General Science I — Physics and Chemistry Fundamentals

Why This Chapter Matters

General Science, particularly foundational Physics and Chemistry, is a staple section in nearly every competitive examination in India, including recruitment for Village/Ward Secretariat posts, because it tests basic scientific literacy that every functionary is expected to possess, especially those in technical roles such as Engineering Assistant or those handling health and welfare duties where basic scientific reasoning is useful. Questions in this section are usually conceptual and everyday-life oriented rather than requiring advanced mathematics, so the goal of this chapter is to build a clear, intuitive understanding of core physics and chemistry principles that recur across question papers, explained in plain language without formulas.

Units, Measurement, and Basic Mechanics

Physics begins with measurement, and the internationally accepted system for this is the International System of Units, commonly called SI units, which defines standard units for fundamental quantities: the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity. Nearly all other physical quantities are derived from combinations of these seven base units — for example, speed is derived from length divided by time, and force is derived from mass multiplied by acceleration. Exam questions sometimes ask which unit measures which quantity, so a general familiarity with common derived units — the newton for force, the joule for energy or work, the watt for power, the pascal for pressure, and the hertz for frequency — is useful to keep in mind.

Motion is described using a small set of interconnected concepts. Speed describes how fast an object covers distance, while velocity additionally specifies direction, making velocity what is called a vector quantity (having both magnitude and direction) as opposed to speed, which is a scalar quantity (having magnitude only). Acceleration describes the rate at which velocity changes over time; an object can accelerate by speeding up, slowing down, or changing direction, since all three represent a change in velocity. These distinctions between scalar and vector quantities are commonly tested conceptually — other vector quantities include displacement, force, and momentum, while other scalar quantities include distance, mass, and energy.

Sir Isaac Newton's three laws of motion form the foundation of classical mechanics and are extremely frequently tested, usually through everyday examples rather than mathematical problems. Newton's First Law, often called the law of inertia, states that an object at rest stays at rest, and an object in motion continues in motion at constant velocity in a straight line, unless acted upon by an external unbalanced force; this explains everyday experiences such as passengers lurching forward when a bus suddenly brakes (their bodies, in motion, resist the sudden change imposed by the braking vehicle) or a tablecloth being pulled quickly from under dishes without disturbing them much (the dishes' inertia resists sudden motion). Newton's Second Law relates force, mass, and acceleration, essentially stating that the force required to accelerate an object is proportional to its mass and to the acceleration desired — heavier objects require more force to achieve the same acceleration as lighter ones, which is why pushing an empty cart is easier than pushing a fully loaded one. Newton's Third Law states that for every action there is an equal and opposite reaction; common illustrative examples include the recoil felt when firing a gun, the forward thrust experienced when jumping off a boat (the boat moves backward as you push off it), and the mechanism by which rockets propel themselves forward by expelling gas backward.

Gravity is the force of attraction between any two masses, and on Earth, this manifests as the familiar downward pull that gives objects weight. It is important to keep clear the distinction, frequently tested, between mass and weight: mass is the amount of matter in an object and remains constant regardless of location, measured in kilograms, while weight is the force exerted on that mass by gravity, measured in newtons, and varies depending on the local gravitational field — an astronaut's mass stays the same on the Moon as on Earth, but their weight is considerably less on the Moon because lunar gravity is weaker than Earth's. Sir Isaac Newton is credited with formulating the law of universal gravitation, while it was later work, notably by Albert Einstein's General Theory of Relativity, that provided a deeper explanation of gravity as a curvature of spacetime caused by mass, though for the level of this exam, Newton's gravitational framework is generally sufficient.

Work, energy, and power are closely related concepts often confused with one another. Work, in the physics sense, is done when a force causes displacement of an object in the direction of that force; simply holding a heavy object stationary, however tiring, does no physics "work" because there is no displacement. Energy is the capacity to do work, and it exists in many interconvertible forms, including kinetic energy (energy of motion), potential energy (stored energy due to position or configuration, such as a raised weight or a compressed spring), thermal energy, chemical energy, electrical energy, and nuclear energy. The Law of Conservation of Energy, one of the most fundamental principles in all of physics, states that energy cannot be created or destroyed, only converted from one form to another or transferred from one object to another, so the total energy in an isolated system remains constant over time; this principle underlies the working of countless real-world systems, from a swinging pendulum continuously converting kinetic and potential energy back and forth, to a hydroelectric dam converting the potential energy of stored water into electrical energy. Power is the rate at which work is done or energy is transferred — the same amount of work done more quickly requires greater power.

