Scroll down for some basic but important chemistry definitions and concepts

Doc Brown's Chemistry KS4 science GCSE (9-1)/IGCSE/O Level Chemistry Revision Notes

 PART 1 Definitions of elements, compounds and mixtures, other definitions of some important words common in chemistry, physical changes and chemical changes, particle pictures of elements, compounds and mixtures, what are atoms, molecules, particle pictures-diagrams-images of elements, compounds, mixtures, symbols, formula, pure substance, impure substance, purification, evidence and explanations  of a chemical change or a physical change, what is a chemical reaction? Iron and sulfur reaction - index of keywords-terms-phrases explained below. These revision notes should prove useful for the new AQA, Edexcel and OCR GCSE (9-1) courses.

The reaction of iron and sulphur/sulfur to make iron sulphide/sulfide (c) doc b

Part 1 Some important definitions in Chemistry, Elements, Compounds & Mixture pictures and Physical & Chemical Changes (this page)

Part 2 Methods of Separating Mixtures of Substances

Part 3 How to write equations, work out formula and name compounds

AND Uses of Chemicals : over 220 examples briefly described

AND Types of Chemical Reactions or Chemical Processes

Alphabetical list of KEYWORDS for Parts 1-3: atom  *  balancing equations (work your way down the section carefully)  *  centrifuges/centrifuging  *  chemical reaction/change  *  chromatography (paper/thin layer)  *  compound  *  covalencycrystallisation  *  decanting/decantation  * displayed formula  *  distillation (simple or fractional)  *  element  *  equations  *  evaporation  *  filtration  *  formula  * gas chromatography  *  impure/pure  *  insoluble  *  ionic equations  *  ionic valence  *  iron-sulphur separation and heating experiment  *  magnet  *  mixture  *  molecule  *  naming compounds and ions  *  particle pictures of elements/compounds/mixtures  *  physical change  *  precipitation  *  products  *  pure substance  *  purification  *  reactants  *  sand/salt separation  *  separating funnel  *  separating mixtures  *  soluble/solution/solvent/solute  *  solvent extraction  *  symbols (for elements, formula, in equations)  *   state symbols  *  EleCmdMix3.htm  * working out formulae  *

Spelling note: sulphur = sulfur, sulphuric acid = sulfuric acid, sulphate = sulfate, sulphite = sulfite, aluminium = aluminum (US)


Section 1.1 Introduction and Some keywords (see also pictures)



a picture of an atom

(i) Even as far back as ancient Greece ~500BC philosophers had considered the concept of what would be formed on continuously dividing matter i.e. what was the smallest 'bit' left of any substance. In 1808 the English scientist-chemist Dalton proposed his 'atomic theory' - that all matter was made up of tiny individual units called atoms which could NOT be subdivided into simpler substances. What is more, he proposed the idea that there were different types of atoms which we now call 'elements' and combinations of them produce all the different substances which exist. The different types of atoms are called elements (examples below).

An ATOM is the smallest particle of a substance, an element, which can have its own characteristic properties AND cannot be split into simpler substances.

Therefore, more simply, 'atoms are the smallest bits of an element that can exist'.

The different types of atoms are called elements, more on elements later.

(ii) BUT, remember atoms are built up of even more fundamental sub-atomic particles - the electron, proton and neutron. The centre of an atom, called the nucleus, consists of proton and neutron particles and the electrons move around the nucleus in 'orbital' energy levels. For more details see the Atomic Structure Notes.

(iii) In chemical reactions, the atoms rearrange themselves in changing from reactants to new products (see chemical change on this page).


MOLECULES and their representation


model of a molecule


A MOLECULE is a larger particle formed by the chemical combination of two or more atoms.

The molecule may be an element e.g. hydrogen formula H2 (H-H, two atoms combined, all atoms the same)


a compound (more examples below) e.g. carbon dioxide formula CO2 (O=C=O, three atoms combined) and in each case the atoms are held together by chemical bonds. (detailed GCSE bonding notes and examples)

You can represent molecule in various styles of diagram. For example, you can colour and size code the atoms of different elements, so in the molecule pictured on the upper left, you can tell there are five types of atom (elements) and six atoms in total in the molecule.

