General Science I — Physics and Chemistry Fundamentals

Introduction: Rebuilding Your Science Foundation

Welcome to the first of our two general science chapters. Many DSC aspirants, especially those who studied arts or commerce streams after school, sometimes feel a flutter of nervousness when they see "Physics" or "Chemistry" in the syllabus. Let me reassure you right away: the general science component of the DSC GK paper is not designed to test cutting-edge research or complex numerical problem-solving. It is designed to test whether you have a sound, school-level understanding of scientific concepts — the kind of understanding that makes you a well-informed citizen and, more importantly for our purposes, an effective teacher who can explain everyday scientific phenomena to your students with confidence and accuracy.

In this chapter, we will revisit the core building blocks of physics and chemistry, moving through each major topic area with a clear, exam-oriented focus. We won't get lost in complicated formulas or advanced derivations; instead, we will build (or rebuild) genuine conceptual clarity, because conceptual clarity, more than memorisation, is what allows you to correctly answer the applied, scenario-based questions that general science sections often contain.

Part One: Physics Fundamentals

Measurement and Units

All of physics begins with measurement — the ability to quantify physical properties like length, mass, time, and more. The internationally accepted system of units used across science today is the International System of Units, commonly abbreviated as SI units. Some of the base SI units you should be comfortable with include the metre (for length), the kilogram (for mass), the second (for time), the kelvin (for temperature), the ampere (for electric current), the mole (for amount of substance), and the candela (for luminous intensity). From these base units, a wide range of derived units are constructed — for example, speed is measured in metres per second (a combination of the length and time units), and force is measured in newtons (a derived unit reflecting mass multiplied by acceleration).

It's useful to remember that temperature has multiple common scales: Celsius (used commonly in everyday life in most of the world, including India), Fahrenheit (used commonly in a few countries, notably the United States), and Kelvin (the scientific/absolute temperature scale, where zero Kelvin, called absolute zero, represents the theoretical point at which all thermal molecular motion ceases — the lowest possible temperature). The relationship between Celsius and Kelvin is straightforward: Kelvin equals Celsius plus 273 (approximately), a simple conversion occasionally tested directly.

Motion, Force, and Newton's Laws

The study of motion, called mechanics, is foundational to physics, and Sir Isaac Newton's three laws of motion form the bedrock of classical mechanics — concepts you should know cold, both by statement and by everyday example.

Newton's First Law (the Law of Inertia): 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 is why passengers in a car lurch forward when the car suddenly brakes — their bodies, in motion, tend to continue moving forward due to inertia even as the car itself decelerates.

Newton's Second Law: The rate of change of momentum of an object is directly proportional to the applied force and occurs in the direction of that force. This is most commonly summarised through the simple relationship: Force equals mass multiplied by acceleration (F = ma). This law explains why a heavier object requires more force to achieve the same acceleration as a lighter object, and it underlies a vast range of practical and everyday physics scenarios.

Newton's Third Law: For every action, there is an equal and opposite reaction. When you push against a wall, the wall pushes back against you with equal force in the opposite direction — this is why you don't simply pass through the wall, and it also explains phenomena like rocket propulsion, where hot gases expelled downward and backward push the rocket upward and forward.

Related concepts worth reviewing include the distinction between speed (a scalar quantity, meaning it has only magnitude, no direction) and velocity (a vector quantity, meaning it has both magnitude and direction), and the distinction between mass (the amount of matter in an object, which remains constant regardless of location) and weight (the force exerted on an object due to gravity, which varies depending on the local gravitational field — for example, an object's weight on the Moon is significantly less than on Earth, even though its mass remains unchanged, because the Moon's gravitational pull is weaker than Earth's).

Gravitation

Sir Isaac Newton's law of universal gravitation states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. This single elegant law explains an enormous range of phenomena, from why objects fall to the ground on Earth, to why the Moon orbits the Earth, to why planets orbit the Sun.

Gravity on Earth causes objects to accelerate toward the ground at a roughly constant rate (in the absence of air resistance), a value commonly denoted "g" and approximately equal to 9.8 metres per second squared near the Earth's surface. This value can vary slightly depending on altitude and location, but for general exam purposes, the approximate value and the underlying concept of uniform gravitational acceleration near Earth's surface (ignoring air resistance) is what matters most.

