Class 9 Chemistry Cheatsheet

Cheatsheet Content

### Matter in Our Surroundings #### DEFINITIONS SNAPSHOT - **Matter** → Occupies space, has mass - **Melting point** → Temp solid melts to liquid at atm pressure - **Fusion** → Change solid to liquid state - **Latent heat of fusion** → Heat to change 1kg solid to liquid at melting point - **Boiling point** → Temp liquid boils at atm pressure - **Latent heat of vaporisation** → Heat to change 1kg liquid to gas at boiling point - **Sublimation** → Solid directly to gas (no liquid state) - **Deposition** → Gas directly to solid (no liquid state) - **Evaporation** → Liquid to vapor below boiling point - **Diffusion** → Intermixing of particles on their own - **Density** → Mass per unit volume #### LAWS / PRINCIPLES - **Law of Conservation of Mass** → Mass cannot be created or destroyed in chemical reaction. #### FORMULAS & CONSTANTS - **Density** = mass / volume - mass → kg - volume → m³ - density → kg m⁻³ - **Temperature conversion** - K = °C + 273.15 (approx 273) - °C = K - 273.15 (approx 273) - **Volume units** - 1 L = 1 dm³ - 1 L = 1000 mL - 1 mL = 1 cm³ #### TABLE MEMORY ZONE | Property | Solid | Liquid | Gas | |---------------------|---------------------------------------|--------------------------------------------|-----------------------------------------| | Shape | Definite | No definite (takes container shape) | No definite (takes container shape) | | Volume | Fixed | Fixed | No fixed | | Compressibility | Negligible | Low | High | | Rigidity/Fluidity | Rigid | Fluid | Fluid | | Interparticle force | Maximum | Intermediate | Minimum | | Interparticle space | Minimum | Intermediate | Maximum | | Kinetic energy | Minimum | Intermediate | Maximum | | Diffusion | Very low | Faster than solids | Very fast | #### NUMERICAL TRIGGERS - **Temperature conversions** - When to use: Converting between Kelvin and Celsius. - Common mistake: Forgetting to add/subtract 273 (or 273.15). - **Density calculations** - When to use: Given mass/volume, find density or vice-versa. - Common mistake: Unit consistency (kg/m³ or g/cm³). #### NCERT TRAPS & CONFUSIONS - **Evaporation vs Boiling** - Evaporation: Surface phenomenon, any temp below boiling point. - Boiling: Bulk phenomenon, fixed temp (boiling point). - **Ice floats on water** → Due to lower density (unique property of water). - **Particles of matter** → Always moving, possess kinetic energy. - **Heating** → Increases kinetic energy, increases diffusion. - **Rubber band, sugar/salt crystals, sponge** → All are solids. - **Gases are highly compressible** → LPG, CNG. - **Condensation** → Water vapor to liquid (e.g., water droplets on cold glass). ### Is Matter Around Us Pure? #### DEFINITIONS SNAPSHOT - **Pure substance** → Single type of particle, fixed composition - **Mixture** → Two or more pure substances, mixed in any proportion - **Homogeneous mixture (solution)** → Uniform composition throughout - **Heterogeneous mixture** → Non-uniform composition, distinct parts - **Solvent** → Component dissolving other (larger amount) - **Solute** → Component dissolved (lesser amount) - **Saturated solution** → Max solute dissolved at given temp - **Unsaturated solution** → Less solute than saturation level - **Solubility** → Amount of solute in saturated solution at given temp - **Suspension** → Heterogeneous, visible particles, settle down, scatter light - **Colloid (colloidal solution)** → Heterogeneous, particles not visible to naked eye, scatter light, stable - **Tyndall effect** → Scattering of light by colloidal particles - **Dispersed phase** → Solute-like particles in colloid - **Dispersion medium** → Medium in which dispersed phase is suspended - **Physical change** → No change in composition/chemical nature (e.g., state change) - **Chemical change (reaction)** → Change