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Chemistry

Rail·Book shelfCore (Papers 1C & 2C)

Elements, compounds and mixtures

Learning objectives · Understand how to classify a substance as an element, compound or mixture; Understand how a chromatogram provides information about the composition of a mixture; Understand that a pure substance has a fixed melting and boiling point, but that a mixture may melt or boil over a range of temperatures; Describe these experimental techniques for the separation of mixtures: simple distillation; fractional distillation; filtration; crystallisation; paper chromatography; Understand how to use the calculation of Rf values to identify the components of a mixture; Practical: Investigate paper chromatography using inks/food colourings

The map of all matter

SB p.14

Look around: nearly everything you meet in everyday life is a mixture. The air you breathe contains elements such as nitrogen and oxygen along with compounds such as carbon dioxide and nitrogen oxides. The food you eat and the drinks you drink are mixtures too. Even a gold ring is not pure gold — an 18-carat ring is only 75% gold, with the rest usually copper. Only a few things — a nugget of pure metal, a diamond, a bottle of distilled water — come close to being the single, uncontaminated substances chemists call pure.

Chemistry sorts every substance into one of three boxes: an element, a compound or a mixture. Getting that classification right is the business of this lesson, and it matters far beyond the classroom. Most forensic work starts by separating a mixture into its components — you cannot analyse a sample until the things mixed into it are apart. By the end of this lesson you will be able to:

classify any substance as an element, compound or mixture;
say why a pure substance melts and boils at a fixed temperature while a mixture melts and boils over a range;
describe the four separation techniques — simple distillation, fractional distillation, filtration and crystallisation — plus paper chromatography;
use a chromatogram and Rf values to work out what a mixture contains.

Key point

Every substance is an element, a compound or a mixture. Elements and compounds are pure; mixtures are not, and their components can be separated by physical means such as filtration or distillation.

Prior knowledge

Open this chapter in the Rail All chapters

Elements — one type of atom

SB p.14

An element is a substance that cannot be split into anything simpler by chemical means. Every element contains only one kind of atom — heat it, react it, electrocute it, and you will never chemically prize an element apart into different substances. In models and diagrams, elements are drawn as atoms of a single colour or size. The classic examples are a pure metal such as magnesium, oxygen gas, and diamond (a form of carbon) — three very different materials, but each contains just one type of atom.

One subtlety to catch early: it is not completely true to say an element contains only one type of atom. Most elements are natural mixtures of isotopes — atoms with the same atomic number but different mass numbers, because they carry different numbers of neutrons. The precise statement is that all the atoms in an element have the same atomic-number. Diagrams and models usually ignore the difference between isotopes; they get their full treatment in Chapter 3.

Key point

The safe definition: all the atoms in an element have the same atomic number. "Only one type of atom" is almost right, but isotopes mean most elements contain atoms of slightly different masses — same protons, different neutrons (Chapter 3).

Common misconception

"Oxygen gas can't be an element — its particles contain two atoms joined together." No — an element is defined by having one type of atom, not one atom per particle. Oxygen gas contains only oxygen atoms, so it is an element; the same is true of nitrogen, hydrogen and chlorine gases.

Exam hint

"State what is meant by an element" (1 mark): mark schemes accept "a substance that cannot be broken down into anything simpler by chemical means" or "contains only one type of atom / atoms with the same atomic number". Avoid circular answers like "an element is a substance made of elements".

Quick check

Question-bank join: spec 1.8 × 3 items — define element; pick the element from a list of substances; explain why a diagram of one-colour atoms represents an element.

Key terms in this section

atom: — The smallest neutral particle of an element; the building block shown in element and compound diagrams. (Full structure: Chapter 3.)
element: — A substance that cannot be split into anything simpler by chemical means; all its atoms have the same atomic number.
atomic number: — The number of protons in the nucleus of an atom. All atoms of the same element share the same atomic number.
isotope: — Atoms of the same element with the same atomic number but different mass numbers, because they contain different numbers of neutrons.

