Water will be absorbed by root hairs when:

The movement of molecules always takes place from a region of higher concentration to a region of lower concentration. The soil has a higher concentration of water as compared to the water in the root hair, this allows the water from the soil to move to the root hair by facilitated diffusion. This process of diffusion and absorption is possible only when the cell sap (the watery fluid in the plant vacuole) has a lower concentration of water and a higher concentration of solute as compared to the root hair so that the root hair absorbs the water.