in chemical composition, new substances formed (e.g., burning) - **Element** → Basic form of matter, cannot be broken down by chemical reactions - **Compound** → Two or more elements chemically combined in fixed proportion - **Metal** → Lustrous, conductor, ductile, malleable, sonorous - **Non-metal** → Variety of colors, poor conductor, non-lustrous, non-ductile, non-malleable, non-sonorous - **Metalloid** → Intermediate properties between metals and non-metals #### FORMULAS & CONSTANTS - **Concentration of a solution** - Mass by mass % = (Mass of solute / Mass of solution) × 100 - Mass by volume % = (Mass of solute / Volume of solution) × 100 - Volume by volume % = (Volume of solute / Volume of solution) × 100 - **Solution mass** = Mass of solute + Mass of solvent - **Particle size (solution)** → < 1 nm (10⁻⁹ m) #### TABLE MEMORY ZONE | Property | Solution (Homogeneous) | Suspension (Heterogeneous) | Colloid (Heterogeneous) | |-------------------|--------------------------------------|------------------------------------------|----------------------------------------------| | Appearance | Clear, transparent | Opaque | Translucent | | Particle size | < 1 nm | Visible to naked eye | 1 nm - 100 nm | | Settling | Stable (particles don't settle) | Unstable (particles settle) | Stable (particles don't settle) | | Tyndall effect | No | Yes | Yes | | Filtration | Cannot be separated by filtration | Can be separated by filtration | Cannot be separated by filtration (use centrifugation) | | Examples | Salt in water, sugar in water, air | Chalk in water, muddy water | Milk, fog, smoke | | Property | Mixture | Compound | |-------------------|------------------------------------------|------------------------------------------| | Formation | Elements/compounds mix, no new compound | Elements react, new compound formed | | Composition | Variable | Fixed | | Properties | Shows properties of constituents | Totally different properties from constituents | | Separation | Easy physical methods | Chemical/electrochemical reactions only | | Type of Colloid | Dispersed Phase | Dispersion Medium | Example | |-----------------|-----------------|-------------------|-------------------------| | Aerosol | Liquid | Gas | Fog, clouds, mist | | Aerosol | Solid | Gas | Smoke, automobile exhaust | | Foam | Gas | Liquid | Shaving cream | | Emulsion | Liquid | Liquid | Milk, face cream | | Sol | Solid | Liquid | Milk of magnesia, mud | | Foam | Gas | Solid | Foam, rubber, sponge | | Gel | Liquid | Solid | Jelly, cheese, butter | | Solid Sol | Solid | Solid | Coloured gemstone | #### NUMERICAL TRIGGERS - **Concentration calculations** - When to use: Given solute/solvent mass/volume, find percentage concentration. - Common mistake: Correctly identifying mass of solution vs. mass of solvent. #### NCERT TRAPS & CONFUSIONS - **Alloys** → Homogeneous mixtures of metals (or metal + non-metal). Considered mixtures, not compounds. - **Milk** → A colloid, heterogeneous mixture. - **Air** → A solution (homogeneous mixture of gases). - **Physical vs. Chemical changes** → Focus on whether new substance is formed. - **Burning** → Always a chemical change. - **Elements known** → >100 (92 naturally occurring). - Most elements are solid. - 11 elements are gaseous at room temp. - 2 elements are liquid at room temp (Hg, Br). - Ga, Cs liquid above 303 K. ### Atoms and Molecules #### DEFINITIONS SNAPSHOT - **Atom** → Smallest particle of an element, participates in chemical reactions - **Molecule** → Group of two or more atoms chemically bonded, independent existence - **Atomicity** → Number of atoms in a molecule - **Ion** → Charged species (single atom or group of atoms) - **Cation** → Positively charged ion - **Anion** → Negatively charged ion - **Polyatomic ion** → Group of atoms carrying a net