The IB specification demands absolute precision in terminology. Mark schemes strictly enforce exact wording for core definitions. The difference between full marks and zero often hinges on a single vital phrase.
Core Classifications of Matter
Elements
Elements are the primary constituents of matter, which cannot be chemically broken down into simpler substances. They consist entirely of atoms possessing the identical atomic number (number of protons). Some elements exist as individual monoatomic entities (e.g. Noble gases like Xenon), while others bond into elemental molecules (e.g. Diatomic \(O_2\), \(I_2\)).
Compounds
Compounds are formed when atoms of different elements are chemically bonded together in a fixed, whole-number ratio. The resulting substance possesses chemical and physical properties entirely distinct from its constituent elements.
The law of constant composition dictates that the elemental ratio within a specific compound remains invariable regardless of its origin, method of synthesis, or environmental history. Every molecule of pure water is invariably \(H_2O\).
Mixtures
Mixtures contain more than one element or compound in no fixed ratio. The constituents are not chemically bonded to one another: they retain their discrete chemical identities and can be separated by physical methods alone.
Alloys are Mixtures, Not Compounds
Despite exhibiting strong metallic bonding and appearing uniform, alloys (e.g. Brass, steel) are strictly classified as mixtures. They contain metallic elements in a variable, non-fixed ratio dependent on the manufacturing process, not stoichiometric laws.
Homogeneous vs Heterogeneous Mixtures
Homogeneous Mixtures
Uniform composition throughout. Single visible phase.
Examples: aqueous solutions, atmospheric air, alloys
Heterogeneous Mixtures
Non-uniform composition. Visible phase boundaries.
Examples: sand and water, oil and water emulsion
Physical Separation Techniques
Because the components of mixtures are not chemically bonded, we exploit their differing physical properties to separate them. Candidates must justify the selection of a specific method based on the nature of the mixture.
| Technique | Application & Physical Principle |
|---|---|
| Filtration | Separates an insoluble solid from a liquid. Relies on the disparity in particle size. The porous filter allows fluid to pass while trapping larger solid particulates. |
| Evaporation | Separates a soluble non-volatile solid from a volatile solvent. Exploits a vast difference in boiling points. The solvent vaporizes, leaving the solid solute behind. |
| Distillation | Separates components of a liquid mixture based on differential boiling points. Unlike evaporation, the vaporized component is condensed and collected as a purified distillate. |
| Recrystallization | Purifies a solid contaminated with impurities using differential solubility across a temperature gradient. Target dissolves in hot solvent and selectively crystallizes out on cooling. |
| Chromatography | Separates solutes based on differential affinity for a stationary phase (paper) vs a mobile phase (solvent). Components with stronger mobile-phase affinity travel further. |
| Solvation | Preliminary step: treat a mixture with a solvent that selectively dissolves only one component, then separate via filtration. |
Physical Separation vs Chemical Decomposition
Mixtures can be separated by physical methods (no bond breaking). Compounds can only be separated into elements by chemical reactions that break bonds. For example, the electrolysis of water:
2H₂O(l) → 2H₂(g) + O₂(g)
Always Include State Symbols
The omission of state symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous, is a pervasive error that consistently loses marks. Examiners expect state symbols as a non-negotiable part of every chemical equation.
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