Cambridge IGCSE0620

Elements, compounds and mixtures

Chemistry 0620 Chapter Notes

What this chapter covers

Elements, compounds and mixturesAtomic structure and the Periodic TableIsotopesIons and ionic bondsSimple molecules and covalent bondsGiant covalent structuresMetallic bonding
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In this chapter

  1. 1Elements: The Simplest Substances
  2. 2Compounds: New Substances Formed
  3. 3Mixtures: A Physical Combination
  4. 4Compounds vs. Mixtures: The Key Differences
  5. 5Recognising Chemical Changes

1. Elements: The Simplest Substances

An element is a pure substance that contains only one type of atom. They are the fundamental building blocks of all matter. For example, the element helium (He) consists only of helium atoms, and a piece of pure iron (Fe) contains only iron atoms. Elements cannot be broken down into anything simpler by chemical reactions. All the known elements are organised in the Periodic Table.

Key term

Element: A pure substance consisting of only one type of atom, which cannot be broken down into anything simpler by chemical means.

Worked example 12 marks

Explain why aluminium is classified as an element.

  1. 1

    An element is a substance made up of only one type of atom.

  2. 2

    Aluminium is made up entirely of aluminium atoms.

  3. 3

    Therefore, aluminium is an element. It cannot be broken down into simpler substances by chemical methods.

Recap

  • An element contains only one type of atom.
  • Elements are listed on the Periodic Table, each with a unique symbol.
  • Elements are the fundamental building blocks of all other substances.
  • An element cannot be broken down into simpler substances by a chemical reaction.

Quick check

  1. What is the chemical symbol for the element potassium?1 mark

2. Compounds: New Substances Formed

A compound is a pure substance formed when two or more different elements are chemically bonded together. The atoms are joined in a fixed, definite ratio. This ratio is shown by the compound's chemical formula, for example, water is always H₂O (two hydrogen atoms bonded to one oxygen atom). Crucially, the properties of a compound are completely different from the properties of its constituent elements. For instance, hydrogen and oxygen are both gases at room temperature, but when they combine chemically, they form water, a liquid.

Key term

Compound: A pure substance formed from two or more elements chemically bonded together in a fixed ratio.

Examiner insight

Examiners reward answers that clearly state that compounds have properties that are different from their constituent elements.

Common pitfall

Stating that a compound is just a 'mix' of elements, without mentioning the crucial chemical bonds and fixed ratio.

Fun fact

Table salt (sodium chloride, NaCl) is a stable, edible compound made from a highly reactive, silvery metal (sodium) and a poisonous, green gas (chlorine)!

Worked example 13 marks

Carbon dioxide has the chemical formula CO₂.(a) Name the elements present in carbon dioxide.(b) State the ratio of atoms of each element.(c) Explain why carbon dioxide is a compound.

  1. 1

    (a) The elements are Carbon (C) and Oxygen (O).

  2. 2

    (b) The ratio is 1 carbon atom to 2 oxygen atoms.

  3. 3

    (c) It is a compound because it is made of two different elements (carbon and oxygen) that are chemically bonded together in a fixed ratio.

Recap

  • A compound contains atoms of two or more different elements.
  • The atoms in a compound are chemically bonded together.
  • The elements in a compound are present in a fixed ratio, shown by its formula.
  • A compound has chemical and physical properties that are different from its constituent elements.

Quick check

  1. How many atoms of each element are in one formula unit of calcium nitrate, Ca(NO₃)₂?3 marks

3. Mixtures: A Physical Combination

A mixture consists of two or more substances (elements or compounds) that are not chemically bonded together. Because there are no chemical bonds between the components, they can be mixed in any proportion. Each substance in a mixture retains its own individual properties. For example, in a mixture of iron filings and sulfur powder, the iron is still magnetic and the sulfur is still yellow. A key feature of mixtures is that they can be separated into their original components by physical means, such as filtration, evaporation, or using a magnet.

Key term

Mixture: Consists of two or more substances (elements or compounds) that are not chemically combined together.

Common pitfall

Forgetting that the components of a mixture keep their original properties, which is a key difference from compounds.

Worked example 13 marks

A beaker contains a mixture of sand and water.(a) Explain why this is a mixture.(b) Suggest a physical method to separate the sand from the water and briefly describe it.

  1. 1

    (a) It is a mixture because the sand and water are not chemically bonded. The sand and water retain their own properties and can be separated by physical means.

  2. 2

    (b) The method is filtration. The mixture is poured through filter paper in a funnel. The water (filtrate) will pass through the paper, while the sand (residue) will be trapped by it.

Recap

  • A mixture contains different substances that are not chemically bonded.
  • The components of a mixture can be mixed in any proportion.
  • Each substance in a mixture keeps its own chemical and physical properties.
  • Mixtures can be separated into their components using physical methods.

Quick check

  1. Is air an element, compound or mixture? Give one reason.2 marks

4. Compounds vs. Mixtures: The Key Differences

It is essential to distinguish between compounds and mixtures. The key differences relate to their formation, composition, properties, and how they can be separated. For example, mixing iron powder and sulfur creates a mixture; they can be separated with a magnet. However, heating this mixture causes a chemical reaction, forming the compound iron(II) sulfide (FeS). This new black solid is not magnetic and its components cannot be separated with a magnet; a chemical reaction would be needed to separate them.

