Diffraction pattern
The pattern obtained on a photographical plate when a beam of X-rays or electrons is passed through a crystal. It is used for the study of crystal structures.

Dilatometer
An apparatus for measuring small changes of volume of a liquid, solution or solid immersed in a liquid.

Dilute
Denoting a solution in which the amount of solute is low relative to that of the solvent. The term is always relative and includes dilution at trace level as well as the common term ‘bench dilute acid’, which usually means a 2M solution.

Dinitrogen tetraoxide (N2O4)
A colourless gas that changes to a pale yellow liquid below 21ºC and solidifies below 11ºC. It is used a nitrating agent.

Diodine hexachloride
Refers to a powder made by reacting excess chlorine with iodine. It is a strong oxidizing agent and at 70ºC it dissociates into iodine monochloride and chlorine.

Diol (dihydric alcohol, glycol)
Refers to an alcohol that has two hydroxyl groups (-OH) per molecule of compound. Dioxygen O2 fomally the oxygen molecule but used for Dioxygen complexes and dioxygenyl, (O2)+, salts.