Why Unpeeled Grapes Float in Mineral Water
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Peeled Grapes Do Not Float In Mineral Water
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Unpeeled grapes float in mineral water while peeled grapes do not. This truth is the basis of a classic grade school or middle school science experiment. A glass of mineral water, soda water, club soda, or any clear carbonated beverage is poured, and two grapes presented, one peeled and one not. The students are invited to speculate what will happen, and give reasons for their theories.
The Dancing Grape.
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When the two grapes are dropped into the soda, the peeled grape will sink to the bottom of the beaker. The unpeeled grape will quickly accumulate a coating of shining bubbles, and will rise and fall repeatedly, seeming to dance in the sparkling soda water.
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Tiny Bubbles.
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The bubbles clinging to the grape are the cause of the effect. The peeled grape's surface is hydrophilic: it bonds to water, giving the bubbles no place to latch on. The unpeeled grape has a hydrophobic surface: it repels water, leaving a dry surface for the carbon dioxide bubbles to cling. The bubbles are lighter than the water, and act like little balloons lofting the grape up through the mineral water. As it reaches the surface or as the bubbles burst and break loose, the grape will drop down in the beaker again until it accumulates a new collection of bubble-balloons. This is why the peeled grape sinks but the unpeeled grape dances and floats.
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