Work, Energy, and Power

Work, in the physics sense, is done when a force causes displacement (movement) of an object in the direction of the applied force. Simply exerting force without causing any movement (such as pushing against an immovable wall) technically does no "work" in the physics sense, even though it may feel effortful — this distinction between the everyday and the scientific meaning of "work" is a classic conceptual point worth understanding clearly.

Energy is the capacity to do work, and it exists in many forms, including:

• Kinetic energy: The energy an object possesses due to its motion, dependent on both its mass and its velocity.
• Potential energy: The energy an object possesses due to its position or configuration — for example, gravitational potential energy, which depends on an object's height above a reference point, or elastic potential energy stored in a stretched or compressed spring.
• Other forms of energy include thermal (heat) energy, chemical energy (stored in the bonds of chemical compounds), electrical energy, and nuclear energy (released through nuclear reactions).

The Law of Conservation of Energy, one of the most important principles in all of physics, states that energy cannot be created or destroyed, only converted from one form to another. When you drop a ball, its gravitational potential energy converts into kinetic energy as it falls; when it hits the ground and eventually comes to rest, that kinetic energy has been converted into heat and sound energy, but the total amount of energy in the system remains constant throughout — none has been created or destroyed, only transformed.

Power is the rate at which work is done or energy is transferred, measured in watts. A more powerful engine or device is one that can do the same amount of work in less time, or more work in the same amount of time.

Light and Optics

Light is a form of electromagnetic radiation that is visible to the human eye, and its behaviour is governed by several important, testable phenomena:

Reflection: Light bouncing off a surface, governed by the law of reflection, which states that the angle of incidence (the angle at which light strikes a surface) equals the angle of reflection (the angle at which it bounces off), both measured from an imaginary line called the "normal," perpendicular to the surface at the point of contact.

Refraction: The bending of light as it passes from one transparent medium into another (for example, from air into water, or from air into glass), caused by the change in the speed of light as it moves between media of different optical densities. This is why a straw appears to bend at the point where it enters a glass of water — the light travelling from the submerged part of the straw to your eye bends as it exits the water and enters the air, creating the visual illusion of a bent straw.

Dispersion: The splitting of white light into its constituent colours (the visible spectrum, commonly remembered by the acronym VIBGYOR — violet, indigo, blue, green, yellow, orange, and red) when passed through a prism, because different wavelengths (colours) of light bend by slightly different amounts when refracted. This same principle explains the natural formation of a rainbow, where sunlight is refracted, dispersed, and reflected within countless tiny water droplets suspended in the atmosphere after rainfall.

Lenses and mirrors: Convex lenses (thicker in the middle than at the edges) converge light rays, while concave lenses (thinner in the middle) diverge them; similarly, concave mirrors converge reflected light while convex mirrors diverge it. These principles underlie the functioning of everyday optical devices such as spectacles (eyeglasses, which use lenses to correct vision defects), magnifying glasses, cameras, microscopes, and telescopes. A common everyday application worth remembering: convex lenses are typically used to correct hypermetropia (long-sightedness, or difficulty seeing nearby objects clearly), while concave lenses are used to correct myopia (short-sightedness, or difficulty seeing distant objects clearly).

Sound

Sound is a form of energy that travels as a mechanical wave, meaning it requires a medium (a substance such as air, water, or a solid) to travel through — unlike light, sound cannot travel through a vacuum (empty space with no matter). Sound waves are produced by vibrating objects and travel through a medium as a series of compressions (regions where particles of the medium are pushed closer together) and rarefactions (regions where particles are spread further apart).

Key properties of sound worth knowing include frequency (the number of vibrations per second, measured in hertz, which determines the pitch of a sound — higher frequency means higher pitch), amplitude (which determines the loudness or volume of a sound), and the speed of sound, which varies depending on the medium it travels through — generally, sound travels fastest through solids, slower through liquids, and slowest through gases like air, because particles in solids are more tightly packed and can transmit vibrations more efficiently than the more loosely packed particles in liquids and gases.

