Comprehensive Study Guide for Class 10 Science: Science Concepts and Principles

Chemical Reactions and Equations

A chemical reaction is defined as the transformation of one chemical substance into another chemical substance. Common examples of these reactions in everyday life include the rusting of iron and the setting of milk into curd. These reactions are represented by chemical equations, which use symbols and formulae to represent the substances involved. In these equations, the substances that undergo change are called reactants, and the new substances formed are called products. A balanced chemical equation is one where the number of atoms of each element remains the same on both the reactants' side and the products' side, adhering to the Law of Conservation of Mass, which states that the mass of reactants must equal the mass of products.

Several characteristics identify a chemical reaction. A change in colour is observed in the reaction where iron ($Fe$) reacts with blue copper sulphate ($CuSO_4$) to produce blue-green iron sulphate ($FeSO_4$) and copper ($Cu$). A change in temperature occurs when calcium oxide ($CaO$) reacts with water ($H_2O$) to form calcium hydroxide ($Ca(OH)_2$), releasing significant heat. A change in state is seen when gaseous hydrogen ($H_2(g)$) and oxygen ($O_2(g)$) combine to form liquid water ($H_2O(l)$). The evolution of gas is demonstrated when zinc ($Zn(s)$) reacts with sulphuric acid ($H_2SO_4(aq)$) to form zinc sulphate ($ZnSO_4(aq)$) and hydrogen gas ($H_2(g)$). Finally, the formation of a precipitate, which is an insoluble solid, is seen when aqueous lead nitrate ($Pb(NO_3)_2$) reacts with potassium iodide ($KI$) to form a yellow precipitate of lead iodide ($PbI_2$).

Types of Chemical Reactions and Energy Changes

Chemical reactions are classified into several types based on how atoms are rearranged. A combination reaction ($A + B \rightarrow AB$) occurs when two or more reactants combine to form a single product, such as the reaction of magnesium ($Mg$) with oxygen ($O_2$) to form white magnesium oxide ($MgO$) powder with a dazzling white flame. Decomposition reactions ($AB \rightarrow A + B$) involves a single reactant breaking down into two or more products. This can occur via thermal decomposition using heat (e.g., $CaCO_3 \rightarrow CaO + CO_2$), photolytic decomposition using sunlight (e.g., $2AgBr \rightarrow 2Ag + Br_2$, used in black and white photography, where white silver chloride becomes grey), or electrolytic decomposition using electrical energy (e.g., $2H_2O \rightarrow 2H_2 + O_2$). During electrolysis of water, hydrogen gas at the cathode produces a popping sound with a candle, while oxygen at the anode makes a flame burn brighter.

Displacement reactions occur when a more reactive element displaces a less reactive element from its compound ($A + BC \rightarrow AC + B$). For example, iron nails in copper sulphate solution become covered in a red-brown layer of copper while the blue solution fades to light green. Double displacement reactions ($AB + CD \rightarrow AD + CB$) involve an exchange of ions between reactants, such as the reaction between sodium sulphate ($Na_2SO_4$) and barium chloride ($BaCl_2$) forming a white precipitate of barium sulphate ($BaSO_4$) and sodium chloride ($NaCl$).

Energy-wise, reactions are categorized as exothermic, which release heat energy (e.g., the reaction of $CaO$ with water, digestion, and respiration), or endothermic, which absorb heat energy (e.g., the decomposition of calcium carbonate or photosynthesis). Redox reactions involve simultaneous oxidation (gain of oxygen or loss of hydrogen) and reduction (loss of oxygen or gain of hydrogen). An oxidizing agent accepts electrons and gets reduced, while a reducing agent loses electrons and gets oxidized. For instance, when copper is heated, black copper oxide ($CuO$) forms, but hydrogen gas can reduce it back to copper.

