Class 9 Science

Here you can find Class 9 Science Notes. These notes include important revision points, simple explanations.

Chapter 1. Matter in Our Surroundings  

This chapter explains that everything around us is made of matter, which has mass and occupies space. It introduces the particle nature of matter and explains that matter is made up of extremely small particles that are always in motion. The chapter discusses the characteristics of particles of matter, such as having spaces between them and attracting each other. It further explains the three states of matter—solid, liquid, and gas—and compares their properties. The chapter also describes how matter can change its state by changing temperature or pressure, introduces concepts like melting point, boiling point, latent heat, and explains evaporation and its cooling effect with daily-life examples.

Key Points

• Matter is anything that has mass and occupies space.

• Matter is made up of tiny particles that are too small to see.

• Particles of matter have spaces between them.

• Particles of matter are in continuous motion.

• Particles of matter attract each other with varying force.

• Matter exists in three states: solid, liquid, and gas.

• Solids have fixed shape and volume and are rigid.

• Liquids have fixed volume but no fixed shape.

• Gases have neither fixed shape nor fixed volume and are highly compressible.

• The state of matter depends on particle arrangement and energy.

• Matter can change state by changing temperature or pressure.

• Melting point is the temperature at which a solid changes into a liquid.

•  Boiling point is the temperature at which a liquid changes into gas.

• Latent heat of fusion is required to change solid into liquid.

• Latent heat of vaporisation is required to change liquid into gas.

• Sublimation is direct change from solid to gas.

• Evaporation occurs at all temperatures below boiling point.

• Evaporation causes cooling.

• Rate of evaporation depends on surface area, temperature, humidity, and wind speed.

• Cotton clothes help keep the body cool in summer due to evaporation of sweat.

👉 👉Matter behaves differently under different conditions of temperature and pressure. By understanding the nature of matter and its changes, we can explain many everyday phenomena and use scientific knowledge wisely in daily life.

Chapter 2. Is Matter Around Us Pure?  

This chapter explains that most of the substances around us are mixtures, even though they may appear pure. It clearly distinguishes between pure substances and mixtures from a scientific point of view. The chapter introduces different types of mixtures, such as solutions, suspensions, and colloids, and explains their properties with everyday examples. It also discusses physical and chemical changes, and classifies pure substances into elements and compounds, helping students understand how matter behaves and how substances can be separated.

Key Points

• Scientifically, a pure substance contains only one type of particle.

• Most materials around us are mixtures, not pure substances.

• Mixtures can be homogeneous or heterogeneous in nature.

• Solutions are homogeneous mixtures with uniform composition.

• Suspensions are heterogeneous mixtures with visible particles.

• Colloids appear uniform but are actually heterogeneous mixtures.

• Solutions do not scatter light, while colloids show the Tyndall effect.

• Concentration of a solution depends on the amount of solute dissolved.

• A saturated solution cannot dissolve more solute at a given temperature.

• Physical changes do not create new substances.

• Chemical changes result in new substances with different properties.

• Pure substances are classified as elements and compounds.

• Elements cannot be broken down chemically, but compounds can.

👉 👉This chapter teaches us that appearances can be misleading. By understanding the true nature of matter, we learn how science helps us classify, separate, and use substances wisely. Scientific thinking enables us to observe carefully, reason logically, and apply knowledge responsibly in daily life and industry.

Chapter 3. Atoms and Molecules 

This chapter explains that all matter is made up of extremely small particles called atoms, which combine to form molecules. It traces the development of the idea of atoms from ancient philosophers to John Dalton’s atomic theory. The chapter introduces the laws of chemical combination, which explain how substances react and combine in fixed ways. It also explains important concepts such as atomic mass, molecular mass, ions, valency, and chemical formulae, helping students understand the basic building blocks of matter and chemical reactions.

Key Points

• Matter is made up of very tiny particles called atoms.

• Ancient Indian and Greek philosophers proposed the idea of indivisible particles.

• Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction.

• Law of Constant Proportions states that elements in a compound are always present in fixed mass ratios.

• John Dalton explained these laws using his atomic theory.

• According to Dalton’s atomic theory, atoms are indivisible and identical for a given element.

• Atoms of different elements have different masses and properties.

• Atoms combine in simple whole number ratios to form compounds.

