This chapter focuses on atoms, molecules, chemical combinations and classification of matter.
Created & Curated By S.K. Sinha
Enhanced Theory with Definitions, Examples & Concepts
Atom: The smallest indivisible particle of an element that retains all the chemical properties of that element. From Greek word "atomos" meaning indivisible.
Molecule: The smallest particle of a substance (element or compound) that can exist independently and retains all properties of that substance.
Element: A pure substance made up of only one type of atom. Cannot be broken down into simpler substances by chemical methods.
🧪 CLASSIFICATION OF MATTER 🧪
Based on Composition:
PURE SUBSTANCES
ELEMENTSCOMPOUNDS
IMPURE SUBSTANCES
MIXTURES
Each category has unique properties and composition!
⚡ ATOMIC NUMBER: Number of protons = Number of electrons
Atoms are electrically neutral (equal protons and electrons)
Examples of Molecules
Water (H₂O):HOH
Carbon Dioxide (CO₂):OCO
Sodium Chloride (NaCl):NaCl
1. Elements
Properties:
• Made of only one type of atom
• Cannot be broken down chemically
• 118 known elements
• Each has unique atomic number
• Represented by symbols
Properties:
• Made of two or more different atoms
• Chemically combined in fixed ratio
• Properties different from elements
• Can be decomposed chemically
• Represented by chemical formulas
Examples: Water (H₂O), Salt (NaCl), Sugar (C₁₂H₂₂O₁₁), Carbon dioxide (CO₂)
3. Mixtures
Properties:
• Two or more substances mixed physically
• Components retain their properties
• No fixed composition
• Can be separated by physical methods
• Variable composition
Elements arranged in Periodic Table by atomic number!
Property
Metals
Non-metals
Metalloids
Physical State
Mostly solid (except Hg)
Solid, liquid, or gas
Mostly solid
Lustre
Metallic shine
Dull (no shine)
Semi-metallic
Conductivity
Good conductor
Poor conductor
Semiconductor
Malleability
Malleable
Brittle
Varies
Examples
Fe, Cu, Au, Ag, Al
C, N, O, S, Cl
Si, Ge, As, Sb
Chemical Formulas and Equations
Chemical Formula: A symbolic representation showing the types and numbers of atoms present in a compound.
Reading Chemical Formulas:
• H₂O: 2 atoms of Hydrogen + 1 atom of Oxygen
• CO₂: 1 atom of Carbon + 2 atoms of Oxygen
• CaCO₃: 1 Ca + 1 C + 3 O atoms
• NH₃: 1 Nitrogen + 3 Hydrogen atoms
Definition: Mixtures with uniform composition throughout. Components cannot be distinguished visually.
Examples:
• Salt water (solution)
• Sugar water
• Air
• Alloys (brass, steel)
• Gasoline
Characteristics:
• Uniform composition
• Same properties throughout
• Also called solutions
• Cannot see individual components
Heterogeneous Mixtures
Definition: Mixtures with non-uniform composition. Components can be distinguished and often seen separately.
Examples:
• Oil and water
• Sand and water
• Soil
• Granite rock
• Salad
Characteristics:
• Non-uniform composition
• Visible boundaries
• Different properties in regions
• Can see individual components
Separation Techniques
⚗️ METHODS TO SEPARATE MIXTURES ⚗️
Physical methods based on different properties
Choice depends on nature of components!
🔬 SEPARATION METHODS 🔬
Filtration: Separates insoluble solids from liquids using filter paper
Evaporation: Separates dissolved solids from liquids by heating
Distillation: Separates liquids with different boiling points
Magnetic Separation: Separates magnetic materials from non-magnetic
Decantation: Separates immiscible liquids or settled solids
Chromatography: Separates substances based on different movement rates
Method
Principle
Used For
Example
Filtration
Particle size difference
Insoluble solid + liquid
Sand from water
Evaporation
Boiling point difference
Soluble solid + liquid
Salt from seawater
Distillation
Boiling point difference
Two miscible liquids
Water from alcohol
Magnetic Separation
Magnetic property
Magnetic + non-magnetic
Iron filings from sand
Decantation
Density difference
Immiscible liquids
Oil from water
Solutions
Solution: A homogeneous mixture of two or more substances where one substance (solute) is completely dissolved in another (solvent).
