How do elements, compounds and mixtures differ, and how do we tell a chemical change from a physical one?
Distinguish between elements, compounds and mixtures, interpret chemical formulae, and tell physical changes apart from chemical reactions.
A focused answer to WJEC GCSE Chemistry topic 1.1, covering the difference between elements, compounds and mixtures, reading chemical formulae, and distinguishing physical changes from chemical reactions with evidence such as colour change and gas production.
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What this topic is asking
WJEC topic 1.1 starts with the language of chemistry. You must be able to classify substances as elements, compounds or mixtures, read a chemical formula to say which elements are present and in what proportions, and decide whether a change is physical or chemical using observable evidence. These ideas underpin every reaction you meet later in the course.
Elements, compounds and mixtures
A compound is formed when the atoms of two or more elements are chemically bonded together. The elements in a compound are joined in a fixed ratio, so the compound always has the same composition: water is always , with two hydrogen atoms for every oxygen atom. The properties of a compound are usually very different from the elements it contains; for example sodium (a reactive metal) and chlorine (a poisonous gas) react to form sodium chloride, which is ordinary table salt.
A mixture contains two or more substances (elements or compounds) that are not chemically bonded together. Because they are not bonded:
- the substances keep their own properties,
- they can be present in any proportion, and
- they can be separated by physical methods such as filtering, distillation or chromatography (no chemical reaction needed).
Air is a mixture of nitrogen, oxygen, argon and carbon dioxide; sea water is a mixture of water and dissolved salts.
Reading a chemical formula
A chemical formula tells you which elements are in a compound and how many atoms of each are present. The symbols are the element symbols; the small subscript numbers give how many atoms of the element just before them.
- : 2 hydrogen atoms and 1 oxygen atom.
- : 1 carbon atom and 2 oxygen atoms.
- : 2 hydrogen, 1 sulfur and 4 oxygen atoms.
- : the bracket means everything inside is multiplied by 2, so 1 calcium, 2 oxygen and 2 hydrogen atoms.
Physical changes versus chemical reactions
In a physical change the substance itself does not change; only its state or form changes. Melting ice, dissolving sugar and boiling water are physical changes - the same substance is still there and the change is usually easy to reverse (you can refreeze the water).
In a chemical reaction, the reactants are changed into new substances (the products) with different properties. The atoms are rearranged and joined by new chemical bonds. Chemical reactions are usually difficult to reverse.
Reactants and products
We describe reactions using the words reactant and product. The reactants are the starting substances on the left of an equation; the products are the new substances on the right. For example, when methane burns:
Methane and oxygen are the reactants; carbon dioxide and water are the products.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample4 marksAir, water and iron are three substances. Classify each as an element, a compound or a mixture, and explain your choice for each one.Show worked answer →
A Unit 1.1 classification question. Reward: iron is an element because it is made of only one type of atom and cannot be broken down chemically. Water is a compound because it is two elements (hydrogen and oxygen) chemically bonded in a fixed ratio. Air is a mixture because it contains several substances (nitrogen, oxygen, argon, carbon dioxide) that are not chemically bonded and are present in no fixed ratio. Markers credit one mark for each correct classification and reward an explanation that links elements to one type of atom, compounds to chemically bonded elements in fixed proportions, and mixtures to substances that are not bonded together. A common slip is to call air a compound.
WJEC sample3 marksWhen magnesium burns in air it produces a white powder and gives out light and heat. Explain whether this is a physical change or a chemical reaction, giving two pieces of evidence.Show worked answer →
A Unit 1.1 reasoning question. Reward: this is a chemical reaction because a new substance (magnesium oxide, a white powder) is formed and the change is difficult to reverse. Two pieces of evidence: a new substance with different properties is made (white solid, different from grey magnesium), and energy is given out (light and heat), which is typical of a chemical reaction. Markers credit the conclusion plus two valid pieces of evidence. A common error is to give only one piece of evidence or to call it physical because it "just changed colour".
Related dot points
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A focused answer to WJEC GCSE Chemistry topic 1.1, covering the method for paper chromatography, why components separate, how to interpret a chromatogram, and how to calculate Rf values to identify substances.
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A focused answer to WJEC GCSE Chemistry topic 1.2, covering protons, neutrons and electrons and their relative masses and charges, atomic number and mass number, working out particle numbers for atoms and ions, isotopes, and calculating relative atomic mass.
- Describe the arrangement of electrons into shells for the first twenty elements and relate electronic structure to group and period in the Periodic Table.
A focused answer to WJEC GCSE Chemistry topic 1.2, covering how electrons fill shells, writing the electronic structures of the first twenty elements, and relating the number of outer electrons to group number and the number of shells to period number.
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A focused answer to WJEC GCSE Chemistry topic 1.2, covering how the Periodic Table is arranged by atomic number, the difference between metals and non-metals, and the properties and trends of the alkali metals (Group 1), halogens (Group 7) and noble gases (Group 0).
Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)