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1.1.1Reactivity 1 — What drives chemical reactions?

Chemical Elements, Compounds and Mixtures — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 5 of 12 Unit quiz

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Pure Substances

Matter is either a pure substance, with fixed composition, or a mixture of two or more substances.

Element
  • Element A pure substance made of atoms with the same atomic number, which cannot be broken down chemically into simpler substances.
Compound
  • Compound A pure substance formed from two or more elements chemically bonded in a fixed ratio, such as H₂O or NaCl.
Properties
  • Properties A compound has properties unlike those of the elements in it;
    • sodium and chlorine are very reactive, but sodium chloride is stable.
Separation
  • Separation Compounds can only be split into elements by chemical reactions such as electrolysis, not by physical methods.

Part 2 of 4Mixtures

Mixtures
Definition
  • Definition Two or more substances physically combined, with no fixed ratio and with each keeping its own chemical properties.
Homogeneous
  • Homogeneous Uniform composition throughout, so one phase is seen, as in salt dissolved in water or air without dust.
Heterogeneous
  • Heterogeneous Non-uniform composition with visible separate phases, as in sand and water or oil and vinegar.
Separable
  • Separable The components can be separated by physical processes that use differences in physical properties.
FeatureCompoundMixture
CompositionFixed ratioVariable
Bonding between partsChemical bondsNo new bonds formed
SeparationChemical reactionPhysical methods
PropertiesDifferent from componentsCombination of components

Part 3 of 4Classifying Examples

Work through these questions to classify any sample of matter.

1
Ask whether the sample is uniform throughout; if not, it is a heterogeneous mixture.
2
If it is uniform, ask whether its composition can change by simply adding more of one component, which identifies a homogeneous mixture.
3
If composition is fixed, check whether all the atoms are of the same element, which identifies an element.
4
If the atoms are of two or more elements in a fixed ratio, the sample is a compound.
5
Use the formula to confirm: O₂ is an element, CO₂ a compound, and air a homogeneous mixture of gases.

Part 4 of 4Exam Checklist & Tips

1.1.1 – Key Points for the Exam

  • Element A substance of atoms having the same atomic number.
  • Compound A substance of two or more elements chemically combined in fixed proportions.
  • Homogeneous mixture A mixture with the same composition throughout.
  • Heterogeneous mixture A mixture with visibly different regions of composition.

Common Mistakes

  • Calling a molecule such as O₂ a compound when it contains only one element.
  • Saying that a homogeneous mixture is a pure substance, when it contains more than one substance.
  • Describing the bonds in a mixture as being broken during separation.
  • Thinking that a compound keeps the properties of its elements.

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