropic of Capricorn (23/½°S). It is winter season in the Northern Hemisphere and summer season in the Southern Hemisphere. The position of the Earth on December 22 is called 'winter solstice' in the Northern Hemisphere, which experiences the shortest day of the year. It is called 'summer solstice for the Southern Hemisphere, which experiences longer days and shorter nights.

The positions on March 21 and September 23

On these two dates, neither of the Poles is inclined towards the Sun. The rays of the Sun fall vertically over the Equator. As a result, on these days, days and nights are of equal duration throughout the world. This is called an equinox. The weather conditions in both the hemispheres are almost similar. It is neither hot nor cold in either hemisphere.

There is spring season in one hemisphere and autumn in the other. March 21 is called 'spring equinox' in the Northern Hemisphere and 'autumn equinox' in the Southern Hemisphere. September 23 is called 'autumn equinox' in the Northern Hemisphere and 'spring equinox' in the Southern Hemisphere.

Revision

Rotation: Movement of the Earth on its axis.

Axis: An imaginary line running north to south across the centre of the Earth.

Circle of illumination: An imaginary circle that divides the half of the Earth that experiences day from that which experiences night.

Dawn: Time of the day when the Sun is not visible and yet there is some light.

Sunrise: The time when the Sun appears on the eastern horizon.

Noon: Also known as mid-day; time of the day when the sun is directly overhead a place.

Sunset: The time when the Sun appears to go down on the western horizon.

Dusk: Also known as twilight; time of the day when a place gets some diffused light from the Sun before it gets completely dark.

Rotation causes day and night. Revolution and earth's titled axis causes unequal lengths of day and night, change of seasons, and changes in temperatures.

Revolution: Movement of the Earth on its orbit around the Sun.

Perihelion: When the distance between the Earth and the Sun is minimum.

Aphelion: When the distance between the Earth and the Sun is maximum.

Orbital plane: An imaginary plane formed by the Earth's orbit.

Season: Refers to a period of the year when general weather conditions remain the same.

Solstice: Refers to that time when the Sun is directly overhead one of the tropics.

Equinox: Refers to the time when days and nights are of equal duration throughout the world.

Regions lying in the Northern Hemisphere experience the longest day on June 21 and the shortest day on December 22.

On March 21 and September 23, days and nights are of equal duration throughout the world.

Exercise

 

1. What is the cause of day and night on Earth?

a) The Earth's revolution around the Sun
b) The Earth's rotation on its axis
c) The Sun moving from east to west
d) The Earth’s tilt

Answer: b) The Earth's rotation on its axis

 

2. In which direction does the Earth rotate on its axis?

a) East to west
b) West to east
c) North to south
d) South to north

Answer: b) West to east

 

3. How long does it take for the Earth to complete one full rotation?

a) 12 hours
b) 24 hours
c) 365 days
d) 48 hours

Answer: b) 24 hours

 

4. What imaginary line divides the part of the Earth experiencing day from the part experiencing night?

a) Axis of rotation
b) Equator
c) Circle of illumination
d) Tropic of Cancer

Answer: c) Circle of illumination

 

5. Why does the Sun appear to move from east to west in the sky?

a) The Sun is moving around the Earth
b) The Earth rotates from east to west
c) The Earth rotates from west to east
d) The Earth is stationary

Answer: c) The Earth rotates from west to east

 

6. When it is daytime on one half of the Earth, what is happening on the other half?

a) It is also day
b) It is night
c) It is sunset
d) It is dawn

Answer: b) It is night

 

7. Which of the following best describes the axis of the Earth?

a) A real line from the North to the South Pole
b) An imaginary line from the North to the South Pole
c) A real line from east to west
d) An imaginary line running around the Equator

Answer: b) An imaginary line from the North to the South Pole

 

 

8. In which dynasty was Aryabhatta born?

a) Maurya Dynasty
b) Gupta Dynasty
c) Chola Dynasty
d) Mughal Dynasty

Answer: b) Gupta Dynasty

 

9. What significant discovery did Aryabhatta make about Earth's motion?

a) The Earth revolves around the Sun
b) The Earth rotates about its axis daily
c) The stars move across the sky
d) The Sun revolves around the Earth

