(b) Elements, compounds and mixtures

Learning objectives

1.8Elements, compounds and mixturesUnderstand how to classify a substance as an element, compound or mixture 1.9Pure substances and mixturesUnderstand that a pure substance has a fixed melting and boiling point, but that a mixture may melt or boil over a range of temperatures 1.10Separation techniquesDescribe these experimental techniques for the separation of mixtures:• simple distillation• fractional distillation• filtration• crystallisation• paper chromatography.1.11ChromatogramsUnderstand how a chromatogram provides information about the composition of a mixture 1.12Rf valuesUnderstand how to use the calculation of Rf values to identify the components of a mixture 1.13Paper chromatography practicalPractical: investigate paper chromatography using inks/food colourings

Classify a substance by asking how many types of atom or substance are present and whether different elements are chemically bonded together.

Class	What it contains	Chemical bonding and composition	Examples
Element	One type of atom only	May contain single atoms or bonded atoms of the same element	Diamond, copper, $\ce{Cl2}$
Compound	Atoms of two or more different elements	Chemically bonded in a fixed ratio; represented by a chemical formula	$\ce{CO2}$ , $\ce{NaCl}$ , $\ce{H2O}$
Mixture	Two or more elements and/or compounds	Not chemically bonded to one another; proportions can vary and components can be separated physically	Air, petrol, salt water

In a particle diagram, identical particles made from one atom type represent an element. Identical particles containing different atom types bonded together represent a compound. More than one kind of particle in the same box represents a mixture.

Two atoms in one particle do not automatically make a compound: $\ce{Cl2}$ is an element because both atoms are chlorine. A mixture may contain compounds, elements or both; the key is that its different substances are not chemically bonded to each other.

A pure substance has a fixed, sharp melting point and a fixed boiling point at a stated pressure. A mixture usually melts or boils over a range of temperatures because its different components change state under different conditions.

Observation while heating	Conclusion
Temperature remains at one characteristic value while the sample melts or boils	Evidence that the substance is pure
The sample melts or boils across a temperature interval	Evidence that the sample is a mixture or contains an impurity

Measure the melting point of a solid or boiling point of a liquid and compare both the value and whether the transition is sharp. Adding an impurity commonly lowers and broadens a solid's melting point, so the range is as important as a single recorded temperature.

A substance is not proven pure merely because it looks uniform. Use a measured physical constant. Keep pressure comparable when using boiling point, because boiling temperature also depends on pressure.

Mixtures are separated by differences in physical properties such as solubility, boiling point or attraction to a solvent and paper. Choose the method from the component you need to collect and the property that differs.

Technique	Use it to separate	Essential method
Filtration	An insoluble solid from a liquid	Pour through filter paper in a funnel; the solid is the residue and the liquid passing through is the filtrate.
Simple distillation	A solvent from a solution	Heat so the solvent boils; cool its vapour in a condenser and collect the liquid distillate. Non-volatile solute remains in the flask.
Fractional distillation	A mixture of miscible liquids with different boiling points	Heat the mixture through a fractionating column; repeated vaporisation and condensation enrich the lower-boiling liquid before its vapour is condensed and collected.
Crystallisation	A soluble solid from a solution	Evaporate some solvent, cool the concentrated solution to form crystals, filter the crystals and dry them.
Paper chromatography	Soluble components such as dyes in ink	Let a solvent rise through spotted paper; components separate because they move different distances.

For sea water, simple distillation collects pure water while salt remains. Filtration cannot remove dissolved salt. To obtain dry salt instead, crystallise it; heating every solution to dryness can decompose the solid or produce poor crystals.

Simple distillation and fractional distillation are not interchangeable names: use a fractionating column for liquids whose boiling points must be separated. A condenser cools vapour so it condenses; it does not filter the mixture.

A chromatogram separates the soluble components of a sample into spots. Each separated spot is evidence for one component that moved in that solvent.

Pattern	What it supports
One moved spot	The sample may contain one soluble component.
Several spots from one start position	The sample is a mixture containing at least that many soluble components.
Spots from different samples at the same height	They may contain the same component, if the chromatography conditions are the same.
A sample spot with no match among references	It may contain an unidentified component.
Material remains on the start line	It is insoluble in that solvent; it may still contain more than one component.

A spot that travels farther is usually more soluble in the mobile solvent, relative to its attraction to the stationary paper. The dye travelling nearest the solvent front therefore has the greatest movement under those conditions.

Same colour alone does not identify a component, and the same height is meaningful only when solvent, paper and run conditions match. A stationary spot does not prove purity because insoluble components have not separated.

$R_f$ compares how far a component travels with how far the solvent front travels in the same chromatogram. Measure both distances from the start line and use the centre of the spot.

$R_f=\frac{\text{distance moved by component}}{\text{distance moved by solvent front}}$

Worked example: the spot moves 9.7 cm from the start line and the solvent front moves 12.0 cm. $R_f = 9.7/12.0 = 0.808\ldots = 0.81$ to two significant figures. $R_f$ has no unit because it is a ratio of two distances in the same unit.

Compare an unknown's $R_f$ with reference values obtained under the same conditions. A matching value supports an identification; different solvents, papers or temperatures can change the value.

A valid spot cannot travel beyond the solvent front, so $0 \le R_f \le 1$ . Do not use the top of the paper as the denominator unless that is exactly where the marked solvent front reached.

Paper chromatography separates dyes in inks or food colourings so their number, movement and possible identities can be compared.

Order	Method and reason
1	Draw a horizontal start line in pencil near the bottom of the chromatography paper; pencil is insoluble and will not add moving ink spots.
2	Place small, concentrated spots of the samples and any references on the line, allowing each spot to dry.
3	Stand the paper in a shallow solvent with the start line and spots above the solvent level, so samples do not dissolve directly into the reservoir.
4	Let the solvent rise until it is near the top, then remove the paper before the front reaches the edge.
5	Mark the solvent front immediately in pencil and let the chromatogram dry before comparing spots or calculating $R_f$ .

For a fair comparison, use the same solvent, type of paper and start-line arrangement. If a sample does not move, repeat with a different suitable solvent because the component may be insoluble in the first solvent.

Never draw the baseline in ink or place it below the solvent surface: ink can dissolve and contaminate the pattern, while submerged sample spots wash into the solvent instead of travelling with the front.

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—(c) Atomic structureRead

(b) Elements, compounds and mixtures

Learning objectives

Classify elements, compounds and mixtures

Use melting and boiling points to test purity

Choose and describe a separation technique

Read a chromatogram as composition evidence

Calculate and use Rf values

Carry out paper chromatography reliably

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