Describe the differences between elements, compounds and mixtures

Elements, Compounds and Mixtures – IGCSE Chemistry (0620)

Learning objective

Describe the differences between elements, compounds and mixtures, and explain how each can be identified, separated and characterised using the techniques required by the Cambridge IGCSE syllabus.

Key definitions

• Element: A pure substance that cannot be broken down into simpler substances by chemical means. It consists of only one type of atom and is represented by a unique chemical symbol (e.g., \(\mathrm{H}\), \(\mathrm{O}\), \(\mathrm{Fe}\)).
• Compound: A pure substance formed when two or more different elements combine **in a fixed, definite proportion by mass** (law of definite proportions). Compounds have chemical and physical properties that differ from those of the constituent elements and are written with a chemical formula (e.g., \(\mathrm{H_2O}\), \(\mathrm{NaCl}\), \(\mathrm{CO_2}\)).
• Mixture: A physical combination of two or more substances (elements, compounds, or both) that are not chemically bonded. Each component retains its own chemical identity and can be separated by physical methods.

Law of definite proportions

In a given compound the masses of the constituent elements are always in the same ratio, regardless of how the compound is prepared.

Example: Water contains 2 g of hydrogen for every 16 g of oxygen (mass ratio 1 : 8). Using the molar masses (H = 1 g mol⁻¹, O = 16 g mol⁻¹):

This fixed ratio underlies the statement that compounds have a *definite composition*.

Classification of mixtures

• Homogeneous mixture (solution): The components are uniformly distributed and the mixture appears as a single phase (e.g., salt water, air).
• Heterogeneous mixture: The components are not uniformly distributed and can be seen as separate phases (e.g., sand + iron filings, oil + water).

Identifying elements, compounds and mixtures

Separation & purification techniques (why they work)

Physical vs chemical changes

1. Physical change: No new substances are formed; the chemical identity of each component remains unchanged. • Example (mixture): Dissolving table salt in water – the salt can be recovered by evaporation. • Example (pure substance): Melting ice – water remains H₂O.
2. Chemical change: Bonds are broken and new ones formed, producing different substances. • Example (compound): Electrolysis of water → \(\mathrm{2H_2O \rightarrow 2H_2 + O_2}\). • Example (element): Burning magnesium ribbon → \(\mathrm{2Mg + O_2 \rightarrow 2MgO}\).

Cross‑reference to other syllabus sections

Understanding elements, compounds and mixtures underpins later topics such as:

• Stoichiometry and the mole concept (Section 4)
• Energy changes in reactions (Section 5)
• Organic chemistry – functional groups are compounds built from elements (Section 9)
• Acids, bases and salts – preparation and properties of compounds (Section 8)

Suggested diagram

Quick revision checklist

• Can I write the correct chemical symbol for an element?
• Do I know the fixed mass ratio of elements in a given compound (law of definite proportions) and can I use it in a simple calculation?
• Can I list at least two physical methods to separate a mixture and explain the principle behind each?
• Do I understand why the properties of a compound differ from those of its constituent elements?
• Am I able to distinguish a physical change from a chemical change?
• Do I remember the key safety precautions for the common apparatus (gloves, goggles, heat‑resistant mat, etc.)?

Practice questions

1. Identify whether each of the following is an element, a compound or a mixture: \(\mathrm{O_2}\), \(\mathrm{H_2SO_4}\), air, brass (copper & zinc alloy).
2. Explain why copper sulphate (\(\mathrm{CuSO_4}\)) is a compound but a mixture of copper sulphate crystals and water is not.
3. Describe a physical method you could use to separate sand from a mixture of sand and sugar, and state why each step works.
4. Give one example of a chemical change involving a compound and one physical change involving a mixture.
5. State the principle behind the separation of ethanol from water by simple distillation.
6. Briefly outline how you could separate a mixture of two soluble salts using recrystallisation.

Answers to practice questions

1. • \(\mathrm{O_2}\) – Element (diatomic molecule of oxygen).
   • \(\mathrm{H_2SO_4}\) – Compound (sulphuric acid).
   • Air – Mixture (gases in varying proportions, homogeneous).
   • Brass – Mixture (alloy of copper and zinc, heterogeneous at the atomic level).
2. Copper sulphate consists of copper, sulphur and oxygen atoms chemically bonded in a fixed 1 : 1 : 4 mass ratio. Its properties (blue crystals, solubility in water) are different from metallic copper or elemental sulphur. When the crystals are dissolved, water molecules are not chemically bonded to the \(\mathrm{CuSO_4}\) units; the resulting solution is therefore a mixture that can be separated by evaporation or crystallisation.
3. 1. Add a small amount of water – sugar dissolves, sand does not.
   2. Filter the mixture – sand is retained on the filter paper while the sugar solution passes through (size‑based separation).
   3. Evaporate the filtrate – water boils away, leaving solid sugar behind (exploits the lower boiling point of water).
4. Chemical change: Electrolysis of water → \(\mathrm{2H_2O \rightarrow 2H_2 + O_2}\) (new substances formed).
   Physical change: Dissolving table salt in water – the salt can be recovered by evaporation; no new substances are formed.
5. Ethanol has a lower boiling point (78 °C) than water (100 °C). On heating, ethanol vapourises first; the vapour is condensed and collected, leaving most of the water behind. The separation relies on the difference in boiling points.
6. Heat the mixture so that one salt (e.g., NaCl) becomes highly soluble while the other (e.g., AgCl) remains only sparingly soluble. Cool the solution; the less‑soluble salt crystallises out and can be filtered, leaving the more‑soluble salt in the filtrate. This uses the temperature‑dependence of solubility (recrystallisation).