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Class 8th Science NCERT Notes and Solutions (Non-Rationalised)
1. Crop Production And Management
This chapter provides a comprehensive overview of the systematic agricultural practices required for large-scale food production. It details the entire farming process, beginning with the preparation of soil through ploughing and tilling. It covers the selection and sowing of healthy seeds and the crucial step of replenishing soil nutrients by adding organic manure and chemical fertilisers. The chapter explains various irrigation methods, from traditional to modern systems like drip irrigation. It also discusses the protection of crops from unwanted plants (weeds) and pests, the process of harvesting the mature crop, and the critical need for proper storage of grains in granaries or silos to protect them from moisture and pests, ensuring food security.
2. Microorganisms : Friend And Foe
Microorganisms are tiny living beings, often invisible to the naked eye, that are found everywhere. This chapter explores their dual nature, categorising them into major groups: bacteria, fungi, protozoa, algae, and viruses. As friends, they are vital for processes like the fermentation of curd and bread (by Lactobacillus and yeast), the commercial production of antibiotics (like penicillin), and fixing atmospheric nitrogen to enrich soil fertility (e.g., Rhizobium bacteria). As foes, these microbes, known as pathogens, cause numerous diseases in humans (like typhoid, cholera), plants, and animals. The chapter also discusses food preservation techniques (like pasteurization, salting, sugaring) to prevent spoilage by harmful microbes.
3. Synthetic Fibres And Plastics
This chapter introduces the world of man-made materials. Synthetic fibres are polymers made from chemical substances, with examples like rayon (artificial silk), strong and elastic nylon, and wrinkle-resistant polyester. It also provides a detailed look at plastics, categorizing them into thermoplastics (which can be remoulded, e.g., polythene) and thermosetting plastics (which cannot be remoulded, e.g., bakelite). While highlighting their useful properties like being lightweight and durable, the chapter raises serious environmental concerns due to their non-biodegradable nature. It strongly emphasizes the 4R principle – Reduce, Reuse, Recycle, and Recover – for responsible plastic use.
4. Materials : Metals And Non-Metals
This chapter classifies materials into two broad categories based on their properties: metals and non-metals. It details their distinguishing physical properties. Metals are typically lustrous, hard, malleable (can be beaten into thin sheets, e.g., gold foil), ductile (can be drawn into wires, e.g., copper), and are good conductors of heat and electricity. Non-metals, in contrast, are generally non-lustrous, brittle, and poor conductors. The chapter also explores their chemical properties, such as the reaction with oxygen to form basic oxides (metals) or acidic oxides (non-metals). The concept of a displacement reaction, where a more reactive metal displaces a less reactive one from its salt solution, is also explained.
5. Coal And Petroleum
Coal and petroleum are vital fossil fuels, which are exhaustible natural resources formed from the degradation of buried dead organisms over millions of years. This chapter explains the formation of coal through the slow process of carbonisation and its various products like coke and coal tar. It describes how petroleum (crude oil) is extracted and then refined through fractional distillation to obtain a range of useful products, including petrol, diesel, kerosene, LPG, and paraffin wax. The chapter strongly emphasizes that these energy sources are finite and their combustion contributes to air pollution, highlighting the urgent need for their conservation and the exploration of alternative energy sources.
6. Combustion And Flame
Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. This chapter explains the three essential conditions required for combustion: the presence of a fuel, a supply of oxygen, and reaching the substance's ignition temperature. It discusses different types of combustion (rapid, spontaneous, explosion) and methods of fire control. The chapter provides a detailed examination of a candle flame, describing its three distinct zones: the dark innermost zone (unburnt vapours), the yellow luminous middle zone (incomplete combustion), and the blue non-luminous outer zone (complete combustion).
7. Conservation Of Plants And Animals
This chapter highlights the critical need to protect our planet's rich biodiversity. It discusses the severe consequences of deforestation, such as soil erosion and climate change. It defines key terms like endemic species (found exclusively in a particular area) and endangered species (facing a high risk of extinction). The chapter explains various conservation strategies, differentiating between a Wildlife Sanctuary, a National Park, and a Biosphere Reserve. It underscores the importance of initiatives like Project Tiger in India to protect our flora and fauna and maintain ecological balance for future generations.
8. Cell — Structure And Functions
This chapter introduces the cell as the fundamental structural and functional unit of all living organisms. It discusses the diversity of cells in terms of number, shape, and size. The chapter provides a detailed look at the main components of a cell: the cell membrane (outer boundary), the cytoplasm (jelly-like substance), and the nucleus (the control centre containing chromosomes). It also describes key organelles like mitochondria (powerhouse), vacuoles, and plastids (including chloroplasts in plant cells for photosynthesis). A clear distinction is made between plant cells (which have a cell wall and chloroplasts) and animal cells.
