Chemical purity: a pure substance is a single element or a single compound
Definition
Pure substance
A single element or a single compound with nothing else mixed into it.
Definition
Element
A substance made of atoms that all have the same number of protons.
Definition
Compound
A substance containing atoms of two or more different elements chemically bonded together.
- In chemistry, a pure substance contains only one element or only one compound, with nothing else mixed in.
- A pure element is made only of atoms that all have the same number of protons, such as copper or oxygen.
- A pure compound contains only that one compound, even though the compound holds two or more elements chemically bonded together.
- Water is a pure compound: every particle is H2O\text{H}_2\text{O}H2O, so the sample is still one substance.
- A substance becomes impure as soon as any other substance is mixed into it.
Key Idea
- Purity describes what is in the sample, not whether it looks clean or is safe to drink.
- A pure compound can hold more than one element because those elements are joined as a single compound.
Melting and boiling data: pure substances change state at a sharp, fixed temperature
Definition
Melting point
The melting point is the temperature at which a substance changes from a solid to a liquid.
- A pure element or compound has a specific melting point and a specific boiling point.
- A pure solid melts sharply at one temperature rather than softening over a range.
- A pure liquid boils at one fixed temperature rather than across a spread of temperatures.
- A mixture usually melts or boils over a range because its different substances change state at different temperatures.
- An impurity in a sample also shifts the measured temperature away from the accepted value, and often lowers a melting point.
Example
- Pure ice melts sharply at 0 ∘C0\,^\circ\text{C}0∘C and pure water boils sharply at 100 ∘C100\,^\circ\text{C}100∘C.
- A sample that melts from 76 ∘C76\,^\circ\text{C}76∘C to 79 ∘C79\,^\circ\text{C}79∘C changes state over a range, so it is impure.
Everyday purity: on a label, pure often just means nothing has been added
Definition
Mixture
Two or more substances present together that are not chemically joined, so each keeps its own properties and can be separated by physical means.
- In everyday language, pure often means a product is in its natural state with nothing added to it.
- A carton labelled pure orange juice only tells you no extra ingredients were added.
- That juice still contains water, sugars, acids and flavour compounds, so chemically it is a mixture.
- Unless a question is clearly about everyday language, use the chemical meaning of pure.
Common Mistake
- Do not judge a sample to be chemically pure just because its packaging uses the word pure.
- Check whether it is a single element or compound, or whether pure only means nothing has been added.
Using data to decide: compare the measured temperature with the accepted value
- Find the accepted melting point or boiling point of the substance named in the question.
- Compare the measured temperature with that accepted value.
- A sharp change at the accepted temperature supports the conclusion that the sample is pure.
- A range of temperatures, or a value away from the accepted one, tells you the sample is impure.
Exam technique
- Quote the numbers: say the sample melts over a range or below the accepted value, rather than only writing that it is impure.
- A high melting point does not prove purity; matching the accepted value is what counts.
Self review
- What is a pure substance in chemistry?
- Why can a compound such as water be pure even though it contains two elements?
- How does the melting behaviour of a pure solid differ from that of a mixture?
- How would you use boiling point data to decide whether a sample of water is pure?
- Why can pure orange juice be pure in everyday language but a mixture in chemistry?
