A single plant cell is placed in an open beaker containing deionised water. Very slowly, solutes are added to the water, which is gently stirred. The cell is observed through a microscope. Eventually, the plasma membrane begins to pull away from the cell wall.

Briefly explain why this occurred.

This occurred because water moved out of the plant as the concentration of solutes increased in the beaker water. As the beaker water became hypertonic to the plant cell due to increasing solute concentration, plant cell starts to shrink, the cell wall is a rigid structure and maintain it's shape but the plasma membrane is very flexible as compared to the cell wall and moved away from the cell wall when cell started shrinking in the hypertonic beaker water. 

When a plant cell was placed in the deionized water, water moved inside the cell because beaker water was hypotonic as compared to the cell's inner environment. The inward flow of water pushed the cell membrane towards the cell wall.

But as the solutes were added gradually added to the beaker water, tonicity of this water increased and beaker water became hypertonic to the plant cell because the cell wall is not permeable to the solute molecules, as result water started moving out of the plant cell and plant cell started to shrink.

The cell wall is a rigid structure and able to maintain its shape, but the plasma membrane is a very flexible structure. That's why, when the cell started to shrink (because the beaker water became hypertonic) the plasma membrane begins to pull away from the cell wall.