Class 8 Science Chapter 11 The Earth and Space

 The Earth:

• The third planet from the Sun, only known planet to support life.
• Made up of lithosphere (land), hydrosphere (water), atmosphere (air), and biosphere (life).
• Age: about 4.5 billion years.
• Universe:
  • All matter and energy, including stars, galaxies, planets, and cosmic dust.
  • Believed to have formed from the Big Bang (~13.8 billion years ago).
• Earth is a tiny part of the vast universe, but it provides unique conditions (water, atmosphere, temperature) necessary for life.

Exam Tip: Be clear about the distinction – Earth = one planet; Universe = everything.

 Minerals

• Definition: Naturally occurring inorganic solid substances with definite chemical composition and crystal structure.
• Examples: Gold, Iron, Coal, Salt, Gypsum, Quartz.
• Minerals are the building blocks of rocks and essential for daily human use.

 Types of Minerals

1. Metallic Minerals

• Definition: Minerals that contain metals in raw form.
• Examples: Iron ore, Copper, Bauxite, Gold, Silver.
• Uses:
  • Iron → steel production, construction.
  • Copper → wires, electrical appliances.
  • Gold & Silver → jewelry, electronics, currency reserves.

2. Non-Metallic Minerals

• Definition: Minerals without metallic elements; usually insulators, non-conductive.
• Examples: Limestone, Gypsum, Mica, Salt, Diamond.
• Uses:
  • Limestone → cement, construction.
  • Gypsum → plaster, fertilizers.
  • Mica → electrical insulation.

3. Energy Minerals

• Definition: Minerals used as fuels to produce energy.
• Examples: Coal, Petroleum, Natural gas, Uranium.
• Uses:
  • Coal → thermal power, industries.
  • Petroleum → fuel, plastics.
  • Uranium → nuclear energy.

 Properties of Minerals

1. Hardness – ability to resist scratching (e.g., diamond is hardest).
2. Luster – how a mineral reflects light (metallic, glassy, dull).
3. Color – external appearance, though sometimes misleading.
4. Streak – color of powdered mineral when rubbed on porcelain.
5. Cleavage/Fracture – tendency to split (cleavage) or break irregularly (fracture).
6. Density/Specific Gravity – heaviness compared to water.

 Uses of Minerals

• In Agricultural Sector:
  1. Phosphate & potash → fertilizers.
  2. Gypsum → soil treatment.
• In Industrial Sector:
  1. Coal, petroleum → energy for industries.
  2. Iron, copper → manufacturing tools, machinery.
• In Construction Sector:
  1. Limestone → cement, roads.
  2. Granite, marble → buildings, monuments.
• In Energy Sector:
  1. Uranium → nuclear power.
  2. Petroleum & coal → electricity, transport fuels.

 Minerals in Nepal

• Nepal has a variety of metallic, non-metallic, and energy minerals due to its complex geology.
• Valuable deposits:
  • Iron ore (Phulchoki, Dhauwadi, Labdhi).
  • Limestone (Udayapur, Chovar, Hetauda) → cement industries.
  • Marble (Godawari, Makwanpur).
  • Slate (Sindhupalchok, Baglung).
  • Coal (Dang, Rolpa, Palpa).
  • Copper (Arghakhanchi, Tanahun, Okhaldhunga).
• Current mining is limited due to lack of technology, investment, and infrastructure.

Interesting Facts

• Diamond is the hardest known natural mineral.
• Salt was once so valuable it was used as currency (“salary” comes from “sal”, Latin for salt).
• Nepal still imports most minerals despite having deposits.

Mnemonics / Memory Aids

• Types of Minerals: MEN → Metallic, Energy, Non-metallic.
• Properties: HLCSDC → Hardness, Luster, Color, Streak, Density, Cleavage.

Summary / Quick Revision

• Earth: only habitable planet; Universe: everything.
• Minerals: naturally occurring, definite composition.
• Types: Metallic (iron, copper), Non-metallic (limestone, gypsum), Energy (coal, uranium).
• Properties: hardness, luster, color, streak, cleavage, density.
• Uses: agriculture (fertilizers), industry (machinery), construction (cement, marble), energy (coal, uranium).
• Nepal: deposits of iron, limestone, marble, coal, copper, but underutilized.

 Origin and Age of the Earth

• Age of Earth: about 4.5 – 4.6 billion years (determined using radiometric dating of rocks and meteorites).
• Theories suggest how Earth and planets formed from gases, dust, and cosmic materials after the formation of the Sun.

 Nebular Hypothesis

• Proposed by Immanuel Kant and developed by Laplace (1796).
• States that:
  • The solar system formed from a rotating nebula (cloud of gas and dust).
  • Due to gravity, the nebula collapsed, flattened into a disc, and the Sun formed at the center.
  • Planets, including Earth, formed from the remaining material.
• Importance: First scientific explanation for solar system formation.

Planetesimal Hypothesis

• Proposed by Chamberlin and Moulton (1905).
• Suggests:
  • A star passed close to the Sun → gravitational forces pulled material from the Sun.
  • Ejected materials condensed into small bodies called planetesimals.
  • These combined to form planets (Earth included).

