A particular colorless organic liquid is known to be one of the following compounds: butan-1-ol diethyl ether, methyl propyl ether, butyraldehyde, or propionic acid. Can you identify which it is, based on the following tests? If not, what additional test would you perform? 

(1) A 2.50 g sample dissolved in 100.0 g water has a freezing point of -0.7 °C. 

(2) An aqueous solution of the liquid does not change the color of blue litmus paper. 

(3) When alkaline KMnO₄(aq) is added to the liquid and the mixture is heated, the purple color of the MnO₄^– disappears.