Heat and Thermodynamics Basics

Heat is a form of energy that flows from a region of higher temperature to a region of lower temperature. It's important to distinguish clearly between heat (a form of energy transfer) and temperature (a measure of the average kinetic energy of the particles in a substance, essentially how "hot" or "cold" something is on a defined scale). Heat can be transferred through three main mechanisms:

• Conduction: The transfer of heat through direct contact between particles, most effective in solids where particles are closely packed and can efficiently pass on vibrational energy to neighbouring particles — this is why a metal spoon left in a hot cup of tea quickly becomes hot to the touch.
• Convection: The transfer of heat through the actual physical movement of a fluid (liquid or gas), as warmer, less dense fluid rises and cooler, denser fluid sinks, creating a circulating current — this is how heat spreads through boiling water or through air in a heated room.
• Radiation: The transfer of heat through electromagnetic waves, which does not require any medium at all and can even travel through the vacuum of space — this is how heat from the Sun reaches the Earth across the vacuum of outer space.

Electricity and Magnetism Basics

Electricity, in its basic form, involves the flow of electric charge, most commonly the flow of electrons through a conductor (a material that allows electric charge to flow through it relatively easily, such as most metals) as opposed to an insulator (a material that resists the flow of electric charge, such as rubber or glass). Key concepts include:

Electric current: The rate of flow of electric charge, measured in amperes.

Voltage (or potential difference): The "push" that drives electric current through a circuit, measured in volts.

Resistance: The opposition a material offers to the flow of electric current, measured in ohms.

Ohm's Law: A fundamental relationship stating that, for many conductors, the current flowing through them is directly proportional to the voltage applied across them and inversely proportional to their resistance (commonly expressed as V = IR, where V is voltage, I is current, and R is resistance).

Magnetism and electricity are deeply interconnected phenomena, a relationship called electromagnetism. A moving electric charge (an electric current) creates a magnetic field around it, and conversely, a changing magnetic field can induce an electric current in a nearby conductor — this second principle, called electromagnetic induction, is the basic operating principle behind electric generators (which convert mechanical energy into electrical energy) and transformers (which change the voltage of alternating current electricity). Electric motors, conversely, use the interaction between electric current and magnetic fields to convert electrical energy into mechanical (motion) energy — the reverse process of a generator.

Part Two: Chemistry Fundamentals

Matter and Its States

Chemistry is fundamentally the study of matter — anything that has mass and occupies space — and the changes it undergoes. Matter commonly exists in three familiar states: solid (definite shape and definite volume, with particles tightly packed and only vibrating in fixed positions), liquid (definite volume but no definite shape, taking the shape of its container, with particles able to move past one another while remaining relatively close together), and gas (neither definite shape nor definite volume, expanding to fill any available space, with particles far apart and moving freely and rapidly). A fourth state, plasma (an ionized gas-like state, found in phenomena like lightning and stars), is sometimes mentioned in more advanced discussions but is a less common exam focus at the school GK level.

Matter can change between these states through processes that generally involve the addition or removal of heat energy: melting (solid to liquid), freezing (liquid to solid), evaporation or boiling (liquid to gas), condensation (gas to liquid), sublimation (solid directly to gas, without passing through the liquid state, as seen with substances like dry ice or naphthalene balls/camphor), and deposition (gas directly to solid).

Atoms, Elements, and the Periodic Table

All matter is composed of atoms, the basic building blocks of chemistry. Each atom consists of a dense central nucleus, containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons that occupy the space around the nucleus in defined energy levels or "shells." The number of protons in an atom's nucleus defines its atomic number and determines which chemical element that atom belongs to — for example, every atom with exactly one proton is hydrogen, and every atom with exactly six protons is carbon, regardless of how many neutrons or electrons might vary.

An element is a pure substance made up of only one type of atom, and cannot be broken down into simpler substances by ordinary chemical means. All known elements are organised into the periodic table, a systematic arrangement first substantially developed by the Russian chemist Dmitri Mendeleev in the nineteenth century, who arranged elements in order of increasing atomic weight (later refined to atomic number, following the development of atomic theory) and noticed that elements with similar chemical properties recurred at regular intervals — a pattern called periodicity, which gives the periodic table its name and its powerful predictive utility.

The periodic table is organised into rows called periods and columns called groups (or families). Elements within 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 determines how an element behaves chemically. Broadly, elements are classified into metals (generally good conductors of heat and electricity, malleable and ductile, and tend to lose electrons in chemical reactions), non-metals (generally poor conductors, brittle in solid form, and tend to gain electrons in chemical reactions), and metalloids (elements with intermediate properties between metals and non-metals).

