Lettuce or spinach placed in water becomes firm and crisp due to the osmotic movement of water into their cells.
What is osmosis?
Osmosis is the movement of water molecules in living cells through a permeable membrane from the region of high to the region of low water potentials.
Lettuce or spinach have cells whose cell sap has lower water potential when compared to that of ordinary water. Thus, if either of them is placed in water, water will move from their outside environment into the cells.
Thus, the cells of the lettuce or spinach will become turgid and give the lettuce/spinach an overall firm and crisp look.
More on osmosis can be found here: https://brainly.com/question/1799974
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