Pure Substances, Mixtures and Separation
Every time the tide goes out on Dublin Bay, it leaves behind sand, seaweed and salt crystals drying on the rocks. Sea water, sand and salt look very different, yet chemists treat each one differently. How would you get pure salt out of a bucket of sea water, and how could you prove that you had really made something new when you burn a piece of magnesium? This page gives you the tools to answer both questions.
The kinetic theory explains much of what follows: particles in a solid vibrate in fixed positions, in a liquid they slide past each other, and in a gas they move freely. Evaporation and distillation are changes of state in which particles gain energy and escape from the liquid. Energy, like mass, is conserved: it is transferred, never created or destroyed.
Elements, compounds and mixtures
All matter is either a pure substance or a mixture.
-
An element contains only one type of atom and cannot be broken down into simpler substances by chemical means (copper, Cu; oxygen, O₂).
-
A compound contains two or more elements chemically bonded in a fixed ratio (sodium chloride, NaCl; water, H₂O). It can be broken down only by a chemical reaction.
-
A mixture contains two or more substances that are not chemically bonded. The composition can vary and each component keeps its own properties.
Mixtures are divided into two kinds:
-
Homogeneous: the composition is uniform throughout, and you cannot see the separate parts (salt dissolved in water, brass, air). Another name is a solution.
-
Heterogeneous: the composition is not uniform and the parts can be seen or separated by eye or a simple filter (sand in water, muesli, granite).
Classify each of the following as an element, a compound, a homogeneous mixture or a heterogeneous mixture: a) sodium chloride b) sea water from Dublin Bay c) copper d)…
Classify each of the following as an element, a compound, a homogeneous mixture or a heterogeneous mixture:
- a) sodium chloride
- b) sea water from Dublin Bay
- c) copper
- d) muesli
Separation techniques
The components of a mixture are not chemically bonded, so they can be separated by physical methods. The rule is simple: choose a technique that uses a property in which the components differ.
| Technique | Property used | Example |
|---|---|---|
| Filtration | Particle size (insoluble solid vs liquid) | Sand from water |
| Evaporation / crystallisation | Dissolved solid is non-volatile, solvent evaporates | Salt from sea water |
| Simple distillation | Different boiling points (large difference) | Pure water from salt solution |
| Fractional distillation | Boiling points close together | Ethanol from water; crude oil fractions |
| Magnetic separation | Only one component is magnetic | Iron filings from sand |
| Decanting / separating funnel | Immiscible liquids of different density | Oil and water |
| Chromatography | Different solubility in the mobile phase and attraction to the stationary phase | Inks and dyes |
Filtration and evaporation
Sand does not dissolve in water, and its particles are too big to pass through the pores of filter paper. The sand stays behind as the residue, and the water passes through as the filtrate. If salt is dissolved in the filtrate, heating drives off the water and leaves the salt.
Distillation
In simple distillation the solution is boiled, the vapour of the lower-boiling component is condensed in a Liebig condenser and the distillate is collected. It works well when the boiling points are far apart (water 100 °C, salt does not boil).
In fractional distillation a fractionating column is placed above the flask. Vapour rises up the column, condenses and re-evaporates many times. Each cycle enriches the vapour in the component with the lower boiling point, so liquids with close boiling points can be separated. Crude oil is separated into fractions (petrol, kerosene, diesel) by boiling point in this way. Ethanol boils at 78 °C and water at 100 °C: too close for simple distillation to give high-purity ethanol, because both evaporate to some extent.
Chromatography
In paper chromatography a spot of the mixture is placed on a pencil baseline on the paper, and the bottom edge is dipped in a solvent. The paper is the stationary phase and the solvent is the mobile phase. As the solvent rises, each component is carried upward to a different extent. Components that are more soluble in the solvent and held less strongly by the paper travel further.
The retention factor is
$$R_f = \frac{\text{distance moved by the spot}}{\text{distance moved by the solvent front}}$$$R_f$ has no units and is always between 0 and 1. Each pure substance has a characteristic $R_f$ in a given solvent. Several spots mean the sample is a mixture; one spot suggests (but does not prove) a pure substance.
A student has a mixture of sand and water. Name the separation technique that would be used to separate the sand from the water, and state the property of the sand that…
A student has a mixture of sand and water. Name the separation technique that would be used to separate the sand from the water, and state the property of the sand that makes this technique work.
A sample of crude oil from an Irish refinery is separated by fractional distillation into petrol, kerosene and diesel. a) Explain why crude oil is a mixture and not a…
A sample of crude oil from an Irish refinery is separated by fractional distillation into petrol, kerosene and diesel.
- a) Explain why crude oil is a mixture and not a compound.
- b) Explain, in terms of boiling point, how fractional distillation separates the components.
