Usually solvent extraction: by sequentially washing with solvents of different polarities and volatility, you can remove solvent from the product more efficiently. Depending on the method of synthesis, different washing schemes are warranted (e.g. water is great for residual salts; solvents for copolymers are used to remove residual template agents, etc.) If you see surface scientists or high vacuum technicians washing down hardware (e.g. sequential water, ethanol, methanol, acetone, bake-out...) it is a similar methodology as washing particulates to leave as close to a 'clean' surface as possible. Hope this helps, and best of luck with experiments!

Thank you Curtis Guild ! The procedure I followed is to dissolve stoichiometic amounts of FeCl3.6H2O and FeCl2.4H2O in water, followed by adding NaOH solution. After the reaction and the solid settling down, the top clear solution was removed, and then water and ethanol were used for washing.

The reaction happened here is:

2FeCl3.6H2O + FeCl2.4H2O + 8NaOH -> Fe3O4 + 14 H2O + 8NaCl.

So I can understand that water is used to remove NaCl, the excess FeCl3, FeCl2 or NaOH. But why using ethanol? I didn't see any residue that can be removed by ethanol...

Likely just a solvent exchange since ethanol has a higher vapor pressure than water. Nano-particles (NPs) can aggregate and hold residual solvent, so washing with more volatile solvents can improve drying characteristics.

If you're interested, take a look at the FTIR of dried NPs washed only with water versus dreid NPs with water/ethanol sequence. Washing and drying studies like these are quite common with oxides on tool coatings, additives and catalysts. A lot of good work there. Best of luck on experiments!

Added general reference: vapor pressures of common solvents: https://www.engineeringtoolbox.com/vapor-pressure-d_312.html