`Questions and Answers
Chapter
1:
Exploring
the Investigative World of Science
Based on
1. Why does a puri
puff up while frying?
Answer:
When a puri is placed in hot oil, the water present inside the dough
turns into steam. This steam pushes the two layers of dough apart, causing the
puri to puff up like a balloon.
2. Why is one side
of a puri thinner than the other?
Answer:
One side becomes thinner because steam pressure inside the puri pushes
more strongly on one side during frying. The side that expands more becomes
thinner. Oil temperature, dough thickness, and the way the puri is dropped into
oil also affect this.
3. What is meant by
scientific investigation?
Answer:
Scientific investigation is a systematic method of finding answers to
questions through observation, experiments, and careful analysis. Scientists
change one factor at a time and observe the results.
4. Why should only
one thing be changed during an experiment?
Answer:
Only one factor should be changed at a time so that we can clearly
understand its effect on the result. If many things are changed together, it
becomes difficult to know which factor caused the change.
5. Why are
observations important in science?
Answer:
Observations help scientists collect information and understand what is
happening during an experiment. Careful observation leads to better
explanations and new discoveries.
6. What are some
examples of scientific questions from everyday life?
Answer:
Examples include:
• Why does dough
rise?
• Why does a puri
puff up?
• Why does the
Moon change shape?
• Why is the Earth
getting warmer?
7. Why is science
called an investigative world?
Answer:
Science is called an investigative world because scientists continuously
ask questions, perform experiments, make observations, and improve their
understanding about nature and the universe.
8. What is the
importance of curiosity in science?
Answer:
Curiosity motivates us to ask questions and explore new ideas. It is the
starting point of all scientific discoveries.
9. How are forces
useful in daily life?
Answer:
Forces help objects move, stop, speed up, or change direction. They are
involved in activities like throwing a ball, applying brakes in a car, and
running machines.
10. What causes
winds and cyclones?
Answer:
Differences in air pressure cause the movement of air. Small pressure
differences create breezes, while large differences can cause storms and
cyclones.
11. Why are
microorganisms important?
Answer:
Some microorganisms
help in digestion, medicine production, and decomposition,
while others may cause diseases.
12. Why is Earth
suitable for life?
Answer:
Earth is suitable for life because:
• It is at the
right distance from the Sun.
• Water remains
liquid.
• The atmosphere
contains oxygen.
• The atmosphere protects
us from harmful
ultraviolet rays.
Chapter
2:
The
Invisible Living World – Beyond Our Naked Eye
Questions and Answers
Based on
1. What are
microorganisms?
Answer:
Microorganisms are very tiny living organisms that cannot be seen with
the naked eye. They can only be seen with a microscope.
2. Name the main
types of microorganisms.
Answer:
The main types of microorganisms are:
• Bacteria
• Fungi
• Protozoa
• Algae
• Viruses
3. Who discovered
cells?
Answer:
Robert Hooke discovered cells in 1665 while observing a thin slice of
cork under a microscope.
4. Who is known as
the Father of Microbiology?
Answer:
Antonie van Leeuwenhoek is known as the Father of Microbiology because he
first observed bacteria and other tiny living organisms.
5. What is a cell?
Answer:
A cell is the basic structural and functional unit of life. All living
organisms are made up of cells.
6. What are the
basic parts of a cell?
Answer:
The three basic parts of a cell are:
• Cell membrane
• Cytoplasm
• Nucleus
Plant cells also have a cell wall.
7. What is the
function of the cell membrane?
Answer:
The cell membrane protects the cell and controls the movement of
substances into and out of the cell.
8. What is the
function of the nucleus?
Answer:
The nucleus controls all activities of the cell and regulates growth.
9. What is the
function of cytoplasm?
Answer:
Cytoplasm contains different cell components, and most life processes
occur in it.
10. What is the role
of the cell wall in plants?
Answer:
The cell wall gives strength, support, and rigidity to plant cells.
11. What is the
difference between plant and animal cells?
Answer:
Plant cells have:
• Cell wall
• Chloroplasts
• Large vacuole
Animal cells do not have a cell wall or chloroplasts, and their vacuoles
are small.
12. Why do cells
have different shapes?
