Anhydrous (Greek an- without + -hydor water) means “containing no water”. Ether that contains no dissolved water is called anhydrous ether. When we remove the water of crystallization from a hydrate such as MgSO₄•7H₂O, we call the MgSO₄ anhydrous magnesium sulfate.
Hydrous (Greek hydor water) means “containing water”. It usually refers to containing water of crystallization. Thus, CuSO₄•5H₂O is sometimes called hydrous copper sulfate.
Undissociated (Old English un- not) means not dissociated into simpler ions or molecules. In aqueous solution, NaCl dissociates into Na⁺ and Cl⁻ ions. Glucose does not dissociate. It remains as undissociated glucose molecules.
