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1.4Atomic Structure and Properties

Composition of Mixtures — AP CHEMISTRY Revision Notes & Topical Questions

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Lesson summary

1.4 Composition of Mixtures — Notes — AP CHEMISTRY notes: Distinguish pure substances from mixtures and use elemental analysis to find the composition and purity of a mixture.Pure substances contain atoms, molecules or formula units of a single type.Mixtures contain two or more t…

  • Homogeneous mixture
  • Heterogeneous mixture
  • Mass percent of a component
  • Mixtures contain two or more types, and their relative proportions can vary
  • Examples — aluminium, water and sodium chloride are pure substances; air and oil with water are mixtures
  • The particles that make up a mixture are its components

Section 1 of 9Pure substances and mixtures

Pure substances contain atoms, molecules or formula units of a single type.
Mixtures contain two or more types, and their relative proportions can vary.
Examples
  • Examples aluminium, water and sodium chloride are pure substances; air and oil with water are mixtures.
The particles that make up a mixture are its components.

Section 2 of 9Types of mixture

Types of mixture
Homogeneous mixture
Homogeneous mixture uniform composition throughout, such as a solution or air.
Heterogeneous mixture
Heterogeneous mixture composition varies between regions, such as oil and water.
At the particle level a pure substance shows one type of particle, a mixture shows two or more.

Section 3 of 9Why composition varies

Why composition varies
The mass percent of an element in a mixture depends on the proportion of each component, so it is not fixed.
Components are not chemically bonded and keep their own properties,
  • so they can be separated by physical methods such as filtration, distillation and chromatography.

Section 4 of 9Elemental analysis

Elemental analysis
Elemental analysis determines the relative numbers of atoms in a substance and its purity.
Mass percent of a component
(mass of component ÷ total mass of mixture) × 100.
If the measured percent of an element differs from the value for the pure compound, the sample is impure.
An impurity containing a lower percent of the element makes the measured percent too low, and a higher one makes it too high.

Section 5 of 9Composition of a two-component mixture

1
Identify which component contains the analysed element.
2
Calculate the mass fraction of that element in the pure component.
3
Divide the measured mass of the element by this fraction to find the mass of the component.
4
Divide by the total sample mass and multiply by 100.
5
Compare with the pure-substance value to judge purity.
  • This relationship holds when only that component contains the element: mass of X in compound = mass of compound × (mass of X in formula ÷ molar mass).

Section 6 of 9Purity

Purity
(mass of desired compound ÷ total mass of sample) × 100.
It can also be found from the percentage composition of the desired compound.

Section 7 of 9More from the specification

More from the specification
Molar ratios and shared covalent bonds
  • Molar ratios and shared covalent bonds calculating constituent element moles confirms stoichiometric ratios of atoms joined by covalent bonds.
Matter
  • Everything you can see and feel; it exists as solid, liquid or gas.
Subscript
  • The small number after a symbol that gives how many atoms.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
Empirical formula
  • The simplest whole-number ratio of atoms or ions.
Ionic compound
  • A compound made of positive and negative ions held together by electrostatic attraction.
Covalent bond
  • A shared pair of electrons between two atoms, giving noble gas configurations.

Section 8 of 9Key terms

TermMeaning
Pure substanceMatter of fixed composition with a single type of particle.
MixtureMatter containing two or more types of particle in variable proportions.
Homogeneous mixtureMixture with uniform composition.
Heterogeneous mixtureMixture whose composition varies between regions.
Elemental analysisTechnique that finds the relative amounts of elements in a sample.
PurityPercentage by mass of the desired compound in a sample.

Section 9 of 9Exam technique

1
Describing a mixture as having fixed composition — only pure substances do
2
Describing a mixture as a compound — the components are not chemically joined
3
Using the mass of the whole sample as the mass of one component
4
Applying the single-component method when both components contain the analysed element

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