Pure SubstancesOxford AQA IGCSE Chemistry: Flashcards

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Define a pure substance in chemistry.

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Define a pure substance in chemistry.: A single element or compound that contains no other substance mixed in with it.
How does the everyday meaning of 'pure' differ from the chemistry meaning?: Everyday 'pure' often means natural/unprocessed; chemistry 'pure' means made of only one element or compound with nothing else mixed in.
At what kind of temperature does a pure substance melt?: A single, sharp, fixed temperature.
How does a mixture's melting behaviour differ from a pure substance's?: A mixture melts over a range of temperatures, rather than sharply at one fixed temperature.
How does an impurity typically affect melting point?: It lowers the melting point and widens the melting range compared to the pure substance.
Name two types of data used to assess the purity of a substance.: Melting point data and boiling point data.
Why is purity important in foodstuffs?: To check the product has not been contaminated or adulterated with cheaper substitute substances.
Why is purity important in drugs/medicines?: An impure drug may not work as intended or may contain harmful contaminants, so purity is critical for safety and effectiveness.
A solid melts sharply at exactly its known reference melting point. What does this suggest?: The sample is likely pure.
A solid starts melting below its known reference value and melts gradually over a range. What does this suggest?: The sample is likely impure.