MYP Chemistry · Topic 5
Pure and impure substances
Almost nothing around you is pure. Know the kinds of mixture, and choose the separation that exploits the property the components do not share.
Topic: Pure and impure substances (types of mixtures [solutions, oils, alloys, emulsions]; separation techniques, including: filtration, distillation [including crude oil], chromatography). The chapter distinguishes pure substances from mixtures, describes the main kinds of mixture, and explains each separation technique by the property it relies on.
Prior knowledge: particles and changes of state (Topic 4); alkanes (Topic 2).
Learning objectives
Objectives and contextOverview
By the end of this chapter you should be able to:
- distinguish pure substances from mixtures, including by melting and boiling points A
- describe solutions, immiscible oils, emulsions and alloys, and explain why alloys are harder than pure metals A
- select and outline a suitable separation technique for a given mixture, and justify the choice from the properties involved A B
- explain the fractional distillation of crude oil A
- interpret chromatograms and calculate Rf values A C
- calculate percentage composition by mass from experimental data C
“Pure orange juice” on a carton means nothing has been added. To a chemist it is a mixture of water, sugars, acids, vitamins, colours and flavour compounds. In chemistry a pure substance contains only one element or one compound. Most materials we use are mixtures — air, sea water, crude oil, steel, milk — and a great deal of practical chemistry is about separating them into useful parts. The key to every separation is the same question: which property do the components not share?
02 / Pure or not
Pure substances and mixturesTypes of mixtures
An element contains only one kind of atom. A compound contains two or more elements chemically bonded in fixed proportions, and has properties different from those elements. A mixture contains two or more substances that are not chemically bonded; its composition can vary, and each component keeps its own properties, so it can be separated by physical methods.
Purity can be tested by melting and boiling points. A pure substance melts and boils at a sharp, fixed temperature: pure water boils at 100 °C at normal atmospheric pressure. Impurities lower and widen the melting range and raise the boiling point. That is why salt is spread on icy roads — the salt water freezes below 0 °C — and why comparing a measured melting point with the data-book value is a standard purity check for medicines.
Solution: a mixture in which a solute is dissolved in a solvent, giving a uniform mixture whose particles are too small to see or to be filtered out.
Soluble / insoluble: able / unable to dissolve in a particular solvent.
Saturated solution: one that contains as much dissolved solute as possible at that temperature.
Miscible / immiscible: liquids that do / do not mix to form a single layer.
03 / Kinds of mixture
Solutions, oils, emulsions and alloysTypes of mixtures
Solutions
In a solution the solute particles — atoms, molecules or ions — are spread uniformly among the solvent particles. The solute can be a solid (salt in water), a liquid (ethanol in water) or a gas (carbon dioxide in a fizzy drink; oxygen in water, which fish depend on). Water dissolves many ionic substances because the ions separate and spread through it; that is why salt solution conducts electricity while pure water barely does (Topic 6). The solubility of most solids increases with temperature; the solubility of gases decreases with temperature and increases with pressure.
Oils and immiscible liquids
Oil and water do not mix: they are immiscible, and the less dense oil floats as a separate layer. This is why an oil spill spreads as a slick on the sea surface and can be collected by skimming. Two immiscible liquids can be separated in a separating funnel by running off the lower layer.
Emulsions
If oil and water are shaken hard, tiny droplets of one spread through the other, but they soon separate again. An emulsifier stabilises the droplets so they stay dispersed, forming an emulsion. Milk is droplets of fat in water; mayonnaise is oil droplets in vinegar stabilised by egg yolk; many creams and paints are emulsions. Emulsions look cloudy or opaque because the droplets are large enough to scatter light, unlike the particles in a true solution.
Alloys
An alloy is a mixture of a metal with one or more other elements, usually other metals or carbon. Steel is iron with a small amount of carbon (and often manganese, chromium or nickel); brass is copper and zinc; jewellery gold is gold alloyed with silver and copper, because pure gold is too soft to wear well.
Alloys are usually harder and stronger than the pure metal. In a pure metal the atoms are all the same size and are arranged in regular layers, which can slide over each other when a force is applied — that is why pure metals are malleable. In an alloy, atoms of a different size distort the layers, so they cannot slide as easily. The proportion matters: in carbon steels, more carbon gives a harder, stronger steel, but too much makes it brittle.
