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School Chemistry Notes: Important chemistry definitions explained with examples

Scroll down for lots of basic, but important, chemistry definitions and concepts

(Suitable for AQA, Edexcel and OCR GCSE chemistry students)


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

What next? Associated Pages

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

USE THIS Alphabetical list of KEYWORDS for Parts 1-3: CLICK for definitions - meaning - examples explained etc.  (work your way down the section carefully!)

atom  *  balancing equations  *  centrifuges/centrifuging  * chemical bond  * chemical equation  *  chemical reaction/change  *  chromatography (paper/thin layer)  *  compound  *  covalencycrystallisation  *  decanting/decantation  *  displayed formula  *  distillation (simple and fractional)  *  element  *  equations  *  evaporation  *  filtration  * formula  * formulation * gas chromatography  *  impure substance  *  insoluble  *  ion  * ionic equations  *  ionic valence  *   iron-sulfur separation and heating experiment  *  magnet  *  melting point and boiling point  mixture  *  moleculenaming compounds (basics)  *  naming compounds including ions  *  paper chromatography  *  particle pictures of elements/compounds/mixtures  *  physical change  *  precipitation  *  products  *  pure substance  *  purification of a substancereactants  *  sand/salt separation  *  separating funnel  *  separating mixtures  *  soluble/solution/solvent/solute  *  solvent extraction  *  symbols (for elements, formula, in equations)  *   state symbols  * working out formulae  *

Section 1.1 Introduction and Some keywords (see also pictures)

Many of these terms overlap with each other, so there is some repetition - no harm in that?



a picture of an atom

Why can't we trust atoms? Because they make up everything!

(i) Even as far back as ancient Greece ~500 BC 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, namely an element, which can have its own characteristic properties AND cannot be split into simpler substances by chemical means.

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

This also means it is the smallest part of an element that can take part in a chemical reaction.

(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, usually involving at least two elements and often many more (see chemical change on this page).

The mass of atoms varies from about 1 x 10-23 to 1 x 10-21 g

The diameter of atoms varies from about 1 x 10-10 to 5 x 10-10 m (0.1 to 0.5 nm)



ELEMENTs 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) A basic definition of an element
  • An element consist of one type of atom only and all the atoms of an element  have the same atomic number (number of protons in nucleus).
  • Therefore, elements are the simplest substances that we can use and investigate in chemistry because an element cannot be split into other substances (unlike compounds).
  • Since an ELEMENT is by definition, a pure substance made up of only one type of atom, unlike a compound, it cannot be split into simpler substances.
    • 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 sulfur S8. (more examples applied to equations and see note about 'formula of elements')
  • Particle model pictures of elements - one type of atom only
  • a particle picture model of gaseous helium atoms
  • a particle picture model of gaseous O2 oxygen molecules
  • a particle picture model of liquid bromine molecules
  • a particle picture model of solid iron atoms

(ii) Extended ideas on elements

  • *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 - part of which is shown below.
  • All of these points are discussed in detail on the GCSE Periodic Table summary notes page. e.g. explaining what groups, periods and series of elements are.

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


Note the diagonal zig-zag line from B-Al to At-Ts represents the approximate division between the majority metals on the left of the periodic table and non-metals on the right. BUT, silicon Si, germanium Ge, arsenic As, antimony Sb, tellurium Te are described as 'metalloids' because they have a mixture of metallic and non-metallic properties.


MOLECULES and FORMULAE and their representations


model of a molecule


and a


shows which atoms are in a compound and how many of each element are present in the molecule

A formula can summarise the atoms present

e.g. C2H6

or show how the atoms are connected by their chemical bonds e.g. alkanes structure and naming (c) doc b




and short note on


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

The atoms can be the same element (still the element) or different elements (a compound).

Any molecule must have a chemical FORMULA - whether it is an element or a compound.

The molecule may be made up of atoms of a single element e.g. hydrogen formula H2 (H-H, two atoms combined, all atoms the same). Other molecular examples are oxygen O2, nitrogen N2, one form of phosphorus is P4 and one form of sulfur consists of S8 ring molecules, and another form of carbon is a C60  molecule!

OR if two different symbols appear in the formula, it must denote a compound and it must have a formula!

A compound (a chemical combination of two or more different elements and NOT a mixture - 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.

the fully displayed structural formula of ethanol

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

CH3CH2OH for the structural formula and the even shorter C2H5OH, (9 atoms in molecule) but using these styles requires much more experience than is required when first learning the basic principles of chemistry.