- c) Is this separation a physical or a chemical process? Justify your answer.
A mixture contains salt (sodium chloride), iron filings and sand. Describe a sequence of separation steps, with the technique used at each step, that would give all…
A mixture contains salt (sodium chloride), iron filings and sand. Describe a sequence of separation steps, with the technique used at each step, that would give all three components in separate and dry form. Justify the technique chosen at each step by referring to a property of the component being separated.
A water sample from a river contains dissolved salt, suspended clay particles and a small amount of dissolved ethanol from an industrial discharge. a) Design a sequence…
A water sample from a river contains dissolved salt, suspended clay particles and a small amount of dissolved ethanol from an industrial discharge.
- a) Design a sequence of separation techniques to obtain pure water, clay and ethanol. Justify each step.
- b) Explain why simple distillation would be unsuitable for separating the ethanol from the water if a high-purity ethanol sample were required.
- c) Suggest how a student could show that the final water sample is pure.
Paper chromatography is used to analyse a green food dye. Three spots appear on the chromatogram, with $R_f$ values of $0.25$, $0.55$ and $0.80$. The solvent front has…
Paper chromatography is used to analyse a green food dye. Three spots appear on the chromatogram, with $R_f$ values of $0.25$, $0.55$ and $0.80$. The solvent front has moved $12.0\ \text{cm}$ from the baseline.
- a) Calculate the distance moved by each spot.
- b) Explain why the dye is a mixture.
- c) Explain, in terms of the stationary and mobile phases, why different components travel different distances.
- d) Explain why the baseline is drawn in pencil and not in ink.
Physical and chemical change
Signs of a chemical change: a new substance with different properties, a colour change, a gas given off, a precipitate, a temperature change (heat or light released), and difficulty reversing the change. No single sign is proof on its own: boiling gives bubbles and ice melting absorbs heat, and both are physical.
| Change | Type | Reason |
|---|---|---|
| Ice melting | Physical | Still H₂O |
| Sugar dissolving in tea | Physical | Sugar can be recovered by evaporation |
| Iron rusting | Chemical | Iron(III) oxide forms |
| Magnesium burning | Chemical | Bright light, white solid MgO forms |
State whether each of the following is a physical change or a chemical change: a) ice melting on a lake in Connemara b) iron rusting on a gate c) magnesium ribbon…
State whether each of the following is a physical change or a chemical change:
- a) ice melting on a lake in Connemara
- b) iron rusting on a gate
- c) magnesium ribbon burning in air
- d) sugar dissolving in tea
Iron filings $(7.0\ \text{g})$ are heated with sulfur $(4.0\ \text{g})$. The product is iron(II) sulfide, $\mathrm{FeS}$, and the iron and sulfur react in the mass ratio…
Iron filings $(7.0\ \text{g})$ are heated with sulfur $(4.0\ \text{g})$. The product is iron(II) sulfide, $\mathrm{FeS}$, and the iron and sulfur react in the mass ratio $7:4$.
- a) What mass of iron(II) sulfide forms?
- b) A magnet is passed over the mixture before heating and again after heating. Describe and explain what is observed each time.
- c) State two pieces of evidence that a chemical change has taken place.
Conservation of mass
The law of conservation of mass states that in a chemical reaction, mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products, because atoms are only rearranged and none are lost or gained.
When $2.43\ \text{g}$ of magnesium is burned completely in air, $4.03\ \text{g}$ of magnesium oxide forms. a) Calculate the mass of oxygen that combined with the…
When $2.43\ \text{g}$ of magnesium is burned completely in air, $4.03\ \text{g}$ of magnesium oxide forms.
- a) Calculate the mass of oxygen that combined with the magnesium.
- b) Explain how this result illustrates the law of conservation of mass.
- c) A student finds that the product weighs $3.90\ \text{g}$ when the experiment is performed in an open crucible. Suggest two reasons for the discrepancy.
- d) Justify why the burning of magnesium is classified as a chemical change.
Back to the start: salt is recovered from sea water by evaporation (a physical separation, using the fact that salt is non-volatile), and burning magnesium is a chemical change because a new substance, MgO, forms and the masses balance once the oxygen is counted.
Summary
-
Matter is a pure substance (element or compound) or a mixture (homogeneous or heterogeneous). A compound has fixed composition and chemically bonded elements; a mixture does not.
-
Mixtures are separated by physical methods that use a difference in a property: filtration (particle size), evaporation, distillation (boiling point), magnet, decanting (density, immiscibility) and chromatography.
-
In chromatography $R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$; several spots show a mixture, and the baseline is in pencil.
-
A physical change makes no new substance; a chemical change does (new properties, gas, colour change, energy released).
-
Mass is conserved in chemical reactions; apparent changes come from gases entering or leaving the system.