Question: explain why rings are made from 18-carat gold (gold alloyed with silver and copper) rather than pure gold.
Reasoning: pure gold is very soft because its identical atoms form regular layers that slide easily, so a ring would scratch and bend out of shape. The silver and copper atoms have different sizes and distort the layers, so the alloy is harder and keeps its shape, while keeping gold's colour and resistance to corrosion.
Structure of the answer: property needed → property of the pure metal → particle-level reason the alloy is better.
04 / Filtration
Filtration and crystallisationFiltration
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel: the liquid and anything dissolved in it (the filtrate) pass through the tiny holes, while the solid particles are too large and stay behind as the residue. Filtration cannot separate a dissolved solute from its solvent, because dissolved particles are as small as the solvent particles.
To recover a dissolved solid, the solution is heated to evaporate some of the solvent until it is nearly saturated, then left to cool. Crystals form because the solid is less soluble in the cooler, smaller volume of solvent (crystallisation). They are filtered off and dried. Evaporating to dryness also recovers the solid, but can decompose it or leave soluble impurities mixed in.
Precipitation and filtration: removing ions from water
Filtration can remove dissolved ions if they are first turned into an insoluble solid. “Hard” water contains Ca2+ and Mg2+ ions, which stop soap from lathering. Solubility data show which negative ion will precipitate which positive ion:
| Negative ion | with Mg2+ | with Ca2+ |
|---|---|---|
| Cl−, Br−, I− | soluble | soluble |
| SO42− | soluble | insoluble |
| OH− | insoluble | insoluble |
| CO32−, PO43− | insoluble | insoluble |
To remove both ions, add carbonate or phosphate ions: both form precipitates, which are filtered off. The table can also be used to remove one ion selectively — look for the row where the two ions behave differently. Using the formulas in the table also lets you write the formula of a precipitate: magnesium phosphate is Mg3(PO4)2, because three 2+ ions balance two 3− ions (Topic 6).
Because oil and water are immiscible, spilled oil can be skimmed from the surface. Burning it off removes it quickly but produces smoke and fumes; chemical dispersants break it into droplets that stay in the water and can harm aquatic life. Evaluating clean-up methods means linking each to the science and weighing its advantages and disadvantages.
05 / Distillation
Simple and fractional distillationDistillation
Simple distillation separates a solvent from a solution — for example, pure water from salt water. The solution is heated; the water boils, and its vapour passes into a condenser, where cold water around the tube cools it back to a liquid (the distillate). The salt, which has a far higher boiling point, stays in the flask. The thermometer bulb sits at the side arm to measure the temperature of the vapour that is actually distilling.
Fractional distillation separates liquids with different but closer boiling points, such as ethanol (78 °C) and water (100 °C), or the many compounds in crude oil and in liquid air (Topic 3). A fractionating column is placed between the flask and the condenser. The column is hot at the bottom and cooler at the top. Vapour rises, condenses and re-boils many times on the way up; each time, the lower-boiling component is concentrated in the vapour, so the substance with the lowest boiling point reaches the top first.
Crude oil
Crude oil is a mixture of hundreds of hydrocarbons, mostly alkanes, with chains from one to more than forty carbon atoms. It is far too complex to separate into single compounds, so it is separated into fractions — groups of hydrocarbons with similar chain lengths and boiling points. The crude oil is heated until most of it vaporises and is fed into the bottom of a tall column. As the vapours rise the temperature falls; each fraction condenses at the level where the temperature has dropped below its boiling point and is drawn off. The smallest molecules, with the lowest boiling points, leave at the top as gases; the largest do not vaporise at all and leave at the bottom as bitumen.
Boiling point increases with chain length because larger molecules have stronger forces of attraction between them, so more energy is needed to separate them. Longer chains are also more viscous (thicker), less volatile and harder to ignite — which is why petrol is a car fuel while lubricating oil is not.
Which fraction?
Choose the number of carbon atoms in a straight-chain alkane. The model looks up its boiling point (data for C1–C20; longer chains are estimated) and finds the level of the column where it condenses, using the tray temperatures in Figure 5.3.
