C10.1 Pure Substances and Mixtures

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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. For a pure substance it is a fixed, specific value.
• Boiling point: The temperature at which a liquid boils. For a pure substance it is a fixed, specific value.
• 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 bottle of “pure” spring water has nothing added, but it still contains dissolved calcium, magnesium and other ions, so it is a mixture.

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

• nitrogen gas from a cylinder; Pure in chemistry? yes; Reason one element, N₂ molecules only
• a diamond; Pure in chemistry? yes; Reason one element, carbon atoms only
• distilled water; Pure in chemistry? yes; Reason one compound, H₂O only
• spring water; Pure in chemistry? no; Reason water with dissolved ions
• “pure” milk; Pure in chemistry? no; Reason natural, but a mixture of water, fats, proteins and sugars

Using melting and boiling points

A pure substance melts and boils at specific, fixed temperatures. While it is melting or boiling, its temperature stays the same. We can compare a measured value with the value in a data book to check purity. For example, pure water melts at 0 °C and boils at 100 °C.

• A mixture melts and boils over a range of temperatures.
• An impure solid usually melts at a lower temperature than the pure substance.
• An impure liquid usually boils at a higher temperature than the pure substance.

Worked example: is it pure?

A student’s sample of stearic acid melts between 61 °C and 66 °C. Pure stearic acid melts at 69 °C.
The sample melts below 69 °C and over a range of 5 °C, not at a single temperature.

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

Useful mixtures. Salt is spread on icy roads because salty water freezes below 0 °C. Antifreeze mixed with the water in a car engine lowers its freezing point even more. Petrol is a mixture of hydrocarbons, so it boils over a range of temperatures.

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 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), filler (bulk), binder (holds the tablet together), coating
• fertiliser; Examples of components and their jobs compounds of nitrogen, phosphorus and potassium in set proportions
• toothpaste; Examples of components and their jobs abrasive (cleans), fluoride compound (protects teeth), humectant (stops it drying out), flavour, water
• others; Examples of components and their jobs fuels, cleaning agents, alloys, foods, cosmetics

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

Exam tip: To decide whether a product is a formulation, look for two clues: it is a useful product, and its components are in measured amounts.

Calculations with formulations

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

Worked example: percentage by mass

A 120 g tube of toothpaste contains 30 g of abrasive and 24 g of humectant. The rest is water and other ingredients.
mass of the rest = 120 − 30 − 24 = 66 g
percentage of abrasive = (30 ÷ 120) × 100 = 25%

Answer: rest 66 g; abrasive 25%

Worked example: mass from a percentage

The toothpaste contains 0.32% of a fluoride compound by mass. Find the mass of fluoride compound in the 120 g tube.
mass = (0.32 ÷ 100) × 120 = 0.384 g

Answer: 0.384 g (384 mg)

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.

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, Fresh orange juice, Sea water, Air.)

Answer: Distilled water

Working: Distilled water contains only H₂O molecules. Orange juice, sea water and air are mixtures of several substances.

Question 2. The graph shows heating curves for two samples of the same solid. Which sample is pure? (Choose from: Sample A, Sample B, Both samples, Neither sample.)

Answer: Sample A

Working: Sample A has a flat section: it stays at 58 °C while it melts, so it is pure. Sample B melts over a range (about 49–55 °C), so it is impure.

Question 3. True or false? An impure solid usually melts at a lower temperature than the pure solid, and over a range of temperatures.

Answer: True

Working: True. Impurities lower the melting point and make the solid melt over a range instead of at one temperature.

Related lessons

• Previous lesson: C9.4 Cracking Hydrocarbons
• Next lesson: C10.2 Analysing Chromatograms
• In this chapter: C10.3 Testing for Gases
• In this chapter: C10 RP Required Practical: Chromatography

Continue in the full course

This lesson is section C10.1 (AGCSC2701) of our free GCSE Combined Science 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 Combined Science course.