Heat, Temperature, and States of Matter

Matter commonly exists in three familiar states — solid, liquid, and gas — distinguished by how tightly and how freely their constituent particles are arranged and able to move. In solids, particles are packed closely together in a fixed, ordered arrangement, vibrating in place but not moving freely, which gives solids a definite shape and volume. In liquids, particles are close together but can move past one another, which is why liquids have a definite volume but take the shape of their container. In gases, particles are widely spaced and move freely and rapidly in all directions, which is why gases have neither a definite shape nor a definite volume, expanding to fill whatever container they occupy. A fourth state, plasma — an ionised gas containing free-moving charged particles, found for instance in lightning, neon signs, and stars including our Sun — is sometimes mentioned in more advanced questions but is generally beyond the core focus of these exams.

Matter changes between these states through processes with specific names that are commonly tested: melting (solid to liquid), freezing (liquid to solid), vaporisation or boiling/evaporation (liquid to gas), condensation (gas to liquid), sublimation (solid directly to gas, without passing through the liquid state, as seen with substances like naphthalene or dry ice/solid carbon dioxide), and deposition (gas directly to solid). These changes of state are driven by changes in temperature and/or pressure, and they involve the absorption or release of latent heat — heat energy that changes a substance's state without changing its temperature, which is why water at its boiling point stays at 100 degrees Celsius (at standard atmospheric pressure) throughout the boiling process rather than continuing to rise in temperature until all of it has turned to steam.

Temperature and heat, while related, are conceptually distinct: temperature is a measure of the average kinetic energy of particles in a substance (essentially, how "hot" or "cold" something is on a defined scale), while heat is the energy that flows from a hotter object to a colder one due to their temperature difference, continuing until thermal equilibrium is reached. India commonly uses the Celsius scale for everyday temperature measurement, where water freezes at 0 degrees and boils at 100 degrees under standard atmospheric pressure, while the Kelvin scale, used in scientific contexts, sets its zero point at absolute zero — the theoretical temperature at which particle motion is minimal and no further heat can be extracted — with 0 degrees Celsius corresponding to 273.15 Kelvin.

Heat transfers between objects or through materials by three mechanisms, each commonly tested with everyday examples. Conduction is heat transfer through direct contact between particles, typically significant in solids, such as a metal spoon left in a hot cup of tea becoming warm at the handle end. Convection is heat transfer through the actual movement of a fluid (liquid or gas), such as warm air rising and cooler air sinking to create convection currents that help heat a room, or water heating from the bottom of a pot circulating upward. Radiation is heat transfer through electromagnetic waves and does not require any medium at all, which is why heat from the Sun can reach Earth across the vacuum of space; the warmth felt standing near a fire even without touching it is largely due to radiation.

Light and Sound

Light is a form of electromagnetic radiation that travels as a wave but also exhibits particle-like behaviour, a duality that is central to modern physics though generally beyond the scope of what this exam tests directly. What is commonly tested is the behaviour of light in simpler, everyday terms: light travels in straight lines (rectilinear propagation), reflects off surfaces (with the well-known law that the angle of incidence equals the angle of reflection), and refracts, or bends, when it passes from one transparent medium into another of different density, such as from air into water, which is why a straight stick appears bent at the point it enters water. The splitting of white light into its constituent colours — the familiar rainbow sequence of violet, indigo, blue, green, yellow, orange, and red — occurs through refraction and dispersion, most commonly observed either through a glass prism or naturally in a rainbow, where sunlight is refracted, internally reflected, and dispersed by countless tiny water droplets suspended in the air after rain.

Human vision and common eye defects are also frequently tested in a practical, everyday-science context. Myopia (nearsightedness) occurs when a person can see nearby objects clearly but distant objects appear blurred, generally because the eyeball is too elongated or the eye's lens focuses images in front of rather than directly on the retina, and it is corrected using concave (diverging) lenses. Hypermetropia or hyperopia (farsightedness) is the opposite condition, where distant objects are seen clearly but nearby objects appear blurred, corrected using convex (converging) lenses. Presbyopia is an age-related loss of the eye's ability to focus on nearby objects, common in older adults, and is also corrected with convex lenses, often through bifocal glasses.

