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2.1.1 Elements, compounds and mixtures

Elements

Definition

Atom

The smallest particle of an element that can take part in a chemical reaction.

Definition

Element

A substance made of only one type of atom, which cannot be broken down into simpler substances by chemical reactions.

  1. All matter is built from extremely small particles called atoms.
  2. An element is a substance that contains only one type of atom, so every atom in a pure sample of copper is a copper atom.
  3. An element cannot be broken down into anything simpler by a chemical reaction, because it already contains only one type of atom.
  4. Every known element is listed in the Periodic Table, and each one is represented by a chemical symbol of one or two letters.
    1. Carbon is C\text{C}C, oxygen is O\text{O}O and sodium is Na\text{Na}Na.
    2. The first letter of a symbol is always a capital and any second letter is always lower case, so Co\text{Co}Co is the element cobalt while CO\text{CO}CO is carbon monoxide.
  5. Some elements exist as single atoms, such as helium, He\text{He}He, and argon, Ar\text{Ar}Ar.
  6. Other elements exist as small groups of identical atoms joined together, such as oxygen gas, O2\text{O}_2O2​, and chlorine gas, Cl2\text{Cl}_2Cl2​.
  7. A sample of O2\text{O}_2O2​ is still an element, because both atoms in each particle are oxygen atoms.

Compounds

Definition

Compound

A substance made of two or more different elements chemically combined in a fixed ratio.

  1. A compound forms when atoms of two or more different elements join together in a chemical reaction.
  2. The atoms in a compound are held together by chemical bonds, so the elements are chemically combined rather than simply sitting side by side.
  3. The elements in a compound are always present in a fixed ratio, which is shown by its formula.
    1. Every sample of water, H2O\text{H}_2\text{O}H2​O, contains two hydrogen atoms for every one oxygen atom.
    2. Every sample of carbon dioxide, CO2\text{CO}_2CO2​, contains one carbon atom for every two oxygen atoms.
  4. Because the ratio is fixed, a compound always has the same composition no matter where it comes from or how it was made.
  5. A compound has different properties from the elements that formed it, because the atoms are now bonded in a new arrangement.
    1. Sodium is a soft, highly reactive metal and chlorine is a toxic green gas, yet sodium chloride, NaCl\text{NaCl}NaCl, is the white crystalline solid sprinkled on chips.
    2. Hydrogen is a flammable gas and oxygen is a gas that helps things burn, yet water, H2O\text{H}_2\text{O}H2​O, is a liquid that is used to put fires out.
  6. A compound can only be split back into its elements by another chemical reaction, such as passing an electric current through it or heating it strongly.

Mixtures

Definition

Mixture

Two or more substances that are present together but are not chemically combined.

  1. A mixture contains two or more substances that are mixed together but are not chemically combined.
  2. The substances in a mixture can be elements, compounds or both.
    1. Air is a mixture of the elements nitrogen, oxygen and argon together with the compound carbon dioxide.
    2. Seawater is a mixture of the compound water with dissolved compounds such as sodium chloride.
  3. The proportions of the substances in a mixture can vary, so a cup of tea can be made weaker or stronger and is still a mixture of the same substances.
  4. Each substance in a mixture keeps its own properties, because no chemical bonds have formed between the different substances.
  5. A mixture can therefore be separated by physical methods, such as using a magnet, filtration or distillation, without any chemical reaction.
  6. No new substance forms when a mixture is made, so there is usually no colour change, no gas given off and little or no temperature change.

Comparing the three types of substance

  1. Particles: an element contains one type of atom, a compound contains atoms of different elements bonded together, and a mixture contains different particles that are not bonded to each other.
  2. Composition: the ratio of elements in a compound is fixed, while the proportions of substances in a mixture can change.
  3. Properties: a compound has new properties that are different from those of its elements, while a mixture shows the properties of the substances in it.
  4. Separation: an element cannot be broken down by chemical reactions, a compound needs a chemical reaction to split it into its elements, and a mixture can be separated by physical methods.
  5. To classify a sample, first check whether it contains only one type of atom, because if it does the sample is an element.
  6. Next check whether different elements are bonded together in a fixed ratio, because if they are the sample is a compound.
  7. Finally check whether two or more substances are simply mixed with no bonding between them, because if so the sample is a mixture.
Example

