12.1 Pure Substances and Mixtures

A free GCSE Chemistry lesson for AQA (8462) from OBA School: what “pure” means to a chemist, using melting and boiling points to test purity, and formulations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain the difference between a pure substance in chemistry and a “pure” product in everyday language
• Use melting point and boiling point data to tell pure substances from impure ones
• Describe what a formulation is and identify formulations from information about them
• Calculate the percentage or mass of each component in a formulation

Key words

• Pure substance: A single element or a single compound, not mixed with any other substance.
• Mixture: Two or more substances that are not chemically joined. Each keeps its own properties.
• Impurity: A small amount of another substance mixed into the main substance.
• Melting point: The temperature at which a solid turns into a liquid (boiling point: liquid to gas). For a pure substance each is a fixed, specific value.
• Fixed points: The melting point and boiling point of a pure element or compound. They can be looked up in data books or databases.
• Formulation: A mixture designed as a useful product. Each component is added in a measured quantity and has a particular job.

Pure in chemistry and in everyday life

In everyday language, “pure” usually means that nothing has been added, or that a product is natural. A carton of “pure” apple juice has nothing added, but it is still a mixture of water, sugars, acids, vitamins and flavour compounds.

In chemistry, a pure substance contains only one element or one compound. Every particle in it is the same.

• oxygen gas from a cylinder; Pure in chemistry? yes; Reason one element, O₂ molecules only
• distilled water; Pure in chemistry? yes; Reason one compound, H₂O only
• sea water; Pure in chemistry? no; Reason water with many dissolved salts
• air; Pure in chemistry? no; Reason mixture of nitrogen, oxygen, argon, carbon dioxide and more
• “pure” honey; Pure in chemistry? no; Reason natural, but a mixture of sugars, water and other compounds

Using melting and boiling points

A pure substance melts and boils at specific, fixed temperatures (at a given pressure), called its fixed points. We can compare a measured value with the value in a data book or database to identify the substance and check its purity. For example, pure water melts at exactly 0 °C and boils at exactly 100 °C.

• An impure solid usually melts at a lower temperature and over a range of temperatures.
• An impure liquid usually boils at a higher temperature and over a range of temperatures.
• The more impurity there is, the bigger the change.

Worked example: is it pure?

A student’s sample of benzoic acid melts between 114 °C and 119 °C. Pure benzoic acid melts at 122 °C.
The sample melts below 122 °C and over a range of 5 °C, not at a single temperature.

Answer: the sample is impure (it contains another substance)

Finding a melting point. Pack a little of the dry, powdered solid into a thin capillary tube sealed at one end. Fix it beside the bulb of a thermometer and heat it slowly in an oil bath (or an electric melting point apparatus). Record the temperature when the solid starts to melt and when it has completely melted. Oil is used because it can be heated well above 100 °C without boiling.

Key fact: Sea water freezes at about −2 °C and boils at just above 100 °C, while pure water freezes at 0 °C and boils at 100 °C. The dissolved salts are impurities.

Formulations

A formulation is a mixture that has been designed as a useful product. Chemists choose each component for a job and add it in a measured quantity, so that the product works in the same way every time.

• paint; Examples of components and their jobs pigment (colour), binder (sticks the pigment to the surface), solvent (makes it runny, then evaporates)
• medicine tablet; Examples of components and their jobs active ingredient (the drug – often only a small percentage of the mass), filler (bulk), binder (holds the tablet together), coating (easy to swallow)
• fertiliser; Examples of components and their jobs compounds of nitrogen, phosphorus and potassium in set proportions
• washing-up liquid; Examples of components and their jobs surfactant (the detergent that lifts grease), water (thins it so it pours), colouring and perfume (appeal to buyers), rinse aid (helps water drain from plates)
• others; Examples of components and their jobs fuels with additives, cleaning products, alloys, foods, cosmetics, sun creams

Not every mixture is a formulation. Air, sea water and crude oil are mixtures that occur naturally. Nobody designed them, so they are not formulations.

Exam tip: If a question asks whether a product is a formulation, look for two clues: it is a useful product, and its components are in measured / fixed amounts.

Calculations with formulations

percentage of a component = (mass of component ÷ total mass) × 100

Worked example: percentage by mass

A 250 g tin of paint contains 60 g of pigment and 85 g of binder. The rest is solvent.
mass of solvent = 250 − 60 − 85 = 105 g
percentage of pigment = (60 ÷ 250) × 100 = 24%

Answer: solvent 105 g; pigment 24%

Worked example: mass from a percentage

A cleaning spray contains 2.5% surfactant by mass. Find the mass of surfactant in a 750 g bottle.
mass = (2.5 ÷ 100) × 750 = 18.75 g

Answer: 18.75 g (about 19 g)

Common mistakes

• Thinking “pure” means natural or “nothing added”. In chemistry it means one substance only.
• Saying impurities raise the melting point. They usually lower it and make it melt over a range.
• Saying a pure substance has a “high” melting point. The key idea is a sharp, fixed melting point.
• Calling every mixture a formulation. A formulation is designed, with each component in a measured amount.
• Forgetting to work out the “rest” of a formulation before finding its percentage.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is a pure substance in chemistry? (Choose from: Distilled water, Air, Sea water, “Pure” apple juice.)

Answer: Distilled water

Working: Distilled water contains only H₂O molecules. Air, sea water and apple juice are all mixtures.

Question 2. True or false? Impurities usually raise the melting point of a solid.

Answer: False

Working: Impurities usually lower the melting point and make the solid melt over a range of temperatures.

Question 3. Pure stearic acid melts at 69 °C. A student’s sample melts between 61 °C and 66 °C. What can she conclude? (Choose from: The sample is impure, The sample is pure, The thermometer must be broken, The sample is a different pure compound.)

Answer: The sample is impure

Working: The sample melts below 69 °C and over a range of 5 °C, so it contains impurities.

Related lessons

• Previous lesson: 11.4 DNA
• Next lesson: 12.2 Analysing Chromatograms
• In this chapter: 12.3 Testing for Gases
• In this chapter: 12.4 Tests for Positive Ions
• In this chapter: 12.5 Tests for Negative Ions

Continue in the full course

This lesson is section 12.1 (AGCCH1201) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.