06 / Chromatography
ChromatographyChromatography
Paper chromatography separates and identifies small amounts of dissolved substances such as inks, food colourings or plant pigments. A pencil line is drawn near the bottom of the paper (pencil, because ink would itself separate), and a small spot of each sample is placed on it. The paper stands in a solvent whose level is below the line, so the spots do not dissolve into the solvent reservoir. The solvent rises up the paper and carries the substances with it. Each substance moves at its own rate, depending on how strongly it is attracted to the paper compared with how soluble it is in the solvent, so the components separate into spots.
A pure substance gives a single spot. A mixture gives several. A substance is identified by comparing its spots with those of known reference substances run on the same paper, or by its Rf value:
Rf = distance moved by the substance ÷ distance moved by the solvent front
Both distances are measured from the pencil (start) line — to the centre of the spot and to the solvent front. Rf has no unit and is always between 0 and 1. For a given solvent and paper it is characteristic of the substance.
Given: a spot 3.2 cm from the start line; solvent front 8.0 cm from the start line. Find: Rf.
Rf = 3.2 ÷ 8.0 = 0.40 (no unit).
Check: the spot travelled less than half as far as the solvent, so Rf must be less than 0.5.
Students who understand chromatography still lose marks by measuring from the bottom of the paper instead of the start line, or to the top of the paper instead of the solvent front. Mark the solvent front as soon as the paper is removed; it disappears as the paper dries.
Chromatogram lab
Run the solvent and watch the spots separate. Each dye moves a fixed fraction (its Rf) of the distance travelled by the solvent front. Stop at any time, measure, and compare the mixture's spots with the reference dyes.
07 / Choosing a method
Choosing a separationSeparation techniques
| Mixture | Technique | Property that differs |
|---|---|---|
| Insoluble solid + liquid (sand + water) | Filtration | Particle size: solid cannot pass through the filter |
| Dissolved solid + solvent, keep the solid | Evaporation / crystallisation | Solvent evaporates; solute does not |
| Dissolved solid + solvent, keep the solvent | Simple distillation | Very different boiling points |
| Miscible liquids (ethanol + water; crude oil) | Fractional distillation | Different boiling points |
| Immiscible liquids (oil + water) | Separating funnel; skimming | Do not mix; different densities |
| Dyes or pigments in solution | Chromatography | Different attraction to paper vs solubility in solvent |
| Iron/steel from other solids | Magnet | Magnetism |
| Solid that dissolves in one solvent but not another | Dissolve, then filter | Solubility in a chosen solvent |
| Solid that floats or sinks | Flotation / sinking in water | Density |
Match the mixture to the method
Choose the technique, then read which property makes it work.
Percentage composition
Analysing a mixture often ends with a percentage by mass: percentage of X = (mass of X ÷ mass of sample) × 100. A 2.00 g rock sample containing 0.994 g of silicon dioxide is (0.994 ÷ 2.00) × 100 = 49.7% SiO2. Keep the same number of significant figures as the data (here three).
08 / Practice
Examination practicePast examination tasks
These tasks are adapted from past on-screen examinations; the chemistry, data and marks are unchanged. Attempt each one before opening the marking guidance.
A Criterion A · Knowing and understanding
Hard water contains Ca2+ and Mg2+ ions. Using the solubility data in Table 5.1, explain how you would remove only the Ca2+ ions from hard water.
Marking guidance
- Sulfate ions identified as the reagent (adding sulfuric acid also accepted).
- Calcium sulfate is insoluble (so Ca2+ is precipitated).
- Magnesium sulfate is soluble, or Mg2+ ions remain in solution.
- Filter off the calcium sulfate precipitate (remove the solid).
Why: sulfate is the only negative ion in the table that behaves differently with the two metal ions.
The first toy spring of its kind was made from high carbon steel. Four carbon steels are compared below.
| Steel | % carbon | % manganese | % iron | Properties |
|---|---|---|---|---|
| Low carbon | 0.25 | 0.4 | 99.35 | Loses its shape easily |
| Medium carbon | 0.54 | 1.65 | 97.81 | Ductile and long-wearing |
| High carbon | 0.95 | 0.9 | 98.15 | Strong and holds shape memory well |
| Very high carbon | 2.1 | 0 | 97.9 | Brittle |
Marking guidance
(a) Alloy.
