Contains nitrogen - usually derived from an amino acid. They give a precipitate with heavy metal iodides. Cream coloured precipitate. Caffeine, a purine derivative, does not precipitate like most alkaloids. Alkaloids are basic - they form water soluble salts.
Most alkaloids are well-defined crystalline substances which unite with acids to form salts.
In plants, they may exist in the free state, as salts or as N-oxides. Occur in a limited number of plants. Nucleic acid exists in all plants, whereas, morphine exists in only one plant species.