Unit 3: Diversity of Matter by Chemical Composition (Notes)

Unit 3: Diversity of Matter by Chemical Composition

Element and Atoms

• Elements are pure substances of one atom type; cannot be broken down chemically.
• Each has unique properties; fundamental building block of matter.
• Exist as: monoatomic (e.g., He), molecules of elements (e.g., $\text{H}<em>2$, $\text{O}</em>2$).

Elements by mass in Earth's crust and human body

• Earth’s crust: Oxygen, Silicon, Aluminium, Iron, Calcium are most abundant.
• Human body: Oxygen (~65%), Carbon (~18%), Hydrogen (~9–10%), Nitrogen (~3%), Calcium (~3%).

Monoatomic vs. molecules of elements

• Monoatomic: Pure substance, single atoms (e.g., He).
• Molecule of an element: Pure substance, two or more same atoms bonded (e.g., $\text{H}_2$).

Periodic Table: organisation and purpose

• Elements arranged by increasing proton number (atomic number).
• Rows are periods, columns are groups (similar chemical properties).
• Helps predict trends and properties.
• Symbols: 1-2 letters, first capitalized.

Classification of elements

• Metals: Shiny, good conductors, malleable, ductile, high density/melting points (e.g., Cu, Fe).
• Non-metals: Dull, poor conductors, brittle, low density/melting points (e.g., I, S).
• Metalloids: Properties of both (e.g., Si, B).

Applications of elements

• Aluminium (Al): Light, low density; aircraft, cans.
• Copper (Cu): Good electrical/thermal conductivity; wires.
• Iron (Fe): Strong; construction, stainless steel.
• Silicon (Si): Metalloid; semiconductors.

Chemical formulas and naming conventions

• Compounds: Different elements chemically bonded in fixed ratios (e.g., $\text{NaCl}$, $\text{CO}_2$).
• Metal + non-metal name ends in -ide (e.g., Sodium chloride).
• Example reaction: $\text{2 H}<em>2 + \text{O}</em>2 \rightarrow \text{2 H}_2\text{O}$.

Separation and composition: elements, compounds, and mixtures

• Compound: Requires chemical processes to separate elements (e.g., electrolysis); fixed formula/ratio.
• Mixture: Substances not chemically bonded; retain properties; can be separated by physical means (filtration, distillation); composition not fixed.

Mixtures and examples

• Alloys (solid): brass (Cu + Zn).
• Gas mixtures: Air (N₂, O₂).
• Solutions (homogeneous): salt in water.

Solutions: solute and solvent

• Solution: Homogeneous mixture where solvent dissolves solute.
• Solute: Substance that dissolves; lesser quantity.
• Solvent: Substance that does the dissolving; greater quantity.
• Aqueous solution: Water is the solvent.
• Example: $\text{NaCl (s)} \rightarrow \text{Na}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$.

Properties of solutions

• Clear, transparent; particles too small to scatter light.
• Water is the universal solvent; ~75–85% of human body is water.

Concentration of solutions

• Measures solute amount in solvent.
• Dilute: Low solute; Concentrated: High solute; Saturated: Max solute dissolved at a given temperature.
• Calculation: $C = \frac{m<em>{\text{solute}}}{V</em>{\text{solvent}}}$ (mass in grams, volume in cm³).
  • Example: 10 g solute in 250 cm³ solvent, $C = \frac{10}{250} = 0.04 \text{ g cm}^{-3}$.

Rate of dissolving and solubility

• Rate of dissolving: How fast solute dissolves.
• Factors increasing rate: stirring, smaller particles, higher solvent temperature, more solvent.
• Solubility: Ability of solute to dissolve.
• Solubility curve: Shows max solute in 100 g water vs. temperature for a saturated solution.
  • Above line: oversaturated; On line: saturated; Below line: unsaturated.
  • For many, solubility increases with temperature.

Suspensions

• Mixture with insoluble substances dispersed; solute does not dissolve.
• Large, visible particles; settle over time.
• Separated by filtration (residue on filter paper).
• Scatter light; not transparent (e.g., muddy water, calamine lotion).

Key comparisons

• Elements: Pure, one atom type, cannot be broken down chemically.
• Compounds: Pure, two+ elements chemically joined in fixed ratio; new properties.
• Mixtures: Two+ substances not chemically bonded; separated by physical means; composition not fixed; components retain properties.

Important formulas to remember

• Concentration: $C = \frac{m<em>{\text{solute}}}{V</em>{\text{solvent}}}$
• Water: $\text{H}_2\text{O}$
• Salt: $\text{NaCl}$
• Carbon dioxide: $\text{CO}_2$
• Magnesium oxide: $\text{MgO}$

Key terms to recall

Element, atom, proton, monoatomic, diatomic, polyatomic, molecule, compound, mixture, solution, solvent, solute, solubility, concentration, saturated, unsaturated, supersaturated, filtration, distillation, electrolysis.