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
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.
Section 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.
Section 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.
| Feature | Compound | Mixture |
|---|---|---|
| Composition | Fixed ratio | Variable |
| Bonding between parts | Chemical bonds | No new bonds formed |
| Separation | Chemical reaction | Physical methods |
| Properties | Different from components | Combination of components |
Section 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.
Section 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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