Keep goingElement, Compound or Mixture 1-2-elements-compounds-and-mixtures

Compounds — elements in fixed proportions

SB p.15

A compound is formed when two or more elements chemically combine — and the moment that reaction happens, the proportions lock. The elements always combine in fixed proportions: the same compound, made in London or on Mars, always contains exactly the same ratio of its elements. Hydrogen and fluorine always combine to form hydrogen fluoride, formula HF; magnesium and fluorine always form magnesium fluoride, MgF₂; carbon dioxide is always CO₂ and methane always CH₄. You cannot make a "stronger" or "weaker" carbon dioxide by using more carbon or more oxygen — chemistry does not negotiate on ratios.

In diagrams, a compound shows more than one type of atom bonded together. That picture is worth pausing on: the atoms are not just mixed side by side as in a mixture — they are bonded, which is precisely why undoing a compound needs a chemical reaction (§s06).

Key point

Compounds = two or more elements chemically combined in fixed proportions. Hydrogen fluoride is always HF; magnesium fluoride is always MgF₂; carbon dioxide is always CO₂. Diagrams show two or more atom types bonded together.

Common misconception

"You can make carbon dioxide with different proportions of carbon and oxygen, like mixing more or less squash in water." No — that is mixture thinking. In a compound the ratio is fixed by the bonding: CO₂ is always two oxygen atoms per carbon atom, never more, never fewer. Change the ratio and you are either making a different compound or leaving unreacted excess behind.

Exam hint

"Explain why magnesium oxide is a compound" (2 marks): mark schemes want both halves — (i) it contains two (or more) elements chemically combined, and (ii) the elements are present in fixed proportion. Writing only "it has different types of atom" describes a mixture of atoms just as well — the "chemically combined" phrase is what scores.

Quick check

Question-bank join: spec 1.8 × 3 items — identify the compound from particle diagrams; explain fixed proportions; distinguish compound vs mixture from a description.

Key terms in this section

compound: — Two or more elements chemically combined, always in fixed proportions; it can only be split apart by chemical reactions.
formula: — The fixed ratio of atoms in a compound written with symbols and subscript numbers, e.g. HF, MgF₂, CO₂, CH₄.

Keep goingElement, Compound or Mixture 1-2-elements-compounds-and-mixtures

Mixtures — anything goes

SB p.15

A mixture is what you get when substances are simply put together — no chemical reaction occurs. The components can be elements, compounds, or both at once, and (unlike a compound) they can be present in any proportion: you can put in as much or as little of each component as you like, and it is still the same mixture. Nothing locks the ratio in, because nothing has reacted.

Three possible recipes cover every mixture, and they are worth memorising as a classification toolkit:

a mixture of elements — nitrogen and oxygen (this is essentially the air);
a mixture of compounds — carbon dioxide and water vapour;
a mixture of an element with a compound — carbon dioxide and nitrogen.

Because no reaction happens, the components keep their own properties and can be got back apart by physical means — the theme of §s06 and §s09.

Key point

In a mixture no chemical reaction occurs; the components (elements and/or compounds) can be present in any proportion. Mixtures of elements, of compounds, and of an element with a compound are all possible.

Common misconception

"A mixture has to be made of either all elements or all compounds." No — the third combination is just as legal: an element mixed with a compound (carbon dioxide and nitrogen). Classification asks what is in it and has it reacted, not which single category the contents belong to.

Exam hint

2-mark "explain why X is a mixture" answers score with: (i) the substances are not chemically joined (no reaction has occurred) and (ii) they are not in fixed proportions / can be mixed in any ratio. Either half alone usually banks 1 mark; both halves bank both.

Quick check

Question-bank join: spec 1.8 × 3 items — classify the three mixture recipes; mixture vs compound from proportion statements; spot the mixture from a diagram of different atoms not bonded.

Key terms in this section

mixture: — Two or more substances (elements and/or compounds) mixed together with no chemical reaction; the components can be in any proportion and keep their own properties.

Keep goingElement, Compound or Mixture 1-2-elements-compounds-and-mixtures

Mixture or compound — the simple differences

SB p.15

Water is the cleanest contrast of all. In water (a compound), every single water molecule has two hydrogen atoms combined with one oxygen atom — it never varies. In a mixture of hydrogen and oxygen gases, the two can be present in any proportion you fancy. The same story with iron and sulfur: as a mixture you can combine them in any ratio, but iron sulfide (FeS) — the compound — always contains iron and sulfur in exactly the same proportion.