The second molecule (lower left) shows the molecular structure of ethanol ('alcohol') which consists of two carbon atoms, six hydrogen atoms and one oxygen atom.

You will also come across shorthand versions of this diagrammatic style written like

CH3CH2OH and the even shorter C2H5OH, but using these styles requires much more experience than is required when first learning the basic principles of chemistry.

Why they are combined in this particular number and order depends primarily on an atoms combining power (its valence) an advanced concept dealt with in Part 3 equations, formula and valence.

There are also styles to give a much greater '3D' impression of the shape of a molecule and they attempt to show the '3D' spatial arrangement of the atoms in a molecule and how the bonds connect them together.

alkanes structure and naming (c) doc bOn the left is the displayed formula of ethane, which shows all the atoms and individual bonds. Below left is stylised '3D' version of the displayed formula and you can see how it fits in with the 'ball and stick' (below it) and 'space-filling' model (below right) diagrams.

Below are three '3D' representations in 2D of the hydrocarbon molecule called ethane.

alkanes structure and naming (c) doc b

The lower image is an example of a 'ball-and-stick' diagram and shows the chemical bonds which hold the atoms together. topis an example of a 'space-filling' diagram which gives a more accurate representation of the space the molecule actually occupies.


ELEMENT and symbols

nuclide symbol for cobalt-60

H I Th Er Ho W Ar U?

Element Symbol-name quizzes: easier-pictorial! or harder-no pictures!

Metals and non-metals

(i) Basic definition
  • An ELEMENT is a pure substance made up of only one type of atom, 92 occur naturally and can be 'summarised' in the Periodic Table (detailed notes) i.e. from element 1 hydrogen H to number 92 uranium U which occur naturally.
    •  Over 100 elements are now known, and ' man-made' or 'synthesised' up to 118, though in many cases only a few atoms!
  • Note that each element has a symbol which is a single capital letter eg
    • H hydrogen, N nitrogen, O oxygen, S sulfur, K potassium, U uranium etc.
    • or a single capital letter plus a small lower case letter e.g.
    • cobalt Co, calcium Ca, Fe iron or sodium Na etc.
    • and have you spotted the 'text style' message on the left! and can you name the elements!
  • Each element has its own unique set of properties but the Periodic Table is a means of grouping similar elements together to produce important patterns in the physical. and especially the chemical behaviour of elements.
  • Elements may exist as atoms like the Noble Gases e.g. helium He or as molecules e.g. hydrogen H2 or sulphur S8. (more examples applied to equations and see note about 'formula of elements')

(ii) Extended ideas

  • *At a higher level of thinking, all the atoms of the same element, have the same atomic or proton number. This number determines how many electrons the atom has, and so ultimately its chemistry. Any atom with 27 protons and electrons is cobalt! The diagram 60-Co-27 uses advanced notation - all explained on the atomic structure page.
  • See also picture diagrams of elements/compounds and mixtures.
  • Elements are broadly divided by physical and chemical character into metals and non-metals. A few elements display characteristics of metallic and non-metallic elements and are referred to as semi-metals or metalloids. Elements can be more highly characterised and organised in the form of the Periodic Table.
  • All of these points are discussed on the GCSE Periodic Table summary notes page.

Element symbol & name QUIZ - easier-pictorial  or  Element symbol & name QUIZ harder - no pictures!



  • The variety of chemical substances around you are all due to different combinations of atoms. Atoms combine or 'connect' together by means of chemical bonds of which there are various types, but all chemical bonds are based on the attraction of oppositely charged particles, i.e. the natural attraction of positive and negative particles - a fundamental law of physics!
  • The result is that millions of different substances (molecules/compounds) can exist because of the huge variety of atom combinations possible.
  • KS3 students do NOT need to know about chemical bonding except the idea that they exist.
  • topDetailed GCSE bonding notes and examples for ionic, covalent and metallic bonds,
    • and includes a simplified introduction to chemical bonding