 Binary Hypothesis

• Proposed by Lyttleton (1930s).
• Suggests:
  • The Sun was once part of a binary star system.
  • One star exploded/disappeared, leaving the Sun and a disc of matter.
  • From this disc, planets including Earth were formed.

 Tidal Hypothesis

• Proposed by James Jeans and Harold Jeffreys (1918–1925).
• Suggests:
  • A massive star passed near the Sun.
  • Strong tidal forces pulled matter out from the Sun.
  • The matter cooled, condensed, and formed planets.

Exam Tip: Be ready to compare these hypotheses. Nebular = gas cloud collapse, Planetesimal = small solid bodies, Tidal = matter pulled by passing star, Binary = companion star system.

 History of Development of Living Beings on Earth

• Life developed gradually after Earth cooled and oceans formed (~3.5 billion years ago).
• Evolution of life is divided into Eons and Eras.

 Cryptozoic Eon (Precambrian)

• Time span: From Earth’s origin (~4.5 billion years ago) to about 600 million years ago.
• Features:
  • Earth’s crust solidified.
  • First oceans formed.
  • Primitive life (bacteria, algae) appeared.
  • Atmosphere lacked oxygen initially.

 Phanerozoic Eon

• Started ~600 million years ago – present.
• Divided into three main eras:

a. Paleozoic Era (600 – 225 million years ago)

• Beginning of complex life.
• Three important events:
  1. Explosion of marine life (fishes, corals).
  2. Appearance of first land plants.
  3. First amphibians and reptiles evolved.

b. Mesozoic Era (225 – 65 million years ago)

• Known as “Age of Reptiles”.
• Three important events:
  1. Dinosaurs dominated land.
  2. First birds and mammals appeared.
  3. Breakup of supercontinent Pangaea.

c. Cenozoic Era (65 million years ago – present)

• Known as “Age of Mammals”.
• Four important events:
  1. Extinction of dinosaurs.
  2. Mammals diversified and dominated.
  3. Grasslands expanded → human evolution possible.
  4. Humans appeared (~2 million years ago).

Universe

• Definition: Everything that exists, including matter, energy, stars, planets, galaxies, and cosmic dust.
• Vast and still expanding.

 Origin of Universe – Big Bang Theory

• Proposed by Georges Lemaître (1927); evidence from Edwin Hubble’s expansion of galaxies.
• States:
  • About 13.8 billion years ago, all matter and energy were concentrated in a single point (singularity).
  • A tremendous explosion occurred (Big Bang).
  • Universe expanded rapidly, matter cooled, stars and galaxies formed.
• Evidence:
  • Cosmic Microwave Background Radiation (CMBR).
  • Redshift of galaxies (Hubble’s law).

Asteroids

• Definition: Small rocky objects orbiting the Sun, mostly found in the asteroid belt between Mars and Jupiter.
• Features:
  • Irregular shape, no atmosphere.
  • Size varies from few meters to hundreds of kilometers.
  • Example: Ceres, Vesta, Pallas.

 Comets

• Definition: Icy celestial bodies that release gas and dust forming a glowing head (coma) and tail when near the Sun.
• Features:
  • Orbit is highly elliptical.
  • Made of ice, dust, rock.
  • Tail always points away from the Sun due to solar wind.
• Famous Comets: Halley’s Comet, Hale-Bopp, Shoemaker-Levy 9.

Galaxy

• Definition: A huge system of stars, dust, gas, and dark matter bound by gravity.
• Features:
  • Contains billions of stars.
  • Our galaxy is the Milky Way.
• Examples: Andromeda Galaxy, Whirlpool Galaxy, Triangulum Galaxy.

 Constellation

• Definition: Recognizable group of stars forming imaginary patterns in the night sky.
• Features:
  • Used for navigation in ancient times.
  • Appear fixed but stars are light-years apart.
• Examples:
  • Orion (Hunter),
  • Ursa Major (Great Bear/Big Dipper),
  • Cassiopeia,
  • Leo.

Interesting Facts

• Light from the Sun takes 8 minutes 20 seconds to reach Earth.
• Halley’s comet appears every 76 years.
• The Milky Way galaxy alone has 100–400 billion stars.

Mnemonics / Memory Aids

• Eras of Life: “P-M-C” → Paleozoic, Mesozoic, Cenozoic.
• Big Bang Evidence: R-C → Redshift & Cosmic radiation.
• Constellations: “OULC” → Orion, Ursa Major, Leo, Cassiopeia.

Summary / Quick Revision

• Earth formed ~4.5 billion years ago; different hypotheses explain origin (Nebular, Planetesimal, Binary, Tidal).
• Life history: Cryptozoic (primitive life) → Phanerozoic (Paleozoic: fishes & plants; Mesozoic: dinosaurs; Cenozoic: mammals & humans).
• Universe: everything; formed by Big Bang (~13.8 billion years ago).
• Celestial objects:
  • Asteroids = rocky (Mars–Jupiter belt).
  • Comets = icy, with tails.
  • Galaxies = huge star systems (Milky Way).
  • Constellations = star patterns (Orion, Ursa Major).

Also Check Out :- Chapter 1 , chapter 2 , chapter 3 , chapter 4 , chapter 5 , chapter 6 , chapter 7 , chapter 8