AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Pure substances: Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.
Every pure element and pure compound has a sharp, fixed melting point and boiling point at a given pressure.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every pure element and pure compound has a sharp, fixed melting point and boiling point at a given pressure. For example, pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure, while pure iron melts at about 1538 °C. A mixture usually melts and boils over a range of temperatures rather than at one exact value, because its components have different melting and boiling points. This means melting point and boiling point data can be used to test purity: a sample melting sharply at the expected temperature is likely pure, whereas a sample melting over a range, or at a lower temperature than expected, is likely impure or a mixture. Impurities generally lower the melting point and raise the boiling point of a substance.
What earns the marks
- States that pure elements and compounds have specific, sharp melting and boiling points.
- Explains that mixtures melt and boil over a range of temperatures rather than at one fixed value.
- Uses given melting point or boiling point data to decide whether a sample is pure or a mixture.
- Explains that impurities lower the melting point and raise the boiling point of a substance.
- Compares a measured melting or boiling point with a known value to judge purity.
Common mistakes and how to put them right
Assuming any liquid that boils is pure.
Put it right: A mixture boils over a range of temperatures, not at one sharp value.
Thinking impurities raise the melting point.
Put it right: Impurities usually lower the melting point and widen the melting range.
Ignoring the effect of pressure.
Put it right: Melting and boiling points are quoted at a stated pressure, usually standard atmospheric pressure.
Exam tips
- Quote the actual temperature values from the data when explaining whether a sample is pure.
- Use the phrase 'sharp melting point' or 'melts over a range' to show you understand the difference between pure substances and mixtures.
- If comparing two samples, state which has the sharper or more expected value and link this to purity.
Try the lesson
The start of the Pure substances: Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures. lesson, made from checked teaching notes for this specification point.
Have a go first
A student heats a sample of a pure solid element until it melts. Which statement describes the temperature at which this pure element melts?
Listen
Every pure element and pure compound has a sharp, fixed melting point and boiling point at a given pressure. For example, pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure, while pure iron melts at about 1538 °C. A mixture usually melts and boils over a range of temperatures rather than at one exact value, because its components have different melting and boiling points. This means melting point and boiling point data can be used to test purity: a sample melting sharply at the expected temperature is likely pure, whereas a sample melting over a range, or at a lower temperature than expected, is likely impure or a mixture. Impurities generally lower the melting point and raise the boiling point of a substance. A strong answer earns marks for these points. States that pure elements and compounds have specific, sharp melting and boiling points. Explains that mixtures melt and boil over a range of temperatures rather than at one fixed value. Uses given melting point or boiling point data to decide whether a sample is pure or a mixture. Explains that impurities lower the melting point and raise the boiling point of a substance. A common slip: Assuming any liquid that boils is pure. Instead, a mixture boils over a range of temperatures, not at one sharp value. Exam tip: Quote the actual temperature values from the data when explaining whether a sample is pure.
See it
Key words
Say what each word means before you turn the card.
Fixed melting and boiling points of pure substances
Try answering aloud or in your head before turning the card.
Melting and boiling ranges of mixtures
Try answering aloud or in your head before turning the card.
Using data to test purity
Try answering aloud or in your head before turning the card.
Effect of impurities on melting and boiling points
Try answering aloud or in your head before turning the card.
See every definition
- Fixed melting and boiling points of pure substances
- Pure elements and compounds melt and boil at specific temperatures, giving a sharp, fixed melting point and boiling point at a given pressure.
- Melting and boiling ranges of mixtures
- A mixture usually melts and boils over a range of temperatures rather than at one exact value, because its components have different melting and boiling points.
- Using data to test purity
- Melting point and boiling point data can be used to distinguish pure substances from mixtures, since a sample melting sharply at the expected temperature is likely pure.
- Effect of impurities on melting and boiling points
- Impurities generally lower the melting point and raise the boiling point of a substance.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Pure substances: Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge