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?

1.1a

ATOMS

(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)

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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ELEMENTs and symbols

H I Th Er Ho W Ar U?

Element Symbol-name quizzes: easier-pictorial!

or harder-no pictures!

Metals and non-metals

• 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 sulphur 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

• *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!

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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MOLECULES and FORMULAE and their representations

and a

FORMULA

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.

and short note on

Nanoparticles

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 molecules, and another form of carbon is a C₆₀  molecule!

OR

A compound (a combination of two or more different elements - 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, (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, BUT, it tells you nothing about the molecule's structure i.e. in what way the 9 atoms are joined together.

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.

On 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.
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 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!

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

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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CHEMICAL BOND

• 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)

COMPOUNDS,

FORMULA

and

MOLECULES

• 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
•   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 

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century chemistry examinations.

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More on formulae and COMPOUNDS

methane CH₄

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: FeO,  Fe₂O₃  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 SO₄^2-. 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.
• See also picture diagrams of elements/compounds and mixtures.

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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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.

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CHEMICAL EQUATION

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

picture equation:

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.

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MIXTURE

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.
  • Each component of a mixture retains its individual chemical properties, this can be important when considering formulations.
  • In other words the properties of a mixture are a mix of the properties of the substance in the mixture!
  • Sand and salt solution is a mixture.
  • Crude oil is a mixture of hydrocarbon molecules of different length.
  • Iron filings with sulfur powder is another mixture and they both retain their physical or chemical properties.
  • Alloys are described as mixtures of metals, BUT they are not easily separated because there is actually strong chemical bonding between the different metal atoms.
  • Components of a mixture can usually be separated by physical means e.g. physical processes like filtration, crystallisation, simple distillation, fractional distillation, chromatography etc.
  • Detailed notes on separation methods are described when needed to separate useful materials and purify them eg

    Distillation - Simple and Fractional Distillation 

    Paper & thin layer chromatography (tlc) and gas chromatography (gc, glc)

    Filtration, evaporation, crystallisation, drying and decantation

    Separating funnel, magnet, solvent extraction, centrifuge

  • Examples of mixtures: air, soil, all solutions, all sorts of products in the home (*) both solid (eg baking powder) or liquids (eg milk) etc.
  • See also picture diagrams of elements/compounds and mixtures.
  • * and Formulations
• 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

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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FORMULATION

MIXTURES

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

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

A PURE

SUBSTANCE

and a note on

melting point

and

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 100^oC. 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 100^oC, usually around 102^oC.
  • 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 79^oC (boiling point of alcohol) and the last drops distil over at 100^oC (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 0^oC 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.
• 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 ...
  • Volumetric titration analysis calculations, acid-alkali titrations
  • Instrumental Methods of Chemical Analysis
  • Explaining and calculating % purity of a product
  • ... but these are more advanced pages for GCSE students onwards ...
• 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!

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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IMPURE

• 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.
• a particle picture of a pure liquid
• 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

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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1.1k

PURIFICATION

• 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.

1.1l

IONS

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 SO₄^2-

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 1.3  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.

Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.

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