Ch 1 · Elements, Compounds and Mixtures · IGCSE Chemistry 0620
Topic 1 of 7 in Atoms, Elements and Compounds — IGCSE Chemistry 0620 — 58 lessons.
Introduction
Chemistry classifies every substance as an element, a compound or a mixture. The distinction decides how a substance can be separated, whether it has a formula, and whether making it involved a chemical reaction. Questions on this topic are often short, but they demand precise wording: "made of one type of atom" and "chemically combined" are the phrases that score.
Key ideas — Elements
An element is a substance that contains only one type of atom. It cannot be broken down into anything simpler by chemical means.
🧑🏫 Teacher's explanation
Imagine you have a huge box of LEGO bricks. If every single brick in that box is a red 2 imes 4 brick, you have an element. It doesn't matter if they are scattered individually or snapped together into long chains; as long as there is only one type of brick, it is an element. You cannot break a single red brick into a blue brick using any chemical process.
📖 Definitions
🔍 Worked example — Easy
Identify whether the following is an element, a compound, or a mixture: A sample contains only O_2 molecules.
🔍 Worked example — Standard
A substance consists of H_2O molecules. Is this an element or a compound? Explain.
🔍 Worked example — Exam-style
A sample of pure sulfur is represented by the formula S_8. A sample of air is a mixture of N_2, O_2, and Ar. Compare the composition of these two substances.
⚠️ Common errors
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🎯 Exam question
Key ideas — Compounds
A compound is a substance containing two or more different elements chemically combined (bonded together) in a fixed proportion. Water, H_2O, always contains two hydrogen atoms for every oxygen atom; carbon dioxide, CO_2, always has two oxygens per carbon. Because the proportion is fixed, a compound has a definite chemical formula.
🧑🏫 Teacher's explanation
Imagine you are making a sandwich. If you follow a recipe that says you must use exactly 2 slices of bread and 1 slice of cheese every single time, you are making a 'compound'. No matter how much you shake the sandwich, you cannot get the cheese out without a 'chemical reaction' (like a magic spell). If you just throw bread, cheese, and lettuce into a bowl together, you have a 'mixture'. You can pick the lettuce out with your hands (a physical process), and one sandwich might have more lettuce than the next.
📖 Definitions
🔍 Worked example — Easy
A sample of magnesium oxide contains 24 g of magnesium and 40 g of oxygen. Calculate the mass percentage of magnesium in the compound MgO.
🔍 Worked example — Standard
A compound is formed from 12 g of Carbon and 32 g of Oxygen. Determine the empirical formula of the compound.
🔍 Worked example — Exam-style
A student has a mixture of iron filings and sulfur powder. They heat the mixture strongly in a crucible. Describe the change in properties and the nature of the resulting substance.
⚠️ Common errors
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🎯 Exam question
Key ideas — Mixtures
A mixture contains two or more substances (elements, compounds or both) that are not chemically combined. The substances can be present in any proportion, so a mixture has no formula, and they can be separated by physical methods: filtration, evaporation, crystallisation, distillation, chromatography or a magnet.
🧑🏫 Teacher's explanation
Imagine you are making a bowl of fruit salad. You throw in some grapes, some strawberries, and some melon. You can see each fruit clearly, and you can easily pick out the grapes with your fingers if you don't like them. This is a mixture. The fruit pieces are still 'themselves'; a strawberry doesn't turn into something else just because it is touching a grape.
📖 Definitions
🔍 Worked example — Easy
Identify whether the following is an element, a compound, or a mixture: A container holds particles of pure Iron (Fe) and pure Sulfur (S) mixed together.
🔍 Worked example — Standard
A student has a solution of salt in water. Explain why this is a mixture and not a compound.
🔍 Worked example — Exam-style
A sample contains 48 g of Magnesium (Mg) and 32 g of Oxygen (O_2). If these were to form a compound, they would react in a fixed mass ratio. If they are simply mixed, what is the total mass of the mixture?