charge - **Valency** → Combining power/capacity of an element/atom - **Atomic mass unit (u)** → 1/12th mass of one carbon-12 atom - **Molecular mass** → Sum of atomic masses of all atoms in a molecule - **Formula unit mass** → Sum of atomic masses of all atoms in a formula unit (for ionic compounds) #### LAWS / PRINCIPLES - **Law of Conservation of Mass** → Mass can neither be created nor destroyed in a chemical reaction. - **Law of Constant Proportions (Law of Definite Proportions)** → In a chemical substance, elements are always present in definite proportions by mass. - **Dalton's Atomic Theory Postulates** - All matter made of tiny particles called atoms. - Atoms indivisible, cannot be created or destroyed. - Atoms of given element identical in mass and properties. - Atoms of different elements have different masses and properties. - Atoms combine in small whole number ratios to form compounds. - Relative number and kinds of atoms constant in a compound. #### FORMULAS & CONSTANTS - **Atomic radius** - 1 nm = 10⁻⁹ m - 1 m = 10⁹ nm - **Ratio by number of atoms** → (Mass / Atomic Mass) for each element, then simplify ratio. #### TABLE MEMORY ZONE | Element (Non-Metal) | Atomicity | |---------------------|-------------| | Argon (Ar) | Monoatomic | | Helium (He) | Monoatomic | | Oxygen (O₂) | Diatomic | | Hydrogen (H₂) | Diatomic | | Nitrogen (N₂) | Diatomic | | Chlorine (Cl₂) | Diatomic | | Phosphorus (P₄) | Tetra-atomic | | Sulphur (S₈) | Poly-atomic | | Compound | Combining Elements | Ratio by Mass | |-----------------------|--------------------|---------------| | Water (H₂O) | Hydrogen, Oxygen | 1:8 | | Ammonia (NH₃) | Nitrogen, Hydrogen | 14:3 | | Carbon dioxide (CO₂) | Carbon, Oxygen | 3:8 | | Valency | Cation | Anion | Polyatomic Ion | |---------|------------------|--------------|----------------| | 1 | Na⁺, K⁺, Ag⁺, H⁺ | H⁻, Cl⁻, Br⁻, I⁻ | NH₄⁺, OH⁻, NO₃⁻, HCO₃⁻ | | 2 | Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Cu²⁺ | O²⁻, S²⁻ | CO₃²⁻, SO₃²⁻, SO₄²⁻ | | 3 | Al³⁺, Fe³⁺ | N³⁻ | PO₄³⁻ | #### NUMERICAL TRIGGERS - **Law of Conservation of Mass problems** - When to use: Given masses of reactants/products, find unknown mass. - Common mistake: Not accounting for all reactants/products. - **Law of Constant Proportions problems** - When to use: Given mass ratio, find required mass of one element for given mass of another. - Common mistake: Incorrectly applying the ratio. - **Molecular/Formula unit mass calculations** - When to use: Sum atomic masses of all atoms in a molecule/formula unit. - Common mistake: Forgetting to multiply atomic mass by number of atoms. #### NCERT TRAPS & CONFUSIONS - **Dalton's theory limitation** → Atoms are divisible (sub-atomic particles exist). - **Symbols of elements** → First letter capital, second letter small (e.g., Al, Co, not AL, CO). - **Valency of elements** → Can be variable (e.g., Fe, Cu). Roman numerals used. - **Writing chemical formulae** - Charges must balance. - Metal name/symbol first. - Polyatomic ions in brackets if >1. - **Atoms vs Molecules** → Atom is building block, molecule can exist independently. ### Structure of the Atom #### DEFINITIONS SNAPSHOT - **Electron** → Negatively charged sub-atomic particle (J.J. Thomson) - **Proton** → Positively charged sub-atomic particle (E. Goldstein - canal rays) - **Neutron** → Neutral sub-atomic particle (J. Chadwick) - **Atomic number (Z)** → Number of protons in nucleus - **Mass number (A)** → Sum of protons and neutrons (nucleons) - **Nucleons** → Protons and neutrons - **Valence electrons** → Electrons in outermost shell - **Valency** → Combining capacity (number of electrons gained/lost/shared to achieve octet) - **Octet** → 8 electrons in outermost shell - **Isotopes** → Atoms of same element (same Z), different mass numbers (different A, different