Sound, unlike light, is a mechanical wave and therefore requires a medium (solid, liquid, or gas) to travel through — it cannot travel through a vacuum, which is a frequently tested contrast between light and sound. Sound travels by causing particles in the medium to vibrate and pass that vibration along, and its speed varies with the medium, generally travelling fastest through solids (where particles are closely packed and can transmit vibrations efficiently), slower through liquids, and slowest through gases such as air. The pitch of a sound is determined by its frequency (how many vibrations occur per second, measured in hertz) — higher frequency produces a higher-pitched sound — while loudness is related to amplitude, the intensity or size of the sound wave's vibration. The audible range for typical human hearing is roughly 20 hertz to 20,000 hertz; sounds above this range are called ultrasonic (used, for example, in medical ultrasound imaging and in some industrial cleaning applications), while sounds below this range are called infrasonic.

Basic Electricity and Magnetism

Electricity, in its simplest everyday sense, is the flow of electric charge, most commonly carried by electrons moving through a conductor such as a copper wire. Electric current is the rate of flow of this charge, measured in amperes, and it flows through a circuit when there is a difference in electric potential, or voltage, measured in volts, between two points, driving the current from higher to lower potential (in conventional current terms) through the circuit. Resistance, measured in ohms, describes how much a material opposes the flow of current; materials with low resistance, such as copper and silver, are good conductors and are used for wiring, while materials with very high resistance, such as rubber, glass, and most plastics, are insulators used to prevent unwanted current flow, for example as the coating around electrical wires. Ohm's Law, a foundational relationship in basic electricity, states that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to its resistance, meaning that for a fixed resistance, increasing the voltage increases the current, while for a fixed voltage, increasing the resistance decreases the current.

Circuits can be arranged in series, where components are connected end-to-end along a single path so the same current flows through each component in turn (and if one component fails or the path breaks anywhere, the entire circuit stops working, as with old-style decorative light strings where one blown bulb could darken the whole string), or in parallel, where components are connected across multiple separate paths so that each receives the same voltage and the failure of one component does not necessarily interrupt current flow to the others, which is why household electrical wiring is generally arranged in parallel, allowing one appliance to be switched off or to fail without affecting others on the same circuit.

Magnetism and electricity are deeply interconnected, a relationship known as electromagnetism. A moving electric charge or current creates a magnetic field around it, which is the working principle behind electromagnets (used in devices ranging from electric bells to industrial cranes that lift scrap metal) and behind electric motors, which convert electrical energy into mechanical motion using the interaction between magnetic fields and current-carrying conductors. Conversely, a changing magnetic field can induce an electric current in a nearby conductor, a phenomenon called electromagnetic induction, discovered by Michael Faraday, and this principle underlies the working of electric generators, which convert mechanical energy into electrical energy — essentially the reverse process of a motor — and is the basic operating principle behind most large-scale electricity generation, including hydroelectric, thermal, and wind power plants, all of which ultimately use some form of mechanical rotation to turn a generator and produce electric current through electromagnetic induction.

Fundamentals of Chemistry: Atoms, Elements, and the Periodic Table

All matter is composed of atoms, the basic building blocks of chemistry, each atom consisting of a dense central nucleus containing positively charged protons and electrically neutral neutrons, surrounded by negatively charged electrons occupying the space around the nucleus in defined energy levels or shells. The number of protons in an atom's nucleus, called its atomic number, uniquely determines which chemical element that atom belongs to, while the total number of protons and neutrons together gives the atom's mass number. Atoms of the same element can have differing numbers of neutrons, and such variants are called isotopes; a well-known example is carbon, which has isotopes including carbon-12 (the most common and stable) and carbon-14 (a radioactive isotope used in radiocarbon dating to estimate the age of ancient organic materials).

Elements are organised into the Periodic Table, originally devised in a recognisable form by the Russian chemist Dmitri Mendeleev in 1869, who arranged known elements by increasing atomic weight (the modern table instead uses atomic number, a refinement made possible by later discoveries about atomic structure) and left gaps for elements not yet discovered, correctly predicting some of their properties in advance based on the patterns he observed — a striking early triumph of the periodic law, which states that the physical and chemical properties of elements are a periodic function of their atomic number. The modern periodic table arranges elements into horizontal rows called periods (elements in the same period have the same number of electron shells) and vertical columns called groups (elements in the same group tend to share similar chemical properties because they have the same number of electrons in their outermost shell, called valence electrons, which largely determine an element's chemical behaviour). Broad categories worth remembering include metals (generally good conductors of heat and electricity, malleable and ductile, and located mostly on the left and centre of the table), non-metals (generally poor conductors, often brittle if solid, and located mostly on the right of the table), and metalloids or semi-metals (elements such as silicon and germanium with properties intermediate between metals and non-metals, notably valuable in electronics as semiconductors).