Answer: b) The Earth rotates about its axis daily

 

10. Aryabhatta was known for his contributions to which fields?

a) Medicine and Biology
b) Architecture and Art
c) Mathematics, Astronomy, and Physics
d) Music and Literature

Answer: c) Mathematics, Astronomy, and Physics

 

11. What was the prevailing belief before Aryabhatta’s theory about Earth's rotation?

a) The Earth was stationary, and the sky rotated
b) The Earth rotated around the Sun
c) The stars and planets did not move
d) The Sun rotated around the Earth

Answer: a) The Earth was stationary, and the sky rotated

 

12. What would happen if the Earth did not rotate?

a) There would be no change in seasons
b) Half of the Earth would experience continuous day and warmth
c) Both halves of the Earth would experience continuous day
d) The Earth would remain at the same temperature throughout

Answer: b) Half of the Earth would experience continuous day and warmth

 

13. What is the shape of Earth's orbit around the Sun?

a) Circular
b) Elliptical
c) Square
d) Irregular

Answer: b) Elliptical

 

14. How long does it take for the Earth to complete one revolution around the Sun?

a) 24 hours
b) 365 days
c) 365 days and 6 hours
d) 366 days

Answer: c) 365 days and 6 hours

 

15. What is the annual motion of the Earth?

a) The Earth’s rotation on its axis
b) The Earth's revolution around the Sun
c) The movement of the Sun around the Earth
d) The revolution of the Moon around the Earth

Answer: b) The Earth's revolution around the Sun

 

16. When the Earth is closest to the Sun, what is this position called?

a) Aphelion
b) Perihelion
c) Solstice
d) Equinox

Answer: b) Perihelion

 

17. When the Earth is farthest from the Sun, it is said to be in which position?

a) Perihelion
b) Aphelion
c) Solstice
d) Equinox

Answer: b) Aphelion

 

18. How long does it take the Solar System to complete one trip around the Milky Way Galaxy?

a) 225 million years
b) 1 million years
c) 365 days
d) 25,000 years

Answer: a) 225 million years

 

19. What causes the change of seasons on Earth?

a) The Earth’s rotation
b) The Earth’s revolution around the Sun
c) The Earth's distance from the Moon
d) The Earth's movement around the Milky Way

Answer: b) The Earth’s revolution around the Sun

 

20. Why is an extra day added to February every four years?

a) To make up for 6 hours added annually
b) To correct the Earth's orbit
c) To balance the months
d) To adjust for the leap year

Answer: a) To make up for 6 hours added annually

 

21. What is a person born on February 29 called?

a) Leaper
b) Leapling
c) Extra-born
d) Februaryite

Answer: b) Leapling

 

22.At what angle is the Earth's axis tilted with respect to the orbital plane?

a) 45°
b) 90°
c) 66½°
d) 23½°

Answer: c) 66½°

 

23. What is the angle between the Earth's axis and a perpendicular to the orbital plane?

a) 23½°
b) 45°
c) 66½°
d) 90°

Answer: a) 23½°

 

24. What causes the unequal length of days and nights on Earth?

a) The Earth's elliptical orbit
b) The Earth's rotation
c) The tilt of the Earth's axis
d) The Earth’s proximity to the Sun

Answer: c) The tilt of the Earth's axis

 

25.When the North Pole is tilted towards the Sun, which hemisphere experiences summer?

a) Southern Hemisphere
b) Northern Hemisphere
c) Equator
d) Both Hemispheres

Answer: b) Northern Hemisphere

 

26. At the Equator, how do the lengths of day and night compare throughout the year?

a) Day is always longer
b) Night is always longer
c) Day and night are almost equal in length
d) Day is twice as long as night

Answer: c) Day and night are almost equal in length

 

27. As you move from the Equator towards the North Pole during summer, what happens to the length of days?

a) Days become shorter
b) Days become longer
c) Days remain the same length
d) Days and nights become equal

Answer: b) Days become longer

 

 

