Coaching Centre
Classifying and separating matter
Pure substances and mixtures, elements and compounds, homogeneous and heterogeneous mixtures, physical properties used to identify substances, choosing a separation technique, and gravimetric analysis to find the percentage composition of a mixture.
The beaches of eastern and western Australia hold some of the world's richest deposits of heavy mineral sands: ilmenite and rutile (sources of titanium) and zircon (used in ceramics). They make up only a few per cent of the sand, and they are all grey-black grains about the same size. Miners separate them without any chemical reaction. Spiral chutes of water sort grains by density, magnets pull out the weakly magnetic ilmenite, and charged rollers separate grains that conduct electricity (rutile) from those that don't (zircon). Each step exploits a difference in one physical property. That idea, choosing a property that differs, is the whole of this lesson.
Pure substances and mixtures
A pure substance has a fixed composition and fixed properties: every sample of pure water is H₂O, melts at 0 °C and boils at 100 °C at standard pressure. Pure substances are either elements, made of one kind of atom (Cu, O₂, S₈), or compounds, made of two or more elements chemically bonded in a fixed ratio (H₂O, NaCl, CO₂).
A mixture contains two or more substances that are not chemically bonded to each other. Its composition can vary: sea water can be more or less salty. Each component keeps its own properties, which is why mixtures can be separated by physical methods.
- A homogeneous mixture has the same composition throughout, and you can't see separate parts even under a microscope: salt water, air, brass, vinegar. Solutions are homogeneous mixtures.
- A heterogeneous mixture has visibly different parts or phases: sand in water, granite, oil and water, blood (cells in plasma).
A particle model helps. Picture each kind of atom as a coloured ball. An element shows one colour only, either as single atoms or identical molecules. A compound shows identical molecules or a regular lattice, each containing more than one colour. A mixture shows two or more kinds of particle jumbled together in no fixed ratio.
Physical properties
A physical property can be observed or measured without changing the substance into a different substance. A chemical property describes how a substance reacts (whether it burns, corrodes or reacts with acid). Physical properties are used both to identify substances and to separate them.
| Property | What it tells you |
|---|---|
| Melting point and boiling point | Strength of forces between particles; a sharp melting point indicates purity. |
| Density, $\rho = \dfrac{m}{V}$ | Mass per unit volume (g cm⁻³ or g mL⁻¹ for solids and liquids). |
| Solubility | Mass that dissolves in a given amount of solvent at a given temperature. |
| Electrical and thermal conductivity | Whether charged particles are free to move. |
| Magnetism, colour, lustre, hardness, malleability | Identification and some separations. |
| Volatility | How readily a liquid evaporates (volatile liquids have low boiling points). |
A pure substance melts at one sharp temperature. Impurities disrupt the regular packing of particles, so an impure sample starts melting at a lower temperature and melts over a range. Chemists use this as a quick purity test, for example on a newly made pharmaceutical. Dissolved impurities also raise the boiling point of a liquid: salty water boils a little above 100 °C.
There are exceptions to "mixtures melt over a range". Some alloys have a special composition, called the eutectic, that melts sharply. Tin–lead solder with 63% tin melts at a single temperature, 183 °C, while the more common 60/40 solder melts over a range from 183 °C to about 190 °C. (Some textbooks simply quote 190 °C for 60/40 solder; that is the temperature at which it is fully liquid.)
A small metal cylinder has a mass of 44.6 g. When it is lowered into a measuring cylinder, the water level rises from 20.0 mL to 25.0 mL. Identify the metal. Densities (g cm⁻³): Al 2.70, Fe 7.87, Cu 8.96, Pb 11.3.
Volume of the cylinder = 25.0 − 20.0 = 5.0 mL = 5.0 cm³ (displacement method).
$\rho = \dfrac{m}{V} = \dfrac{44.6}{5.0} = 8.9 \text{ g cm}^{-3}$
Closest value: copper. Note that 5.0 cm³ limits the answer to 2 significant figures.
Choosing a separation technique
To separate a mixture, find a physical property in which the components differ, then pick the technique that uses that difference. Often you need several steps in sequence.
- Filtration separates an insoluble solid (the residue) from a liquid (the filtrate).
- Evaporation recovers a dissolved solid by boiling off the solvent; crystallisation lets the solution cool slowly so larger crystals form.
- Simple distillation boils a solution and condenses the vapour, recovering the solvent (the distillate).