• Atoms are extremely small and cannot be seen with naked eyes.

• Each element has a chemical symbol, approved by IUPAC.

• Atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom.

• Atoms usually do not exist independently.

• Atoms combine to form molecules or ions.

• A molecule is the smallest particle that can exist independently.

• Atomicity is the number of atoms present in a molecule.

• Ions are charged particles formed by loss or gain of electrons.

• Cations are positively charged ions, while anions are negatively charged ions.

• Valency is the combining capacity of an element.

• Chemical formulae represent the composition of compounds.

• Molecular mass is the sum of atomic masses of all atoms in a molecule.

• Formula unit mass is used for ionic compounds.

👉 👉Understanding atoms and molecules helps us realise that all matter is structured and organised at a microscopic level. This knowledge forms the foundation of chemistry and allows us to explain chemical reactions, write formulae correctly, and apply science meaningfully in daily life and technology.

Chapter 4. Structure of the Atom  

This chapter explains that atoms are not indivisible but are made up of smaller particles called electrons, protons, and neutrons. It describes how scientists discovered these particles through experiments and proposed different models of the atom to explain its structure. The chapter discusses Thomson’s model, Rutherford’s nuclear model, and Bohr’s model, showing how each model improved our understanding of atomic structure. It also introduces important ideas such as atomic number, mass number, valency, isotopes, and isobars, helping students understand how atoms are arranged and how elements differ from one another.

Key Points

• Atoms are made up of smaller particles called electrons, protons, and neutrons.

• Electrons carry negative charge, while protons carry positive charge.

• Neutrons have no charge and are present in the nucleus.

• J.J. Thomson discovered the electron and proposed the plum pudding model.

• Rutherford’s gold foil experiment showed that atoms have a small, dense nucleus.

• Most of the atom is empty space.

• The nucleus contains almost all the mass of the atom.

• Rutherford’s model could not explain the stability of atoms.

• Bohr proposed that electrons move in fixed energy levels or shells.

• Electrons do not lose energy while revolving in allowed shells.

• The maximum number of electrons in a shell is given by 2n².

• The outermost shell of an atom can have a maximum of 8 electrons.

• Valency depends on the number of electrons in the outermost shell.

• Atomic number is equal to the number of protons in an atom.

• Mass number is the sum of protons and neutrons.

• Isotopes are atoms of the same element with different mass numbers.

• Isobars are atoms of different elements with the same mass number.

• Isotopes have similar chemical properties but different physical properties.

👉 👉The structure of the atom shows that matter is made of well-organised particles. Understanding atomic structure helps us explain the behaviour of elements, their reactions, and their applications in science, medicine, and technology.

Chapter 5. The Fundamental Unit of Life   

This chapter explains that cells are the basic structural and functional units of all living organisms. It describes how cells were discovered and how living organisms can be unicellular or multicellular. The chapter discusses the shape, size, and function of cells, and explains the internal structure of a cell, including the plasma membrane, nucleus, cytoplasm, and cell organelles. It highlights how different organelles perform specific functions and how coordination among them helps the cell survive, grow, and reproduce.

Key Points

• All living organisms are made up of cells.

• The cell is the basic structural and functional unit of life.

• Cells were first discovered by Robert Hooke in 1665.

• Unicellular organisms consist of a single cell.

• Multicellular organisms are made up of many specialised cells.

• Cells differ in shape, size, and function according to their role.

• Every cell arises from a pre-existing cell.

• The plasma membrane surrounds the cell and is selectively permeable.

• Diffusion and osmosis help substances move in and out of the cell.

• Plant cells have an additional outer layer called the cell wall.

• The nucleus controls cell activities and contains genetic material.

• Chromosomes carry hereditary information in the form of DNA.

• Prokaryotic cells lack a true nucleus and membrane-bound organelles.

• Eukaryotic cells have a well-defined nucleus and organelles.

• Cytoplasm is the fluid part where cell organelles are suspended.

• Mitochondria release energy and are called the powerhouse of the cell.

• Ribosomes help in protein synthesis.

• Endoplasmic reticulum helps in transport and synthesis of materials.

• Golgi apparatus modifies, packages, and transports substances.

• Lysosomes digest waste materials and are called suicide bags.

• Plastids are present in plant cells and help in photosynthesis and storage.