Answer:
Cells have different shapes because each type of cell performs a special
function. The shape helps the cell perform its work efficiently.
13. What are
unicellular organisms?
Answer:
Organisms made up of only one cell are called unicellular organisms.
Example: bacteria and Amoeba.
14. What are
multicellular organisms?
Answer:
Organisms made up of many cells are called multicellular organisms.
Example: humans and plants.
15. What are the
levels of organisation in living organisms?
Answer:
The levels of organisation are:
Cell → Tissue → Organ → Organ System → Organism
16. How do
microorganisms help in decomposition?
Answer:
Microorganisms break down dead plants, animals, and waste materials into
simpler substances rich in nutrients. This process is called decomposition.
17. What is manure?
Answer:
Manure is a nutrient-rich substance
formed when microorganisms decompose plant and animal waste. It improves soil
fertility.
18. What is the role
of yeast in bread making?
Answer:
Yeast produces carbon dioxide during respiration. The gas forms bubbles
in the dough, making bread soft and fluffy.
19. Which bacterium
helps in curd formation?
Answer:
Lactobacillus bacteria help convert milk into curd by producing lactic
acid.
20. What is the
function of Rhizobium bacteria?
Answer:
Rhizobium bacteria live in the root nodules of leguminous plants and fix
nitrogen from the air into the soil.
21. Why are
microalgae important?
Answer:
Microalgae:
• Produce oxygen
• Serve as food
for aquatic animals
• Help clean water
• Are used in
medicines and biofuel production
22. Why are viruses
different from other microorganisms?
Answer:
Viruses are acellular and can reproduce only inside living cells.
23. What would
happen if microorganisms did not exist?
Answer:
Dead plants and animals would not decompose, waste would accumulate, soil
fertility would decrease, and many food products like curd and bread could not
be prepared properly.
Chapter - 3
Health: The
Ultimate Treasure
Questions and Answers Very
Short Answer Questions Q1. What is health?
Health is a state of complete physical, mental, and social well-being.
Q2. What are pathogens?
Disease-causing microorganisms are called pathogens.
Q3. Name two communicable diseases.
Cholera and tuberculosis.
Q4. Name two non-communicable
diseases.
Diabetes and asthma.
Q5. What is immunity?
The ability of the body to fight diseases.
Q6. What are vaccines?
Substances that help the body develop protection against diseases.
Q7. Who discovered penicillin?
Alexander Fleming.
Q8. What is antibiotic resistance?
When bacteria stop responding to antibiotics.
Important One-Line
Points • Prevention is
better than cure.
• Vaccines prevent
diseases.
• Cleanliness
helps prevent infections.
• Antibiotics kill
bacteria only.
• Balanced diet is
essential for good health.
• Exercise keeps
the body fit.
• Mosquitoes
spread malaria and dengue.
• Healthy mind and
healthy body are connected.
Quick
Revision Mind Map
Conclusion
Good health is essential for a happy and successful life. Proper
nutrition, cleanliness, exercise, vaccination, and healthy habits protect us
from diseases. A healthy body, peaceful mind, and clean environment together
create a healthy life.
Chapter - 4
Electricity:
Magnetic and Heating Effects
Chapter
Questions and Answers – Grade 8
1. Fill in
the blanks
(i) The solution used in a Voltaic cell is called ________.
Answer: Electrolyte
(ii) A current carrying coil behaves like a _______.
Answer: Magnet
2. Choose
the correct option
(i)
Dry cells are less portable
compared to Voltaic cells.
Answer: False
(ii)
A coil becomes an
electromagnet only when electric current flows through it.
Answer: True
(iii)
An electromagnet, using a
single cell, attracts more iron paper clips than the same electromagnet with a
battery of 2 cells. Answer: False
3. Heating
and Magnetic Effect in Nichrome Wire An electric current flows through a
nichrome wire for a short time.
(i) The wire becomes warm.
(ii)
A magnetic compass placed
below the wire is deflected.
Correct Option:
Both (i) and (ii) are correct Explanation:
The wire becomes warm due to the heating effect of
electric current. The compass needle deflects because current flowing through
the wire creates a magnetic field.