- Fractional distillation separates miscible liquids with boiling points close together. A fractionating column gives many cycles of evaporation and condensation, so the most volatile liquid reaches the top first. It is used for crude oil and for ethanol–water mixtures.
- A separating funnel separates immiscible liquids (liquids that don't mix) by density: the denser layer is run off from the bottom.
- Decanting pours off a liquid from a settled solid; centrifugation spins a mixture so fine solids settle quickly.
- Paper chromatography separates small amounts of dissolved substances by how strongly each is attracted to the paper compared with the solvent.
- Magnetic separation and sieving separate solids by magnetism and particle size.
Gravimetric analysis: percentage composition
Gravimetric analysis means finding the amount of a component by weighing it. For a mixture, separate each component, dry it, weigh it, and express its mass as a percentage of the original sample:
$$\%\text{ by mass of component} = \frac{\text{mass of component}}{\text{mass of sample}} \times 100$$
A student separates 25.00 g of a mixture of iron filings, sand and salt and recovers 4.85 g of iron, 14.32 g of dry sand and 5.61 g of dry salt. Calculate the percentage composition and the percentage of the sample recovered.
Iron: $\dfrac{4.85}{25.00}\times100 = 19.4\%$. Sand: $\dfrac{14.32}{25.00}\times100 = 57.3\%$. Salt: $\dfrac{5.61}{25.00}\times100 = 22.4\%$.
Total recovered = 4.85 + 14.32 + 5.61 = 24.78 g, which is $\dfrac{24.78}{25.00}\times100 = 99.1\%$ of the sample. About 0.22 g was lost, for example as grains left on the filter paper or crystals that spat out of the dish.
The mixture above was made up with exactly 25.0% salt. Calculate the percentage error in the salt result.
$\%\text{ error} = \dfrac{\text{measured} - \text{true}}{\text{true}}\times100 = \dfrac{22.4 - 25.0}{25.0}\times100 = -10.4\%$
The negative sign shows the result is too low, which points to a systematic loss of salt rather than random scatter.
- Run all three steps on Sample A with good technique. You should get 2.75 g iron, 6.60 g sand and 3.15 g salt: 22.0%, 52.8% and 25.2%.
- Restart and switch off Rinse the residue. The sand now reads 6.73 g (53.8%) and the salt 3.02 g (24.2%). Why do the two errors cancel in the total?
- Turn rinsing back on and switch off Dry to constant mass. What happens to the total? (You should get 12.99 g, a recovery of 103.9%.)
- Heat strongly instead of gently. Which result is affected, and in which direction?
- Analyse the Beach sand. Why is a 20 g sample better than a 2 g sample for measuring a component that is only about 3% of the mixture?
Test yourself
Which of the following is a homogeneous mixture?
Bronze is an alloy of copper and tin, uniform throughout but variable in composition. Water and NaCl are compounds; granite has visible grains.
A white solid melts between 118 °C and 124 °C. The pure compound melts at 128 °C. The best conclusion is that the sample
A lower melting point and a melting range are signs of impurity.
Which technique would best separate a mixture of hexane (bp 69 °C) and heptane (bp 98 °C), which are miscible?
Miscible liquids with different boiling points are separated by fractional distillation.
A 12.0 g sample of a sand–salt mixture yields 8.4 g of dry sand. What is the percentage of salt?
Salt = 12.0 − 8.4 = 3.6 g; 3.6 ÷ 12.0 × 100 = 30%.
A student forgets to dry the sand to constant mass. The calculated percentage of sand will be
Retained water adds mass, a systematic error that always makes the sand reading high.
Complete the classification of matter.
Classify each substance.
Put the gravimetric method in order.
Solid left on the filter paper is called the , and the liquid that passes through is the . Liquids that do not mix are and can be separated with a separating funnel. Drying until two weighings agree is called drying to . A mixture can be separated by methods.
A sample of 15.00 g of a mixture contains sand, salt and iron filings. Outline a method to separate the three components, justifying each step in terms of a physical property, and calculate the percentage of salt if 3.42 g of dry salt is recovered.
1. Pass a magnet over the dry mixture: iron is magnetic, sand and salt are not. Weigh the iron.
2. Add warm water and stir: salt is soluble in water, sand is not. Filter; the sand stays on the paper as residue. Rinse the residue so no salt solution is left in it, then dry the sand to constant mass and weigh it.
3. Evaporate the filtrate gently: water is volatile and salt is not, so salt crystals remain. Dry to constant mass and weigh.
% salt = 3.42 ÷ 15.00 × 100 = 22.8%.