Proportions are only half the story. The other half is properties. In a mixture of iron and sulfur, each element keeps its own properties: the iron stays grey, the sulfur stays yellow, and when you add dilute hydrochloric acid the iron fizzes to produce hydrogen while the sulfur does not react at all. The compound iron sulfide behaves completely differently — it reacts with acids in its own way, unlike either element. Compare a mixture of hydrogen and oxygen with the compound water: put a flame to the mixture and it explodes; the compound, water, is a colourless liquid that simply puts the flame out.

Separation follows the same divide. Mixtures can be separated by physical means — methods like changing the temperature or dissolving part of the mixture, which involve no chemical reactions. A magnet pulls the iron out of an iron–sulfur mixture while the sulfur stays behind. The elements in a compound, though, cannot be separated by physical means: turning water back into hydrogen and oxygen requires electrolysis (a chemical process — Chapter 10). You can separate a mixture of hydrogen and oxygen physically, though — oxygen condenses to a liquid at a much higher temperature than hydrogen (−183 °C as opposed to −253 °C), so cooling far enough leaves the oxygen liquid while hydrogen stays gaseous.

Finally, the vocabulary of purity: a mixture is not a pure substance. If a sample contains only a small amount of an unwanted substance, that unwanted substance is called an impurity.

Key point

Mixture: components in any proportion, each keeping its own properties, separable by physical means. Compound: fixed proportion, its own new properties, separable only by chemical reactions (e.g. electrolysis of water).

Common misconception

"Iron sulfide is just iron and sulfur sitting next to each other, so it still reacts with acid like iron does." No — once iron and sulfur are chemically combined, the individual properties are gone. Iron sulfide reacts with acids differently from iron; sulfur's reaction with the acid disappears too. New bonds mean new substance.

Exam hint

The 4–6 mark "compare a mixture of iron and sulfur with the compound iron sulfide" answer earns marks per difference stated: composition (any ratio vs fixed), properties (kept vs new — use the acid test), separation (physical vs chemical). Mark schemes reward precise comparisons, so write "the mixture keeps the properties of iron and sulfur" — not just "they are different".

Quick check

Question-bank join: spec 1.8 × 4 items — water vs hydrogen/oxygen mixture; predict what a magnet separates; which needs electrolysis; iron sulfide vs iron in dilute acid.

Key terms in this section

pure substance: — A single element or single compound, with nothing else mixed in; it melts and boils at fixed temperatures.
impurity: — An unwanted substance mixed with the one you want; impurities lower the melting point and raise the boiling point of a substance.

Keep goingElement, Compound or Mixture 1-2-elements-compounds-and-mixtures

Melting and boiling points — the purity test

SB p.16

Pure substances — elements and pure compounds alike — melt and boil at fixed temperatures. Water melts at 0 °C and boils at 100 °C; every clean sample of water does exactly the same. Mixtures usually melt and boil over a range of temperatures instead. That single contrast is spec point 1.9, and it hands you a practical purity test.

Impurities push both numbers the "wrong" way: they lower the melting point and raise the boiling point. Dissolve 10 g of common salt (sodium chloride) in 1 litre of water and the melting point drops to about −0.6 °C while the boiling point climbs to about 100.2 °C. (This is why salt gritters scatter salt on winter roads — the salty water is harder to freeze than pure water.)

The melting point gives chemists a working purity test. Suppose you make aspirin in a sixth-form practical. Aspirin is a white powder that melts at 138 °C. You record the temperature at which your sample starts to melt and the temperature at which it has fully melted. If your sample melts anywhere in the range 128–134 °C you know it is quite impure — it melts over a wide range, and below the melting point of the pure substance.

Note how this reuses Lesson u1:c01: melting point and boiling point are the same numbers you used to predict the state of a substance — but here they become evidence about what is dissolved in it.

Key point

Pure substance → fixed melting and boiling point. Mixture → melts and boils over a range. Impurities lower the melting point and raise the boiling point. Melting point is the classic lab test for purity.

Maths skills

Reading a purity test is comparison logic, not calculation: (i) does the sample melt at a single temperature or across a range? (ii) is that range below the pure substance's melting point? Sample at 128–134 °C vs pure at 138 °C fails both tests at once — range and depression — so the verdict is "impure" with two pieces of evidence.