(b) Low carbon steel: would not be strong enough, or too malleable, or would not hold its shape. Very high carbon steel: brittle, or not malleable / not easy to shape.
Glass jars with steel lids and plastic labels are to be recycled. Before the glass is processed, the other components must be removed. For each component — the plastic label, the steel lid and the glass jar — select the property that could be used to separate it: dissolves in water; magnetic; dissolves in organic solvent; sinks in water.
Marking guidance
Plastic label: dissolves in an organic solvent. Steel lid: magnetic. Glass jar: sinks in water.
Lava types are classified by their percentage of SiO2 by mass: basaltic 45.5–55.2%; andesitic 52.0–63.7%; rhyolitic 68.3–77.4%. A 2.00 g rock sample from one volcano contained 0.994 g of SiO2 (49.7%). A 2.00 g sample from a second volcano contained 1.16 g of SiO2.
Marking guidance
(a) Basaltic. (b) 1.16 × 100 ÷ 2.00 = 58.0%. (c) Andesitic (follows from your answer to (b)).
Note: 49.7% lies only in the basaltic range; 58.0% lies only in the andesitic range, even though the basaltic and andesitic ranges overlap between 52.0% and 55.2%.
B Criterion B · Inquiring and designing
Children enjoy blowing bubbles. The bubbles do not last a long time and burst easily if they come into contact with another object or if they are made with too much force. Bubbles used in play are made from a mixture of liquid soap and water in a fixed ratio. The basic structure of the bubble consists of two layers of soap film with a layer of water inside. The exact ratio of water to soap will determine its lifespan.
A student has decided to explore the ratio of water to soap to determine the mixture that will produce the bubbles with longest lifespan. The student has liquid soap, distilled water and various pieces of laboratory equipment. A volume of 10 cm3 of each solution was produced in the following dilutions.
| Bubble mixture | Volume of soap / cm3 | Volume of water /cm3 | Total volume / cm3 |
|---|---|---|---|
| 1 | 1.0 | 9.0 | 10.0 |
| 2 | 2.0 | 8.0 | 10.0 |
| 3 | 3.0 | 7.0 | 10.0 |
| 4 | 4.0 | 6.0 | 10.0 |
| 5 | 5.0 | 5.0 | 10.0 |
| 6 | 6.0 | 4.0 | 10.0 |
| 7 | 7.0 | 3.0 | 10.0 |
Select the most appropriate piece of equipment for accurately measuring the volume of each liquid.
Marking guidance
- 1 cm3 pipette
Alloys have different physical properties compared to their component elements. Pure copper has a density of 8.96 g cm–3 and pure zinc has a density of 7.14 g cm–3. Use the data in the table in part (d) to formulate a hypothesis linking the density of a brass alloy to the percentage of zinc it contains.
If: ____________
Then: ____________
Because: ____________
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
If the percentage of zinc in the brass increases, then the density of the alloy decreases, because zinc (7.14 g cm−3) is less dense than copper (8.96 g cm−3), so replacing copper with zinc lowers the mass of each unit volume. The table supports this: 5 % zinc gives 8.86 g cm−3, 30 % zinc gives 8.47 g cm−3.
C Criterion C · Processing and evaluating
State of matter is a physical characteristic which allows materials to be classified and grouped together. Melting point is a useful property which is an important characteristic of materials. Two students wanted to check the melting point of the metal gallium, symbol (Ga). One student used a thermometer and the other used a temperature probe. The samples of gallium are placed in boiling tubes which As the temperature of the water increased, the students observed the change in state of the gallium.
The melting points of mixtures are different to the melting points of individual elements. This can be seen when you look at the melting points of zinc (419.5 °C) and copper (1085 °C) compared to brass, which is a mixture or alloy of zinc and copper.
| Type of brass | Cu / % | Zn / % | Melting point / °C | Density / g cm−3 | Uses |
|---|---|---|---|---|---|
| A | 95 | 5 | 1066 | 8.86 | Pre-1983 coins |
| B | 88 | 12 | 1035 | 8.78 | Jewellery |
| C | 85 | 15 | 1027 | 8.75 | Electrical sockets |
| D | 70 | 30 | 954 | 8.47 | Radiators |
Use the data in the table to state what happens to the melting point of the alloy as the percentage of zinc increases.