28. Why are summers hot and winters cold?

a) Because the Earth’s orbit changes
b) Due to the angle of the Sun’s rays
c) The Sun emits more heat in summer
d) The Earth’s rotation speeds up in winter

Answer: b) Due to the angle of the Sun’s rays

 

29. What happens to the temperature when a hemisphere receives slanting rays of the Sun?

a) Temperature increases
b) Temperature remains constant
c) Temperature decreases
d) Temperature fluctuates

Answer: c) Temperature decreases

 

30. Why is the Equator hot throughout the year?

a) The Equator is always close to the Sun
b) The Sun emits more heat at the Equator
c) The Equator receives slanting rays of the Sun
d) The Equator moves towards the Sun during summer

Answer: a) The Equator is always close to the Sun

 

31. Why does the Equator experience more rain?

a) Warm air holds less water than cold air
b) Warm air rises and cools, leading to condensation
c) The Sun heats up the air quickly
d) Cold air from the poles moves to the Equator

Answer: b) Warm air rises and cools, leading to condensation

 

32. What is the average length of day and night at the Equator?

a) 10 hours of day and 14 hours of night
b) 12 hours of day and 12 hours of night
c) 14 hours of day and 10 hours of night
d) 16 hours of day and 8 hours of night

Answer: b) 12 hours of day and 12 hours of night

 

33. Why do places near the North Pole experience six months of darkness?

a) The North Pole is tilted away from the Sun for six months
b) The Sun does not shine on the North Pole
c) The Earth’s rotation stops during this time
d) The Sun moves to the Southern Hemisphere for six months

Answer: a) The North Pole is tilted away from the Sun for six months

 

34. What is the term 'season' used to describe?

a) A specific month of the year
b) A period of the year with consistent weather conditions
c) The time when the Earth rotates on its axis
d) The day and night cycle

Answer: b) A period of the year with consistent weather conditions

35. What causes the change in seasons on Earth?

a) The Sun’s distance from Earth
b) The tilt of Earth’s axis and its revolution around the Sun
c) The Earth's rotation
d) The movement of the Moon around the Earth

Answer: b) The tilt of Earth’s axis and its revolution around the Sun

 

36. What is the position of the Earth on June 21 called in the Northern Hemisphere?

a) Winter solstice
b) Spring equinox
c) Summer solstice
d) Autumn equinox

Answer: c) Summer solstice

 

37. What happens in the Northern Hemisphere on June 21?

a) The shortest day of the year
b) Equal duration of day and night
c) The longest day and the shortest night of the year
d) Winter season

Answer: c) The longest day and the shortest night of the year

 

38. On which date does the Northern Hemisphere experience the winter solstice?

a) March 21
b) June 21
c) December 22
d) September 23

Answer: c) December 22

 

39.What is the position of the Earth on March 21 and September 23 known as?

a) Solstice
b) Equinox
c) Tilt day
d) Leap day

Answer: b) Equinox

 

40. What happens to the duration of day and night on March 21 and September 23?

a) Days are shorter than nights
b) Days are longer than nights
c) Days and nights are of equal duration
d) Nights are longer than days

Answer: c) Days and nights are of equal duration

 

41. What is March 21 called in the Northern Hemisphere?

a) Autumn equinox
b) Winter solstice
c) Summer solstice
d) Spring equinox

Answer: d) Spring equinox

 

42. On December 22, where do the Sun's rays fall vertically?

a) Tropic of Cancer
b) Tropic of Capricorn
c) Equator
d) North Pole

Answer: b) Tropic of Capricorn

 

43. What season does the Southern Hemisphere experience during the December 22 solstice?

a) Winter
b) Autumn
c) Summer
d) Spring

Answer: c) Summer

 

44. What is "Perihelion"?

a) When the Earth is closest to the Sun
b) When the Earth is farthest from the Sun
c) The point where the Earth rotates fastest
d) When the Sun is directly overhead the Equator

Answer: a) When the Earth is closest to the Sun

 

45. What is the "Orbital plane"?

a) The path the Earth takes while revolving around the Sun
b) The imaginary line passing through the Earth's center
c) The axis of the Earth
d) The imaginary plane formed by the Earth's orbit