Common misconception

"Impurities raise the melting point, because there is more stuff to melt." No — the opposite: dissolving an impurity lowers the melting point (salt water freezes at about −0.6 °C, not above 0 °C) and raises the boiling point (about 100.2 °C, not below 100 °C).

Exam hint

"Substance X melts at 122 °C; substance Y melts between 87 and 93 °C. Explain which is the pure substance" (2–3 marks): name X, then give the reason in mark-scheme language — a pure substance has a fixed/sharp melting point, while Y melts over a range of temperatures, so Y contains an impurity. Both halves are needed: the verdict and the fixed-vs-range rule.

Quick check

Question-bank join: spec 1.9 × 3 items — fixed vs range verdict from melting data; effect of impurity on mp/bp; salt-water boiling point.

Key terms in this section

melting point: — The fixed temperature at which a pure solid melts; impurities lower it and make melting happen over a range.
boiling point: — The fixed temperature at which a pure liquid boils; impurities raise it, so a solution boils above 100 °C.
pure substance: — A single element or single compound, with nothing else mixed in; it melts and boils at fixed temperatures.
impurity: — An unwanted substance mixed with the one you want; impurities lower the melting point and raise the boiling point of a substance.

Watchblock 8

GCSE Chemistry - Purity & Formulations (2027/28 exams)

Purity from melting and boiling points — a pure substance has fixed values; a mixture melts over a range.

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Video © Cognito (cognito.org) — embedded via YouTube

Keep goingPure Substance vs Mixture 1-1-states-of-matter

Separation of mixtures — why and how

SB p.16

Separating mixtures is not a school-only exercise — it is one of the load-bearing walls of the whole chemical industry. Crude oil arriving from a well is a mixture of hundreds of hydrocarbons and is split by distillation before anything useful can be made from it. Fresh water is produced from salt water by distillation on an industrial scale. The enrichment of uranium — separating two forms of the same element — is a separation problem. And in forensic science, the components of a mixture usually have to be separated before they can be analysed: an unseparated sample tells you almost nothing.

The common thread: because mixtures were made by mixing (no reaction), they can be undone by physical means. Which physical method you choose depends on what is mixed with what — an insoluble solid in a liquid needs filtration (sand from water), a dissolved solid needs crystallisation or simple distillation, two miscible liquids need fractional distillation, and dissolved dyes yield to paper chromatography. Spec point 1.10 asks you to describe each of the first four techniques; §s09 builds every one of them, and chromatography follows in §s10.

Key point

Mixtures are separated by physical means because no chemical reaction holds them together. The method is chosen by what is mixed with what: insoluble solid → filtration; dissolved solid → crystallisation or simple distillation; miscible liquids → fractional distillation; dyes → paper chromatography.

Common misconception

"Separating a mixture needs a chemical reaction to pull the substances apart." No — that is compound thinking. Physical changes (boiling, condensing, filtering, evaporating, dissolving) are enough for mixtures; it is compounds that need chemical reactions such as electrolysis to break them up.

Exam hint

"State which separation method you would use to …" (1 mark each): pick the method from the mixture type, not from habit. Mark schemes award the method name only — "filtration", "crystallisation", "simple distillation", "fractional distillation", "paper chromatography" — so spell out the full name; "distillation" alone is ambiguous when fractional is intended.

Quick check

Question-bank join: spec 1.10 × 2 items — match separation to mixture type; why forensics separates before analysis.

Label the diagramblock 5

Name the separation technique each mixture needs.

One technique per mixture - name it the way the spec does.

Watchblock 9

GCSE Chemistry - Separation Techniques - Filtration | Evaporation | Crystallisation

Filtration and crystallisation — separating an insoluble solid, then purifying the filtrate to dry crystals.

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Video © Cognito (cognito.org) — embedded via YouTube

Keep goingSeparation Techniques 1-1-states-of-matter 1-2-elements-compounds-and-mixtures 2-5-extraction-and-uses-of-metals 3-2-rates-of-reaction

Rock salt to pure salt — filtration, crystallisation and distillation

SB p.17

Filtration separates a solid from a liquid. Pour the mixture through filter paper: the solid left behind on the paper is the residue, the liquid that comes through is the filtrate (sand from water). Filtration also separates two solids, provided only one of them is soluble in water — which is exactly the trick used for rock salt below.

