Suppose I've to prepare a 100 mL of 70% ethanol by mixing 70 mL of ethanol and 30 mL of water, I'ld like to know is there any difference between 1st pouring large volume (70 mL) next small volume (30 mL) or 1st small volume (30 mL) next large volume (70 mL)?
There is no difference. There will be better mixing if you pour the large volume into the small volume, but you will still have to do more to complete mixing either way.
If you are diluting concentrated acids or bases, you should add the small volume of acid or base to a large volume of water for safety reasons.
It is good practice to always add the solute (the thing needing dilution) first then add the solvent (the solution doing the diluting....usually water or buffer). This allows for a starter mix, and you can guarantee that you have all of your solute in solution
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