4. Match the Following
Column A Column B
(i)
Voltaic cell (d) chemical reactions
(c) Works
on heating effect of
(ii)
Electric iron
electric
current
Column A Column B
(iii)
Nichrome
(a) Best
suited for electric heater
wire
(iv)
(b) Works on magnetic effect
of
Electromagnet
electric current
5. Why is nichrome wire commonly used in heating
devices?
Correct Option:
(ii) generates more heat for a given current
Explanation:
Nichrome wire has high resistance, so it produces more
heat when electric current passes through it.
6. Why are
electric heating devices more convenient than traditional methods?
Answer:
Electric heating devices are more convenient because:
• They are cleaner and produce less smoke.
• They heat quickly and efficiently.
• They are easy to use.
• They reduce air pollution.
• They save time and effort.
• They are safer than burning wood or charcoal.
7. Questions based on Fig. 4.4a
(i) Draw an arrow to show the direction of current. Answer:
Current flows from the positive terminal of the battery through the coil
towards the negative terminal.
(ii) Why does the compass needle move?
Answer: The compass needle moves because the current
carrying coil produces a magnetic field around it.
(iii)
What happens if battery
terminals are reversed?
Answer: The direction of current changes, so the
magnetic poles of the electromagnet also reverse. The compass needle will
deflect in the opposite direction.
8. Why did the lifting electromagnet stop lifting clips?
Answer:
Possible reasons are:
• The battery became weak after continuous use.
• Less current flowed through the coil.
• The magnetic field became weak.
• The wire became warm due to the heating effect.
• Therefore, the electromagnet could no longer attract the
clips.
9. In which case will the LED glow?
Answer:
The LED will glow in Fig. (a).
Explanation:
Lemon juice acts as an electrolyte and allows electric
current to flow. Pure water in Fig. (b) does not conduct electricity well.
10. What happens if the iron nail is removed from the coil?
Answer:
Yes, the coil will still deflect the compass because
current flowing through the coil still produces a magnetic field.
However, the deflection will be less because the iron
nail makes the electromagnet stronger.
11. Compass needle deflection in different coils
Correct Option: (iv) In all four
circuits Explanation:
|
All
the materials—iron, copper, aluminium, and nichrome—allow electric current to
pass through them. Therefore, all the coils produce magnetic fields and deflect
the compass needle.
Extra
Important Short Questions and Answers
1. What is the magnetic effect of electric current? When
electric current flows through a conductor, it produces a magnetic field around
it. This is called the magnetic effect of electric current.
2. What is an electromagnet?
A current carrying coil that behaves like a magnet is
called an electromagnet.
3. Name two uses of electromagnets.
1. Electric bells
2. Cranes used for lifting heavy iron objects
4. What is the heating effect of electric current? The
production of heat when electric current flows through a conductor is called
the heating effect of electric current.
5. Why is nichrome wire used in heaters? Nichrome wire has high
resistance and produces more heat.
6. What is a Voltaic cell?
A Voltaic cell is a device that produces electricity
using chemical reactions.
7. What is the electrolyte in a lemon cell?
Lemon juice acts as the electrolyte.
8. What is a dry cell?
A dry cell is an electric cell that contains a
paste-like electrolyte.
9. What are rechargeable batteries?
Rechargeable batteries are batteries that can be
charged and reused many times.
10. Why should batteries be recycled?
Batteries contain harmful chemicals and valuable
materials. Recycling protects the environment.
Quick
Revision Notes
• Electric current produces magnetic and heating effects.
• A current carrying coil behaves like a magnet. • Electromagnets work only when
current flows.
• Heating effect is used in electric heaters, irons and
kettles.
• Cells and batteries produce electricity using chemical
reactions.
• Rechargeable batteries can be reused many times.
Chapter - 5
Exploring Forces – Humanized Notes
with
Questions & Answers
Q1. What is
force?
Answer: Force is a
push or pull acting on an object due to interaction with another object.
Q2. What
are the effects of force?
Answer: Force can:
• Start motion
• Stop motion
• Change speed
• Change direction
• Change shape
Q3. What is
muscular force?
Answer: The force
produced by muscles of humans or animals is called muscular force.
Q4. What is
friction?
Answer: Friction
is the force that opposes motion between two surfaces in contact.
