chemical reaction query

Can anyone suggest a liquid-phase reaction where the products have
a significantly different density to the starting reageants (say by
a few percent or more)? Ideally, it would involve readily available
chemicals in aqueous solution.

That's not too difficult a task.

However, to make it harder, I need the time constant governing the
rate of reaction at ambient temperature to be of the order of 1 second.

I'll offer a virtual prize to anyone who comes up with better
suggestions than mine...
.

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