Corrosion and Rancidity

Oxidation has notable effects in daily life, specifically through corrosion and rancidity. Corrosion is the process where metals are gradually destroyed by chemical reactions with environmental substances like moisture and acids. Iron rusts, silver develops a black coating called tarnishing, and copper develops a green coating. This can be prevented by coating metals with protective layers such as paint or through galvanization to block oxygen and moisture. Rancidity refers to the spoilage of fats and oils in food, resulting in unpleasant taste and smell. This oxidation process is slowed by adding antioxidants, using airtight containers, or refrigeration.

Acids, Bases, and Salts

Acids are substances that usually have a sour taste, turn blue litmus paper red, and give hydrogen ions ($H^+$) in solution. Examples include hydrochloric acid ($HCl$) and acetic acid ($CH_3COOH$). Bases are bitter in taste, soapy to the touch, turn red litmus blue, and provide hydroxyl ions ($OH^-$) in solution, such as sodium hydroxide ($NaOH$). Indicators are chemical compounds that change colour or odour in the presence of an acid or base. Olfactory indicators like vanilla, onion, and clove oil change their smell; for instance, the smell remains in acid but disappears in a base. Natural indicators include litmus (a purple dye from lichen), Hydrangea flowers, and turmeric. Synthetic indicators include phenolphthalein (colorless in acid, pink in base) and methyl orange (red in acid, yellow in base).

Strength of acids and bases is measured on the pH scale, which spans from $0$ to $14$ based on hydrogen ion concentration ($P$ stands for 'potenz' or power). A value of $pH < 7$ is acidic, $pH = 7$ is neutral, and $pH > 7$ is basic. Strong acids release more $H^+$ ions, while weak acids release fewer. Dilution of an acid or base involves mixing it with water, which reduces the concentration of $H_3O^+$ or $OH^-$ ions per unit volume. In daily life, the stomach maintains a pH of $1$ to $3$ for digestion using $HCl$. Plants prefer soil with a pH of $6.3$ to $7.3$. Human blood functions best between $7.0$ and $7.8$. Tooth decay starts when the mouth becomes acidic, and basic toothpaste helps neutralize this.

Properties and Importance of Specific Salts

Salts are ionic compounds held by ionic bonds. A neutral salt ($pH = 7$) forms from a strong acid and strong base, an acidic salt ($pH < 7$) from a strong acid and weak base, and a basic salt ($pH > 7$) from a strong base and weak acid. Sodium Chloride ($NaCl$), found in seawater, is used for seasoning and as a raw material for other chemicals. Sodium Hydroxide ($NaOH$) is produced by the electrolysis of brine in the Chlor-alkali process, where chlorine gas ($Cl_2$) is at the anode, hydrogen gas ($H_2$) is at the cathode, and $NaOH$ forms near the cathode.

Bleaching Powder or Calcium Oxychloride ($CaOCl_2$) is produced by reacting chlorine with slaked lime and used in the textile and paper industries. Baking Soda or Sodium Hydrogen Carbonate ($NaHCO_3$) is produced using sodium chloride, water, ammonia, and carbon dioxide; it is used in baking powder, antacids, and fire extinguishers. Washing Soda or Sodium Carbonate Decahydrate ($Na_2CO_3 \cdot 10H_2O$) is produced by recrystallizing sodium carbonate and is used to remove water hardness. Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$) is obtained by heating gypsum ($CaSO_4 \cdot 2H_2O$) and is used for setting fractured bones. Many salt crystals, like blue copper sulphate, contain water of crystallization ($CuSO_4 \cdot 5H_2O$); heating them removes the water, turning the crystals white, while rehydration restores the blue color.

Metals and Non-Metals

Metals are generally solid at room temperature (except mercury), shiny, hard (except sodium and potassium), malleable, ductile, and good conductors of heat and electricity. Non-metals exist in all three states, are dull (except iodine), generally soft (except diamond), brittle, and poor conductors (except graphite). Chemically, metals react with oxygen to form basic metal oxides, though some, like Aluminium ($Al$) and Zinc ($Zn$), are amphoteric—reacting with both acids and bases. Metals react with water to form hydroxides and hydrogen gas, but the vigor varies: potassium and sodium react violently, calcium mildly, magnesium with hot water, and aluminium, iron, and zinc only with steam. Lead, copper, silver, and gold do not react with water at all.