Q5. Why is
friction useful?
Answer: Friction
helps us walk, write, hold objects, and move vehicles.
Q6. Why do
we slip on smooth surfaces? Answer: Smooth surfaces produce less
friction, so our feet cannot grip properly.
Q7. What is
magnetic force?
Answer: The force
exerted by a magnet on another magnet or magnetic material is called magnetic
force.
Q8. What is
electrostatic force?
Answer: The force
exerted by charged objects on other charged or uncharged objects is called
electrostatic force.
Q9. What is
gravitational force?
Answer: The force
with which Earth attracts objects toward itself is called gravitational force.
Q10. What
is weight?
Answer: Weight is
the gravitational force with which Earth pulls an object.
Q11.
Differentiate between mass and weight.
Mass Weight
Amount of matter Gravitational force Constant Changes with gravity
kg N
Q12. Why
does a wooden block float but a coin sink?
Answer: The wooden
block experiences enough buoyant force to float, while the coin’s weight is
greater than the buoyant force.
Q13. Why
does a ball thrown upward come back? Answer: Earth’s gravitational force
pulls the ball back downward.
Q14. State
True or False.
1. Force can change the speed of an object. → True
2. Friction increases motion. → False
3. Gravity is a non-contact force. → True
Match the
Following
Column A Column B
Muscular
force Lifting a bag
Magnetic
force Compass needle
Frictional
force Ball stopping
Gravitational force Fruit falling
Electrostatic force Balloon attracting hair
HOTS Questions
Q1. Why do
vehicles have tyres with grooves? Answer: Grooves increase friction
and prevent slipping.
Q2. Why do
astronauts float in space?
Answer: Because
they experience very weak gravitational pull.
Q3. Why do
ships float though they are heavy? Answer: Ships displace a large
amount of water, producing enough buoyant force to float.
Conclusion
Force plays an important role in every activity around
us. From walking and cycling to floating boats and falling fruits, different
forces work continuously in nature. Understanding these forces helps us explain
many everyday events scientifically.
Chapter - 6
Pressure, Winds, Storms and Cyclones
Solved Questions and Answers
1. Multiple Choice Answers
(i)
Correct Answer: (d) equal in all
three vessels
(ii)
Correct Answer: (c) M will stick
but N will not stick
(iii)
Correct Answer: (a) increase the
height H
(iv)
Correct Answer: (b) PA = PB, FA
< FB
2. True or False
Statement
Answer
Air flows from high pressure to low pressure True
Liquids exert pressure only at bottom
False
Weather is stormy at eye of cyclone False
Car is safer during thunderstorm True
3. Boy on Sand
The boy sinks more when standing vertically because:
•
area of contact is smaller,
•
pressure becomes greater.
4. Elephant Pressure Calculation Given:
•
Weight = 20000 N
•
Area of one foot = 0.25 m²
•
Total area = 4 × 0.25 = 1 m²
Formula
𝑃 = 
Answer
Pressure = 20000 Pa
5. Boat Pressure Problem
Boat A
•
Total force = 5 × 700 = 3500 N
•
Area = 7 m²
𝑃𝐴
= 
=
500 𝑃𝑎
Boat B
•
Total force = 3 × 700 = 2100 N
•
Area = 3.5 m²
𝑃𝐵
= 
=
600 𝑃𝑎
Answer
Boat B exerts more pressure.
Difference = 600 − 500 = 100 Pa
6. Would Lightning Occur if Air Was a Good Conductor?
No.
Because charges would flow continuously through air and would not
accumulate to create lightning.
7. Balloon Bottle Experiment
Yes, both balloons will bulge equally because:
•
water pressure at same height is equal.
8. How Does a Storm Become a Cyclone?
•
Warm moist air rises.
•
Low pressure forms.
•
More air rushes in.
•
Earth’s rotation causes spinning.
•
Strong rotating storm becomes a cyclone. 9. Sea
Coast Question Land is on side B.
Because during afternoon:
•
sea breeze blows from sea to land.
10. Activity Showing Air Moves from High to Low Pressure Take two balloons connected with a
straw: • one inflated,
• one
empty.
Air moves from inflated balloon (high pressure) to empty balloon (low
pressure).