⚠️ Common errors
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🎯 Exam question
(a) Was the original substance a mixture or a compound? (b) Explain your answer using the concept of chemical bonding. (c) State one physical method that could have been used to separate the original mixture if it had been a mixture of sand and salt. [4 marks]
Key ideas — Fixed composition and properties
The most important contrast for the examiner is this: a compound has a fixed composition and properties that are different from those of its elements, while a mixture has neither.
🧑🏫 Teacher's explanation
Imagine you are making a bowl of fruit salad versus baking a cake. A fruit salad is a mixture; you can throw in three strawberries and one banana, or ten strawberries and no bananas, and it is still a fruit salad. The individual fruits keep their original taste and texture.
📖 Definitions
🔍 Worked example — Easy
A sample of a substance contains 12 ext g of Carbon and 32 ext g of Oxygen. If this is a pure compound of Carbon and Oxygen, what is the mass of the compound?
🔍 Worked example — Standard
In a reaction between Iron (Fe) and Sulfur (S), the ratio of atoms is 1:1. If you start with 56 g of Fe and 32 g of S, how many grams of FeS are produced?
🔍 Worked example — Exam-style
A student has a mixture of Iron filings and Sulfur powder. Describe two physical properties that would allow the student to separate the iron from the sulfur.
⚠️ Common errors
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🎯 Exam question
(a) State whether the resulting mixture has a fixed composition. [1] (b) The student heats the mixture strongly. A black solid, iron(II) sulfide, is formed. (i) Is the black solid an element, a compound, or a mixture? [1] (ii) Explain why the properties of the black solid are different from the properties of the original iron and sulfur. [2] [4 marks]
Key ideas — Recognising them from particle diagrams
Particle diagrams show atoms as circles, usually with different colours or shading for different elements.
🧑🏫 Teacher's explanation
Imagine you have a large box of LEGO bricks. If you open the box and find only red 2 imes 2 bricks, you have an element. Even if they are snapped together in identical pairs, if every single unit is exactly the same, it is still just one type of thing: an element.
📖 Definitions
🔍 Worked example — Easy
A particle diagram shows three identical pairs of circles, where each pair consists of one large black circle and one small white circle. Identify if this is an element, a compound, or a mixture.
🔍 Worked example — Standard
A diagram contains four single red circles and two pairs of joined red and blue circles. Identify the substance.
🔍 Worked example — Exam-style
In a sample, every particle is a molecule consisting of two identical grey atoms. Is this an element or a compound?
⚠️ Common errors
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🎯 Exam question
(a) Identify the type of substance shown. [1] (b) Explain your answer by referring to the particles present. [2] [3 marks]
Worked example
Question. A student is given four sealed jars. Jar A contains particles that are all identical pairs of the same atom. Jar B contains particles that are all identical groups made of one atom of one type and two atoms of another. Jar C contains single atoms of one kind together with pairs of atoms of a second kind. Jar D contains only single identical atoms.
🔍 Worked example — Easy
Identify whether the following particle descriptions represent an element, a compound, or a mixture: (i) A container with only He atoms. (ii) A container with H_2 and O_2 molecules mixed together.
🔍 Worked example — Standard
A sample contains only CO_2 molecules. Is this an element, a compound, or a mixture? Explain.
🔍 Worked example — Exam-style
A student observes a beaker containing Ag atoms and AgCl particles. Classify the contents and explain why the composition is not fixed.
⚠️ Common errors
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Practice
Answers Element: a substance containing only one type of atom [B1]; compound: a substance containing two or more different elements chemically combined [B1]. Any two: a compound has a fixed composition, a mixture can have any composition [B1]; the components of a mixture can be separated by physical means, a compound needs a chemical reaction [B1]; a compound has different properties from its elements, a mixture keeps the properties of its components [B1] (max 2). Compound [B1]; every particle is identical and contains two different types of atom joined together [B1]. Any two: heat and light are given out during heating, showing a chemical reaction [B1]; the product is not attracted to a magnet [B1]; the product has a different colour and properties from iron and sulfur [B1]; the product cannot be separated by physical means [B1] (max 2).