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Pure substance

I can describe what a pure substance is, and identify a pure substance from melting and boiling point data.

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Year 10
AQA

Pure substance

I can describe what a pure substance is, and identify a pure substance from melting and boiling point data.

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Lesson details

Key learning points

A 'pure substance' in chemistry is a single element or compound not mixed with any other substance.
The definition of 'pure' means something different in its everyday usage.
Pure elements and compounds have specific melting and boiling points.
Melting point data can be used to recognise substances with specific melting points as pure substances.

Keywords

Pure substance - A pure substance in chemistry is a single element or compound that is not mixed with any other substance.
Property - A property is a feature or characteristic of a substance that can be used to classify it, or describe how it behaves.

Common misconception

Pupils can struggle to decide on the scales needed for an axis on a graph.

Suggest a suitable scale that will fit on the graph paper they will use or use a computer to draw graphs.

To save time, pupils work in pairs and each collect data for one of the substances at the same time or combine results with another group. Test substances prior to lesson to make sure there is enough time to gather results. Sample data is in additional materials.

Teacher tip

Equipment

Paraffin wax, salol or other pure substances that give good cooling curve results, boiling tube, thermometer, water bath to melt substance, stop clock.

Content guidance

Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2026), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Prior knowledge starter quiz

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6 Questions

Q1.
A material is composed of only one type of chemical.

Correct Answer: pure

Q2.
What is the name given to a substance found in a material in very small amounts, but which can alter the material’s properties?

Correct Answer: impurity, an impurity, impurities

Q3.
Which diagram correctly represents a mixture of two metals in the solid state?

Correct Answer: An image in a quiz

Q4.
Which of these statements about mixtures are true?

Mixtures boil at a single, specific temperature.

Mixtures contain a maximum of three types of chemicals.

Correct answer: Mixtures boil over a range of temperatures.

Mixtures always include water.

Correct answer: Mixtures must contain at least two types of chemicals.

Q5.
What is the highest temperature at which all of these substances are in the solid state?

-218$$^o$$C

Correct answer: -210$$^o$$C

0$$^o$$C

2950$$^o$$C

never

Q6.
Starting from a substance in the gas state, can you put the states and the temperatures at which changes of state happen in order?

1 - substance in the gas state

2 - condensing point

3 - substance in the liquid state

4 - freezing point

5 - substance in the solid state

Assessment exit quiz

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6 Questions

Q1.
A pure substance in chemistry is a single element or compound that is not mixed with any other substance. Which of the following are pure substances in chemistry?

pure beeswax

Correct answer: pure gold bars

pure orange juice

Correct answer: pure water in the solid state

Q2.
The image shows the label of a pack of orange juice. What information on the label tells us it is not chemically pure?

Correct answer: it contains at least two types of carbohydrate

Correct answer: it contains 200 mg of potassium per 100 ml

Correct answer: it contains protein and vitamin C

it has one ingredient: orange juice

it is NOT FROM CONCENTRATE

Q3.
Honey has a melting range of 35 to 40$$^o$$C and it decomposes before it boils. Using this information and what you know about pure substances, what makes honey an impure substance?

it decomposes before it gets to boiling

it is melting at 36$$^o$$C

Correct answer: it has a melting range from 35 to 40$$^o$$C

it has another substance added to it

Correct answer: it is made up of more than one type of compound

Q4.
How is the melting point of a chemical substance represented on a graph?

Correct answer: as a horizontal line on a cooling curve

Correct answer: as a horizontal line on a heating curve

as a sloped line on a cooling curve

as a sloped line on a heating curve

Q5.
A melting and freezing experiment is done with stearic acid. Put the stages of the experiment in order:

1 - Set up a beaker with water, heating up above a Bunsen burner to a gentle boil.

2 - Clamp a boiling tube, with solid substance and thermometer below water level.

3 - Record temperature at suitable time intervals until it reaches 70$$^o$$C.

4 - Note the point at which the solid starts to melt.

5 - Lift the clamp with the tube from the hot water.

6 - Record the temperature at suitable time intervals until it reaches 50$$^o$$C.

7 - Note the point at which the liquid starts to solidify.

Q6.
Honey can crystallise at room temperature, and starts to decompose (caramelise and smoke) at around 60$$^o$$C. How could you melt crystallised honey so this doesn't happen? Put the steps in order.

1 - Heat water in a pan until 60$$^o$$C and turn off the heat.

2 - Put honey in a glass bowl.

3 - Place the bowl in the hot water.

4 - Wait until all the honey has just melted and remove from the pan.

5 - Enjoy! You can repeat this if it crystallises again.

Q1 image 3 Pure Florida Orange Juice, Steven Miller, 2014, licensed under CC-BY 2.0

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Pure substance

Pure substance

Lesson slides

Lesson details

Key learning points

Keywords

Common misconception

Equipment

Content guidance

Supervision

Licence

Lesson video

Worksheet

Prior knowledge starter quiz

6 Questions

Q1.A material is composed of only one type of chemical.

Q2.What is the name given to a substance found in a material in very small amounts, but which can alter the material’s properties?

Q3.Which diagram correctly represents a mixture of two metals in the solid state?

Q4.Which of these statements about mixtures are true?

Q5.What is the highest temperature at which all of these substances are in the solid state?

Q6.Starting from a substance in the gas state, can you put the states and the temperatures at which changes of state happen in order?

Assessment exit quiz

6 Questions

Q1.A pure substance in chemistry is a single element or compound that is not mixed with any other substance. Which of the following are pure substances in chemistry?

Q2.The image shows the label of a pack of orange juice. What information on the label tells us it is not chemically pure?

Q3.Honey has a melting range of 35 to 40$$^o$$C and it decomposes before it boils. Using this information and what you know about pure substances, what makes honey an impure substance?

Q4.How is the melting point of a chemical substance represented on a graph?

Q5.A melting and freezing experiment is done with stearic acid. Put the stages of the experiment in order:

Q6.Honey can crystallise at room temperature, and starts to decompose (caramelise and smoke) at around 60$$^o$$C. How could you melt crystallised honey so this doesn't happen? Put the steps in order.

Additional material

Lesson appears in

UnitChemistry / States of matter

AQAChemistry

EdexcelChemistry

OCRChemistry

UnitCombined science / States of matter

AQACombined science

EdexcelCombined science

OCRCombined science

Q1.
A material is composed of only one type of chemical.

Q2.
What is the name given to a substance found in a material in very small amounts, but which can alter the material’s properties?

Q3.
Which diagram correctly represents a mixture of two metals in the solid state?

Q4.
Which of these statements about mixtures are true?

Q5.
What is the highest temperature at which all of these substances are in the solid state?

Q6.
Starting from a substance in the gas state, can you put the states and the temperatures at which changes of state happen in order?

Q1.
A pure substance in chemistry is a single element or compound that is not mixed with any other substance. Which of the following are pure substances in chemistry?

Q2.
The image shows the label of a pack of orange juice. What information on the label tells us it is not chemically pure?

Q3.
Honey has a melting range of 35 to 40$$^o$$C and it decomposes before it boils. Using this information and what you know about pure substances, what makes honey an impure substance?

Q4.
How is the melting point of a chemical substance represented on a graph?

Q5.
A melting and freezing experiment is done with stearic acid. Put the stages of the experiment in order:

Q6.
Honey can crystallise at room temperature, and starts to decompose (caramelise and smoke) at around 60$$^o$$C. How could you melt crystallised honey so this doesn't happen? Put the steps in order.