Which is a smaller molecule: monosaccharide or polysaccharide?

Well it would be monosaccharide.

Explanation: This is so because monosaccharides are simple sugar unit molecules, whereas polysaccharides are enormous, linking thousands of sugar units. Monosaccharides provide cells with short-term energy. Polysaccharides provide long-term energy storage and rigid structure to cell walls and exoskeletons of animals. Hope this helped. :)

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom

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