Classifying particle diagrams

  • A box of identical, separate single circles shows an element made of single atoms, such as argon.
  • A box of identical pairs, where both circles in each pair are the same colour, shows an element made of small groups of atoms, such as nitrogen, N2\text{N}_2N2​.
  • A box of identical groups, each made of one large circle bonded to two small circles of a different colour, shows a compound, such as water, H2O\text{H}_2\text{O}H2​O.
  • A box containing two or more different kinds of particle that are not bonded to each other shows a mixture, such as air.
Practical

Iron and sulfur as a mixture and a compound

  • Aim: to show that iron and sulfur keep their own properties when they are mixed, but form a new substance with different properties when they react.
  • Apparatus: iron filings, powdered sulfur, a balance, a spatula, a sheet of paper for mixing, a bar magnet wrapped in cling film, a watch glass, a hard-glass test tube, mineral wool, a clamp and stand, a Bunsen burner, a heat-proof mat, a fume cupboard and eye protection.
  • Method:
    • Weigh out about 3.5 g3.5\ \text{g}3.5 g of iron filings and 2.0 g2.0\ \text{g}2.0 g of powdered sulfur, so that the two elements are in the correct ratio to react completely.
    • Mix the two powders thoroughly on the sheet of paper until the mixture looks an even grey-yellow colour.
    • Spread a small portion of the mixture on the watch glass and pass the wrapped magnet just above it, noting which particles are pulled out.
    • Place the rest of the mixture in the hard-glass test tube to a depth of about 2 cm2\ \text{cm}2 cm and loosely plug the mouth of the tube with mineral wool.
    • Clamp the tube at an angle inside the fume cupboard and heat the bottom of the mixture strongly with a blue Bunsen flame.
    • Remove the flame as soon as the mixture starts to glow red, then watch the glow spread through the rest of the powder on its own.
    • Leave the tube to cool completely, then your teacher wraps it in a cloth and breaks it so that the solid product can be removed.
    • Hold the wrapped magnet over the black product and compare its behaviour with that of the mixture.
  • Results: the magnet pulls the grey iron filings out of the mixture and leaves the yellow sulfur behind, but the black solid formed on heating is not attracted to the magnet.
  • Conclusion: in the mixture the iron and sulfur keep their own properties and can be separated physically, while heating produces iron(II) sulfide, Fe(s)+S(s)→FeS(s)\text{Fe}(\text{s})+\text{S}(\text{s})\rightarrow\text{FeS}(\text{s})Fe(s)+S(s)→FeS(s), a compound with new properties that a magnet cannot separate.
  • Watch out: if the iron is in excess, some unreacted iron filings remain in the product and are still attracted to the magnet.
  • Safety: heat only in a fume cupboard because some sulfur may burn to form toxic sulfur dioxide, wear eye protection, and never add acid to the product because it releases toxic hydrogen sulfide gas.
Common Mistake
  • Do not call water a mixture of hydrogen and oxygen, because in H2O\text{H}_2\text{O}H2​O the hydrogen and oxygen atoms are chemically combined in a fixed ratio.
  • Do not call air a compound, because its gases are only mixed together and their proportions can change.
  • Do not call oxygen gas, O2\text{O}_2O2​, a compound just because each particle contains two atoms, because both atoms belong to the same element.
Exam technique

Describing the differences

  • When you describe how a compound differs from a mixture, mention both the chemical bonding and the fixed ratio, because each idea can earn its own mark.
  • Write chemically combined or chemically bonded rather than vague words such as stuck or joined.
  • Back up a comparison of properties with a named example, such as iron still being magnetic in a mixture but not in iron(II) sulfide.
  • In a particle diagram, look for bonding between different atoms before deciding whether you are looking at a compound or a mixture.
Self review
  • What is an element?
  • State two differences between a compound and a mixture.
  • Why does sodium chloride have different properties from sodium and chlorine?
  • How can a magnet show that a mixture of iron and sulfur is not a compound?
  • Is carbon dioxide an element, a compound or a mixture?
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2.1.1 Elements, compounds and mixtures Revision Guide

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Revision notes for CIE IGCSE Chemistry 2.1.1 Elements, compounds and mixtures: explanations and worked examples.

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