When 100 mL of water is mixed with 400 mL of ethanol, the total volume is not simply additive — because hydrogen bonding causes volume contraction.

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Reason:

Water and ethanol molecules form strong hydrogen bonds, allowing the molecules to pack more closely.
So the mixture’s final volume is less than 500 mL.

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Experimental result:

When equal amounts of water and ethanol are mixed, the volume contraction is about 4–5%.