Most metals react with dilute acids to produce salt and hydrogen gas, but hydrogen is not produced with nitric acid ($HNO_3$) because it is a strong oxidizing agent that oxidizes hydrogen to water; only magnesium ($Mg$) and manganese ($Mn$) react with very dilute $HNO_3$ to release $H_2$. Aqua regia, a $3:1$ mix of concentrated $HCl$ and $HNO_3$, can even dissolve gold and platinum. When metals react with non-metals, they form ionic compounds through electron transfer. These compounds are solid, hard, have high melting and boiling points, are soluble in water, and conduct electricity in molten or aqueous states.

Metallurgy and Alloying

Metallurgy is the science of extracting metals from their ores. Minerals are naturally occurring compounds in the Earth’s crust, while ores are minerals from which metals can be extracted economically. Gangue refers to impurities like sand or soil in the ore. Extraction depends on reactivity: highly reactive metals are extracted via electrolysis, moderately reactive metals through carbon reduction, and low reactivity metals like gold and silver are found in their native state. Specific processes include roasting (heating sulphide ores in excess air) and calcination (heating carbonate ores in limited air). Electrolytic refining is used for purification, where the impure metal is the anode and pure metal is the cathode. Alloying involves mixing metals with other elements to alter properties; for example, brass is a mix of copper and zinc, and bronze is copper and tin. Pure $24$ carat gold is soft, so it is often alloyed with silver or copper as $22$ carat gold to make jewelry.

Carbon and Its Compounds

Carbon is the 15th most abundant element in the Earth's crust with an atomic number of $6$ and a valency of $4$. It forms covalent bonds by sharing electrons. Carbon exhibits catenation—the ability to form long chains or rings—and tetravalency, which allows it to form four bonds. Allotropes of carbon include Diamond (a very hard 3D network used as an abrasive), Graphite (layers of hexagons that are soft and conduct electricity), and Fullerenes (hollow cage-like structures like $C_{60}$).

Hydrocarbons are divided into saturated (alkanes with single bonds) and unsaturated (alkenes with double bonds and alkynes with triple bonds). Functional groups like alcohols ($-OH$), aldehydes ($-CHO$), ketones ($-CO-$), and carboxylic acids ($-COOH$) determine the property of the organic compound. A homologous series is a group of compounds where each member differs from the next by a $-CH_2$ group (mass difference of $14\,u$). Nomenclature follows IUPAC rules, identifying the longest chain and priority of functional groups. Isomerism occurs when compounds have the same formula but different structures.

Chemical Properties of Carbon Compounds and Cleansing Agents

Carbon compounds undergo combustion, burning in oxygen to produce $CO_2$, $H_2O$, and heat. In oxidation, alcohols are converted to acids using reagents like potassium permanganate ($KMnO_4$). Addition reactions involve unsaturated hydrocarbons adding hydrogen in the presence of a nickel catalyst, while substitution reactions involve alkanes reacting with halogens in sunlight. Ethanol ($C_2H_5OH$) is a widely used solvent and medicine ingredient that reacts with sodium to release hydrogen. Ethanoic acid ($CH_3COOH$), or acetic acid, is a weak acid found in vinegar. It undergoes esterification with ethanol to form sweet-smelling esters and saponification to produce soap.

Soaps are sodium or potassium salts of long-chain carboxylic acids, while detergents are ammonium or sulphonate salts. While soaps form scum in hard water (which contains high calcium and magnesium ions), detergents remain effective. The cleansing action of soap involves micelle formation; the hydrophobic tail attaches to oil/dirt, and the hydrophilic head stays in water. When rinsed, the micelles are washed away, carrying the dirt.

