A student places a group of cells in an unknown solution whose solute is unable to pass through the cell membrane. After an hour, the student notices the cells have lost water. What does this tell the student about the unknown solution?

Water moves across a membrane toward the side with the higher concentration of solute that cannot cross it. Since the cells lost water, water moved from inside the cells into the unknown solution. The solution therefore had a higher concentration of nonpenetrating solute than the cells, so it was hypertonic.

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