🥛 Everyday meaning
Nothing has been added, so it is natural and unadulterated. Example: pure milk.
Purity and Chromatography » Pure Substances and Formulations
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.8.1.1, 4.8.1.2
- What a chemist means by a pure substance
- How melting point and boiling point data show if a sample is pure
- What a formulation is and how to spot one
- Examples of formulations, from fuels to foods
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Unlock This CourseWhat does "pure" mean in chemistry?
In everyday life, "pure" often means that nothing has been added. A carton of pure milk is just milk, in its natural state. Chemists use the word more strictly.
In chemistry, a pure substance is a single element or compound, not mixed with any other substance. Pure milk is not pure to a chemist. Milk contains water, fats, sugars and proteins all mixed together.
Key terms:
- Pure substance: a single element or compound, not mixed with any other substance.
- Impure substance: a substance that has other substances mixed in with it.
⚗ Chemistry meaning
Only one element or compound is present. Example: distilled water, which is only H2O.
Using melting points to test purity
Pure water freezes at exactly 0 °C - add salt and it melts lower, which is why gritters salt icy roads
Pure elements and compounds melt and boil at specific temperatures. For example, pure water melts at exactly 0 °C and boils at exactly 100 °C (at normal air pressure). Every sample of pure water gives the same numbers.
A mixture behaves differently. Its melting point is not sharp. It melts gradually over a range of temperatures, and the temperature is usually different from that of the pure substance. Boiling points of mixtures also change and spread over a range.
This gives chemists a simple test. Heat the sample, measure the melting point or boiling point and compare it with the data for the pure substance.
- If the sample melts or boils at the exact temperature in the data book, it is pure.
- If it melts or boils at a different temperature, or over a range, it is impure.
The more impurity there is, the further the result is from the pure value.
Reading melting point data
Worked example
A data book says that pure compound P melts at 122 °C. Three samples were tested.
- Sample 1 melted at 122 °C.
- Sample 2 melted between 115 °C and 120 °C.
- Sample 3 melted between 119 °C and 123 °C.
Sample 1 is pure, because it melts at exactly the specific temperature. Sample 2 is impure, because it melts over a range and well below 122 °C. Sample 3 is also impure, because it melts over a range, even though its range is close to 122 °C.
The same idea works for boiling points. Pure ethanol boils at 78 °C. A liquid that boils over a range, for example from 78 °C up to 95 °C, is a mixture. It might be ethanol mixed with water.
Here is a salty example. Pure water freezes at 0 °C, but salty water freezes below 0 °C. That is why salt is spread on icy roads in winter.
Common mistakes
Saying "pure means natural" in an exam answer. Orange juice or pure milk is natural, but it is a mixture, so it is not pure in chemistry. Also, do not say that an impure sample has no melting point. It does melt, but over a range of temperatures.
What is a formulation?
Some mixtures are made on purpose. A formulation is a mixture that has been designed as a useful product.
Many products are complex mixtures. Each chemical in the mixture has a particular job. The components are mixed in carefully measured quantities, so that the product has exactly the properties it needs.
Key terms:
- Formulation: a mixture that has been designed as a useful product, made by mixing the components in carefully measured quantities.
Getting the quantities right matters. Too much or too little of one component and the product may not work, or may not be safe.
Examples of formulations
Paints, medicines and cleaners are all formulations - mixtures designed with each ingredient in a carefully measured amount
The spec lists seven types of formulation. You need to know all of them.
⛽ Fuels
Petrol is a mixture of different hydrocarbons, designed to burn well in engines.
🧼 Cleaning agents
Washing-up liquid contains several chemicals, each with a job, such as removing grease.
🎨 Paints
A paint is a mixture of a colour, a liquid to spread it and other components, so it covers well and dries.
💊 Medicines
A medicine contains a drug plus other substances, in measured amounts, so each dose is the right size.
⚒ Alloys
An alloy is a mixture of a metal with other elements, designed to have properties that the pure metal lacks.
🌱 Fertilisers
Many fertilisers contain nitrogen, phosphorus and potassium compounds, mixed to feed plants.
🍪 Foods
A cake recipe is a formulation. Flour, sugar, eggs and fat are measured out, because the wrong amounts give a cake that does not rise or taste right.
📝 Names not needed
You do not need to know the names of the components in products that companies make and sell under their own brand. You only need to know that they are formulations.
Spotting a formulation
You may be given information about a product and asked if it is a formulation. Look for these clues:
- It is a mixture of two or more substances, not a single element or compound.
- It was designed to be useful, with a particular purpose.
- Each component has its own job.
- The components are present in carefully measured quantities.
Worked example
A bottle of plant feed has this label: nitrogen compounds 10%, phosphorus compounds 5%, potassium compounds 5%, the rest water. Is it a formulation?
Yes. It is a mixture of several components, in exact quantities, and each is there to help plants grow. It is a fertiliser, which is one of the types of formulation.
Common mistakes
Thinking every mixture is a formulation. Sea water is a mixture, but nobody designed it for a purpose or measured out its parts. It is not a formulation. Also, an alloy is a mixture, so it does not count as a pure substance.
Exam-style question
A student tests two samples of a white solid. The data book says the pure solid melts at 80 °C. Sample A melts at 80 °C. Sample B melts between 72 °C and 78 °C. Which sample is pure? Explain your answer. (3 marks)
Model answer
Sample A is pure. A pure substance melts at one specific temperature, and sample A melts at 80 °C, which matches the data book. Sample B is impure. It melts over a range of temperatures and below 80 °C, which shows that other substances are mixed with it.
Exam tip
For a purity question, always compare with the data. Say whether the melting or boiling point is sharp and whether it matches the pure value.