Answer: d) The imaginary plane formed by the Earth's orbit

 

46. What is meant by "Season"?

a) A period of one month with fluctuating weather
b) The time when Earth moves closest to the Sun
c) A period of the year when general weather conditions remain the same
d) A point in Earth’s orbit where days are longer than nights

Answer: c) A period of the year when general weather conditions remain the same

 

47. What does "Solstice" refer to?

a) The time when days and nights are of equal duration
b) The time when the Sun is directly overhead one of the tropics
c) When the Earth is at its farthest point from the Sun
d) When the Earth’s rotation slows down

Answer: b) The time when the Sun is directly overhead one of the tropics

 

48. What is an "Equinox"?

a) When the Earth is tilted towards the Sun
b) When the distance between Earth and Sun is equal
c) When days and nights are of equal duration throughout the world
d) The shortest day of the year in the Northern Hemisphere

Answer: c) When days and nights are of equal duration throughout the world

 

49. When does the Northern Hemisphere experience its longest day?

a) December 22
b) March 21
c) June 21
d) September 23

Answer: c) June 21

 

50. When do days and nights have equal duration worldwide?

a) June 21 and December 22
b) March 21 and September 23
c) July 1 and January 1
d) April 15 and October 15

Answer: b) March 21 and September 23

 

51. When does the Northern Hemisphere experience its shortest day?

a) June 21
b) September 23
c) December 22
d) March 21

Answer: c) December 22

 

Sortinge Materials into Groups

 

The objects which we see around us are made of different materials like wood, glass, rubber, plastic, cotton, etc. They are of different shapes, sizes, colours and uses. For example, table, chair, bag, books, clothes, birds, cars, trees are all made of different materials. How do we group such a variety of objects?

We group objects on the basis of certain similarities and differences in their properties. This type of grouping in which similar objects are placed together is known as classification. Therefore, the above mentioned objects may be classified as living or non-living. We can also classify them as naturally occurring or man-made objects.

 

But why is it necessary to classify objects into groups?

1. Classification makes the study of things around us convenient and systematic.
2. It also helps us to have a general idea about the members of a group.
3. We come to know how each member differs from the other in the same group or in different groups.

 

OBJECTS AROUND US

1. An object may be made up of different materials. For example, a chair may be made of wood, steel or plastic.
2. Different objects may be made up of the same material. For example, utensils, parts of vehicles, bridges are all made of steel. 
3. An object may be made of many materials. For example, aerated drinks contain water, sugar and a gas dissolved in it.

All the objects that we see and feel around us have definite mass and occupy space. We can say that these objects are made up of matter. Therefore, matter is defined as anything that has mass and occupies space. The various objects and materials we discussed before, are all matter. Air is also matter since it has mass and occupies space, though we cannot see it. The amount of matter contained in an object is measured by its mass. The space occupied by an object is known as its volume.

Matter is made up of tiny particles called molecules. The arrangement of molecules decides whether an object is a solid, liquid or a gas.

 

Solid: The molecules are tightly packed and a strong attractive force acts between them. So, they have a definite shape and volume.

Liquid:

The molecules in a liquid are loosely packed and the intermolecular attractive forces between them are weak. So, liquids do not have a definite shape but they have a definite volume and take the shape of the container in which they are kept.

Gas: The molecules in a gas are far apart from each other and there is hardly any attractive force acting between them. So, a gas does not have a fixed shape or volume. The molecules move around freely and occupy the entire space available to them.

Materials-All objects are made up of substances called materials.

 

PROPERTIES OF MATERIALS

The characteristics of materials, that enable us to separate one object from the other, are called its properties. The properties of materials decide how and for what purpose they can be used. For example, wood or steel would be the most suitable material for making a cupboard but not clothes. Some of the properties of materials are discussed below.

 

Appearance

It is the external property of matter that distinguishes one material from the other. How things appear to the eyes is called appearance.

Metals like iron, copper, aluminium, gold, etc., have a shining surface. This characteristic of metals is known as lustre. Materials like wood, plastic, paper or rubber do not possess lustre and appear dull. This characteristic of non-metals is termed as non-lustre.When metals are exposed to air, most of them lose their lustre and get tarnished due to the action of air and moisture. 

