AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Aldehydes and ketones (A-level only): The hazards of using KCN.
Potassium cyanide is acutely toxic by swallowing, by skin contact and through the gas it releases.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Potassium cyanide is acutely toxic by swallowing, by skin contact and through the gas it releases. The serious hazard in this preparation is that cyanide salts liberate hydrogen cyanide, a volatile and extremely toxic gas, on contact with acid, including the dilute acid added in the second stage of the reaction and the acid in the stomach. Control the risk rather than just listing equipment: work in a fume cupboard so any hydrogen cyanide is removed, wear gloves because the solid and its solution are toxic on skin contact, use the smallest quantity that will do, add the acid only when the addition step is complete, and dispose of residues in the designated container rather than down the sink.
What earns the marks
- A named precaution, such as working in a fume cupboard.
- The matching reason, such as the release of toxic hydrogen cyanide gas.
- Stating that cyanide releases hydrogen cyanide when it meets acid.
- A second precaution with its reason, such as gloves because the solid and solution are toxic by skin contact.
Common mistakes and how to put them right
Giving a precaution with no reason, or a reason that does not match the precaution given.
Describing potassium cyanide as flammable or corrosive rather than toxic.
Treating only the solid as hazardous, when the solution and the gas released are the greater danger.
Offering "wear goggles" as the whole answer, when eye protection is assumed in every practical.
Exam tips
- Always pair a precaution with the hazard it controls; AQA marks them as a pair.
- Name the toxic gas as hydrogen cyanide and say what releases it.
- Avoid generic safety points that would apply to any experiment.
Try the lesson
The start of the Aldehydes and ketones (A-level only): The hazards of using KCN. lesson, made from checked teaching notes for this specification point.
Have a go first
Which hazard is most associated with using potassium cyanide in the school laboratory?
Listen
Potassium cyanide is acutely toxic by swallowing, by skin contact and through the gas it releases. The serious hazard in this preparation is that cyanide salts liberate hydrogen cyanide, a volatile and extremely toxic gas, on contact with acid, including the dilute acid added in the second stage of the reaction and the acid in the stomach. Control the risk rather than just listing equipment: work in a fume cupboard so any hydrogen cyanide is removed, wear gloves because the solid and its solution are toxic on skin contact, use the smallest quantity that will do, add the acid only when the addition step is complete, and dispose of residues in the designated container rather than down the sink. A strong answer earns marks for these points. a named precaution, such as working in a fume cupboard. the matching reason, such as the release of toxic hydrogen cyanide gas. stating that cyanide releases hydrogen cyanide when it meets acid. a second precaution with its reason, such as gloves because the solid and solution are toxic by skin contact. Exam tip: Always pair a precaution with the hazard it controls; AQA marks them as a pair.
Key words
Say what each word means before you turn the card.
toxicity of cyanide
Try answering aloud or in your head before turning the card.
hydrogen cyanide released by acid
Try answering aloud or in your head before turning the card.
fume cupboard
Try answering aloud or in your head before turning the card.
precaution paired with reason
Try answering aloud or in your head before turning the card.
See every definition
- toxicity of cyanide
- Potassium cyanide is acutely toxic by swallowing, by skin contact and through the gas it releases.
- hydrogen cyanide released by acid
- Cyanide salts liberate hydrogen cyanide, a volatile and extremely toxic gas, on contact with acid, including the dilute acid added in the second stage of the reaction and the acid in the stomach.
- fume cupboard
- Working in one removes any hydrogen cyanide, and it is a named precaution paired with the release of toxic hydrogen cyanide gas as its reason.
- precaution paired with reason
- A precaution must be given with the hazard it controls, such as gloves because the solid and its solution are toxic on skin contact.
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 Aldehydes and ketones (A-level only): The hazards of using KCN.
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