Question: why do things only crystalize when they are pue

Roy Adkin answered on 17 Mar 2014:

Hi Jude…great question…thank you.
Lots of chemicals have crystals…copper sulfate, sugar, table salt (sodium chloride is table salt’s chemical name) etc. The crystals form because the particles (molecules, atoms or ions) can line up and arrange themselves in very ordered and constantly repeating patterns. That’s why crystals have regular shapes with flat surfaces and crisp lines…they look like, well, crystals.
If a substance is not pure then the impurities get in the way of the regular arrangements of the particles and cause the arrangement to be irregular and often stop the crystal from forming at all.
Although things often crystallise better when they are pure, some pure substances don’t crystallise at all, they are ‘amorphous’ for example the gemstone opal. That is pure silica (silicon dioxide, SiO2). Also some substances will crystallise when they are impure…it’s just the crystal will be deformed in some way from the crystal of the pure substance e.g. it will be elongated or flatter.
So really things don’t ONLY crystallise when they are pure but you can place a safe bet that if it does crystallise easily then it has very little impurity.