On polishing or rubbing the surface with sandpaper, they regain their lustre once again. Therefore, we notice the lustre, only on their freshly cut surface. Gold and silver metals are made into ornaments due to their colour and lustre. Gold does not lose its shine on exposure to air, so ornaments made of this metal remain sparkling throughout. 

 

Hardness

Materials are grouped according to their ability to be compressed or squeezed. We can easily squeeze a piece of sponge between our fingertips but we cannot squeeze an iron ball, even if we press it with all our might. Therefore, we say that sponge is soft while iron is hard. Materials which can be compressed or scratched easily are called soft, while those materials which are difficult to compress are called hard. Diamond is one of the hardest substance

 

Metals like lead, aluminium, copper and materials like wood, stone, etc., are hard. Chalk, cotton, wax, etc., are soft materials. The shape of hard objects cannot be changed by applying any amount of force but the shape of soft objects can be altered easily. The property of hardness of a material decides the purpose for which it can be used. For example, the materials used for making vehicles, houses, roads, etc., should be hard while clothes should be made of soft materials. The soft materials can also be scratched with our nails, but not the hard materials. We can easily scratch a candle, made of wax, but it is not possible to scratch a stone.

Materials can also be smooth or rough to touch. The surface of steel utensils, polished wood, etc., are smooth. Bricks, rocks and soil have rough surfaces. Therefore, the materials can also be classified according to their smoothness or roughness.

 

Lustre, hardness, roughness or smoothness are some of the properties which describe the appearance of a material. In addition to this, we can also describe the appearance of a material by its size and weight i.e., whether it is large or small or whether it is heavy or light.

 

Solubility

Materials or substances like sugar or salt, when mixed with water, disappear completely in it. We say that, sugar or salt has dissolved in water. Substances which dissolve in water are known as soluble substances. Sand and chalk powder do not dissolve in water. They are said to be insoluble substances.

 

The substance which dissolves in a liquid is known as a solute. The liquid in which a solute dissolves is known as a solvent. The solute and solvent together make up a solution.

Water is the most commonly used solvent since it readily dissolves a number of substances in it. Therefore, it is called a universal solvent. Being a universal solvent, water plays an important role in the functioning of our body.

 

Some liquids like glycerine, vinegar and lemon juice dissolve in water to form solutions. They are known as miscible liquids. Vinegar and lemon juice or vinegar and glycerine are miscible with each other. In both the cases, the solvent used is not water. 

Cooking oil, kerosene oil, petrol, etc., do not dissolve in water. They separate into two distinct layers when kept undisturbed for some time. The liquids which do not mix with each other are known as immiscible liquids. Sand is insoluble in water other are known as immiscible liquids.

 

Gases like oxygen and carbon dioxide dissolve in water. These gases help the aquatic plants and animals to carry out the process of photosynthesis and respiration. Aerated or fizzy drinks also contain carbon dioxide dissolved in water under pressure.

 

Floatation

Certain materials float on water whereas, others sink. This property is called floatation. Generally, wood, leaves, plastic, paper and feathers float on water whereas rocks glass, honey, sand and metals sink. We can test these materials to see whether they float o sink in oil.

 

The materials which sink in water, will sink in oil, since oil is lighter than water. The materials which float in water, may float or sink in the oil, depending on the density of oil and the density of the materials. Hence, floating and sinking of objects will depend upon density of the liquid and the density of the objects.

Density is a property of matter that describes how heavy an object is for its size. Density compares the weight of a material to its volume. In some matter, the molecules are more closely packed together. These materials have a higher density. Scientists compare everything to water; which has density equal to 1 g/cm³. When the density of a material is less than one, it will float in water. Materials with a density greater than one will sink in water.

 

The shape of an object also affects the amount of liquid it displaces. Certain heavy objects are able to float because of their shape. For example, the shape of a ship allows it to float on water, despite the fact that it is very heavy.

 

Transparency:

The amount of light that passes through an object depends on the material with which it is made. Based on this property, materials are classified as transparent, translucent and opaque.