11. What is a Thunderstorm?
A storm with:
•
thunder,
•
lightning,
•
strong winds,
•
heavy rain is
called a thunderstorm.
12. Process of Lightning • Charges develop inside clouds.
• Opposite charges
attract.
• Sudden electric
discharge occurs.
• Bright flash
produced is lightning.
13. Why Holes Are Made in Hoardings?
Holes allow air to pass through.
Chapter - 7
Particulate Nature of Matter
Solved Questions and Answers
1. Why do gases mix easily while solids do not? Gases mix easily because their
particles move freely and have very weak attraction. Solids do not mix because
their particles are tightly packed.
2. Why do liquids take the shape of the container?
Liquids take the shape of the container because their particles can move
freely within limited space.
3. Why do solids have fixed shape?
Solids have fixed shape because particles are tightly packed with strong
attraction.
4. What is melting point?
The minimum temperature at which a solid changes into liquid is called
melting point.
5. What is boiling point?
The temperature at which a liquid changes into vapour is called boiling
point.
6. Why does perfume smell spread in a room?
Perfume particles mix with air particles and spread due to diffusion.
7. Why is sugar not visible after dissolving in water? Sugar particles occupy spaces between
water particles and become too small to see.
8. Why are gases compressible?
Gases are compressible because there is large space between gas
particles.
9. Why do solids not flow like liquids?
Solid particles are tightly packed and cannot move freely.
10. What is diffusion?
The spreading of particles from one place to another due to their motion
is called diffusion.
Short Revision Points
• Matter is made
of tiny particles.
• Particles have
spaces between them.
• Particles
attract each other.
• Solids →
strongest attraction.
• Gases → weakest
attraction.
• Heating
increases particle movement.
• Liquids and
gases are fluids.
• Diffusion occurs
in liquids and gases.
Very Short Questions and Answers
Q1. Which state has maximum
interparticle space?
Gas
Q2. Which state has fixed shape and
volume?
Solid
Q3. Which state can flow?
Liquids and gases
Q4. Name the force between particles.
Interparticle attraction
Q5. Which particles move freely?
Gas particles
Q6. What is the melting point of ice?
0°C
Q7. What happens during evaporation? Liquid changes slowly into vapour
Q8. Which state is highly
compressible?
Gas
Q9. What is diffusion?
Mixing of particles due to motion
Q10. Why do gases fill the container?
Gas particles move in all directions freely.
Chapter - 8
Nature of Matter — Elements,
Compounds, and Mixtures
Solved Question Answers
1. Which statement is correct?
Answer:
(iv) A and B are elements, C is a compound, and has a fixed composition.
2. Assertion and Reason
Answer:
(i) Both Assertion and Reason are true and Reason correctly explains
Assertion.
3. Water has different properties than hydrogen and
oxygen. Justify.
Answer:
Hydrogen is a combustible gas and
oxygen supports combustion, but water extinguishes fire. Therefore, compounds
have properties different from their constituent elements.
4. Correctly matched examples
Answer:
(ii) Uniform
mixtures— minerals, seawater, bronze, air
(iii) Pure substances—
carbon dioxide, iron, oxygen, sugar
5. Classify the substances
|
Substance Classification
|
|
|
Iron Element Moist air Mixture Iron oxide Compound Magnesium Element Oxygen Element Magnesium
oxide Compound 6. Classify the
following |
|
|
Elements Compounds |
Mixtures |
|
Aluminium
Carbon dioxide |
Sand |
|
Gold Magnesium oxide Seawater Oxygen Rust Muddy
water Nitrogen Iron sulfide Air
Sulfur Glucose Fruit
juice Hydrogen Sodium chloride Baking
soda Water
|
|
Pure Substances
Elements and compounds are pure substances.
7. New substance formed when iron and sulfur are heated
Answer:
Iron sulfide is formed.
Word Equation
Iron + Sulfur → Iron sulfide
8. Can a substance be both element and compound?
Answer:
No.
An element contains only one kind of
atom, whereas a compound contains two or more elements chemically combined.
9. What if water were a mixture?
Answer:
Hydrogen and oxygen could separate easily. Water would not be stable or
safe for life.
