Liquid Soap, Water Content, and Fill

Name: Saen
Status: educator
Grade: 6-8
Location: New Zealand

Question: Why is the last of liquid soaps in their bottle often more
watery than when the bottle is full?
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I have not witnessed this effect, but I can share a thought. Some things
naturally attract water from the air. These materials are called
'hygroscopic'. Some soaps contain a compound known as glycerol, a compound
that is very hygroscopic. There is a chemical attraction between the soap
and the water in the air. If the air is very moist, the absorption of water
happens even faster. Even if your soap does not contain glycerol, other
components of soaps may also be hygroscopic.

Have you considered performing an experiment to test your hypothesis? Take a
carefully measured volume of liquid soap fresh from its unopened container
and measure its mass. Then compare it with an equal amount of soap taken
from a nearly-empty bottle. Is the mass the same? Soap is typically more
dense than water (water's density is 1g/ml, soap's density is greater than 1
g/ml), but if you dilute the soap with water, the density will get closer to
one (since it contains more water). Be careful to use an analytical balance
(should be available in a chemistry lab).

Hope this helps,
Burr Zimmerman
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Funny, I usually find the last dregs to be a little dry and viscous.
So I eventually add some water, to help me get the last part out.
Maybe someone in your place is doing that,
without thinking it is something people would discuss.
Besides, when the bottle is low perhaps you want to make it last as longer
while you try to get a replacement.
Often my use of soap seems a little more accurate and frugal if it is
watered down some optimal degree (and the bottle-spout is not too wide).

Vague possibility the soap picks up water from air over months,
if your environment is over 90% humidity most of the time,
or something keeps the soap bottle cooler than its surroundings.
If a dispenser installed in a sink leaked water-spillage inwards,
that would do it too.   Never seen either happen.

To me it sounds like a phenomenon of the human element.

Jim Swenson
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