Chemical Bonding, Reactions, and Acids, Bases, and Salts

Atoms combine with one another through chemical bonds to form molecules and compounds, primarily in order to achieve a more stable electron configuration, generally by filling their outermost electron shell. Ionic bonding occurs when one atom transfers one or more electrons to another, creating oppositely charged ions (a positively charged cation and a negatively charged anion) that attract each other electrostatically, as seen in common salt, sodium chloride, where sodium loses an electron to become a positive ion and chlorine gains that electron to become a negative ion. Covalent bonding occurs when atoms share electrons rather than transferring them outright, as seen in molecules like water (where oxygen shares electrons with two hydrogen atoms) or the oxygen and nitrogen gases that make up most of the air we breathe. Metallic bonding, found in metals, involves a structure of positively charged metal ions surrounded by a "sea" of freely moving shared electrons, which explains why metals conduct electricity and heat so well and can be shaped (malleability and ductility) without breaking.

Chemical reactions involve the rearrangement of atoms to form new substances, and several broad categories are commonly tested: combination reactions (two or more substances combine to form a single product), decomposition reactions (a single compound breaks down into two or more simpler substances, often requiring heat, light, or electricity as an input), displacement reactions (a more reactive element displaces a less reactive one from its compound), and combustion reactions (a substance reacts rapidly with oxygen, typically releasing heat and light, as in burning). The Law of Conservation of Mass, an important foundational principle, states that in any chemical reaction, matter is neither created nor destroyed, meaning the total mass of the reactants equals the total mass of the products, even though the substances themselves are chemically transformed.

Acids, bases, and salts form a heavily tested everyday-chemistry topic. Acids are substances that release hydrogen ions when dissolved in water, taste sour (in edible contexts, such as citric acid in lemons or acetic acid in vinegar), turn blue litmus paper red, and react with metals to release hydrogen gas and with bases to form salt and water in what is called a neutralisation reaction. Bases are substances that release hydroxide ions when dissolved in water, generally feel slippery, taste bitter, and turn red litmus paper blue; a base that is soluble in water is specifically called an alkali. The pH scale, ranging from 0 to 14, is used to measure how acidic or basic (alkaline) a solution is: a pH of exactly 7 is neutral (as in pure water), values below 7 indicate increasing acidity as the number decreases, and values above 7 indicate increasing alkalinity as the number increases. This scale is practically relevant well beyond the classroom — soil pH affects which crops grow well in a given region, human blood maintains a narrow, tightly regulated pH range for the body to function properly, and everyday substances span the scale widely, from strongly acidic battery acid and gastric (stomach) acid to strongly alkaline substances like sodium hydroxide used in soap-making. Salts are formed from the neutralisation reaction between an acid and a base, and common table salt (sodium chloride) is simply the most familiar everyday example, formed from the reaction of hydrochloric acid and sodium hydroxide, though many other salts exist with widely varying properties and uses, from baking soda (sodium bicarbonate) in cooking to various fertiliser salts used in agriculture.

Finally, it is worth remembering a few commonly tested everyday chemical facts that recur across general science sections: water has the chemical formula H2O, consisting of two hydrogen atoms covalently bonded to one oxygen atom; the air we breathe is a mixture composed mostly of nitrogen (roughly seventy-eight percent) and oxygen (roughly twenty-one percent), with smaller amounts of argon, carbon dioxide, and other trace gases; rusting of iron is a chemical reaction (specifically a slow oxidation reaction) in which iron reacts with oxygen and moisture in the air to form iron oxide, which is why keeping iron and steel dry or coated (through painting, galvanising, or oiling) helps prevent this economically significant form of material degradation; and carbon dioxide, produced by respiration and combustion and absorbed by plants during photosynthesis, plays a central role both in the natural carbon cycle and in the broader discussion of greenhouse gases and climate change, since it is one of the principal gases responsible for trapping heat in the Earth's atmosphere.