Molecules, Compounds, and Chemical Bonding

When two or more atoms combine chemically, they form a molecule. If the atoms involved are of different elements, the resulting substance is called a compound — a pure substance made of two or more different elements chemically combined in a fixed ratio, with properties often very different from the elements that compose it (for example, sodium, a highly reactive metal, and chlorine, a toxic gas, combine to form sodium chloride, which is ordinary, safe table salt).

Atoms combine through chemical bonds, of which two major types are most important for exam purposes:

Ionic bonds: Formed when one atom transfers one or more electrons to another atom, creating oppositely charged ions (a positively charged cation and a negatively charged anion) that are then held together by electrostatic attraction. This typically occurs between metals (which tend to lose electrons) and non-metals (which tend to gain electrons). Table salt (sodium chloride) is a classic example of an ionically bonded compound.

Covalent bonds: Formed when two atoms share one or more pairs of electrons, typically occurring between two non-metal atoms. Water (formed by covalent bonds between one oxygen atom and two hydrogen atoms) is a classic example.

It's worth understanding the basic difference in properties this bonding difference tends to create: ionic compounds generally have high melting and boiling points and conduct electricity when dissolved in water or melted (because the ions are then free to move and carry charge), while covalent compounds generally have lower melting and boiling points and typically do not conduct electricity, since they do not form free-moving charged particles.

Acids, Bases, and the pH Scale

Acids and bases are two important, contrasting categories of chemical substances that are highly relevant to everyday life and are frequently tested in GK papers. Acids are substances that, when dissolved in water, release hydrogen ions (or more precisely, hydronium ions), and are generally characterised by a sour taste (though tasting chemicals is never advisable as a practical identification method), the ability to turn blue litmus paper red, and the ability to react with metals to release hydrogen gas, and to react with bases to form salt and water (a reaction called neutralisation). Common examples of acids include hydrochloric acid (found in the human stomach, where it aids digestion), acetic acid (found in vinegar), and citric acid (found in citrus fruits like lemons and oranges).

Bases, by contrast, are substances that release hydroxide ions when dissolved in water, are generally characterised by a bitter taste and a slippery feel, the ability to turn red litmus paper blue, and the ability to react with acids in neutralisation reactions. Common examples include sodium hydroxide (used in soap-making) and ammonia (found in many cleaning products). A base that is soluble in water is specifically called an alkali.

The pH scale is used to measure how acidic or basic (alkaline) a solution is, running from 0 to 14, with 7 representing neutral (neither acidic nor basic — pure water is the classic example of a neutral substance), values below 7 indicating increasing acidity as the number decreases, and values above 7 indicating increasing alkalinity as the number increases. This scale is logarithmic, meaning each whole number step represents a tenfold change in acidity or alkalinity — a nuance worth remembering for more conceptually detailed questions.

Neutralisation reactions, in which an acid and a base react to cancel out each other's properties, producing a salt and water, are practically important in everyday life — for example, antacid medications work by neutralising excess stomach acid using a mild base, and farmers sometimes add lime (a base) to overly acidic soil to bring its pH closer to neutral for better crop growth.

Chemical Reactions and Their Types

A chemical reaction is a process in which one or more substances (called reactants) are transformed into one or more different substances (called products), involving the breaking and forming of chemical bonds. Chemical reactions are represented using chemical equations, and a fundamental principle governing all chemical reactions is the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction — the total mass of the reactants must equal the total mass of the products, which is why chemical equations must always be balanced (having equal numbers of each type of atom on both sides of the equation).

Common types of chemical reactions worth being familiar with include:

• Combination (or synthesis) reactions: Two or more simple substances combine to form a single, more complex product.
• Decomposition reactions: A single compound breaks down into two or more simpler substances, often requiring an input of energy (such as heat, light, or electricity) to occur.
• Displacement reactions: A more reactive element displaces a less reactive element from a compound.
• Double displacement reactions: Two compounds exchange ions or components to form two new compounds, often seen in reactions occurring in solution, including many precipitation reactions (where an insoluble solid, called a precipitate, forms and separates out from a solution).
• Oxidation and reduction (redox) reactions: Reactions involving the transfer of electrons between substances — oxidation refers to the loss of electrons (or, in simpler everyday terms, often associated with a substance combining with oxygen or losing hydrogen), while reduction refers to the gain of electrons (often associated with the loss of oxygen or the gain of hydrogen). Rusting of iron, a very common everyday example, is fundamentally an oxidation reaction, in which iron reacts with oxygen (and moisture) in the environment over time to form iron oxide.