Crystallisation separates a dissolved solid (a solute) from its solution. Heat the solution in an evaporating basin to boil off some water until an almost saturated-solution solution forms — you can test this by dipping a glass rod in and seeing if small crystals form on its surface. Then turn the Bunsen burner off and let the crystals grow as more water evaporates and the solution cools; then filter them off.

Rock salt is salt contaminated with earthy, rocky impurities that are not soluble in water. Crush it and mix with hot water: the salt dissolves, the impurities do not. Filter — the insoluble impurities stay on the filter paper as residue, and the filtrate is a clean salt solution. Crystallise the filtrate and pure salt drops out. The same three moves (dissolve, filter, crystallise) separate any pair of solids where one is soluble and the other is not.

Simple distillation gets the solvent back instead. Boil the solution: the water evaporates, travels into the condenser, and is condensed back to a pure liquid to collect — while the salt stays in the flask. (Run it long enough and the solution becomes so concentrated that the salt crystallises out too.) One detail examiners love: water always enters the condenser at the lower end, so the jacket fills completely and stays full even if the water flow stops.

Fractional distillation separates two miscible liquids — ones that mix in any proportion to form a single layer — using their different boiling points: ethanol boils at 78 °C, water at 100 °C. With careful heating the column stays at the lower temperature, so all the water condenses inside the column and trickles back to the flask while only ethanol vapour reaches the top and passes into the condenser. (The column is often packed with glass beads to help the separation; the ethanol that comes out is about 96% pure, and the last 4% of water cannot be removed by distillation — both details belong to the world beyond International GCSE.)

Paper chromatography — the technique that finishes this lesson — separates mixtures of dyes. Most inks and food colourings are not single colours but mixtures of dyes; chromatography splits them up the paper, and it even works for colourless substances like sugars if a way is found to make the spots visible. The method (spec 1.13):

draw the baseline in pencil, about 1 cm from the bottom of the paper — never pen, because pen ink would move up the paper with the solvent;
spot the dye mixture on the line and let it dry;
stand the paper in a beaker with a small amount of solvent, below the pencil line — or the inks would simply dissolve into the solvent;
put a lid (a watch glass) on the beaker so the atmosphere saturates with solvent vapour, which stops the solvent evaporating off the paper;
when the solvent has risen to about 1 cm from the top, mark the highest level it reached — the solvent-front — then leave the paper to dry.

The pattern left on the paper is the chromatogram; reading it is §s10.

Safety

Avoid skin contact with the solvents and the dyes, especially if you have sensitive skin. (Book safety note for the chromatography work.)

Key point

Water feeds into the condenser at the lower end: the jacket fills better, and if the flow stops the condenser stays full of water.

Key point

If a dye does not move from the pencil line, it is not soluble in that solvent — try another. If it moves with the solvent front, it is too soluble — try another. Finding the right solvent is done by experimenting.

Worked example

The rock-salt run is the exam's favourite separation story, so rehearse it as a chain: crush + hot water (salt dissolves, rock does not) → filter (residue: insoluble impurities; filtrate: salt solution) → crystallise (heat to near-saturation, rod test, Bunsen off, cool, filter the crystals). Each arrow is a mark: method + what it removes.

Common misconception

"You can get salt back from salt water by filtering the solution." No — filtration only catches solids that are insoluble. Dissolved salt passes straight through filter paper with the water; recovering it needs crystallisation (or simple distillation, which collects the water instead).

Exam hint

Method-choice questions follow the mixture type: insoluble solid + liquid → filtration; dissolved solid from solution → crystallisation (want the solid) or simple distillation (want the solvent); two miscible liquids → fractional distillation; two solids, one soluble → dissolve and filter. Describing the rock-salt method earns marks in order — crush/dissolve, filter, crystallise — so narrate it stepwise, naming residue and filtrate.

Quick check

Question-bank join: spec 1.10 × 3 items — rock salt method order; residue vs filtrate identification; simple vs fractional distillation choice. spec 1.13 × 2 items — why pencil not pen; why the solvent must sit below the pencil line.

