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In chemistry, a pure substance is a single element or compound, not mixed with any other substance. Free notes with key terms and 10 questions answered.
What is a Pure Substance? is part of Purity, formulations and chromatography in GCSE AQA Higher Chemistry.
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In chemistry, a pure substance is a single element or compound, not mixed with any other substance. Free notes with key terms and 10 questions answered.
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Use this lesson to stay inside Purity, formulations and chromatography while connecting the detail back to the wider Topic 8: Chemical analysis topic.
Read the core explanations from What is a Pure Substance? before testing yourself from memory.
When you buy "pure orange juice" from the shop, what does "pure" mean? Most people would say it means nothing artificial has been added - just natural orange juice. But if you took that same juice into a chemistry lab, a chemist might tell you it's actually a complex mixture of water, sugars, acids, vitamins, and hundreds of other compounds.
This creates confusion because the word "pure" means different things to different people. In chemistry, we need precision - so chemists have their own specific definition that's much stricter than everyday language.
This definition is black and white - there's no grey area. A substance is either pure or it isn't. Let's break this down with examples:
Pure elements: A sample containing only iron atoms ($Fe$) is pure iron. A sample containing only oxygen atoms ($O$) is pure oxygen. Even if the iron bar has scratches or the oxygen is in a rusty tank, as long as there are no other types of atoms mixed in, it's chemically pure.
Pure compounds: Water that contains only $H_2O$ molecules is pure water. Salt that contains only $NaCl$ is pure sodium chloride. The key is that every molecule or formula unit is identical.
In everyday language, "pure" usually means "natural" or "nothing artificial added". This is why we talk about pure milk, pure honey, or pure orange juice. These substances haven't been tampered with or had artificial ingredients added.
But here's the key difference: even "pure" milk in everyday terms is actually a complex mixture from a chemistry perspective. Milk contains water, proteins, fats, sugars (lactose), vitamins, and minerals all mixed together. It's pure in the sense that nothing artificial was added, but it's definitely not a single compound.
Understanding the difference between these definitions is crucial for several reasons. First, it affects how we interpret data and results in chemistry. If you're measuring the boiling point of water, you need chemically pure water - not just "pure" tap water.
Second, it helps explain why chemists use techniques like distillation and chromatography. These methods separate mixtures to obtain pure substances in the chemical sense. Even something that seems pure to us (like salt from the sea) needs purification to remove other dissolved compounds.
Third, chemical reactions behave predictably only when we use pure reactants. If your "pure" sodium chloride actually contains other salts, your reaction might not go as expected.
Let's apply both definitions to some common substances you might encounter:
Bottled water: Marketed as "pure" (everyday definition - nothing added), but contains dissolved minerals like calcium and magnesium ions. Chemically, it's a mixture. Only distilled water approaches chemical purity.
Gold jewellery: "Pure gold" in everyday terms often means 24-carat gold with no other metals added. But even 24-carat gold contains tiny amounts of other elements. Chemically pure gold would contain only gold atoms.
Table salt: The salt in your kitchen is mostly sodium chloride, but it also contains anti-caking agents and sometimes iodine compounds. It's not chemically pure, even though it's "pure" enough for cooking.
Exam questions about purity often test whether you understand the difference between chemical and everyday definitions. The key is to read the context carefully.
If the question mentions "in chemistry" or asks about melting points, boiling points, or chemical reactions, use the strict chemical definition. If it's about food, natural products, or everyday contexts, consider the everyday definition.
Worked example: Step-by-Step: Structured Chemistry Explanation
Answer a GCSE-style question in What is a Pure Substance? - Pure substances - Purity, formulations and chromatography using precise terminology.
Answer: High-quality answers combine principle, application, and a precise conclusion.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
In chemistry, a pure substance is a single element or compound, not mixed with any other substance. For example, water containing only H₂O molecules is pure water, and a sample containing only iron atoms is pure iron.
A pure compound is a substance in which every molecule or formula unit is identical, such as water containing only H₂O molecules or salt containing only NaCl. It is not mixed with any other substance.
In everyday terms, 24-carat gold is 'pure gold' with no other metals added, but even 24-carat gold contains tiny amounts of other elements. Chemically pure gold would contain only gold atoms.
Distilled water approaches chemical purity, unlike bottled water, which contains dissolved minerals like calcium and magnesium ions. Chemically pure water contains only H₂O molecules, with no other substances mixed in.
A pure element is a sample containing only one type of atom, such as only iron atoms (Fe). Even if an iron bar has scratches, it is chemically pure as long as no other types of atoms are mixed in.
Milk is not a pure substance in chemistry. It may be pure in the everyday sense because nothing artificial has been added, but it is a mixture of water, proteins, fats, sugars such as lactose, vitamins and minerals.
Bottled water is not chemically pure: it is marketed as pure because nothing has been added, but it contains dissolved minerals such as calcium and magnesium ions. Only distilled water approaches chemical purity.
Table salt is not chemically pure. It is mostly sodium chloride, but it also contains anti-caking agents and sometimes iodine compounds, so it is only 'pure' enough for cooking.
Purity matters because chemical reactions behave predictably only with pure reactants, and measuring the boiling point of water needs chemically pure water. This is why chemists use techniques like distillation and chromatography to separate mixtures.
In everyday language, pure means a substance has had nothing added, so it is unadulterated and in its natural state, such as pure orange juice. In chemistry, pure means a single element or compound only, so pure orange juice is actually a mixture.