CDS Exam: Chemistry Basic Concepts

• The Union Public Service Commission (UPSC) conducts the Combined Defence Services (CDS) exam.
• Recruitment is for the Indian Military Academy, Indian Naval Academy, Air Force Academy, and Officers' Training Academy.
• The CDS exam includes English, General Knowledge, and Elementary Mathematics sections.
• Chemistry is a part of the General Knowledge section
• Chemistry questions focus on basic concepts, applications, and current events.

Basic Concepts of Chemistry

• Atomic number identifies an element.
• Valency is the combining capacity of an element.
• An atom is the basic unit of matter, while a molecule is a group of atoms bonded together.
• Avogadro's number is 6.022 x 10^23, representing the number of entities in a mole.
• Molarity is moles of solute per liter of solution, while molality is moles of solute per kilogram of solvent.
• The SI unit of temperature is Kelvin (K).
• A physical change alters the form of a substance, and a chemical change involves a change in chemical composition.
• The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.
• An element is a pure substance, and a compound is a substance formed from two or more elements chemically combined.
• The periodic table organizes elements based on their properties.

Structure of Atom

• J.J. Thomson discovered the electron.
• A proton has a positive charge.
• Isotopes are variants of an element with different numbers of neutrons like Carbon-12 and Carbon-14.
• An orbit is a fixed path around the nucleus, and an orbital is a region where an electron is likely to be found.
• The M shell can hold a maximum of 18 electrons.
• Sodium (Na) has an electronic configuration of 1s² 2s² 2p⁶ 3s¹.
• Quantum numbers describe the properties of atomic orbitals and electrons.
• Heisenberg Uncertainty Principle states that it is impossible to know both the position and momentum of a particle with perfect accuracy.
• An s-orbital is spherical.
• The Aufbau principle states that electrons fill the lowest energy orbitals first.

Periodic Table and Periodicity

• Dmitri Mendeleev is credited with the modern periodic table.
• Groups are vertical columns and periods are horizontal rows in the periodic table.
• Atomic size decreases across a period due to increasing nuclear charge.
• Fluorine (F) has the highest electronegativity.
• Ionization energy decreases down a group because the outermost electrons are farther from the nucleus.
• Alkali metals are Group 1 elements like lithium (Li), sodium (Na), and potassium (K).
• Halogens are Group 17 elements like fluorine (F), chlorine (Cl), and bromine (Br).
• Fluorine is the most reactive non-metal.
• Metals are generally shiny, and conductive, while non-metals are dull, and poor conductors.
• Transition metals are elements in the d-block of the periodic table.

Chemical Bonding

• An ionic bond involves the transfer of electrons like in sodium chloride (NaCl).
• A covalent bond involves the sharing of electrons like in methane (CH₄).
• Polar covalent bonds have unequal sharing of electrons, while non-polar covalent bonds have equal sharing of electrons.
• A coordinate bond is a covalent bond where one atom provides both electrons like in ammonium ion (NH₄⁺).
• A water molecule (H₂O) is bent or V-shaped.
• VSEPR theory predicts the shapes of molecules based on electron pair repulsion.
• Methane (CH₄) has a bond angle of 109.5°.
• Hybridization is mixing atomic orbitals to form new hybrid orbitals like in carbon atoms in organic compounds.
• Sigma (σ) bonds are single bonds, and pi (π) bonds are multiple bonds formed by sideways overlapping of orbitals.
• Hydrogen bonding is a type of intermolecular force like in water (H₂O).

States of Matter

• The three states of matter are solid, liquid, and gas.
• Evaporation occurs at the surface of a liquid, and boiling occurs throughout the liquid at a specific temperature.
• Sublimation is the transition from solid to gas like in dry ice (CO₂).
• The critical temperature is the temperature above which a gas cannot be liquefied by pressure alone.
• Boyle's law states that the pressure of a gas is inversely proportional to its volume at constant temperature (P₁V₁ = P₂V₂).
• Charles's law states that the volume of a gas is directly proportional to its temperature at constant pressure (V₁/T₁ = V₂/T₂).
• The ideal gas equation is PV = nRT.
• Diffusion is the movement of particles from high to low concentration, and effusion is the escape of gas through a small hole.
• The triple point of water is the temperature and pressure at which water can exist in all three phases (solid, liquid, and gas) in equilibrium.
• The Kelvin scale is an absolute temperature scale where 0 K is absolute zero.

Chemical Reactions and Equations

• A balanced chemical equation has the same number of atoms of each element on both sides of the equation.
• Exothermic reactions release heat, and endothermic reactions absorb heat.
• A redox reaction involves both oxidation and reduction like in combustion reactions.
• A catalyst speeds up a chemical reaction without being consumed.
• A combination reaction combines two or more reactants to form a single product. A decomposition reaction breaks down a single reactant into two or more products.
• Common salt is sodium chloride (NaCl).
• Water is H₂O.
• Glucose is C₆H₁₂O₆.
• Methane is CH₄.
• Ammonia is NH₃.

Acids, Bases, and Salts

• The pH scale measures the acidity or alkalinity of a solution (0-14).
• Pure water has a pH of 7.
• A strong acid completely dissociates in water, and a weak acid partially dissociates in water.
• Sulfuric acid is H₂SO₄.
• Hydrochloric acid is HCl.
• Sodium hydroxide is NaOH.
• Baking soda is sodium bicarbonate (NaHCO₃).
• Washing soda is sodium carbonate (Na₂CO₃).
• Acids donate protons or accept electrons, and bases accept protons or donate electrons.
• Neutralization is the reaction between an acid and a base to form a salt and water.

Metals and Non-Metals

• Aluminum is the most abundant metal in the Earth's crust.
• Oxygen is the most abundant non-metal in the Earth's crust.
• Rust is iron oxide (Fe₂O₃).
• Corrosion is the degradation of a metal due to chemical reactions with its environment.
• Ductility is the ability to be drawn into wires, and malleability is the ability to be hammered into sheets.
• An alloy is a mixture of two or more metals to enhance properties like steel.
• Brass is composed of copper and zinc.
• Bronze is composed of copper and tin.
• Aluminum oxide is Al₂O₃.
• Iron oxide is Fe₂O₃.

Organic Chemistry

• Methane (CH₄) is the simplest organic compound.
• Saturated hydrocarbons have only single bonds, and unsaturated hydrocarbons have double or triple bonds.
• Methane is CH₄.
• Ethane is C₂H₆.
• Ethene is C₂H₄.
• Benzene is C₆H₆.
• Alcohols have a hydroxyl (-OH) functional group.
• Carboxylic acids have a carboxyl (-COOH) functional group.
• Ethanol is C₂H₅OH.
• Acetic acid is CH₃COOH.

Environmental Chemistry

• The greenhouse effect is the trapping of heat by gases in the Earth's atmosphere.
• Primary greenhouse gases include carbon dioxide (CO₂), methane (CH₄), and water vapor (H₂O).
• Ozone is O₃.
• Ozone absorbs harmful UV radiation in the stratosphere.
• Acid rain is rain with high levels of sulfuric or nitric acids, formed from pollutants like sulfur dioxide and nitrogen oxides.
• Carbon dioxide is CO₂.
• Carbon monoxide is CO.
• Nitrogen dioxide is NO₂.
• Sulfur dioxide is SO₂.
• Methane is CH₄.