neutrons) - **Isobars** → Atoms of different elements (different Z), same mass number (same A) #### LAWS / PRINCIPLES - **Bohr's Model Postulates** - Electrons revolve in discrete orbits. - Electrons do not radiate energy in discrete orbits. - Orbits/shells are energy levels (K, L, M, N... or n=1, 2, 3, 4...). - **Rules for Electron Distribution (Bohr-Bury Scheme)** - Max electrons in shell = 2n² (n = orbit number). - Max 8 electrons in outermost shell. - Inner shells filled before outer shells. #### FORMULAS & CONSTANTS - **Max electrons in shell** = 2n² - K-shell (n=1) = 2(1)² = 2 - L-shell (n=2) = 2(2)² = 8 - M-shell (n=3) = 2(3)² = 18 - N-shell (n=4) = 2(4)² = 32 - **Mass number (A)** = Number of protons + Number of neutrons - **Atomic number (Z)** = Number of protons = Number of electrons (in neutral atom) - **Representation of atom** → $^{A}_{Z}X$ - **Atomic particle properties** | Particle | Symbol | Charge | Mass (u) | Location | Discoverer | |-----------|--------|--------|----------|----------|----------------| | Electron | e⁻ | -1 | ~1/1837 | Orbits | J.J. Thomson | | Proton | p⁺ | +1 | 1 | Nucleus | E. Goldstein | | Neutron | n | 0 | 1 | Nucleus | J. Chadwick | #### TABLE MEMORY ZONE | Element | Z | Protons | Neutrons | Electrons | K | L | M | N | Valency | |------------|---|---------|----------|-----------|---|---|---|---|---------| | Hydrogen | 1 | 1 | 0 | 1 | 1 | - | - | - | 1 | | Helium | 2 | 2 | 2 | 2 | 2 | - | - | - | 0 | | Lithium | 3 | 3 | 4 | 3 | 2 | 1 | - | - | 1 | | Beryllium | 4 | 4 | 5 | 4 | 2 | 2 | - | - | 2 | | Boron | 5 | 5 | 6 | 5 | 2 | 3 | - | - | 3 | | Carbon | 6 | 6 | 6 | 6 | 2 | 4 | - | - | 4 | | Nitrogen | 7 | 7 | 7 | 7 | 2 | 5 | - | - | 3 | | Oxygen | 8 | 8 | 8 | 8 | 2 | 6 | - | - | 2 | | Fluorine | 9 | 9 | 10 | 9 | 2 | 7 | - | - | 1 | | Neon | 10| 10 | 10 | 10 | 2 | 8 | - | - | 0 | | Sodium | 11| 11 | 12 | 11 | 2 | 8 | 1 | - | 1 | | Magnesium | 12| 12 | 12 | 12 | 2 | 8 | 2 | - | 2 | | Aluminium | 13| 13 | 14 | 13 | 2 | 8 | 3 | - | 3 | | Silicon | 14| 14 | 14 | 14 | 2 | 8 | 4 | - | 4 | | Phosphorus | 15| 15 | 16 | 15 | 2 | 8 | 5 | - | 3, 5 | | Sulphur | 16| 16 | 16 | 16 | 2 | 8 | 6 | - | 2 | | Chlorine | 17| 17 | 18 | 17 | 2 | 8 | 7 | - | 1 | | Argon | 18| 18 | 22 | 18 | 2 | 8 | 8 | - | 0 | #### NUMERICAL TRIGGERS - **Average atomic mass calculation** - When to use: For elements with isotopes, given isotopic masses and percentages. - Formula: $\sum (\text{Isotopic mass} \times \text{Percentage abundance}) / 100$ - Common mistake: Forgetting to divide by 100 or using incorrect percentages. - **Valency determination** - When to use: Given electron configuration. - Common mistake: Forgetting that valency is 8-valence electrons if valence electrons > 4. #### NCERT TRAPS & CONFUSIONS - **Thomson's Model (Plum Pudding)** → Failed, couldn't explain other experiments. - **Rutherford's Model (Nuclear Model)** - Alpha scattering experiment (gold foil). - Most alpha particles pass straight (atom mostly empty space). - Few deflected (positive charge in small nucleus). - Very few rebound (dense positive nucleus). - Drawback: Electrons in orbit should lose energy and fall into nucleus (atom unstable). - **Bohr's Model** → Overcame Rutherford's drawback by postulating discrete orbits and no energy radiation in them. - **Neutrons** → Absent in protium (hydrogen-1). - **Isotopes vs. Isobars** → Crucial difference in atomic number (Z) and mass number (A). - **Uses of Isotopes** - Uranium-235 → Nuclear fuel. - Cobalt-60 → Cancer treatment. - Iodine-131 → Goitre treatment. - **Valency of elements with full outermost shell** → Zero (e.g., Noble gases He, Ne, Ar). - **Valency calculation for elements with >4 valence electrons** → 8 - (number of valence electrons). - **Atomic mass** is practically due to protons and neutrons (electrons negligible).