9. Reproduction In Animals
Reproduction is the biological process essential for the continuation of a species. This chapter explains the two main modes of reproduction in animals. It details asexual reproduction, where a single parent is involved, with examples like binary fission in Amoeba and budding in Hydra. It then provides a comprehensive look at sexual reproduction, which involves the fusion of specialized male and female cells called gametes (sperm and ovum). The process of fertilisation—the fusion of gametes to form a zygote—is explained, differentiating between internal and external fertilisation. The chapter also introduces metamorphosis, the drastic transformation from a larva to an adult, as seen in frogs.
10. Reaching The Age Of Adolescence
Adolescence is the crucial transitional phase from childhood to adulthood, characterized by the onset of puberty. This chapter explores the profound physical and emotional changes that occur during this period. It explains that these changes are triggered by hormones secreted by endocrine glands. The chapter details the development of secondary sexual characteristics in both boys (e.g., facial hair, deepening voice) and girls (e.g., breast development). It emphasizes the importance of maintaining personal hygiene, a balanced diet, and emotional well-being to navigate this period of rapid growth and development successfully.
11. Force And Pressure
This chapter introduces the fundamental concepts of force and pressure. A force is defined as a push or a pull that can change an object's state of motion or its shape. It classifies forces into contact forces (like muscular force and friction) and non-contact forces (like magnetic, electrostatic, and gravitational forces). The chapter then explains Pressure as the force acting per unit area ($\textsf{Pressure} = \frac{\textsf{Force}}{\textsf{Area}}$). It uses everyday examples, like why a sharp knife cuts better than a blunt one, to illustrate that pressure is high when the area is small. The concept of pressure exerted by liquids and gases, including atmospheric pressure, is also discussed.
12. Friction
Friction is an opposing force that comes into play whenever two surfaces in contact move or tend to move relative to each other. This chapter explains that friction is caused by the interlocking of irregularities on the surfaces. It distinguishes between static, sliding, and rolling friction, highlighting that rolling friction is the smallest. The chapter describes friction as a "necessary evil"—it is essential for actions like walking and writing but also causes undesirable wear and tear. It explores methods to reduce friction using lubricants or ball bearings and ways to increase it for better grip, like the treads on tires.
13. Sound
This chapter explains that sound is a form of energy produced by vibrating objects and that it requires a medium (solid, liquid, or gas) for its propagation—it cannot travel in a vacuum. It introduces the key characteristics used to describe a sound wave: its amplitude, which determines its loudness (measured in decibels), and its frequency, which determines its pitch (measured in Hertz). The chapter explains the functioning of the human ear and distinguishes between audible and inaudible sounds. It also addresses the issue of noise pollution and suggests measures to control it.
14. Chemical Effects Of Electric Current
This chapter explores the chemical reactions that can be induced by passing an electric current through conducting liquids, known as electrolytes. This process of causing chemical decomposition using electricity is called electrolysis. The chapter focuses on a major practical application of this effect: electroplating. This is the process of depositing a thin layer of a desired metal onto another material by means of electricity, used for purposes like preventing corrosion (coating iron with zinc) or for a decorative finish (coating objects with silver or gold).
15. Some Natural Phenomena
This chapter examines two powerful and potentially hazardous natural phenomena. It explains that lightning is a massive electric discharge that occurs between clouds or between clouds and the ground due to the separation of charges. It also discusses the function of a lightning conductor as a safety device for buildings. The chapter then describes earthquakes as a sudden shaking of the Earth's crust caused by disturbances deep inside, often due to the movement of tectonic plates. It introduces the Richter scale for measuring an earthquake's magnitude and highlights crucial safety precautions to be taken during these events.
16. Light
This chapter delves into the phenomenon of light and vision. It discusses the reflection of light and introduces the two fundamental laws of reflection. It explains the characteristics of the image formed by a plane mirror (virtual, erect, and laterally inverted). The chapter also introduces spherical mirrors, differentiating between concave and convex mirrors. It then explains the structure and working of the human eye, including the function of the cornea, iris, lens, and retina. The chapter also covers the concept of vision defects and introduces the Braille system for visually challenged individuals.
17. Stars And The Solar System
This chapter takes us on a tour of the cosmos, exploring the celestial objects that adorn our sky. It introduces stars, which emit their own light, and recognizable patterns of stars called constellations (like Ursa Major). A major focus is on our Solar System, which consists of the Sun at the center and the eight planets revolving around it, along with other members like asteroids, comets, and meteors. The chapter discusses the Moon, our natural satellite, and its various phases. It also explains the difference between natural and artificial satellites and their uses in communication and remote sensing.
18. Pollution Of Air And Water
This chapter addresses the critical environmental issue of the contamination of our natural resources. It explains air pollution, identifying major pollutants like sulfur dioxide, carbon monoxide, and particulate matter, and their harmful effects, including acid rain and the greenhouse effect leading to global warming. It also discusses water pollution, its causes (like sewage and industrial waste), and its impact on aquatic life. The chapter introduces the concept of potable water (safe for drinking) and emphasizes the individual and collective responsibility to reduce pollution for a sustainable future.