As salt molecules do not pass over the cell membrane, the hypertonic solution of salt and water will always have higher concentration of salt and a lower concentration of water than will the inside of the onion cell.

The vacuole in onion cell stores water and the takes up about 95% of the space inside of the onion cell. Therefore, that means about 95% of the cell is filled with water. Therefore, due to the difference in concentrations of water (a higher concentration of water inside the cell's vacuole than is the concentration of water outside the cell), water will diffuse from inside the vacuole to the area outside of the cell and the vacuole will collapse.

This diffusion of water is called osmosis.