10. Identify Gas A Answer:
Hydrogen gas
Word Equation
Iron + Hydrochloric acid → Iron chloride + Hydrogen
11. Two compounds made only from non-metals
(a) Water
Uses:
•
Drinking
•
Cleaning (b)
Carbon dioxide
Uses:
•
Fire extinguishers
•
Soft drinks
12. Why is gold both a mineral and a
metal?
Answer:
Gold occurs naturally in Earth as a mineral and also shows metallic
properties such as shine and conductivity.
Short Revision
Topic Key Point
Mixture Physical combination
Element Simplest substance
Compound Chemical combination
Pure substance One type of particle
Air Mixture
Water Compound
Chapter - 9
The Amazing World of Solutes,
Solvents, and Solutions
Solved Question Answers
Very Short Answer Questions
Q1. What is a solution?
A uniform mixture formed by mixing solute and solvent.
Q2. Define solute.
The substance that dissolves in a solvent.
Q3. Define solvent.
The substance that dissolves the solute.
Q4. What is density?
Mass per unit volume of a substance.
Q5. Name the instrument used to measure volume.
Measuring cylinder.
Q6. Name the SI unit of density.
kg/m³
Short Answer Questions
Q1. Differentiate between saturated and unsaturated solutions.
Saturated Unsaturated
Cannot dissolve more solute Can dissolve more solute Undissolved solute
remains No undissolved solute
Q2. Why does oil float on water?
Oil is less dense than water, so it floats.
Q3. Why do fish survive in water?
Because oxygen gas is dissolved in water.
Q4. Why does hot air rise?
Hot air becomes less dense on heating and rises upward.
Q5. Why does ice float?
Ice has lower density than liquid water.
Numerical Problems
Q1. A stone has mass 225 g and volume
90 cm³. Find density.
Given:
Mass = 225 g
Volume = 90 cm³
𝜌
= 
=
2.5 𝑔/𝑐𝑚3
Answer: Density = 2.5 g/cm³ It will sink in water.
Q2. Mass = 400 g, Volume = 40 cm³
𝜌
= 
=
10 𝑔/𝑐𝑚3
Answer: Density = 10 g/cm³
Q3. Iron block mass = 600 g, density
= 7.9 g/cm³. Find volume.
𝑉
Answer: Volume ≈ 75.9 cm³
True or False
1. Oxygen is more
soluble in hot water. → False
2. Sand and water
form a solution. → False
3. Volume is space
occupied by matter. → True
4. Air is a uniform
mixture. → True
5. Density
decreases on heating. → True
Fill in the Blanks
1. The solid is
immersed in water.
2. Maximum amount
of solute dissolved in solvent is called solubility.
3. Density
decreases with increase in temperature.
4. The solution is
called a saturated solution.
HOTS Questions
Q1. Why are measuring cylinders tall and narrow?
To get more accurate
readings.
Q2. Why does peeled orange sink but unpeeled orange
float?
The peel contains air spaces, reducing overall density.
Q3. How can you make an egg float in water?
By adding salt to increase water density.
Revision Summary
• Solution =
uniform mixture
• Solute dissolves
in solvent
• Saturated
solution cannot dissolve more solute
• Density = Mass ÷
Volume
• Low density
objects float
• High density
objects sink
• Heating usually
decreases density
• Cold water
dissolves more gases
Chapter - 10
Light: Mirrors and Lenses
Solved Question Answers
Very Short Answer Questions Q1. What
is a solution?
A uniform mixture formed by mixing solute and solvent.
Q2. Define solute.
The substance that dissolves in a solvent.
Q3. Define solvent.
The substance that dissolves the solute.
Q4. What is density?
Mass per unit volume of a substance.
Q5. Name the instrument used to
measure volume.
Measuring cylinder.
Q6. Name the SI unit of density.
kg/m³
Short Answer Questions
Q1. Differentiate between saturated
and unsaturated solutions.
Saturated Unsaturated
Cannot dissolve more solute Can dissolve more solute
Undissolved solute remains No undissolved solute
Q2. Why does oil float on water?
Oil is less dense than water, so it floats.
Q3. Why do fish survive in water?
Because oxygen gas is dissolved in water.