  • A COMPOUND is a chemical combination of two or more different elements combined in fixed proportions eg four hydrogen atoms are combine with one carbon atom in methane.
  • Compounds can only be separated into elements by chemical reactions.
  • The chemical composition of a pure substance can be represented using the element symbols, and where necessary, subscripted numbers.
  • A chemical formula represents the relative numbers of atoms of each element in a pure chemical compound eg CH4 above.
  • Chemical formula are important for the construction of chemical equations.
  • Note that the formula of a particular pure substance does NOT change just because it becomes part of a mixture. It retains its own chemical identity in a mixture, i.e. it does not change chemically, then it must have the same formula.
  • You need to be able to read a formula e.g. like the one below - a bit more complicated.
    • (i) and (ii)
    • The first 'picture' (i) is an example of a displayed formula, in which every individual atom is shown and how it is bonded ('connected') with other atoms in the molecule. All the dashes represent the covalent bonds between the atoms in the molecule. The dashes actually represent an electrical attractive force, but no need for any detail at all here.
    • From this diagram you can tell there are four different elements in the molecules and the number of atoms of each element ...
    • ... there are 4 carbon atoms (C), 8 hydrogen atoms (H), 1 bromine atom (Br) and 1 chlorine atom (Cl) and because there are at least 2 different elements chemically combined in the molecule or formula, this also tells you it is a compound (see below for more examples).
    • A summary of all the atoms in the individual molecule is called the molecular formula eg (ii) above and C4H10 on the right. The number of atoms of each element is shown as a subscript number in the formula.
    • There are more examples in the next section which discusses the word compound further.
    • Also, look at an example where they are used in chemical equations - burning methane.
    • A displayed formula is sometimes called a full structural formula or graphic formula.

More on formulae and COMPOUNDS

'3D' structural formula of methane

methane CH4


alkanes structure and naming (c) doc b

ethane C2H6


  • As already stated, a COMPOUND is a pure substance formed by chemically combining at least two different elements in fixed proportions by ionic or covalent bonding.
    • The elements in a compound can ONLY be separated by chemical means.
    • A chemical combination is NOT a physical mixture of substances.
  • Compounds can be represented by a FORMULA, which represents the whole number (integer) ratio of the atoms in a formula, and for molecules, a summary of all the atoms in one molecule. Examples:
    • sodium chloride NaCl, ionic compound, 2 elements, 1 atom of sodium to everyone 1 atom of chlorine).
    • methane CH4, covalent compound molecule, has 2 elements in it, 1 atoms of carbon and 4 atoms of hydrogen. The lines in the diagram of methane represent the four bonds in the molecule, and in this style of diagram attempts to portray a 3D image of methane in 2D!
    • ethane C2H6, two atoms of carbon combined with 6 atoms of hydrogen and note that the same two elements can form two different compounds because of different atom ratios i.e. compared with methane.
    • glucose C6H12O6 (covalent compound molecule, 3 elements in it, 6 atoms of carbon, 12 of hydrogen and 6 of oxygen).
    • carbon monoxide CO (1C + 1O atoms) and
      • carbon dioxide CO2 (1C + 2O atoms)
      • is an example of combining two different elements to produce two different compounds because of different atom ratios in the formula.
      • This also shows that elements can have different combining powers, known as the valence.
    • Another example are the oxides of iron: FeOFe2O3  and  Fe3O4
  • NOTE
    • There must be at least two different types of atom (elements) in a compound.
    • The number of atoms of each element is shown as a subscript number in the formula ...
    • ... except the 1 is never written in the formula, so remember, no number means 1 in a formula.
    • In copper sulfate CuSO4, there are no numbers for Cu copper and S sulfur (1 of each), but a 4 is needed for the four oxygen atoms.
  • Compounds have a fixed composition and therefore a fixed ratio of atoms represented by a fixed formula, however the compound is made or formed.
  • In a compound the elements are not easily separated by physical means, but they are by chemical reactions, though quite often not as easily as you might think.
  • Any compound has physical and chemical properties quite different from the elements it is formed from.
    • For example the two elements soft silvery reactive sodium + reactive green gas chlorine ==> colourless, and not very reactive crystals of the compound sodium chloride.
  • The FORMULA of a compound summarises the 'whole number' atomic ratio of what it is made up of e.g.
    • methane CH4 is composed of 1 carbon atom combined with 4 hydrogen atoms. Glucose has 6 carbon : 12 hydrogen : 6 oxygen atoms, sodium chloride is 1 sodium : 1 chlorine atom.
  • Sometimes, a compound (usually ionic), is partly made up of two or more identical groups of atoms. To show this more accurately ( ) are used e.g.
    • Calcium hydroxide is Ca(OH)2 which makes more sense than CaO2H2 because the OH group of atoms is called hydroxide and exists in its own right in the compound.
      • Make sure you understand the use of (brackets). The subscripted 2 after the brackets doubles everything in the brackets BUT nothing else in the formula.
    • Similarly, aluminium sulphate has the formula
      • Al2(SO4)3 rather than Al2S3O12, because it consists of two aluminium ions Al3+  and three sulphate ions SO42-. The sulphate ion is effectively a molecule that carries an overall surplus electrical charge - a 'molecular ion' if you like.
  • The word formula or molecule can also apply to elements. e.g. hydrogen molecule H2, oxygen molecule O2, ozone molecule O3 (2nd unstable form of oxygen), phosphorus molecule P4, sulphur molecule S8, have respectively 2, 2, 3, 4 and 8 atoms in their molecules. Elements like helium He are referred to as 'monatomic' because they exist as single uncombined atoms.
    • Incidentally, at GCSE level, and mainly at Advanced level too, phosphorus and sulfur are written as P and S respectively. However, in equations
  • There are more examples and comments in equation section.
  • Calculations involving empirical formula and molecular formula are dealt with in sections 5. and 8. on the calculations page.
  • topSee also picture diagrams of elements/compounds and mixtures.


