What do water passes through a membrane by?

It's called osmosis. The water will travel across the membrane to equal out concentrations so they are both the same. This is because water is a small molecule, able to move through the membrane.

Think of it this way. If there were no membrane then the dissolved substance would diffuse throughout the mixture. If you put a membrane there then the same idea works but this time it is only the water that can move. So the water moves through the membrane until the same concentration of water molecules appear on each side.