You can write the molecular formula as C2H6O, which summarises the number of atoms of each element in the molecule, BUT, it tells you nothing about the molecule's structure i.e. in what way the 9 atoms are joined together.

Note that

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 the 8atoms of the ethane molecule, which shows all the atoms and individual bonds (carbon-hydrogen and carbon-carbon in this case). 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

Above is an example of a 'ball-and-stick' diagram and shows the chemical bonds which hold the atoms together, but it doesn't indicate the relative size of the atoms. However, it does give a 3D impression of the spatial positions of the centres of the atoms. Above is an example of a 'space-filling' diagram which gives a more accurate representation of the space the molecule actually occupies by using the real relative size of the atoms - most f the occupied space is where the electrons are whizzing around!.

Watch out for some styles of abbreviated formula

e.g. pentane can be written as CH3CH2CH2CH2CH3,

but, using brackets it can be written as CH3(CH2)3CH3

This abbreviated style is particularly useful for long molecules like decane  CH3(CH2)8CH3

A shorthand way of expressing the 10 carbon atoms and 22 hydrogen atoms of decane!

Some more examples of molecular formulae

You should be able to recognise which are elements and which are compounds.

1. HI hydrogen iodide,  2. CO2 carbon dioxide,  3 He helium atoms, 4. H2O water,  5. H2 hydrogen

6. P4 phosphorus, 7. CH4 methane,  8. NOCl molecule,  9. H2SO4 sulfuric acid

Nanoparticles are bigger than the simple molecules described above. They are typically composed of ~100 atoms and range from 1 nm to 100 nm in size.

See nanochemistry notes

Particle size comparison:

nuclear radius < atomic radius < simple 'small' molecule < nanoparticle < polymer molecules




  • 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
  • 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.
  • Detailed GCSE bonding notes and examples for ionic, covalent and metallic bonds and includes a simplified introduction to chemical bonding
  • Chemical bonds are about the same length as the diameter of atoms ...
    • ... 1 x 10-10 to 5 x 10-10 m (0.1 to 0.5 nm)





  • 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.
  • The formation of a compound e.g.
  • When heated together, atoms of the elements iron and sulfur combine to form iron sulfide
  • (c) doc b  or  Fe + S  ==> FeS
  • See also how to write and work out equations
  • A note on names
    • In most cases the metallic element retains its elemental name in the compound.
    • However, this is not the case with non-metallic elements.
    • When the following non-metals combine with other different elements its name changes e.g.
    • the element oxygen forms an oxide compound e.g. magnesium oxide,
    • the element sulfur forms a sulfide compound e.g. iron sulfide,
    • the element chlorine forms a chloride compound e.g. sodium chloride,
    • the element bromine forms a bromide compound e.g. calcium bromide,
    • the element iodine forms an iodide compound e.g. potassium iodide.
    • There are examples where a non-metal retains its element name e.g.
      • sulfur dioxide, carbon dioxide
  • Particle model pictures of compounds - consisting of two or more types of atom (elements)
  • a particle model picture of gaseous water (vapour or steam)
  • a particle model picture of liquid water molecules H2O
  • a particle model picture of gaseous hydrogen chloride HCl
  • a particle model picture of solid hydrogen chloride
  • 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. if 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. 14 atoms in total
    • 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.
    • See on another page working out formulae 



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 sulfate has the formula
      • Al2(SO4)3 rather than Al2S3O12, because it consists of two aluminium ions Al3+  and three sulfate ions SO42-. The sulfate 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, sulfur 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.
  • See also picture diagrams of elements/compounds and mixtures.


A note on the names of chemicals Notes on the names of compounds

In most cases the metallic element retains its elemental name in the compound.

However, this is not the case with non-metallic elements.

When the following non-metals combine with other different elements its name changes e.g.

the element oxygen forms an oxide compound e.g. magnesium oxide,

the element sulfur forms a sulfide compound e.g. iron sulfide,

the element chlorine forms a chloride compound e.g. sodium chloride,

the element bromine forms a bromide compound e.g. calcium bromide,

the element iodine forms an iodide compound e.g. potassium iodide.