compared to a pure substance described below




  • PURE means that only one substance is present in the material and can be a pure element or pure compound, so by definition cannot be mixed with any other substance.
    • Anything else is a mixture - described above!
  • A simple physical test for purity, and properties that can help identify a substance, is to measure the boiling point or melting point.
  • Every pure substance melts and boils at a specific fixed temperature (though boiling point depends on the ambient air pressure).
  • Divergence from the expected melting point or boiling point can be used to indicate whether a substance is impure.
  • If a liquid is pure it should boil at a constant temperature called the boiling point e.g. water boils at 100oC. Unfortunately, up on a very high mountain, at a lower air pressure, water boils at a constant, but lower temperature and it is difficult to make a good brew of tea!
    • An impure liquid will boil at a higher temperature if it contains a dissolved solid impurity e.g. seawater, containing dissolved salts, boils at over 100oC.
    • An impure liquid can initially boil at a lower than the expected temperature, if it contains a lower boiling point liquid impurity. The boiling then takes place over a range of temperatures. For example, in the distillation of an alcohol-water mixture from a fermented yeast-sugar solution mixture, it boils away within a range starting at about 79oC (boiling point of alcohol) and the last drops distil over at 100oC (boiling point of pure water).
  • If a solid is pure, it melts sharply at its fixed melting point.
    • An impure solid melts below its expected melting point and over a range rather than at one sharp temperature.
    • The more impure the solid, the wider the temperature melting range.
    • tope.g. a water and salt mixture melts below 0oC and butter, a mixture of fats, gradually melts more as the temperature rises on a hot summer's day.
  • If a solid substance is coloured like a dye or a plant extract material, paper chromatography can show whether there is more than one substance present in a material. If there is one spot or peak, then its probably pure, but more than one spot or peak indicates a mixture of at least two substances.
  • Liquids and gases can also be tested for purity using the technique of gas-liquid chromatography, a powerful instrumental method of analysis.
  • Chemical analysis can provide an accurate quantitative measure of how pure a compound is, see ...
  • UNFORTUNATELY, in everyday language, a pure substance can mean a substance that has had nothing added to it, so it is 'unadulterated' and in its natural state, eg pure milk, but in chemistry lessons and chemistry exams take care you stick with the scientific definition of pure!


  • IMPURE usually means a mixture of mainly one substance plus one or more other substances physically mixed in.
  • Some examples are mentioned above, in the discussion on the effect of impurities on the melting/boiling points of pure substances.
  • The % purity of a compound is important, particularly in drug manufacture. Any impurities present are less cost-effective to the consumer and they may be harmful substances.
  • Mixtures of volatile liquids and gases can be analysed using the technique of gas-liquid chromatography, a powerful instrumental method of analysis.