• Vacuoles store food, water, and waste materials.

• Cells divide for growth, repair, and reproduction.

👉 👉Cells work in an organised and coordinated manner to support life. Understanding the structure and function of cells helps us appreciate how living organisms grow, repair themselves, and survive in different environments.

Chapter 6. Tissues   

This chapter explains that in multicellular organisms, similar cells group together to perform specific functions, forming tissues. It highlights how division of labour makes life processes more efficient in plants and animals. The chapter describes different types of plant tissues, including meristematic tissues responsible for growth and permanent tissues that provide support, storage, and transport. It also explains animal tissues such as epithelial, connective, muscular, and nervous tissues, showing how structure is closely related to function. Through examples from daily life, the chapter helps students understand how tissues form the building blocks of organs and organ systems.

Key Points

• A tissue is a group of similar cells performing a specific function.

• Multicellular organisms show division of labour at the tissue level.

• Plants and animals have different types of tissues due to different lifestyles.

• Plant tissues are classified as meristematic and permanent tissues.

• Meristematic tissues are actively dividing and help in plant growth.

• Apical meristem increases length, lateral meristem increases girth, and intercalary meristem helps regrowth.

• Permanent tissues are formed after differentiation of meristematic cells.

• Simple permanent tissues include parenchyma, collenchyma, and sclerenchyma.

• Parenchyma stores food and may help in photosynthesis and buoyancy.

• Collenchyma provides flexibility and mechanical support.

• Sclerenchyma provides strength and rigidity to plant parts.

• Complex permanent tissues include xylem and phloem.

• Xylem transports water and minerals, while phloem transports food.

• Animal tissues are classified as epithelial, connective, muscular, and nervous tissues.

• Epithelial tissue provides protection, absorption, and secretion.

• Connective tissue supports, binds, and transports materials in the body.

• Muscular tissue helps in movement through contraction and relaxation.

• Striated muscles are voluntary, smooth muscles are involuntary, and cardiac muscles work continuously.

• Nervous tissue receives and transmits impulses for coordination.

👉 👉Living organisms are highly organised systems where every tissue has a specific role. Understanding tissues helps us appreciate how structure and function work together to maintain life, growth, and coordination in plants and animals.

Chapter 7. Motion 

This chapter explains motion as the change in position of an object with time and describes how different types of motion can be observed in daily life. It introduces the idea of a reference point to describe position and motion accurately. The chapter explains important concepts such as distance, displacement, speed, velocity, and acceleration, helping students understand how fast and in which direction objects move. It also explains uniform and non-uniform motion, graphical representation of motion, and the equations of motion. Finally, the chapter introduces uniform circular motion, showing that even motion with constant speed can involve acceleration due to change in direction.

Key Points

• Motion is defined as a change in position of an object with time.

• To describe motion, a reference point (origin) is required.

• Distance is the total path covered, while displacement is the shortest distance between initial and final positions.

• Distance has only magnitude, whereas displacement has magnitude and direction.

• An object can have zero displacement but non-zero distance.

• Motion along a straight line is the simplest form of motion.

• Uniform motion occurs when equal distances are covered in equal intervals of time.

• Non-uniform motion occurs when unequal distances are covered in equal intervals of time.

• Speed is the distance travelled per unit time.

• Average speed is total distance divided by total time.

• Velocity is speed in a given direction.

• Velocity can change due to change in speed, direction, or both.

• Acceleration is the rate of change of velocity with time.

• Acceleration can be positive, negative (retardation), uniform, or non-uniform.

• Motion can be represented using distance–time graphs and velocity–time graphs.

• A straight-line distance–time graph represents uniform motion.

• The area under a velocity–time graph gives the displacement.

• Motion with uniform acceleration can be described using three equations of motion.

• In uniform circular motion, speed remains constant but velocity changes due to change in direction.

• Uniform circular motion is an example of accelerated motion.

👉 👉Motion is a fundamental part of nature, from the movement of tiny particles to planets in space. Understanding motion helps us describe, predict, and control movement in daily life, transport, sports, and technology, encouraging safer and more efficient use of science.