There are examples where a non-metal retains its element name e.g.

sulfur dioxide, carbon dioxide

You might also come across the following 'parts' of names ..

mono meaning one e.g. carbon monoxide (CO, one oxygen)

di meaning two e.g. carbon dioxide (CO2, two oxygen atoms), sulfur dioxide (SO2, two oxygen atoms)

oxygen can be called dioxygen O2 molecules

tri meaning three e.g. phosphorus trichloride (PCl3, three chlorine atoms), sulfur trioxide (SO3, 3 O atoms)

ozone O3 can be formally called trioxygen

You can get 'mixed' numerical names e.g. Fe3O4 can be called triiron tetroxide !!

There is another context in which mono..., di... and tri... etc. are used.

It is used as a prefix to indicate the number of atoms in a molecule irrespective of the name.

The noble gases exist as single atoms e.g. He, Ar, Ne etc. are sometimes referred to as monatomic molecules - but at this level best to think of them as elements consisting of single atoms.

Diatomic molecules consist of two atoms e.g. H2, HCl, O2, N2, CO

Triatomic molecules consist of three atoms e.g.  O3, H2O, CO2

 and note in both cases they can be elements or compounds.




A chemical change can be represented by a chemical equation, either in words or chemical symbols e.g.

word equation: sodium hydroxide + hydrochloric acid ==> sodium chloride + water

picture equation: (c) doc b

symbol equation: NaOH + HCl ==> NaCl + H2O

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

The chemicals on the left-hand side of the equation are called the REACTANTS.

The chemicals on the right-hand side of the equation are called the PRODUCTS.

To summarise the essence of chemistry is:

to change stuff called reactants into stuff called products!

For lots more examples and how to balance equations see HOW TO WRITE CHEMICAL EQUATIONS with lots of worked examples explained in detail.


1.1h (1)


compared to a pure substance described below

  • A MIXTURE is a material made up of at least two substances which may be elements or compounds physically mixed together but are NOT chemically combined together, so the elements and compounds retain their original chemical properties.
  • Note: Although the above definition of a mixture applies to most mixtures, there are examples of materials described as 'mixtures' where there is chemical bonding between the components of the mixture e.g. alloys are described as mixtures of metals. Although alloys don't have a definite chemical formula, there is strong metallic bonding between all metals atoms and as a consequence it is very difficult to separate out the different metals in an alloy by physical means!
  • Particle model pictures of mixtures
  • particle model picture of a gaseous mixture e.g. nitrogen and water vapour
  • particle model of a liquid mixture e.g. bromine dissolved in water
  • particle model picture of a solid mixture of two substances
  • Homogeneous mixtures
    • A homogeneous mixture (of at least two substances) is a gaseous, liquid or solid mixture that has the same proportions (ratios) of its components throughout a given sample of the material. A homogeneous mixture is uniform in composition throughout the whole sample.
    • Typical examples are mixtures of gases, miscible liquids and solutions.
    • e.g.   and 
    • Air and salt solutions are homogeneous mixtures.
    • Two miscible liquids e.g. ethanol (alcohol) dissolves in water to make a clear homogeneous mixture.
    • Petrol is a homogeneous mixture of several hydrocarbon liquids.
  • Heterogeneous mixtures
    • A heterogeneous mixture (of at least two substances) is a mixture in which the composition is not uniform throughout the whole sample of the material mixture.
    • e.g. a mixture of several solid substances like soil.
    • A mixture of two immiscible liquids e.g. oil and vinegar in salad dressing, would be an example of a heterogeneous mixture no matter how much you shake before use.


1.1h (2)



compared to a pure substance described below or a mixture described above

Formulations are useful mixtures made up of measured out precise amounts (precise ratio) of two or more substances.

They are special mixtures designed for some particular purpose.

Each component in the mixture contributes to the properties of the formulation.

Commercially, they are made up from recipes of tried and tested formulations for that particular purpose.

If you look around the house, especially in the kitchen and bathroom, you will lists of ingredients on the side of the bottle or package of products - percentages or relative amounts of the components in the formulation.