  • PURIFICATION: Materials are purified by various separation techniques.
  • The idea is to separate the desired material in as pure a form as possible from unwanted material or impurities, hence to produce the desired useful product or just a substance whose physical and chemical properties are to be investigated as part of a research project.
  • Detailed examples of methods-examples of separating mixtures are described in later sections on this page.
  • but they include:
    • Filtration to separate a solid from a liquid. You may want the solid or the liquid or both!
    • Simple distillation to separate a pure liquid from dissolved solid impurities which have a very high boiling point.
    • Fractional distillation to separate liquids with a range of different boiling points, especially if relatively close together.
    • Evaporation to remove a solvent to leave a solid behind.
    • Crystallisation to get a pure solid out of a solvent solution of it.
    • Chromatography can be used on a larger scale than spots' to separate out pure samples from a mixture.
  • Methods of collecting gases are on a separate web page.



  • CHEMICAL EQUATION is an expression in words or symbols that describes precisely a particular chemical change

  • eg the reaction in water between the alkali sodium hydroxide and the acid hydrochloric acid

  • sodium hydroxide + hydrochloric acid ==> sodium chloride + water  (word equation)

  • NaOH + HCl ==> NaCl + H2O  (symbol equation)

  • NaOH(aq) + HCl(aq) ==> NaCl(aq) + H2O(l)   (symbol equation with state symbols)

  • Detailed examples on How to write word & symbol equations


Section 1.2 Particle Picture examples of Elements, Compounds and Mixtures - useful visual images

The particle model for gases, liquids and solids is fully described and discussed on the States of Matter page

Particle model diagrams of gases, liquids, solids, elements, compounds, mixtures

Detailed notes on the physical 'States of Matter' - gases, liquids and solids - structure and properties


what a state I'm in! Section 1.3  PHYSICAL CHANGES - no new substance formed

These are changes which do not lead to new substances being formed. Only the physical state of the material changes. The substance retains exactly the same chemical composition. Examples ...

Melting/fusing, solid to liquid, easily reversed by cooling e.g. ice and liquid water are still the same H2O molecules.

Dissolving, e.g. solid mixes completely with a liquid to form a solution, easily reversed by evaporating the liquid e.g. dissolving salt in water, on evaporation the original salt is regained.

So freezing, evaporating, boiling, condensing are all physical changes and may be involved in separating a mixture.

Separating a physical mixture e.g. chromatography, e.g. a coloured dye solution is easily separated on paper using a solvent, they can all be re-dissolved and mixed to form the original dye.

Distillation, filtering are also physical changes.

See also '3 States of Matter' - gases, liquids and solids for more examples and particle theory models to explain state changes and the properties of gases, liquids and solids.


Section 1.4 CHEMICAL CHANGES - REACTIONS - reactants and products

The reaction of iron and sulphur/sulfur to make iron sulphide/sulfide (c) doc b

Heating iron and sulphur is classic chemistry experiment to illustrate what is meant by CHEMICAL CHANGE and you can adapt the general conclusions described at the end of this section to any chemical reaction.

A mixture of silvery grey iron filings and yellow sulphur powder is made.

The iron can be plucked out with a magnet i.e. an easily achieved physical separation because the iron and sulphur are not chemically combined yet!

They are still the same iron and sulphur.

However, on heating the mixture, it eventually glows red on its own and a dark grey solid called iron sulphide is formed. Both observations indicate a chemical change is happening i.e. a new substance is being formed.

We no longer have iron or sulphur BUT a new compound with different physical properties (e.g. colour) and chemical properties (unlike iron which forms hydrogen with acids, iron sulphide forms toxic nasty smelling hydrogen sulphide!).

word equation: iron + sulphur (sulfur) ==> iron sulphide (iron sulfide)

or in a symbol chemical equation: Fe + S ==> FeS

(c) doc b

AND it is no longer possible to separate the iron from the sulphur using a magnet!

Note that chemical changes can be expressed in a chemical equation.

In chemical reactions, the atoms cannot be made or destroyed, but atoms rearrange themselves in changing from reactants to new products. There is no loss or gain of mass - see 'Law of Conservation of Mass'

Further proof of a new substance formed: The original reactant iron, and the iron sulphide product, can be shown to be different substances by their reactions with dilute acid.