Q4. Why does hot air rise?
Hot air becomes less dense on heating and rises upward.
Q5. Why does ice float?
Ice has lower density than liquid water.
Numerical Problems
Q1. A stone has mass 225 g and volume
90 cm³. Find density.
Given:
Mass =
225 g
Volume = 90 cm³
𝜌
= 
=
2.5 𝑔/𝑐𝑚3
Answer: Density = 2.5 g/cm³ It will sink in water.
Q2. Mass = 400 g, Volume = 40 cm³
𝜌
= 
=
10 𝑔/𝑐𝑚3
Answer: Density = 10 g/cm³
Q3. Iron block mass = 600 g, density
= 7.9 g/cm³. Find volume.
𝑉
Answer: Volume ≈ 75.9 cm³
True or False
1. Oxygen is more
soluble in hot water. → False
2. Sand and water
form a solution. → False
3. Volume is space
occupied by matter. → True
4. Air is a uniform
mixture. → True
5. Density
decreases on heating. → True
Fill in the Blanks
1. The solid is immersed in water.
2. Maximum amount
of solute dissolved in solvent is called solubility.
3. Density decreases with increase in
temperature.
4. The solution is
called a saturated solution.
HOTS Questions
Q1. Why are measuring cylinders tall
and narrow?
To get more accurate readings.
Q2. Why does peeled orange sink but
unpeeled orange float?
 |
The peel contains air spaces,
reducing overall density.
Q3. How can you make an egg float in
water?
By adding salt to increase water density.
Revision Summary
• Solution =
uniform mixture
• Solute dissolves
in solvent
• Saturated
solution cannot dissolve more solute
• Density = Mass ÷
Volume
• Low density
objects float
• High density
objects sink
• Heating usually
decreases density
• Cold water
dissolves more gases
Chapter
– 10:
Light: Mirrors and Lenses
Introduction Answers to "Keep the
Curiosity Alive"
1.
Angle with normal = 40°
Angle with mirror = 90° − 40°
= 50°
Answer: (ii)
50°
2.
(i)
Incident ray along normal.
Angle of incidence = 0°
Angle of reflection = 0°
(ii)
Still along normal.
Angle of incidence = 0°
Angle of reflection = 0°
(iii)
Angle of incidence = 20°
Angle of reflection = 20°
3.
Matching:
• Enlarged erect
image → Concave mirror
• Same size image
→ Plane mirror
• Smaller erect
image → Convex mirror
4.
Matching:
• Same size image
→ Flat glass piece
• Enlarged image →
Convex lens
• Smaller image →
Concave lens
5.
When light falls along normal:
Angle of incidence = 0°
Answer: (ii)
Angle of incidence is 0°
6.
Identification:
• Enlarged image →
Concave mirror
• Same size image
→ Plane mirror
• Smaller image →
Convex mirror
7.
As woman approaches concave mirror:
Image becomes larger and finally erect and magnified.
Answer: (iii)
8.
Observation:
• Text appears
bigger at a certain distance.
• On moving away,
image becomes inverted.
Magnifying glass is a:
Convex
Lens
9. Matching
Column I Column II
(i) Concave mirror
(a)
(ii) Convex mirror
(b)
(iii) Convex lens (c)
(iv) Concave lens (d)
10.
Assertion: Correct
Reason: Correct
Reason explains Assertion.
Answer: (i)
11.
Figure (a) → Plane mirror
Figure (b) → Convex mirror
Answer: (iv)
12.
Observation
The pencil appears bent or broken at the water surface.
Reason
Light changes direction when passing from water to air.
This bending of light is called refraction.
Most Important Exam Questions
1. Define spherical
mirrors.
2. Differentiate
between concave and convex mirrors.
3. State the two
laws of reflection.
4. Why are convex
mirrors used as rear-view mirrors?
5. Why does a
concave mirror burn paper in sunlight?
6. Define
lens.
7. Differentiate
between convex and concave lenses.
8. Why is a convex
lens called a converging lens?
9. State uses of
concave mirrors.
10. State uses of
convex lenses.
Chapter - 11
Keeping Time with the Skies
Solved Question Answers
ANSWERS TO ALL TEXTBOOK QUESTIONS
1.