Transparent Materials: These materials allow light to pass through them easily. Air, glass, clear water and some plastics are transparent materials. We can see the objects clearly through these materials.

 

Translucent Materials:These materials allow light to pass through them partially. Ground glass, oiled or waxed paper are translucent materials. We cannot see the objects clearly through these materials. When you cover the palm of your hands with a torch, you can see a dull red glow. Here, the palm acts as a translucent object.

 

Opaque Materials: These materials do not allow light to pass through them at all. Metals, rocks, wood and cardboard are opaque materials. The objects are not visible through opaque materials.

So, all the materials can be grouped in these three categories without any confusion.

 

Thermal and Electrical Conductivity

Metals like silver, copper, iron and aluminium become hot when they are heated over a flame. Thus, they conduct heat. These materials are said to be good conductors of heat.

 

Materials like plastic, rubber, wood, etc., do not allow heat to pass through them. These materials are called poor conductors or bad conductors of heat. The conduction of heat through a substance is called its thermal conductivity.

 

Both pure water and air are bad conductors of heat. Pure water is bad conductor of electricity. Impurities like acids and bases present in it make it a conductor of electricity.

 

You must have noticed that some cooking vessels like pressure cookers and frying pans are made of metals but their handles are made of a plastic like material, bakelite. This is because, metals being good conductors of heat warm up the food quickly. Bakelite, being a bad conductor Lear of heat, does not get heated. Therefore, the handles can be held easily while cooking food in them.

Metals allow electric current to pass through them. So, they are known as good conductors of electricity. Plastic, rubber, wood and paper are poor conductors or bad conductors of electricity.

Materials which do not conduct electricity are known as bad conductors or insulators.

 

Magnetic property

Certain substances have the property of being attracted by a magnet. Such materials are called magnetic materials. Metals like iron, cobalt and nickel are magnetic materials. Objects made of these metals will get attracted to the magnet. The ends of a magnet are called the north pole and south pole.

The materials that are not attracted by a magnet are called non- magnetic materials. Aluminium, copper, brass, wood, plastic, paper and rubber are some of the non-magnetic materials.

 

Thus, materials can be arranged into groups according to any of  the above discussed properties. This type of grouping makes it easier to find things when they are required. It also facilitates the study of their properties and also observe any patterns in these properties.

 

IMPORTANT TERMS

 

Materials: The substances with which objects are made.

 

Classification: The process of grouping things based on similarities and differences.

 

Matter: Matter is anything that has mass and occupies space.

 

Appearance: How things appear to the eyes.

 

Lustre: The shiny appearance of materials.

Transparent materials: The materials which allow light to pass through them

Translucent materials : The materials which allow light to pass through them partially

Opaque materials : These materials do not allow light to pass through them at all)

 

Placing similar things together is called grouping.

Grouping makes it easier to find things when they are required.

Things are grouped together for convenience and to study their properties.

 

Objects are classified based on similarities and differences. Some materials are shiny in appearance while others are not. Some are hard and some soft. Similarly, some materials are rough, others are smooth.

 

Substances which dissolve in water are called soluble substances while substances which do not dissolve are called insoluble. Some materials float on water whereas some sink in it.

 

 

 

Exercise

1. What is the process of grouping objects with similar properties called?

Identification

Classification

Separation

Observation

Answer: (b) Classification

Explanation: Classification is the process of grouping objects based on their similarities and differences in properties. It helps in organizing a wide variety of objects, making them easier to study.

 

Q2. Which of the following can be used as a criterion to classify objects?

Size and Color

Similarities and Differences

Price and Value

Age and Weight

Answer: (b) Similarities and Differences

Explanation: Objects are classified based on similarities and differences in their properties such as material, shape, size, or function. This helps us place similar objects together and understand their relationships.

 

3. Why is it necessary to classify objects into groups?

To create confusion

To make the study of objects easier and systematic

To eliminate some objects

To separate them into equal parts

Answer: (b) To make the study of objects easier and systematic

Explanation: Classification helps in the systematic study of objects by grouping similar items together, making it convenient to understand and compare their characteristics.