The products of a reaction are completely different from the reactants, at least one different substance is formed in ANY chemical reaction.

This is illustrated by their different physical properties and different chemical reactions.

There is also usually a detectable energy change, often, but not always, energy is released producing a temperature rise in the system.

topReactant: The silvery grey metallic iron forms a pale green solution of the salt iron sulphate with dilute sulphuric acid and evolves odourless hydrogen gas which gives a squeaky pop with a lit splint. The word and symbol equations are as follows ...

iron + sulphuric acid ==> iron sulphate + hydrogen

Fe + H2SO4 ==> FeSO4 + H2

Reactant: Yellow, solid non-metallic sulfur burns in air to form the choking acidic gas sulfur dioxide.

sulfur + oxygen ==> sulfur dioxide

S + O2 ==> SO2

Iron sulphide also fizzes and dissolves in dilute sulphuric acid to form iron sulphate BUT produces the 'rotten eggs' smelly gas hydrogen sulphide which gives a black colour with lead ethanoate paper (old name lead acetate).

iron sulphide + sulphuric acid ==> iron sulphate + hydrogen sulphide

FeS + H2SO4 ==> FeSO4 + H2S

This is NOT to be done by students out in the laboratory, hydrogen sulphide gas is highly poisonous!

If hydrochloric acid is used, the same two colourless gases are produced but the salt formed would be iron chloride.

So signs that a chemical reaction has happened include:

change in appearance e.g. change in colour or texture.

temperature changes because an energy change has taken place,

topchange in mass e.g.

some solids when burned in air gain mass in forming the oxide e.g. magnesium forms magnesium oxide.

some solids lose mass when heated, e.g. carbonates lose carbon dioxide in thermal decomposition.

and change in the chemical properties of the products compared to the original reactants.

Therefore a chemical change is one in which a new substance is formed, by a process which is not easily reversed and usually accompanied by an energy (temperature) change.

This is summarised as reactants ==> products as expressed in chemical equations in words or symbols.

It doesn't matter how complex the reaction, the atoms you end up with are just the same as the ones you started with in terms of elements and number of atoms, BUT they have rearranged themselves into different substances with different properties e.g.

(c) doc b

copper carbonate + sulfuric acid ==> copper sulfate + water + carbon dioxide

Check out the above reaction 12 atoms, 1Cu, 7O, 1C, 2H, 1S, and five completely different substances, all with different physical and chemical properties, definitely a big chemical change!

Apart from experiments and preparations in the laboratory, plenty of chemical changes occur in the home. For a start, you are an extremely complex chemical structure with lots of reactions going on in your body all the time, but others in the home include ...

Cooking involves both physical and chemical changes, e.g. meat and potato change in both taste and texture and breakdown chemically to some extent, baking powder breaks down to release carbon dioxide gas which gives the 'rising action' in the production of cakes etc..

Acidic reagents dissolve limescale in the toilet.

Candles burning at birthdays and Christmas and gas fire also involve combustion of hydrocarbons like methane.

More advanced ideas [see GCSE notes on atomic structure and chemical bonding]: Atoms are held together in molecules or compounds by electrical forces of attraction between the positive nucleus and the outer negative electrons. Therefore, Atoms, ions or molecules react with each other to become electronically more stable. When chemical reactions occur chemical bonds are broken in the reactants and new bonds made in the formation of the products.


See other web page for:

(c) doc b KS3 Science GCSE/IGCSE Chemistry States of Matter - kinetic particle theory of Gases, Liquids and Solids revision notes

(c) doc b KS3-GCSE/IGCSE Types of Chemical Reaction revision notes

(c) doc b GCSE/IGCSE Chemical Bonding revision notes (ionic, covalent, metallic etc.)


GCSE balancing and completing equation exercises:

(1) multiple choice * (2) number/word multi-fill

GCSE 'name and formula' of a compound quizzes

(1) pick the name given the formula * (2) pick the formula given the chemical name

GCSE/IGCSE formula quiz given the name, type in the formula

GCSE/IGCSE name quiz given the formula, type in the name

KS3 Science-GCSE/IGCSE Elements, Compounds & Mixtures m/c QUIZ

KS3 Science-GCSE/IGCSE element name/symbol quiz

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