Life Processes: Nutrition and Respiration

Life processes include the basic functions required to maintain life. Nutrition involves organisms obtaining food, categorized into autotrophic (making own food via photosynthesis) and heterotrophic (dependent on others). Photosynthesis in plants involves absorbing light with chlorophyll, splitting water molecules, and reducing carbon dioxide to carbohydrates. Heterotrophic nutrition includes holozoic (ingesting whole food like humans or amoeba), saprophytic (feeding on decaying matter like fungi), and parasitic (living on a host like lice).

In human nutrition, the alimentary canal processes food through several stages: ingestion, digestion (using pepsin, $HCl$, and mucus in the stomach), absorption (in the small intestine via villi), and egestion. Respiration is the process of burning food in mitochondria to release energy in the form of ATP. Aerobic respiration occurs in the presence of oxygen, yielding approximately $36\,ATP$ and producing $CO_2$ and $H_2O$. Anaerobic respiration occurs without oxygen; in yeast, it produces alcohol and $CO_2$, while in human muscles, it produces lactic acid, leading to cramps. The human respiratory system includes nostrils, pharynx, larynx, trachea (windpipe), bronchi, and alveoli where gas exchange occurs.

Transportation and Excretion

Transportation in humans is facilitated by blood, which contains Red Blood Cells for oxygen transport, White Blood Cells for immunity, Plasma for nutrients, and Platelets for clotting. The heart is a muscular organ with four chambers to prevent the mixing of oxygenated and deoxygenated blood, ensuring efficient oxygen delivery through double circulation. In plants, the Xylem conducts water and minerals unidirectionally, while the Phloem moves food bidirectionally.

Excretion in humans is managed by the kidneys, where nephrons act as filtration units. The process involves glomerular filtration of waste from blood, selective reabsorption of useful substances like glucose and salts, and tubular secretion of urea and excess water to form urine. Urine travels through ureters to the bladder and is excreted via the urethra. Plants excrete waste by releasing oxygen through stomata, removing excess water via transpiration, and storing other wastes in vacuoles, old xylem (resins and gums), or falling leaves.

Control and Coordination

Coordination in animals is managed by the nervous and endocrine systems. The nervous system uses neurons (nerve cells) with dendrites to receive signals, axons to transmit electrical impulses, and synapses to bridge gaps chemically. Reflex actions are sudden, automatic responses monitored by the spinal cord through a reflex arc. The human brain consists of the Fore-brain (thinking and voluntary control), Mid-brain (involuntary actions like pupil size), and Hind-brain (cerebellum for balance, medulla for blood pressure, and pons for respiration). Chemical coordination involves hormones from endocrine glands like the Pituitary (growth), Thyroid (metabolism), Adrenal (fight or flight), and Pancreas (blood sugar).

Plants coordinate through tropisms (directional growth toward light, gravity, water, or chemicals) and nastic movements (non-directional responses like the folding of Mimosa leaves). Phytohormones include Auxins (cell elongation), Gibberellins (stem growth), Cytokinins (cell division), Abscisic Acid (growth inhibition), and Ethylene (fruit ripening).

Reproduction, Heredity, and Evolution

Reproduction ensures the continuity of life through DNA copying, though inaccuracies lead to variations essential for evolution. Asexual reproduction modes include fission, fragmentation (e.g., Spirogyra), regeneration (e.g., Planaria), budding (e.g., Hydra), and spore formation. Vegetative propagation allows new plants to grow from roots, stems, or leaves. Sexual reproduction in plants involves pollination (self or cross) and fertilization in the ovary to produce seeds. In humans, sexual maturity occurs at puberty. Male reproductive organs include testes (producing sperm and testosterone) and the vas deferens, while female organs include ovaries (releasing eggs), fallopian tubes (site of fertilization), and the uterus (site of embryo development). If no fertilization occurs, menstruation follows a roughly $28$-day cycle.