 

4. How can objects be classified besides being living or non-living?

According to their uses

As naturally occurring or man-made

Based on their weight

Based on their color

Answer: (b) As naturally occurring or man-made

Explanation: Another common method of classification is grouping objects into naturally occurring (e.g., trees, animals) and man-made (e.g., plastic, cars) objects, making it easier to understand their origin and purpose.

 

5. Which of the following is a correct statement about matter?

Matter has no mass or volume

Matter occupies space and has mass

Matter only exists in liquid form

Matter is always visible to the human eye

Answer: (b) Matter occupies space and has mass

Explanation: Matter is defined as anything that has mass and occupies space. This includes solids, liquids, gases, and even air, which, though invisible, is still considered matter.

 

6. Which of the following is true about solids?

They have a definite shape but no definite volume

They have neither definite shape nor volume

They have a definite shape and volume

They do not occupy space

Answer: (c) They have a definite shape and volume

Explanation: In solids, the molecules are tightly packed, resulting in a definite shape and volume. The strong intermolecular forces keep the structure intact.

 

7. What property of liquids differentiates them from solids?

Liquids have a definite shape

Liquids do not have a definite shape but have a definite volume

Liquids do not have volume

Liquids are made of tightly packed molecules

Answer: (b) Liquids do not have a definite shape but have a definite volume

Explanation: Liquids have molecules that are loosely packed, allowing them to take the shape of their container. However, they still retain a definite volume.

 

8. Why do gases occupy the entire space available to them?

Because their molecules are tightly packed

Because there is a strong force between the molecules

Because their molecules are far apart and move freely

Because gases have a definite shape and volume

Answer: (c) Because their molecules are far apart and move freely

Explanation: In gases, the molecules are far apart, and there is little to no attractive force between them. This allows them to move freely and fill any container they are placed in, occupying all available space.

 

9. Which of the following objects is made of more than one material?

A wooden chair

A steel bridge

An aerated drink

A plastic bottle

Answer: (c) An aerated drink

Explanation: An aerated drink is composed of water, sugar, and dissolved gas, making it an example of an object made from multiple materials. Other examples like a wooden chair or steel bridge are made from a single material.

 

10. What is the term used for the external property of matter that distinguishes one material from another?

Solubility

Hardness

Appearance

Miscibility

Answer: (c) Appearance

Explanation: Appearance refers to how materials look or appear to the eyes. It is an external property that helps distinguish one material from another, such as the shiny surface of metals versus the dull appearance of non-metals.

 

11. Which of the following materials is known for retaining its lustre even after exposure to air?

Iron

Gold

Copper

Aluminium

Answer: (b) Gold

Explanation: Gold does not lose its lustre when exposed to air, which is why it is used for making ornaments. Unlike most metals, gold remains shiny without tarnishing.

 

12. What is the property of materials that describes their ability to be compressed or scratched?

Solubility

Hardness

Appearance

Roughness

Answer: (b) Hardness

Explanation: Hardness is the property of materials that determines how easily they can be compressed or scratched. Hard materials like iron and diamond cannot be easily scratched, while soft materials like wax or sponge can be compressed and scratched easily.

 

13. Which of the following substances is not soluble in water?

Sugar

Salt

Sand

Vinegar

Answer: (c) Sand

Explanation: Sand is an insoluble substance and does not dissolve in water, whereas sugar, salt, and vinegar are soluble in water and form solutions.

 

14. What is the term used for liquids that do not mix with each other?

Miscible liquids

Immiscible liquids

Soluble liquids

Insoluble liquids

Answer: (b) Immiscible liquids

Explanation: Immiscible liquids are liquids that do not mix with each other and separate into distinct layers, such as oil and water. Miscible liquids, on the other hand, mix completely to form solutions.

 

15. Which of the following gases dissolve in water and help aquatic plants and animals in photosynthesis and respiration?

Nitrogen

Hydrogen

Oxygen and Carbon dioxide

Methane

Answer: (c) Oxygen and Carbon dioxide

Explanation: Oxygen and carbon dioxide dissolve in water, enabling aquatic plants and animals to perform photosynthesis and respiration. These dissolved gases are essential for life in aquatic ecosystems.

