Bubble Chemistry
Adapted from Jeremy Conn
Question: How do glycerin and oil affect the size of a bubble? How is a bubble similar to the
functions of a cell membrane?
Background Information: A bubble is gas or air trapped inside a liquid sphere. When the
soap molecule goes to the surface of water it is held by the water molecules. When the soap
reaches the surface of the water the hydrogen and carbon atoms in the soap react
differently. One starts attracting water and one starts to repel it. The surface tension in the
water decreases when the water molecules start separating because of the soap molecules.
With lower surface tension, bubbles form.
Soap molecules are made of long chains of carbon and hydrogen atoms. At one end of the
chain are atoms which like to be in water (hydrophilic—water loving). The other end wants
to stay away from water (hydrophobic—afraid of water) but likes grease. In washing, the
"greasy" end of the soap molecule attaches itself to the grease on your dirty plate, letting
water seep in underneath. The particle of grease is pried loose by the water and
surrounded by soap molecules, to be carried off by a flood of water.
In a soap-and-water solution the hydrophobic (greasy) ends of the soap molecule do not
want to be in the liquid at all. Those that find their way to the surface squeeze their way
between the surface water molecules, pushing their hydrophobic ends out of the water.
This separates the water molecules from each other and spreads them apart. This makes
the surface tension less because the water molecules are farther apart and can’t pull as
hard on each other. The water fearing soap molecules decrease the surface tension. If that
over-filled cup of water mentioned earlier were lightly touched with a slightly soapy finger,
the pile of water would immediately spill over the edge of the cup; the surface tension
"skin" is no longer able to support the weight of the water because the soap molecules
separated the water molecules, decreasing the pull between them, or in other words, their
ability to hold together.
Because the greasy end of the soap molecule sticks out
from the surface of the bubble, the soap film is somewhat
protected from evaporation (grease doesn't evaporate)
which helps the bubble not pop as quickly.
Now, let’s talk about how your additives affected bubble
strength. Dish soap makes bubbles, but they quickly dehydrate and pop. The secret
ingredient that you can add for stronger bubbles is glycerin. When bubbles pop, they often
pop because the water in the bubble evaporates into the environment. Glycerin helps
bubbles stay strong because it’s naturally good at preventing dehydration. How come?
It’s hydroscopic, which means it adds water from the air to the bubble! It also forms a
weak bond with hydrogen, which further delays water evaporation. Corn syrup works by
making the bubble solution more viscous, creating a thicker and tougher bubble. Making
bubbles cold also helps them last longer because evaporation slows down in colder
temperatures. What do you think would happen to your bubbles if you put them in conditions
that are colder than your freezer?
To get big bubbles the solution should contain water, soap and glycerin. The soap makes
the actual bubble form. The glycerin helps because it lets the bubble last longer without
evaporating. If you just had the soap and water the bubble would be too thick and you
would not be able to blow a big bubble. The water helps to make the solution less thick so
that you can blow the bubbles bigger. Oil, on the other hand, won't mix well with the water,
and many tiny oil droplets are formed. The oil droplets at the surface act as bubble
breakers. When a bubble of foam starts to form, it encounters an oil droplet. The part of the
bubble that encountered the oil droplet has a much different surface tension than the rest
of the bubble, and the stress pops the bubble before it gets very large.
Water exhibits a force that holds its molecules together. If you place something light onto
the surface of the water, it may be able to float on top because of this force, known
as surface tension. Compared to some other liquids, water has a particularly high surface
tension. Maybe now you understand how water is capable of forming bubbles—but why
are soap bubbles so much stronger?
First of all, soap molecules are capable of doing something somewhat similar to what water
does—they line up side-by-side to make a layer. In addition, soap is a great cleaning agent
because each of its molecules has two sides: one side likes to stick to water, and the other
side likes to stick to dirt and oil. So why does this matter if we’re talking about bubbles?
The side of the soap molecule that likes to stick to dirt and oil also likes to stick to air!
When you make a soap bubble using a mixture of soap and water, your bubble actually has
three layers: two layers of soap with a layer of water sandwiched in between. The outer
soap layer of the bubble has its air-liking ends sticking outward and its water-liking ends
sticking inward towards the layer of water. Likewise, the innermost layer of the soap
bubble has its water-liking ends sticking out, and its air-liking ends facing the air on the
inside of the bubble.
A Bubble is a lot like a Cell Membrane
A major component of cell membranes is phospholipids. Phospholipids have a love-hate
relationship with water. One end, the “head,” is attracted to water and the other end, the
“tail,” is repelled by water. Place phospholipids in water, and they quickly form a double
layer with the heads facing out. A soap molecule has the same split personality. The “head”
of a soap molecule is charged (ionic) and attracts to water, which exhibits positive and
negative regions of charge (polar). The hydrocarbon tail of the soap molecule is not
charged and is repelled by water’s polar personality. This explains why we use soap to
clean. The hydrocarbon tail of soap mixes with and dissolves in other hydrocarbons, like
oils and fats, while the head region grabs a hold of passing water molecules and follows
them down the drain. The surface of a bubble has three layers. The middle layer is a thin
film of water. On both sides of this film is a layer of soap molecules with hydrophilic heads
facing water and hydrophobic tails pointing out. The diagram below provides a nice
illustration of this comparison.
Diagram comparing the structure of a bubble with that of a cell membrane.
Conclusion and Analysis:
For this experiment, we added glycerin and oil to regular bubble solution to see if either
one would increase the size of the bubble. While doing the experiment, I made a few
observations. With the first solution of just bubble solution, they would pop the second it
touched your hands and the bubbles were small. With the second solution of bubbles with
glycerin the bubbles were bigger and a lot stronger. Some did not last very long but we did
have a bubble which lasted a long time and its diameter was 38.84 cm. That was the biggest
bubble I made. With the last solution of detergent and oil, the bubbles were strong but
small.
Now, write your own conclusion: 5- 8 sentences
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References:
http://pubs.acs.org/cen/whatstuff/stuff/8117sci3.html
http://www.ehow.com/how-does_5421403_glycerin-make-bubbles-last-longer.html
http://www.lecciagroup.com/bubbles/
Conn, Jeremy. "Cell Membrane Bubble Lab - Clear Biology." Clear Biology. WordPress, 27
Oct. 2012. Web. 01 Sept. 2014.