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
The melting point decreases as the percentage of zinc increases — from 1066 °C at 5 % zinc to 954 °C at 30 % zinc.
Metallic elements can be combined to form alloys that have different properties from the pure elements. The
Scandium is added to many different alloys to increase yield strength. The yield strength is the pressure at which the material starts to deform or crack.
Use information about the elements to suggest why scandium would not be suitable to use on its own when building mountain bikes.
Marking guidance
- Sc reacts with air or water
D Criterion D · Reflecting on the impacts of science
Crude oil is a mixture of hydrocarbons of different sizes and other chemicals. It is separated into useful products by fractional distillation (Figure 5.3). Outline the process of fractional distillation.
Marking guidance
- More than two compounds (or mixtures of compounds) can be separated.
- Separation depends on boiling point (which depends on the size of the molecules), or on the change of state from liquid to gas.
- Molecules with the smallest mass or chain length have the lowest boiling points (or the reverse argument).
Urbanization has resulted in a large increase in the number of building and road construction projects. This has led to removal of sand from beaches. The main chemicals in sand are silicon oxides. Beach sand has rough edges and is ideal for construction whereas the sand particles in desert sand are rounded and are not suitable for construction. One possible solution is to replace the lost beach sand with silicon oxides from recycled glass.
Glass bottles can be recycled to make sand which can be used to replace beaches which have eroded, and plastic bottles can be recycled to make plastic pellets for use in the construction of roads and pathways. Suggest why using recycling glass and plastics would be better for the environment than burying used bottles as waste.
Marking guidance
- Accept any three reasonable points (max 3), for example
- save beaches
- preserve biodiversity
- conserve raw materials
- improves sustainability
Retrieval: mixtures and separation
Original practice questions for retrieval — not past examination items.
09 / Examination feedback
Examiner's overall observationEvidence from examination feedback
Examiner's overall observation
Students have been well prepared for the technique of chromatography and for calculating an Rf value from a suitable chromatogram, and most could label the bands on a chromatogram correctly. The difficulty came earlier in the calculation: most found it hard to measure the positions of the solvent front and of the spot, which are the two values the Rf depends on. Almost all students could identify the property that allows liquids to be separated by distillation and could name the changes of state involved, but struggled to describe those changes in terms of particle spacing and energy. Ideas about solutions of ionic compounds were weak: most could not link salt, as an ionic compound, to the ions it releases in water, or those ions to the electrical conductivity of the solution. When choosing between processes or materials from data, students tended to list information rather than compare the options directly.
10 / Summary
Summary and knowledge organiserRevision
Essential knowledge
- A pure substance is one element or compound, with sharp melting and boiling points; impurities lower and widen the melting range.
- Solutions are uniform; oils are immiscible with water; emulsions are droplets of one liquid dispersed in another, stabilised by an emulsifier; alloys are metals mixed with other elements and are harder because layers cannot slide.
- Filtration: insoluble solid from liquid. Crystallisation: dissolved solid from solution. Simple distillation: solvent from solution. Fractional distillation: liquids with different boiling points. Chromatography: dyes and pigments.
- Crude oil fractions: shorter chains → lower boiling point, leave higher up the column.
- Rf = distance moved by substance ÷ distance moved by solvent front, both from the start line.
Definitions
- Solute / solvent / solution
- Immiscible — do not mix
- Emulsion — droplets dispersed in another liquid
- Alloy — metal mixed with other elements
- Fraction — hydrocarbons with similar boiling points
Calculations
- Rf = dspot ÷ dfront (no unit)
- % by mass = (mass of part ÷ mass of sample) × 100
Must-remember distinctions
- Filtrate (passes through) vs residue (stays)
- Evaporation keeps the solute; distillation keeps the solvent
- Pencil start line; solvent below the line
- Pure: one spot; mixture: several
Examination checklist
- Name the property the separation uses
- Measure Rf distances from the start line
- Link chain length → forces → boiling point
- Compare options directly when evaluating
Other chapters: Criteria A–D · 1 · Periodic table · 2 · IUPAC naming · 3 · Atmosphere · 4 · Matter · 5 · Pure and impure · 6 · Bonding · 7 · Types of reaction
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