Elements, Compounds and Mixtures
Classification of Substances
All substances can be classified as an element, compound or mixture.
| Type | Definition | Example |
|---|---|---|
| Element | A pure substance made from only one type of atom; cannot be split into anything simpler | Copper (Cu), oxygen (O2), iron (Fe) |
| Compound | A pure substance made from two or more elements chemically combined in fixed proportions | Water (H2O), sodium chloride (NaCl), copper(II) sulfate (CuSO4) |
| Mixture | Two or more substances (elements and/or compounds) not chemically combined | Salt water, air, crude oil |
Compounds cannot be separated into their elements by physical means; chemical reactions are required. Mixtures, on the other hand, can be separated by physical methods.
Pure Substances vs Mixtures
In chemistry, a pure substance is a single element or compound with no other substances mixed in. This is different from everyday language, where “pure” often just means natural or unadulterated.
Pure substances melt and boil at a single, sharp temperature. For example, pure water boils at exactly 100 °C and melts at exactly 0 °C.
Mixtures melt and boil over a range of temperatures because they contain different substances, each with different melting and boiling points. Generally, impure substances also have:
- Lower melting points than the pure substance
- Higher boiling points than the pure substance
This means melting point and boiling point data can be used to assess purity. If a sample melts over a range of temperatures (or at a different temperature from the known value), it is impure.
Separation Techniques
Because the components of a mixture are not chemically combined, they can be separated by physical methods. The correct method depends on the properties of the components.
Simple Distillation
Used to separate a pure liquid from a solution (e.g. water from salt water) or to separate liquids with very different boiling points.
Process: The solution is heated. The liquid with the lower boiling point evaporates first, rises through the condenser, cools, condenses and is collected as a pure liquid. The sequence is: heating, evaporating, cooling, condensing.
Fractional Distillation
Used to separate miscible liquids (liquids that mix) with different but close boiling points (e.g. ethanol from water, or separating fractions of crude oil).
Process: The mixture is heated in a fractionating column. The component with the lowest boiling point evaporates first and condenses at the top. Other components condense at different heights as the temperature rises. This allows the mixture to be separated into fractions continuously.
Example: Ethanol (boiling point 78 °C) and water (boiling point 100 °C) can be separated by fractional distillation. The mixture is heated to 78 °C; the ethanol boils and condenses first.
Exam Tip
When drawing a distillation diagram, the water must enter the condenser at the end nearest the collecting beaker and exit at the end nearest the flask. This ensures the water flows in the opposite direction to the vapour, cooling it efficiently.
Filtration
Used to separate an insoluble solid from a liquid (e.g. sand from sand and water).
Process: The mixture is poured through filter paper in a funnel. The liquid (filtrate) passes through the tiny holes in the filter paper. The insoluble solid (residue) is too large to pass through and remains on the filter paper.
Crystallisation
Used to separate a dissolved (soluble) solid from a solution (e.g. copper sulfate crystals from copper sulfate solution).
Process:
- Warm the solution in an open evaporating basin to evaporate the solvent and concentrate the solution.
- Test with a cold glass rod; if crystals form on the rod, the solution is saturated.
- Allow the saturated solution to cool slowly.
- Crystals grow as the solid comes out of solution.
- Filter off the crystals, wash with cold distilled water, and allow to dry.
Paper Chromatography
Used to separate substances based on their different solubilities in a solvent (e.g. separating the dyes in food colourings or inks).
How it works:
- A pencil line (baseline) is drawn on chromatography paper. Pencil is used because ink would run and contaminate the chromatogram.
- Spots of sample are placed on the baseline.
- The paper is lowered into solvent so the baseline is above the solvent level (the spots must not touch the solvent directly).
- The solvent travels up the paper by capillary action, carrying the substances with it.
- Different substances travel at different rates depending on their solubility. More soluble substances travel further.
Interpreting Chromatograms
A chromatogram is the visual result of a paper chromatography run.
- A pure substance produces only one spot on the chromatogram.
- An impure substance or mixture produces more than one spot.
- If two substances produce spots at the same height (same Rf value), they are likely to be the same substance.
- A reference (known) compound is often run alongside an unknown sample for comparison.
Rf Values
Rf values (retention factors) are used to identify substances in a chromatogram. For a given substance and solvent, the Rf value is constant.
- Rf values have no units.
- Rf values are always between 0 and 1 (the substance can never travel further than the solvent front).
- Both distances are measured from the baseline to the centre of the spot (or to the solvent front).
Example: If a spot moved 3 cm and the solvent front moved 6 cm, then Rf = 3/6 = 0.5.
Exam Tip
Measure distances carefully with a ruler from the baseline. The Rf value must always be less than 1; if you get a value greater than 1, recheck your measurements as you may have switched numerator and denominator.
Exam Questions
Q: State two differences between a compound and a mixture.
A compound has its components chemically combined in fixed proportions, whereas in a mixture they are not chemically combined. A compound can only be separated by chemical reactions; a mixture can be separated by physical methods.
Q: A student tests the melting point of a sample of sulfur. It begins to melt at 100 °C and finishes melting at 113 °C. Explain whether the sample is pure or impure.
The sample is impure. Pure sulfur melts at a single sharp temperature (114 °C). This sample melts over a range of temperatures, which indicates it contains impurities.
Q: Name the separation technique used to separate an insoluble solid from a liquid.
Filtration.
Q: Calculate the Rf value of a substance that moved 4.5 cm when the solvent front moved 9.0 cm.
Rf = 4.5 / 9.0 = 0.5
Q: Explain why pencil is used to draw the baseline in paper chromatography rather than a pen.
Pen ink would dissolve in the solvent and travel up the paper along with the samples, contaminating the chromatogram.
Exam Tip
The question often distinguishes between separating the liquid from a mixture (use distillation) and separating the solid from a solution (use crystallisation). Read carefully which component is being separated.