True or False
(i)
True
(ii) False
(iii) True
(iv) False
2.
No.
A
birthday on 6 May may not fall on a Full Moon every year because Gregorian
calendar is based on solar year while Full Moon follows lunar cycle.
3.
Two
mistakes:
1.
New Moon and Full Moon
positions are incorrect.
2.
Illuminated side of
Moon is not facing the Sun correctly.
4(i)
Phase
Picture
Three
days after New Moon Crescent
Full
Moon Full
circle
Three
days after Full Moon Waning Gibbous
Week
after Full Moon Half Moon
Day
of New Moon Dark Moon
(Depends
on textbook figure labels.)
4(ii)
Phases
not seen from Earth:
The
completely non-illuminated side facing Earth.
5
(i)
Moon overhead at
sunset:
Half Moon (First Quarter)
(ii) Waxing
Phase
6
Both
Ravi and Kaushalya are correct.
Crescent
Moon can be seen near sunset.
Gibbous
Moon may be visible in afternoon.
7
Less
often.
Because
Moon's revolution becomes slower, lunar months become longer.
8
37
Full Moons occur in 36 months.
By
the Pigeonhole Principle, at least one month must contain two Full Moons.
Hence
at least two Full Moons occur in the same month.
9
Full Moon
Because
it remained visible from sunset to sunrise.
10
Without
leap years:
Error
= 1 day every 4 years.
To
shift about 90 days (summer to winter):
90
× 4 = 360 years
Approximately
360 years.
11
Purpose
of Artificial Satellites:
• Communication
• Navigation
• Weather
forecasting
• Disaster
management
• Scientific
research
12
Measure
Based On
Day
Earth's Rotation
Month
Moon's Phases
Year
Earth's Revolution around Sun
One-Line Exam Summary
The
Moon’s phases, Earth’s rotation, and Earth’s revolution are natural
astronomical cycles that helped humans develop calendars and measure time,
while artificial satellites use these principles to support communication,
navigation, weather forecasting, and scientific research.
Astronomical
Cycles and Time Units
Natural
cycles used for measuring time.
cycle
duration
Day
1
Month
29.5
Year
365.25
Chapter - 12
How Nature Works in Harmony
Solved Question Answers
Answers to "Keep the Curiosity
Alive"
1. Wrong statement
Answer: (iii) An ecosystem is part of
a community. Reason: Community is part of an ecosystem, not vice versa.
2. What if decomposers disappear?
• Dead plants and animals would accumulate.
• Nutrients would not return to soil.
• Plant growth would reduce.
• Food chains would collapse.
Decomposers are essential because
they recycle nutrients.
3. How did mangroves protect the village?
Mangrove forests reduce the force of
waves and storm surges. Their dense roots act as natural barriers against
tsunamis and floods.
4. If frogs disappear?
• Grasshopper population increases.
• Snake population decreases because of lack of
food.
5. Why fewer butterflies?
Possible reasons:
• Fewer flowering plants.
• Pollution.
• Pesticides.
• Habitat destruction.
Steps:
• Plant flowering plants.
• Avoid pesticides.
• Create butterfly gardens.
6. Why can't an ecosystem have only producers?
Without consumers and decomposers:
• Food chains stop functioning.
• Dead matter accumulates.
• Nutrients are not recycled.
Hence the ecosystem collapses.
7. Differences between park and roadside
|
Park |
Roadside |
|
More trees |
Fewer trees |
|
More birds |
Fewer birds |
|
Cleaner air |
Dust and pollution |
Higher biodiversity Lower
biodiversity
8. Comment on sustainability of agricultural fields
Agricultural fields are necessary for
food production. However, excessive use of fertilisers and pesticides damages
soil and biodiversity. Sustainable farming methods should be adopted.
9. If Indian hare population decreases?
• Fox population decreases.
• Eagle population decreases.
• Grass and plants increase due to less grazing.
Thus the whole food web gets
disturbed.
Exam Important Definitions
1. Habitat
2. Population
3. Community
4. Ecosystem
5. Producer
6. Consumer
7. Decomposer
8. Food Chain
9. Food Web
10. Mutualism
11. Commensalism
12. Parasitism