Volume/volume percentage

If both the solute and solvent are liquids, using a volume/ volume percentage is convenient. With volume/volume percentages, both the solute and solution are expressed in milliliters:

Ethyl alcohol (the drinking alcohol) solutions are commonly made using volume/volume percentages. If you want to make 100 milliliters of a 50 percent ethyl alcohol solution, you take 50 milliliters of ethyl alcohol and dilute it to 100 milliliters with water. Again, it’s a case of dissolving and diluting to the required volume.

You can’t simply add 50 milliliters of alcohol to 50 milliliters of water — you’d get less than 100 milliliters of solution. The polar water molecules attract the polar alcohol molecules. This tends to fill in the open framework of water molecules and prevents the volumes from simply being added together.