Key term

Physical Separation: A method used to separate a mixture into its components without a chemical reaction occurring, such as filtration or evaporation.

Examiner insight

Marks are often awarded for creating a clear, point-by-point comparison between compounds and mixtures, especially regarding separation methods and properties.

Worked example 13 marks

The table below lists some statements. For each one, tick (✓) the box to show if it describes a compound, a mixture, or both.

StatementCompoundMixtureBoth
Contains more than one element
Has a fixed melting point
Can be separated by filtration
  1. 1

    Contains more than one element: Both (A compound MUST have >1 element, a mixture CAN have >1 element e.g. salt water).

  2. 2

    Has a fixed melting point: Compound (Pure compounds have sharp, fixed melting points; mixtures melt over a range).

  3. 3

    Can be separated by filtration: Mixture (Filtration is a physical separation method for insoluble solids from liquids).

Recap

  • Compounds are formed by chemical reactions; mixtures are formed by physical mixing.
  • Compounds have a fixed composition by mass; mixtures have a variable composition.
  • Compounds have new properties; components of a mixture retain their original properties.
  • Compounds can only be separated by chemical reactions; mixtures can be separated by physical methods.
  • Energy changes (like heat or light) often accompany the formation of a compound, but not a mixture.

Quick check

  1. State two differences between a mixture of hydrogen and oxygen gas, and the compound water (H₂O).2 marks

5. Recognising Chemical Changes

A chemical change, or chemical reaction, is a process that results in the formation of new chemical substances. This is different from a physical change (like melting ice or dissolving sugar), where no new substances are formed and the change is usually easily reversible. There are several observable signs that a chemical change has taken place. These include: a significant energy change (e.g., heat or light is given out, or the surroundings get noticeably colder), a permanent colour change, fizzing or bubbling (indicating a gas is being produced), or the formation of a solid (a precipitate) when two solutions are mixed.

Key term

Chemical Change: A process that involves the formation of new chemical substances with different properties; it is not easily reversed.

Examiner insight

Examiners look for specific observations as evidence for a chemical reaction, such as 'effervescence' or 'a precipitate formed', rather than just saying 'a reaction happened'.

Fun fact

The browning of food when cooked, like searing a steak or toasting bread, is a complex set of chemical reactions called the Maillard reaction, which creates hundreds of new flavour compounds.

Worked example 12 marks

When a piece of magnesium ribbon is added to a test tube of hydrochloric acid, the test tube gets warm and bubbles of gas are seen. Explain how these two observations indicate that a chemical change has occurred.

  1. 1

    The test tube getting warm shows that energy is being released (an exothermic reaction). A significant energy change is a sign of a chemical change.

  2. 2

    The bubbles of gas show that a new substance (hydrogen gas) is being formed. The formation of a new substance is the definition of a chemical change.

Recap

  • A chemical change results in the formation of one or more new substances.
  • A physical change, like a change of state, does not create new substances.
  • Common signs of a chemical change include energy release, gas production, or a precipitate forming.
  • Chemical changes are generally difficult to reverse.
  • Burning a fuel is a chemical change, while boiling water is a physical change.

Quick check

  1. When bread is toasted, it turns brown. Is this a physical or chemical change? Give a reason.2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'element'.1 mark
  2. State the chemical symbols for the elements present in sulfuric acid, H₂SO₄.3 marks
  3. Explain, using the example of salt dissolved in water, why it is classified as a mixture and not a compound.3 marks
  4. A paper chromatogram is produced from a sample of green food colouring. It shows two separate spots, one yellow and one blue. What does this result tell you about the green food colouring? Explain how paper chromatography works to separate the colours.3 marks
  5. When solid ammonium chloride is heated, it breaks down to form two gases, ammonia and hydrogen chloride. Is this a physical or chemical change? Give two reasons for your answer.3 marks
  6. Compare and contrast a compound and a mixture by describing two differences. For each difference, describe how it applies to water (a compound) and a mix of hydrogen gas and oxygen gas (a mixture).4 marks
  7. A student is given a sample of a white crystalline solid. They are told it is either pure sodium chloride or sodium chloride mixed with a small amount of sand. Describe an experiment they could perform to determine if the solid is pure. Explain the principle behind your method.4 marks
  8. Methane has the formula CH₄. Is methane an element, compound or mixture? Explain your choice, referring to the types of atoms and bonding present.3 marks
  9. Seawater is a complex mixture containing dissolved salts (like NaCl) and suspended solids (like sand). Propose a sequence of two separation techniques to obtain pure, dry salt from a sample of seawater. For each technique, name the key piece of apparatus you would use and explain why the technique works.5 marks
  10. 'The properties of a compound are very different from the properties of its constituent elements.' Justify this statement using water (H₂O) as an example. You should describe one physical property of hydrogen, one of oxygen, and one of water at room temperature.3 marks

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