See also lots of detailed descriptions of examples of formulations




and a note on

melting point


boiling point

  • PURE means that only one substance is present in the material and can be a pure element or pure compound AND nothing else, so by definition is NOT mixed with any other substance.
    • By definition, it means anything else is a mixture - described above!
    • See below under IMPURE for particle pictures of pure and impure substances.
    • Don't confuse the use of the term pure, as used in chemistry, with its everyday use to imply a material in its natural produced state e.g. pure butter, pure flour or pure milk - which are obviously mixtures!
  • A simple physical test for purity, and properties that can help identify a substance, is to measure its boiling point or melting point.
    • You can compare the measured melting point or boiling point of a substance and compare the observed values with those listed in a standard data table.
    • If the observed and listed values do not match, the substance is impure.
  • 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, usually around 102oC.
    • 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 - butter is a good example.
    • The more impure the solid, the wider the temperature melting range.
    • e.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.
    • The closer the melting point or boiling point is to the sharp values of the pure substance, the purer the sample.
    • These are important criteria to bear in mind if the material is part of a formulation for some specific purpose.
    • You need special apparatus to determine a melting point safely and accurately.
    • How to determine a melting point is described on an advanced chemistry page
  • 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 gas, liquid or solid mixture of mainly one substance plus one or more other substances physically mixed in.
    • The term often implies something you might not want in the material.
    • Impure substances have lower and less sharp melting points i.e. the melting range is a few degrees and lower than the specific higher melting point of the pure substance.
  • 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.
  • Mixtures of coloured solids can be investigated using paper chromatography.
  • (c) doc b a particle picture of a pure liquid
  • (c) doc b) a particle picture of an impure liquid, effectively a dilute solution.
  • particle picture of a pure solid crystal
  • a particle picture of a slightly impure solid crystal




  • 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.



An ion is an atom or a group atoms that carries an overall positive or negative electric charge.

In other words they are NOT a neutral particle like a molecule of water H2O or methane CH4.

Examples of positive ions: sodium ion Na+, ammonium ion NH4+, aluminium ion Al3+

Examples of negative ions: chloride ion Cl-, sulfate ion SO42-

The charge is notated as a superscript at the top right of the chemical symbol.

Apart from a charge of single +/- a number must be used to indicate the overall charge e.g. 2+, 2-, 3+ 3-

For ionic equations see a section HOW TO WRITE CHEMICAL EQUATIONS

Easy revision quizzes on some basic chemistry concepts

4 linked easy quizzes on "Separation of Mixtures" 7Hwf1 * 7Hwf2 * 7Hwf3 * 7Hwf4

Easy matching pair quiz based on "Solubility and Solutions" 7Hmp1 (important words and definitions)

4 Easy linked word-fill quizzes on "Atoms, Elements and Compounds" 8Ewf1 * 8Ewf2 * 8Ewf3 * 8Ewf4

5 Easy linked word-fill quizzes on "Compounds and Mixtures" 8Fwf1 * 8Fwf2 * 8Fwf3 * 8Fwf4 * 8Fwf5

Easy matching pair quiz on "Elements, Compounds and Mixtures" ecm1mp (matching particle model pictures)


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 SAME material substance changes. The substance retains exactly the same chemical composition - the same atoms, ions or molecules. 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 - CHEMICAL REACTIONS - reactants and products

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

A 'chemical change' involves the formation of at least one NEW substance AND there is always an energy change - which might, in some circumstances, be detectable with a thermometer. There are often colour changes and the products may look quite different from the reactants

A chemical change involves: STARTER REACTANTS  === energy change ===> NEW PRODUCTS

Heating iron and sulfur 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 sulfur powder is made.

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

They are still the same iron and sulfur e.g. even in the mixture iron will dissolve in acids to form a salt and hydrogen, sulfur will burn to form sulfur dioxide gas.

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

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

word equation: iron + sulfur (sulfur) ==> iron sulfide (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 sulfur 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 sulfide 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 NEW 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.

The iron reactant before reaction with sulfur: The silvery grey metallic iron forms a pale green solution of the salt iron sulfate with dilute sulfuric acid and evolves odourless hydrogen gas which gives a squeaky pop with a lit splint. The word and symbol equations are as follows ...

iron + sulfuric acid ==> iron sulfate + hydrogen

Fe + H2SO4 ==> FeSO4 + H2

The sulfur reactant before reaction with iron: Yellow, solid non-metallic sulfur burns in air to form the choking acidic gas sulfur dioxide.

sulfur + oxygen ==> sulfur dioxide

S + O2 ==> SO2

The iron sulfide product formed after the reaction has taken place: Iron sulfide also fizzes and dissolves in dilute sulfuric acid to form iron sulfate BUT produces the 'rotten eggs' smelly gas hydrogen sulfide which gives a black colour with lead ethanoate paper (old name lead acetate). This shows the product was no longer iron.

iron sulfide + sulfuric acid ==> iron sulfate + hydrogen sulfide

FeS + H2SO4 ==> FeSO4 + H2S

This is NOT to be done by students out in the laboratory, hydrogen sulfide 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,

change 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.


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