Solute: The substance that gets dissolved (usually present in smaller amount).
Solvent: The substance that does the dissolving (usually present in larger amount).
Types of Solutions
Based on Physical State:
• Solid in Liquid: Salt in water
• Liquid in Liquid: Alcohol in water
• Gas in Liquid: CO₂ in water (soda)
• Solid in Solid: Alloys (brass, steel)
• Gas in Gas: Air (mixture of gases)
Based on Concentration:
• Dilute Solution: Less solute
• Concentrated Solution: More solute
• Saturated Solution: Maximum solute dissolved
• Supersaturated: More than maximum (unstable)
Properties of Solutions
Characteristics:
• Homogeneous mixture
• Transparent (can see through)
• Solute particles are very small
• Cannot be separated by filtration
• Stable composition
Common Solutions:
• Sugar water
• Saline water
• Lemon juice
• Vinegar
• Soft drinks
⚠️ Important Note: All solutions are mixtures, but not all mixtures are solutions. Solutions are specifically homogeneous mixtures with very small dissolved particles.
Section B: Short Answer Questions
Q1. Define an atom and molecule with examples.
Answer: Atom is the smallest indivisible particle of an element (e.g., H, O, C). Molecule is the smallest particle of a substance that can exist independently and retains all properties (e.g., H₂O, CO₂, O₂).
Q2. Distinguish between elements and compounds.
Answer: Elements: Made of one type of atom, cannot be broken down chemically (H₂, O₂, Fe). Compounds: Made of two or more different atoms chemically combined in fixed ratio (H₂O, NaCl, CO₂).
Q3. What are mixtures? Give three examples.
Answer: Mixtures are substances formed by physically mixing two or more components without chemical combination. Examples: Air, seawater, soil, alloys, oil and water.
Q4. Differentiate between homogeneous and heterogeneous mixtures.
Answer: Homogeneous: Uniform composition throughout, components not visible (salt water, air). Heterogeneous: Non-uniform composition, components visible and distinguishable (oil-water, sand-water).
Q5. Define solution, solute, and solvent with examples.
Answer: Solution: Homogeneous mixture of solute and solvent (salt water). Solute: Substance that dissolves (salt). Solvent: Substance that does dissolving (water). Water is universal solvent.
Q6. Name four methods to separate mixtures.
Answer: Filtration (sand from water), Evaporation (salt from water), Distillation (alcohol from water), Magnetic separation (iron filings from sand), Decantation (oil from water).
Q7. What is the difference between metals and non-metals?
Answer: Metals: Shiny, good conductors, malleable, mostly solid (Fe, Cu, Au). Non-metals: Dull, poor conductors, brittle, can be solid/liquid/gas (C, O₂, S).
Q8. Explain chemical formula with examples.
Answer: Chemical formula shows types and numbers of atoms in a compound using symbols and subscripts. Examples: H₂O (2H + 1O), CO₂ (1C + 2O), NaCl (1Na + 1Cl).
Q9. What are noble gases? Name three.
Answer: Noble gases are chemically inert elements that rarely react with other elements due to complete outer electron shells. Examples: Helium (He), Neon (Ne), Argon (Ar).
Q10. Why is air considered a mixture and not a compound?
Answer: Air is a mixture because its components (N₂, O₂, CO₂, etc.) retain individual properties, have variable composition, and can be separated by physical methods like fractional distillation.
Section C: Long Answer Questions
Q1. Explain the structure of an atom and how atoms combine to form molecules.
Solution: An atom consists of a central nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in orbital shells. Atoms combine to form molecules through chemical bonding: ionic bonding (transfer of electrons) forms compounds like NaCl, while covalent bonding (sharing electrons) forms molecules like H₂O, CO₂. The number of electrons in the outermost shell determines bonding behavior.