Everyday Chemistry: Common Substances and Their Uses

Given the practical, general-studies orientation of the DSC science section, it is worth being familiar with a range of common chemical substances encountered in daily life, along with their basic chemical identity and common uses — this kind of applied knowledge is frequently tested:

• Baking soda (sodium bicarbonate): Used in cooking as a leavening agent, and also has mild antacid properties.
• Washing soda (sodium carbonate): Used in cleaning and in the glass and detergent industries.
• Bleaching powder (calcium oxychloride/hypochlorite): Used for bleaching and disinfection, including water purification in some contexts.
• Common salt (sodium chloride): Used in food, and also as a raw material for producing various other important industrial chemicals.
• Vinegar (dilute acetic acid): Used in cooking and food preservation.
• LPG (Liquefied Petroleum Gas): A mixture of hydrocarbon gases (mainly propane and butane), widely used as a domestic cooking fuel.
• CNG (Compressed Natural Gas): Primarily methane, used increasingly as a cleaner-burning transportation fuel compared to conventional petrol or diesel.

Understanding not just the names but the basic chemical composition and everyday application of such substances is exactly the kind of practical science knowledge DSC papers value, because it reflects the kind of science a teacher should be comfortable explaining to curious students.

Bringing Physics and Chemistry Together

You'll notice that physics and chemistry, while distinct disciplines, share deep conceptual connections — the same atoms and energy principles that chemistry examines at the level of chemical bonds and reactions are governed by the same fundamental physical laws of energy conservation and electromagnetic interaction that physics studies at a broader scale. As a teacher, appreciating these connections will help you present science not as a set of disconnected facts to memorise, but as a coherent, logical way of understanding how the physical world works — an approach that will serve both your exam preparation and your future teaching career very well.

Common Exam Traps

• Confusing mass and weight: Mass is constant and measures the amount of matter; weight is a force due to gravity and varies with location. Many candidates use these terms interchangeably in casual speech, but exam questions often specifically test this distinction.
• Confusing speed and velocity, or distance and displacement: Speed and distance are scalar (magnitude only); velocity and displacement are vector (magnitude and direction). A question describing circular or back-and-forth motion often exploits this distinction.
• Believing sound can travel through a vacuum like light can: Sound requires a medium; light (and other electromagnetic radiation) does not. This is a very commonly tested contrast.
• Mixing up convex and concave lens/mirror effects: Convex lenses/mirrors generally converge; concave lenses/mirrors generally diverge (for lenses) or converge (for concave mirrors, in the case of reflecting light) — remember that the naming convention and effect can differ slightly between lenses and mirrors, so learn each separately rather than assuming a shared rule.
• Forgetting that the pH scale is logarithmic: A change of one whole number on the pH scale represents a tenfold change in acidity/alkalinity, not a simple linear change — occasionally tested in more conceptually rigorous questions.
• Confusing atomic number and atomic mass/weight: Atomic number is the number of protons (defines the element); atomic mass reflects the combined mass of protons and neutrons (and can vary between isotopes of the same element, which share the same atomic number but differ in neutron count).
• Mixing up oxidation and reduction: A simple memory aid used by many students is "OIL RIG" — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons) — useful for quickly resolving this common point of confusion.
• Assuming all metals are good conductors and all non-metals are poor conductors without exception: While this is a generally reliable rule, exam questions sometimes test edge cases (such as graphite, a non-metallic form of carbon that is unusual in being a good conductor of electricity) — be aware that general rules in chemistry often admit specific, well-known exceptions.
• Forgetting the Law of Conservation of Mass when balancing equations: Chemical equations must always be balanced, reflecting that matter is neither created nor destroyed in ordinary chemical reactions — a foundational principle worth keeping in mind even if you are not asked to literally balance an equation in the exam.

With a solid grounding in physics and chemistry fundamentals now in place, we move next to the second half of our general science coverage: biology, health, and everyday science — a chapter that will feel immediately relevant to daily life and, quite fittingly, to the health and well-being of the very students you will one day teach.