Key terms in this section

filtration: — Separating an insoluble solid from a liquid by pouring through filter paper; the solid left is the residue, the liquid passing through is the filtrate.
residue: — The solid left on the filter paper after filtration.
filtrate: — The liquid that passes through the filter paper during filtration.
crystallisation: — Separating a solute from its solution by evaporating solvent until the solution is nearly saturated, then cooling so crystals form; crystals are filtered off.
saturated solution: — A solution holding the maximum amount of dissolved solute at that temperature; crystals form on its surface or on a dipped glass rod.
simple distillation: — Separating the solvent from a solution: the solvent boils, is condensed in the condenser and collected, while dissolved solids stay in the flask.
fractional distillation: — Separating a mixture of miscible liquids using their different boiling points in a fractionating column, e.g. ethanol (78 °C) from water (100 °C).
miscible: — Describes liquids that mix completely in any proportion to form a single layer, e.g. ethanol and water.
baseline: — The starting line on chromatography paper, drawn in pencil about 1 cm from the bottom, where spots are placed.
solvent front: — The highest level the solvent reaches on the chromatography paper before the run is stopped.

Watchblock 12

Chromatography - GCSE Science Required Practical

The required practical: running and analysing a paper chromatogram of inks and food colourings.

Watch on YouTube

Keep goingSeparation Techniques Practical: Investigating Paper Chromatography 1-1-states-of-matter 1-2-elements-compounds-and-mixtures 2-5-extraction-and-uses-of-metals

Using paper chromatography in analysis — Rf values

SB p.20

Chromatography does more than split a mixture — it tells you what is in it. Suppose your unknown mixture (m) might contain dyes d1, d2, d3 and d4. Draw a pencil line on a larger sheet, spot the mixture at one mark and small spots of the four known dyes at the others, then develop the chromatogram as in §s09. Then compare — and measure carefully: always measure to the centre of the spot.

The comparison rule: if a spot in the mixture has the same colour and travelled the same distance as a known dye, they match. Here, mixture m produces spots corresponding to d1, d3 and d4. Dye d2 is the trap: its colour matches a spot in the mixture, but it travelled a different distance — so it must be a different compound.

Raw distances are unreliable, because each run's solvent moves a different distance up the paper. So we report a ratio instead — the rf-value:

Rf = distance moved by the spot (from the pencil line) ÷ distance moved by the solvent front (from the pencil line)

In the chromatogram just described, the solvent front moved 3.6 cm, so for dye d3: Rf = 2.9 ÷ 3.6 = 0.81. The mixture's three spots give blue 0.9 ÷ 3.6 = 0.25, orange 2.0 ÷ 3.6 = 0.56, green 2.9 ÷ 3.6 = 0.81 — and the known dyes give d1 = 0.56, d2 = 0.36, d3 = 0.81, d4 = 0.25. Matching values identify the mixture's contents: d1, d3 and d4.

Two honesty rules about Rf: it must fall between 0 and 1 (a value bigger than 1 means you divided the wrong way round), and it has no units. And Rf values only compare fairly on the same paper and same solvent — here every spot shared one sheet, so the comparison was safe; separate sheets are fine too, as long as paper type and solvent match.

This lesson's practice is wired to the question bank below.

Key point

Rf value = distance moved by spot ÷ distance moved by solvent front, measured to the centre of each spot. Rf always lies between 0 and 1, has no units, and only compares fairly on the same paper with the same solvent.

Worked example

Mixture m vs dyes d1–d4 (solvent front 3.6 cm): m's spots — blue 0.25, orange 0.56, green 0.81. Known dyes — d1 0.56, d2 0.36, d3 0.81, d4 0.25. Match: 0.25 = d4, 0.56 = d1, 0.81 = d3 → m contains d1, d3 and d4. d2's colour matches but its Rf (0.36) appears nowhere in m, so d2 is absent — colour alone never decides; distance (Rf) does.

Maths skills

Rf is a ratio of two lengths, so the skill is division with sense-checking: spot distance (smaller) on top, solvent-front distance (larger) below. Sanity-check the answer — if Rf > 1 you inverted the fraction. Quote Rf to 2 decimal places (0.25, 0.56, 0.81 in the example above).

Cross-curricular links

Maths: an Rf value is a scale-invariant ratio — the same idea as a scale factor on a map, which is exactly why it works when raw distances change between runs. Forensics: the anonymous-letter problem is real forensic science — comparing an unknown ink against a library of known inks by matching Rf values on the same chromatogram.