Q2. Compare and contrast elements, compounds, and mixtures with detailed examples.
Solution: Elements are pure substances with one type of atom (H₂, O₂, Fe), cannot be broken down chemically, have unique properties. Compounds are pure substances with two or more different atoms chemically combined in fixed ratios (H₂O, NaCl, CO₂), can be decomposed chemically, properties differ from constituent elements. Mixtures are physical combinations of substances (air, seawater, alloys), retain individual properties, variable composition, separated by physical methods. Key difference: chemical vs physical combination.
Q3. Describe various separation techniques for mixtures with principles and applications.
Solution: Separation techniques exploit different physical properties: Filtration uses particle size differences (sand from water using filter paper). Evaporation uses boiling point differences (salt from water by heating). Distillation separates liquids with different boiling points (pure water from seawater). Magnetic separation uses magnetic properties (iron filings from sand). Decantation uses density differences (oil from water). Chromatography separates based on different movement rates through medium. Choice depends on physical properties of components.
Q4. Explain the classification of elements in the periodic table with properties.
Solution: Elements are classified as: Metals (left side) - shiny lustre, good conductors of heat/electricity, malleable, ductile, mostly solid except mercury (examples: Fe, Cu, Au, Ag). Non-metals (right side) - dull appearance, poor conductors, brittle, exist in all three states (examples: C, N₂, O₂, S). Metalloids (diagonal band) - intermediate properties, semiconductors (Si, Ge). Noble gases (Group 18) - chemically inert, complete electron shells (He, Ne, Ar). Properties change systematically across periods and down groups.
Q5. Discuss solutions in detail including types, properties, and concentration terms.
Solution: Solutions are homogeneous mixtures where solute dissolves completely in solvent. Types by state: solid in liquid (salt-water), liquid in liquid (alcohol-water), gas in liquid (CO₂-water), solid in solid (alloys). Properties: transparent, homogeneous, stable, cannot be filtered, solute particles are molecular/ionic size. Concentration types: dilute (less solute), concentrated (more solute), saturated (maximum dissolved), supersaturated (excess, unstable). Water is universal solvent due to polar nature. Solubility depends on temperature, pressure, nature of solute-solvent.
Q6. Explain chemical formulas and how to interpret them with examples.
Solution: Chemical formulas represent composition using element symbols and subscripts indicating atom numbers. Reading: H₂O means 2 hydrogen atoms + 1 oxygen atom. CO₂ means 1 carbon + 2 oxygen atoms. Complex examples: CaCO₃ (1 calcium + 1 carbon + 3 oxygen), H₂SO₄ (2 hydrogen + 1 sulfur + 4 oxygen). Subscripts multiply only the preceding element. Parentheses with subscripts multiply everything inside: Ca(OH)₂ means 1 calcium + 2 oxygen + 2 hydrogen atoms. Formulas show fixed ratios in compounds.
Q7. Describe the differences between physical and chemical properties of matter.
Solution: Physical properties can be observed without changing chemical composition: color, odor, melting point, boiling point, density, hardness, conductivity, malleability. These help identify substances and determine separation methods. Chemical properties involve composition changes during reactions: flammability, reactivity with acids/bases, oxidation tendency, decomposition behavior. Examples: Iron's physical properties include metallic lustre, high density, magnetic nature. Chemical properties include rusting in moist air, reaction with acids to produce hydrogen gas. Understanding both types is crucial for material identification and applications.
Q8. Explain why some mixtures can be separated easily while others require complex methods.
Solution: Separation ease depends on physical property differences between components. Large differences enable simple separation: magnetic/non-magnetic materials (magnetic separation), different particle sizes (sieving), immiscible liquids (decantation). Small differences require complex methods: similar boiling points need fractional distillation, dissolved salts need evaporation/crystallization. Homogeneous mixtures generally harder to separate than heterogeneous. Solutions require methods like distillation, evaporation, or chromatography. Multiple separation steps often needed for complex mixtures like crude oil refining or water purification.