Heredity is the passing of traits via genes. Gregor Mendel, the father of genetics, discovered that traits are determined by dominant and recessive alleles through his experiments with garden pea plants ($Pisum\,sativum$). His laws include the Law of Dominance, Law of Segregation, and Law of Independent Assortment. Sex determination in humans is genetic; females have $XX$ chromosomes and males have $XY$ chromosomes, where the father’s contribution determines the child's sex ($X$ or $Y$). Inherited traits are passed to future generations, while acquired traits (like muscle mass from exercise) are not.

Light: Reflection and Refraction

Light travels in straight lines at a speed of $3 \times 10^8\,m/s$. Reflection follows two laws: the incident ray, reflected ray, and normal lie in the same plane, and the angle of incidence equals the angle of reflection ($\angle i = \angle r$). Spherical mirrors are either concave (converging) or convex (diverging). The mirror formula is $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$, and magnification is $m = \frac{h'}{h} = -\frac{v}{u}$. Concave mirrors are used in searchlights and dental tools, while convex mirrors are used as rear-view mirrors in vehicles due to their wide field of view.

Refraction is the bending of light when passing between media, governed by Snell’s Law: $\frac{\sin(i)}{\sin(r)} = \text{constant}$. The refractive index measures this bending. Lenses follow the lens formula $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$ and power $P = \frac{1}{f}$, measured in Dioptres ($D$). A convex lens has positive power, while a concave lens has negative power. Refraction through a prism causes dispersion, splitting white light into the VIBGYOR spectrum. This explains rainbows, which involve refraction, dispersion, and internal reflection within water droplets.

Human Eye and Atmospheric Phenomena

The human eye adjusts its focal length through the power of accommodation via ciliary muscles. Common defects include Myopia (nearsightedness, corrected by concave lenses), Hyperopia (farsightedness, corrected by convex lenses), Presbyopia (age-related focus loss), and Astigmatism (irregular cornea shape). Atmospheric refraction causes stars to twinkle and leads to advanced sunrise and delayed sunset. Scattering of light, where particles re-emit light, explains the Tyndall effect, why the sky appears blue (short wavelength blue light scatters more), and why the sun appears red at sunset (long wavelength red light travels further).

Electricity and Magnetism

Electric current ($I = \frac{Q}{t}$, measured in Amperes) is the flow of charge. Ohm's Law states $V = IR$, where $R$ is resistance, which depends on length ($l$), area ($A$), and resistivity ($\rho$) via the formula $R = \rho \frac{l}{A}$. In series circuits, current is constant; in parallel circuits, potential difference is constant. Joule's Law of Heating states $H = I^2Rt$. Electric power $P = VI$ is measured in watts, and the commercial unit of energy is the kilowatt-hour ($kWh$).

Magnetism involves field lines that move from the north to the south pole outside a magnet. Hans Christian Oersted proved the link between electricity and magnetism. Maxwell’s Right Hand Thumb Rule determines field direction around a current-carrying wire, while Fleming’s Left Hand Rule gives the direction of force on a conductor in a magnetic field. Solenoids are coils used to create strong magnetic fields. Domestic circuits in India use $220\,V$ at $50\,Hz$ and include live, neutral, and earth wires, with fuses and grounding for safety.

Important Terms and Practical Guidance

When solving physics problems, it is crucial to identify given values and target variables before selecting formulas like the Mirror Formula or Ohm’s Law. In balancing chemical equations, drawing boxes around formulas ensures they are not changed while adjusting coefficients. For biology diagrams, understanding the pathway of blood in the heart or the structure of a nephron is essential for identifying functions like selective reabsorption. The reactivity series is a vital tool for predicting whether a displacement reaction will occur, with metals like Potassium at the top being highly reactive and Gold at the bottom being least reactive. Understanding the Chlor-alkali process is key to industrial chemistry, as it produces three highly useful substances—hydrogen, chlorine, and sodium hydroxide—from simple salt water.