A pure substance contains one element or compound
Pure substance
A single element or a single compound, with nothing else mixed in.
Mixture
A combination of two or more substances that are not chemically bonded together.
- Every particle in a pure element, such as a copper wire, is an atom of that one element.
- Every particle in a pure compound, such as distilled water, is the same H2O\text{H}_2\text{O}H2O molecule.
- A pure compound still contains more than one element, because its atoms are bonded together in a fixed ratio.
- Any sample with two or more substances mixed together is not pure in the chemical sense.
- Sea water is not pure, because it contains water with dissolved salts such as sodium chloride.
Do not call a compound impure because it contains more than one element, since its elements are bonded into a single substance.
Everyday 'pure' is looser than the scientific meaning
Purity
The extent to which a sample contains only one substance.
- In everyday language, pure usually means natural or with nothing added.
- A carton labelled pure orange juice has nothing added, yet the juice is a mixture of water, sugars, acids and many other substances.
- Bottled spring water is sold as pure, but it contains dissolved calcium and magnesium compounds.
- In chemistry, a sample is pure only if it contains a single substance.
- Medicines and food additives are made to high purity, because other substances could change how they work or cause harm.
A chemically pure sample contains one substance only, however natural or unprocessed it is.
A pure substance melts at a sharp, fixed temperature
Melting point
The temperature at which a solid changes to a liquid at a stated pressure.
- Every pure substance has its own fixed melting point at a given pressure.
- Pure ice melts at 0 ∘C0\ ^{\circ}\text{C}0 ∘C at normal atmospheric pressure.
- A pure substance melts sharply, changing from solid to liquid over a very narrow temperature range.
- All its particles are the same, so the attractions holding the solid together all need the same energy to overcome.
- Comparing a measured melting point with a data book value can identify a substance.
Pure benzoic acid melts at 122 ∘C122\ ^{\circ}\text{C}122 ∘C, so a white solid that melts sharply at 122 ∘C122\ ^{\circ}\text{C}122 ∘C is very likely to be pure benzoic acid.
Impurities lower the melting point and widen its range
Impure substance
A sample that contains more than one substance.
- Impurities usually lower the melting point of a substance.
- An impure solid also melts over a range of temperatures rather than at one sharp value.
- The impurity particles disrupt the regular arrangement of the main substance, so some parts of the solid break down at lower temperatures than others.
- The more impurity a sample contains, the lower and wider its melting range usually becomes.
- Salt is spread on icy roads for the same reason, because salty water freezes below 0 ∘C0\ ^{\circ}\text{C}0 ∘C.
- A sample of aspirin melts between 128 ∘C128\ ^{\circ}\text{C}128 ∘C and 133 ∘C133\ ^{\circ}\text{C}133 ∘C, while pure aspirin melts at 136 ∘C136\ ^{\circ}\text{C}136 ∘C.
- The sample melts below the pure value and over a range, so it is impure.
- What does 'pure' mean in chemistry?
- Why is a carton of pure orange juice not pure in the chemical sense?
- How does the melting of a pure substance differ from the melting of an impure one?
- A sample melts between 76 ∘C76\ ^{\circ}\text{C}76 ∘C and 80 ∘C80\ ^{\circ}\text{C}80 ∘C, and the pure substance melts at 82 ∘C82\ ^{\circ}\text{C}82 ∘C; is the sample pure?
- Why does an impurity make a solid melt over a range of temperatures?