Chapter 8. Force and Laws of Motion 

This chapter explains how forces affect the motion of objects and why objects start moving, stop, or change their speed or direction. It introduces the concept of force as a push or pull and distinguishes between balanced and unbalanced forces. The chapter discusses inertia, mass, and momentum, and presents Newton’s three laws of motion, which form the foundation of classical mechanics. These laws help us understand everyday phenomena such as walking, braking of vehicles, catching a ball, recoil of a gun, and motion caused by interactions between objects.

Key Points

• A force is a push or pull that can change the state of motion, direction, or shape of an object.

• Forces can be balanced or unbalanced depending on their net effect.

• Balanced forces do not change the state of rest or uniform motion of an object.

• Unbalanced forces cause a change in speed, direction, or state of motion.

• Inertia is the tendency of an object to resist changes in its state of motion or rest.

• Mass is a measure of inertia; heavier objects have greater inertia.

• Newton’s First Law of Motion states that an object remains at rest or in uniform motion unless acted upon by an unbalanced force.

• The First Law is also known as the Law of Inertia.

• Momentum is the product of mass and velocity of an object.

• Newton’s Second Law of Motion relates force to the rate of change of momentum.

• According to the Second Law, force is equal to mass multiplied by acceleration (F = ma).

• The SI unit of force is newton (N).

• Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction.

• Action and reaction forces act on different objects, not on the same object.

• Everyday activities like walking, jumping, firing a gun, and rowing a boat follow Newton’s laws of motion.

👉 👉Newton’s laws of motion help us understand how and why objects move the way they do. From daily activities like walking and travelling to advanced technologies like vehicles and machines, these laws explain the cause-and-effect relationship between force and motion. Understanding them builds a strong foundation for physics and helps us appreciate the science behind everyday life.

Chapter 9. Gravitation 

This chapter explains the force of gravitation, which is the force of attraction between any two objects having mass. It describes how gravitation governs the motion of planets, moons, and satellites, and also affects objects on Earth. The chapter introduces Newton’s universal law of gravitation and explains how the same force causes objects to fall towards the Earth. It further discusses important concepts such as acceleration due to gravity, mass, weight, free fall, and thrust and pressure. The chapter also explains why objects float or sink in liquids using Archimedes’ principle, helping students connect gravitation with everyday experiences.

Key Points

• Gravitation is the force of attraction between any two masses in the universe.

• Every object in the universe attracts every other object.

• Newton’s universal law of gravitation states that the force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

• The gravitational force keeps planets in their orbits around the Sun.

• Objects fall towards the Earth due to its gravitational pull.

• Acceleration due to gravity (g) is the acceleration of an object falling freely under gravity.

• The value of g on Earth is approximately 9.8 m/s².

• Free fall is the motion of an object under the influence of gravity alone.

• Mass is the amount of matter in an object and remains constant everywhere.

• Weight is the force with which an object is attracted by the Earth and depends on gravity.

• Weight changes with location, but mass does not.

• Thrust is the force acting perpendicular to a surface.

• Pressure is thrust per unit area.

• Pressure in liquids increases with depth.

• Archimedes’ principle explains the buoyant force acting on objects in fluids.

• Objects float or sink depending on their density compared to the liquid.

• Buoyancy is the upward force exerted by a fluid on an immersed object.

👉 👉Gravitation shows that natural laws govern both the universe and everyday life. By understanding gravity, we learn why objects fall, why planets move in fixed paths, and how forces shape our world. This knowledge helps us apply science thoughtfully, from simple activities to advanced technologies like satellites and space exploration.

Chapter 10. Work and Energy 

This chapter explains the scientific meaning of work, energy, and power, and how they are closely connected. It clarifies that in science, work is done only when a force causes displacement in its direction. The chapter introduces energy as the capacity to do work and discusses its different forms such as kinetic energy and potential energy. It also explains how energy can change from one form to another while the total energy remains conserved. The concept of power is introduced to compare how fast work is done or energy is transferred. Through everyday examples, the chapter helps students understand how these ideas apply in real life.

Key Points

• In science, work is done only when a force produces displacement.

• Work is defined as the product of force and displacement in the direction of force.

• Work done can be positive, negative, or zero.

• The SI unit of work is joule (J).

• Energy is the capacity of an object to do work.

• The SI unit of energy is also joule (J).

• Objects in motion possess kinetic energy.

• Kinetic energy depends on mass and square of velocity.

• Energy stored due to position or shape is called potential energy.

