States of Matter: Meanings of Purity

A pure substance contains only one type of substance. For example, 100% water or 100% copper are pure substances. They are not mixtures.
A pure substance has a specific melting and boiling point that does not change. The mean kinetic energy is directly proportional to the temperature, so the temperature remains constant during a state change.
Purity can be verified by finding the substance’s boiling or melting point and comparing it to the known values for that pure substance.
If the substance’s boiling or melting point differs, it means it’s not pure.
Mixtures, which contain more than one substance, may not have a specific boiling or melting point, and instead display a range of temperatures as different components react.
Chromatography can be used for both qualitative and quantitative methods to determine and confirm the purity of a substance.
The RF value (retention or retardation factor) in chromatography can help with the identification of substances and thus gauge the level of purity. RF values are calculated using the formula: RF = (distance travelled by solute)/(distance travelled by solvent).
Formation of crystals can also indicate the purity of a substance. Larger crystals imply fewer impurities as impurities often interrupt crystal formation.
Impurities in a substance may cause it to have different properties from the pure substance. This includes a lower melting point and a higher boiling point, known as the “melting and boiling point depression” or “elevation”.
Impurities can impact a variety of factors including colour, taste, melting/boiling point, reactivity, and more, based on their nature and host substance’s properties.