 

16. What is a universal solvent?

Vinegar

Water

Kerosene

Alcohol

Answer: (b) Water

Explanation: Water is called the universal solvent because it dissolves a large number of substances, making it essential for various biological and chemical processes, including its role in the human body.

 

17. Which of the following is the hardest material?

Wax

Chalk

Diamond

Lead

Answer: (c) Diamond

Explanation: Diamond is one of the hardest known substances and is extremely difficult to compress or scratch. It is used in cutting tools because of its superior hardness.

 

18. What property of a material determines whether it will float or sink in water?

Shape

Volume

Density

Hardness

Answer: (c) Density

Explanation: The ability of a material to float or sink in water depends on its density. If the density of the material is less than that of water, it will float. If it is greater, it will sink.

 

19. Why does a metal ship float on water even though it is made of metal, which normally sinks?

The density of metal decreases when shaped into a ship

The shape of the ship allows it to displace enough water to stay afloat

The ship is made of a special type of metal

Ships have internal heat that keeps them afloat

Answer: (b) The shape of the ship allows it to displace enough water to stay afloat

Explanation: Even though metal has a high density, the shape of a ship allows it to displace enough water, reducing its overall density compared to water, which allows it to float.

 

20. Which of the following is an example of a translucent material?

Clear glass

Wood

Ground glass

Iron

Answer: (c) Ground glass

Explanation: Translucent materials allow some light to pass through but do not provide a clear image. Ground glass is a good example of a translucent material.

 

21. Which of the following materials would be classified as opaque?

Clear water

Plastic wrap

Cardboard

Transparent glass

Answer: (c) Cardboard

Explanation: Opaque materials do not allow any light to pass through them. Cardboard is an opaque material, meaning objects cannot be seen through it.

 

22. Why are cooking vessels made of metals but their handles made of materials like Bakelite?

Metals do not heat up quickly

Bakelite is stronger than metal

Metals are good conductors of heat, while Bakelite is a poor conductor of heat

Bakelite conducts heat better than metal

Answer: (c) Metals are good conductors of heat, while Bakelite is a poor conductor of heat

Explanation: Metals conduct heat efficiently, which helps in cooking, while Bakelite, being a poor conductor of heat, is used for handles to prevent them from getting too hot to touch.

 

23. Which of the following materials is a good conductor of electricity?

Rubber

Copper

Plastic

Wood

Answer: (b) Copper

Explanation: Copper is a metal and a good conductor of electricity. Materials like rubber, plastic, and wood are poor conductors and are often used as insulators.

 

24. What is the term used for materials that are not attracted by a magnet?

Magnetic materials

Non-magnetic materials

Conductors

Insulators

Answer: (b) Non-magnetic materials

Explanation: Non-magnetic materials, such as wood, plastic, and rubber, are not attracted to magnets. Magnetic materials, like iron, cobalt, and nickel, are attracted to magnets.

 

25. What is the name of the property that allows certain materials to float on water?

Transparency

Floatation

Conductivity

Magnetic property

Answer: (b) Floatation

Explanation: Floatation is the property of materials that determines whether they float on water. This is influenced by the density of the material compared to the density of the water.

 

26. Which of the following substances is an immiscible liquid when mixed with water?

Vinegar

Glycerine

Cooking oil

Lemon juice

Answer: (c) Cooking oil

Explanation: Cooking oil does not mix with water and forms a separate layer when kept undisturbed. It is an example of an immiscible liquid.

 

28. What is matter?

Anything that has color and shape

Anything that has mass and occupies space

Only solid substances

Substances that dissolve in water

Answer: (b) Anything that has mass and occupies space

Explanation: Matter is defined as anything that has mass and occupies space. This includes solids, liquids, and gases.

 

29. What is lustre?

The roughness of a material

The hardness of a material

The shiny appearance of materials

The softness of a material

Answer: (c) The shiny appearance of materials

Explanation: Lustre refers to the shiny or glossy appearance that some materials, especially metals like gold and silver, exhibit.