• Gravitational potential energy depends on mass, height, and gravity.

• Energy can change from one form to another.

• According to the law of conservation of energy, energy is neither created nor destroyed.

• The sum of kinetic and potential energy is called mechanical energy.

• Power is the rate at which work is done or energy is transferred.

• The SI unit of power is watt (W).

• Greater power means work is done faster.

👉 👉Understanding work, energy, and power helps us recognise how nature and machines function efficiently. By learning how energy is used and conserved, we can use resources wisely, avoid wastage, and adopt responsible practices for sustainable living.

Chapter 11. Sound  

This chapter explains how sound is produced, how it travels through different media, and how it is heard by humans. It introduces sound as a form of energy produced by vibrating objects and describes how these vibrations travel as waves through solids, liquids, and gases. The chapter discusses important characteristics of sound such as amplitude, frequency, time period, and speed, which help us distinguish between loudness and pitch. It also explains concepts like echo, reverberation, and the range of hearing, connecting sound with real-life experiences such as music, communication, and noise pollution.

Key Points

• Sound is produced due to the vibration of objects.

• Vibrations travel through a medium in the form of sound waves.

• Sound needs a material medium to travel and cannot move through vacuum.

• Sound waves are longitudinal waves consisting of compressions and rarefactions.

• Compressions are regions of high pressure, while rarefactions are regions of low pressure.

• The distance between two consecutive compressions or rarefactions is called wavelength.

• The number of vibrations per second is called frequency.

• The SI unit of frequency is hertz (Hz).

• Time period is the time taken to complete one vibration.

• Frequency and time period are inversely related.

• Amplitude of a sound wave determines its loudness.

• Higher amplitude means louder sound.

• Pitch of sound depends on its frequency.

• Speed of sound depends on the medium and temperature.

• Sound travels fastest in solids, slower in liquids, and slowest in gases.

• Echo is the repetition of sound caused by reflection from a distant surface.

• Reverberation is the persistence of sound due to multiple reflections.

• The audible range of sound for humans is from 20 Hz to 20,000 Hz.

• Sounds above this range are ultrasonic, and below are infrasonic.

• Excessive sound causes noise pollution, which is harmful to health.

👉 👉Sound plays a vital role in communication and daily life, but its misuse can harm health and the environment. By understanding how sound works and controlling noise, we can create a healthier and more peaceful living environment for ourselves and others.

Chapter 12. Improvement in Food Resources 

This chapter explains how food production can be increased to meet the needs of a growing population while protecting natural resources. It discusses scientific methods used to improve crop yields and animal-based food production. The chapter covers crop variety improvement, crop production management, and crop protection management. It also introduces animal husbandry, including cattle farming, poultry farming, fisheries, and bee-keeping. Emphasis is given to sustainable practices so that food security can be achieved without harming the environment.

Key Points

• Food is required for growth, development, and health of living organisms.

• India’s increasing population creates a higher demand for food.

• Increasing food production must be done without damaging the environment.

• Crop yield improvement depends on variety improvement, production management, and protection from pests.

• Crop variety improvement aims at higher yield, better quality, and resistance to diseases and climate stress.

• Hybridisation is used to combine desirable traits of crops.

• Crop production management includes nutrient management, irrigation, and cropping patterns.

• Plants need macro-nutrients and micro-nutrients for healthy growth.

• Manure improves soil structure and fertility naturally.

• Fertilisers provide nutrients quickly but should be used carefully.

• Irrigation ensures water supply at critical stages of crop growth.

• Mixed cropping, intercropping, and crop rotation help improve yield and reduce risks.

• Crop protection management controls weeds, pests, and diseases.

• Proper storage prevents loss of harvested grains.

• Animal husbandry involves scientific care, breeding, feeding, and disease control of animals.

• Cattle farming provides milk and draught power.

• Poultry farming is done for egg and meat production.

• Fisheries provide a cheap source of animal protein.

• Fish production includes marine fisheries and inland fisheries.

• Composite fish culture increases fish yield by using different feeding habits of fish.

• Bee-keeping provides honey and wax and supports farmers’ income.

👉 👉Improving food resources through scientific and sustainable methods is essential for feeding the growing population. By using natural resources wisely and adopting responsible farming practices, we can ensure food security while protecting the environment for future generations.