Common misconception

"The dye that moved furthest up the paper is simply 'the best' dye — and a spot can travel further than the solvent." No — a spot can never overtake the solvent front, so Rf can never exceed 1. And distance moved is not a property to quote: Rf (on the same paper and solvent) is, because raw distances change from run to run.

Exam hint

Chromatogram questions have three stock mark lines: (1) same dye ⇔ same colour and same distance/Rf; (2) a "minimum of N dyes" answer — count the spots, but note two dyes could coincide in one spot (re-run with a different solvent to check); (3) always measure to the centre of the spot. And keep Rf unitless — writing "0.81 cm" throws the mark away.

Quick check

Question-bank join: spec 1.11 × 2 items — what a chromatogram tells you about composition; why a matching colour is not enough. spec 1.12 × 3 items — calculate an Rf from distances; spot the inverted-ratio error; explain why Rf beats raw distance. spec 1.13 × 1 item — the minimum-number-of-dyes argument.

Summary

Every substance is an element, a compound or a mixture; diagrams show it: one atom type (element), two or more atom types bonded in fixed proportions (compound), different substances not chemically joined in any proportion (mixture).
Mixtures keep their components' properties (iron stays grey, sulfur yellow) and separate by physical means; compounds have their own new properties and need chemical reactions (e.g. electrolysis) to undo.
A pure substance melts and boils at fixed temperatures; a mixture melts and boils over a range — impurities lower the melting point and raise the boiling point.
Separation methods: filtration (insoluble solid; residue vs filtrate), crystallisation (dissolved solid; evaporate to near saturation, cool), simple distillation (collect the solvent; condenser water enters at the bottom), fractional distillation (miscible liquids by boiling point: ethanol 78 °C vs water 100 °C).
Paper chromatography: pencil baseline, solvent below the line, lid on, mark the solvent front; a chromatogram identifies dyes by matching spots; Rf = spot distance ÷ solvent-front distance, always 0–1, no units, same paper and solvent to compare.

Can you…?

classify a substance as element, compound or mixture — and say why?
explain why a compound's proportions are fixed but a mixture's are not?
compare the properties and separation of a mixture (iron + sulfur) with its compound (FeS)?
use melting-point data to decide which sample is pure?
describe filtration, crystallisation, simple distillation and fractional distillation, and pick the right one for a given mixture?
carry out paper chromatography correctly (pencil line, solvent below the line, lid, solvent front)?
calculate an Rf value and use Rf matching to identify the dyes in a mixture?

Key terms in this section

chromatogram: — The pattern of spots left on the chromatography paper after the solvent has moved up and the paper has dried.
Rf value: — Distance moved by a spot divided by distance moved by the solvent front, both measured from the pencil line; always between 0 and 1, with no units.

Examiner note - Rf language

Rf = distance moved by the substance divided by distance moved by the solvent. It has NO units and is always between 0 and 1. Pure substances give ONE spot; a mixture gives several - that is how purity is judged from a chromatogram.

Step throughblock 4

A chromatogram: the substance moves 3.2 cm, the solvent front moves 4.0 cm. Find Rf.

1•Substance distance (cm)
(cm)
2•Solvent front distance (cm)
3•Rf = substance / solvent (no units)

Watchblock 10

GCSE Chemistry - Paper Chromatography

Paper chromatography and Rf values — reading a chromatogram the way mark schemes want it.

Watch on YouTube

Watchblock 11

GCSE Chemistry - Distillation - Simple & Fractional Distillation (2027/28 exams)

Simple vs fractional distillation — which apparatus suits which mixture, and why the fractionating column matters.

Watch on YouTube

Keep goingInterpreting Chromatograms Practical: Investigating Paper Chromatography 1-2-elements-compounds-and-mixtures

Elements, compounds and mixtures

The map of all matter

Elements — one type of atom

118 elements — and where they live

Compounds — elements in fixed proportions

Mixtures — anything goes

Mixture or compound — the simple differences

Melting and boiling points — the purity test

Separation of mixtures — why and how

Rock salt to pure salt — filtration, crystallisation and distillation

Using paper chromatography in analysis — Rf values